Synchronizing Carburetors, as on BMW Airhead Motorcycles, but useful for many others
The following information specifically applies to carbureted twin cylinder two-valve BMWs, of the type known as “Airheads”, as manufactured 12/1969->1995. Some of the information may be applicable to many other motorcycles, and even to some fuel-injected models. The method is also applicable to BMW models before the these dates, and while the shorting method works for MOST ALL, you CAN use the vacuum methods for those earlier models with, or install, vacuum ports.The author has used these methods on a wide variety of engines. The author is NOT responsible for any electrical shocks, nor other consequences, for any ineptness on your part in failing to understand and perform procedures properly and safely.
Quite a few decades ago many mechanics used a device called a Uni-Syn to balance carburetors. It was usable on almost any carburetor, and it was usable on most injected models (rare, then). Sometimes a minor modification was needed to allow the Uni-Syn to properly mate with the input area. The Uni-Syn was a specially machined or cast metal plate, rubber on one side to allow some pressure-sealing against the air intake side of the carburetor or injector, and an ADJUSTABLE venturi built into the plate, and attached to the venturi was a glass or plastic tube that had a floating ball in it. Properly used, this device would take accurate relative readings of the airflow through a carburetor or injection intake; then the device was transferred to the other carburetor (usable on engines with many carburetors too). I do not know if they are still available, I have one, and treasure it, as an antique! Treasuring some old tool does NOT mean I use it nowadays!
I do not! I have not seen one used, except in my own shop for some sports cars or 4 or 6 cylinder motorcycles, since the mid-1970’s, except to demonstrate its use. Mechanics had a lot of pride on their ability, without instruments, to listen to the engine; adjusting the idle mixture screw for a smooth idle, then setting idle rpm, and going back and forth until they ended up with the proper idle rpm and idle mixture adjustment. Some very experienced mechanics would even take bets on being able to synchronize a BMW Airhead engine without any special tools….and often made up stories about what they were doing….which, in truth was simply listening to the transmission internals rattling around at idle rpm (which they do, when the oil is hot). The throttle cables were synchronized by eyeball on lifting at the same time, and sometimes by transmission rattle. These methods are not good with multiple cylinder engines such as a V-6 or V-8 or inline 4, particularly those engines with more than two carburetors. The method is tricky to do with a BMW airhead boxer engine, and I do NOT recommend it. It is also NOT EVER as accurate as the vacuum nor shorting methods.
The primary method on the /2 bikes was to adjust the carburetor throttle stops for equal engine speed with first one plug cap, then the other, removed; you alternated between cylinders firing and not firing, back and forth. This was USUALLY safe on those magneto equipped /2 bikes because the magneto incorporated a safety gap, if the plug cap was pulled off the spark plug, the spark could jump across the safety gap. If the safety gap was missing, damage to the ignition coil on these bikes was possible. Unfortunately, sometimes the metal cap shell would give you an electric shock. This ‘lifting the cap’ method is still used today on the /2 bikes; although the shorting method is MUCH better.
On bikes of the /5 series and later, one must NOT!! remove a spark plug cap during ignition-on, although shorting to ground method is perfectly acceptable. It is a matter of both electrical shock AND possible damage to components. When the BMW Airhead’s cylinders are properly balanced by doing carburetor adjustments correctly, the low rpm idle is vastly better, the bike may start easier, the point of transitioning from idle to ~¼th throttle is much better, and the high rpm vibration is noticeably less. Irregular and jumping “unstable idle” is HARD on the timing chain and the cam sprocket.
Too low an rpm for idle is hard on the chain and chain sprockets for another reason: MINIMAL, IF ANY, OILING. The early BMW boxer bikes (before 1981) had heavy flywheels/clutches, and the engines could be made to idle VERY slowly. This was particularly so on those models with mildest camshafts and heaviest flywheels, like the R50 and R60 series. It was thought that this was good (primarily for show-off, IMO); that the engines would idle smoothly and so very slowly…but, in fact, many WAY overdid this, and the oil pressure dropped off a lot at idle, especially with many miles on the machine, which was not good, nor was the effective extra pressure on the rod bearings and the greatly reduced oiling of the timing chain, chain guide, and the sprockets. It was possible on a near perfect motor, to have a quarter (American 25 cent coin) sit on edge on the top of the gas tank, engine idling. After the /2 models, and especially after the flywheel/clutches were lightened in 1981, and with slightly more radical cam timing, it became IMPOSSIBLE to obtain an idle as smooth as the /2….or even the /5 or /6 and /7! The /5 and later models should NOT be idled under 850, as you will not get good off-idle throttle response, and, it is not good for the motor parts as mentioned. I greatly prefer idle RPM on Airhead motorcycles to be 1025.
The /5 Airheads and all later Airhead models use the oil over-pressure relief valve output to lubricate the timing chain area…and that output can be rather low with hot oil and a low idle rpm. Keep in mind that the oil not only lubricates, but carries away heat. The rod journals require a reasonable oil pressure and rpm to avoid damage, so these things are more reasons to NOT use too low an idle rpm.
After the /2 models, with each passing generation of models, it seems that the bikes have become ever more susceptible to poor off-idle response, especially with low throttle openings, from even very slight imbalances in the carburetors. Some of this is due to the leaner jettings used for the bikes (especially those shipped to the United States). I am sure that some of you have noticed that the parts lists show the carburetor jettings to be a bit different for bikes shipped to, say, England. Even so, there are usually only minor jetting differences for the idle passages (if any), and many things contribute to a less smooth idle on the later bikes.
For the /2, the output of the magneto was adequate to fire the relatively rich mixtures used by the early bikes. The /2 magneto was equipped with a safety spark gap at each high voltage output, that allowed only so much voltage to build up, say with the spark plug cap removed from the spark plug, before the gap was bridged by a spark. This avoided damage to the magneto coil. Magneto coil problems on the /2 are generally those of old-age. NOTE that magneto output increases with rpm; the opposite of later ignitions. Thus, lean-burning engines need more powerful ignitions, and magnetos are generally not a good idea, at least for production engines. Note also that many a BMW magneto-equipped bike is seen with no or damaged safety spark gap parts.
When BMW changed to the points and coil ignition in late 1969 with the introduction of the /5, the spark plug voltages rose due to the better design that was required, particularly at the low rpm area, and the spark energy output also was increased, and the system became more dangerous to be played with. In addition, there was no safety spark gap, and lifting the spark plug caps could or would damage a coil. That damage might not show up for a long time. Because of all these various things, I recommend against lifting the caps on ANY engine. In 1979-1980, BMW used an even slightly higher energy system in the points canister models,,,and, in 1981, the ignition system was beefed-up even more.
When BMW changed to the electronic ignition in 1981, which was triggered by semiconductor devices located in the canister at the end of the camshaft in the timing chest, previously occupied for two years of production by points and condenser, the new under-fuel-tank-located ‘black box’ switched a high current through the coils, instead of by mechanical points. These changes were likely done to help fire the leaner mixtures being required by world-wide smog laws, and also to improve combustion firing in the larger engines. With electronic ignition BMW was able to lower the resistance of the primary of the ignition coils, allow more primary current to flow, and this made possible another increase in the high voltage energy output. With the particular characteristics of that voltage, things now became flat-out dangerous. There was no safety spark gap, and there never was one from late 1969 onwards. BMW changed the coils and electronics module a few times until the end of production, with the last coils having a 0.5 ohm primary winding, and these coils are very powerful electrically.
On the electronics models, an electronics device called a Hall Sensor, which is a type of transistor that is sensitive to magnetic fields, is the triggering sensor, and it is susceptible to stray electric fields damage. Thus, the plug caps must NEVER be removed and never allowed is an open circuit condition, while the ignition key is on. On both canister styles of ignition (1979-1980 points; and 1981+ Hall Sensor), the coils (one coil some models) certainly can be injured by doing this opening of the circuit, by such as lifting the spark plug cap off the spark plug. While the coil can be damaged from the extreme voltages that can be had with an open secondary circuit, from such as lifting the spark plug cap, the larger problem is with the Hall sensor in that canister. They are damaged by wire-radiated and air-radiated energies from the coil(s) if the spark plug caps are removed from the spark plugs and those 5000 ohm caps are not shorted to ground. The damage MAY NOT SHOW UP IMMEDIATELY!! Rather typically it will show up when you are 200 miles from a telephone, on your way to a rally you have been looking forward to for a year, and you are running a tight schedule to get to your motel which won’t hold your reservation much longer. OH, YES, it is probably 3 a.m. too! (and raining). IT IS PERFECTLY ACCEPTABLE TO SHORT CIRCUIT ONE OR BOTH SPARK PLUG CAPS TO THE ENGINE METAL (CALLED ‘GROUNDING’), ENGINE RUNNING, SO LONG AS THIS IS DONE BY A CONNECTION TO THE SPARK PLUG END OF THE CAP (that is, INSIDE the cap, where the spark plug threads push into the cap!!), WHICH KEEPS THE CAP RESISTOR IN THE CIRCUIT. THIS SHORTING OF THE SPARK PLUG CAP TO THE METAL OF THE ENGINE (CYLINDER OR HEAD) IS PERFECTLY ACCEPTABLE ON DUAL-PLUG CONVERTED MODELS AS WELL. ** What is NOT acceptable, besides the open-circuit condition previously described (or NOT having the 5000 ohm spark plug cap in the circuit being shorted on electronics models), is to get an electrical shock from the system. It may just be uncomfortable…then again…someone might be killed someday…likely someone with a bad heart… SO, be CAREFUL!!
IMPORTANT, PLEASE READ: Before getting into how to synchronize the carburetors without use of any vacuum port takeoff or similar devices, and/or read the spark plugs, note that it will be a waste of time to do these things, if the ignition timing is not set correctly, if the automatic advance does not work properly, if the valves are not set correctly, if the carburetor floats are not in good condition and set correctly, or slide needles not in good condition and equally set; that is, to the same and proper ‘notch’. There must be no leaks in the rubber adaptors from the carburetors to the cylinders. The ignition wires, coils, spark plugs, and caps, must all be in good condition. It may be helpful to have fresh spark plugs. The carburetors should have identical jets and needles on both cylinders… AND, the aircleaner MUST be in good condition, not so dirty as to cause the mixture to be richened!!…although the appearance can be rather dirty without bad effect.
If you intend to synchronize your carburetors using CarbStix, or Walus differential gauge; or the best synchronizer I know of that is presently for sale, the Harmonizer;….OR, any one of several types of instruments that connect to the carburetor vacuum ports, keep in mind that while these CAN produce good results, there are variations between carburetors due to manufacturing tolerances, and some differences between engine cylinders in certain ways that make these devices ‘not perfect’. I am not prejudiced one way or the other. I have and use a Walus gauge (especially on dual-plugged engines); BUT, I often use the shorting method. The shorting method, done correctly, is ALWAYS better, …that is, more accurate, …than ANYthing else, IF DONE PROPERLY. The basic adjustment for use of vacuum devices is done by adjusting for equal or balanced readings, and the adjusting is done at idle, and then, usually, the throttle cables are adjusted at 1300 to 1500 rpm. It is quite possible for an improperly installed CV carburetor butterfly valve to cause the vacuum readings on one carburetor vastly different than the other; and, the butterfly-to-idle-ports area to NEVER work properly….and thus you will never get a vacuum gauge method…or ANY method … to work truly well, at idle balancing. Yes, that happens. The shorting method illustrated in this article you are reading is not only quite cheap, but is usually BETTER at the job, than vacuum devices! The shorting method is NOT a cure-all for a wrongly installed butterfly valve. Photos of correct, and incorrect, installation of the CV carburetor butterfly valves are in the author’s website in the two Bing CV articles.
For the “shorting method”, you need a method of shorting the spark plugs to the engine. You can possibly purchase a ‘box’ to do this; or, do the following (or, something similar): BMW Airheads use a push-on spark plug cap, that contains an almost-hidden internal retaining clipwire, which applies pressure to the spark plug top end threads. BMW does NOT use the screw-onto-the-sparkplug ‘barrels’ that some spark plugs come with. You will need to obtain a short piece of stiff bare (at least at the ends) copper wire….perhaps a few inches long…for each adapter. Longer if you have dual plugging. That means two pieces of wire for 2 plug models; 4 pieces for dual plug models. The wire size that seems to work well is the solid copper wire used for wiring homes, and a foot or four can be purchased for pennies at any hardware store. For dual plug models you will NOT get away with doing the procedure on JUST the top plugs.
I have seen quite a few different type of these home-brew adaptors. You can improvise on these. One of the simplest would be what I am first describing, a piece of wire that is very solidly crimped or soldered to one of those small screw on barrels that fit on the spark plug. You fit the other end of that barrel to the spark plug, by just screwing it on. The other end of the wire MUST fit tightly (securely) into the BMW spark plug cap.
One of the neatest methods is this one: Use a modified piece of screw thread, or, a wheel spoke. A piece of spoke and spoke nipple are PERFECT for an adaptor, as the spoke threads fit the nipple (of course!), and the nipple threads are the same as the spark plug top threads!! I suggest that you DO put a tiny shallow notch, in the non-threaded spark plug cap end of your adaptor, I do that with the sharp edge of a common shop file, to ensure the stock spark plug cap internal wire spring grabs it well.
It is CRITICAL on the electronics models, and I consider it important on ALL models except, somewhat on the magneto equipped /2, that in no possible way can this tool fall apart OR DISCONNECT AT EITHER END, and thereby give an open circuit! This shorting tool MUST be secure, mechanically. Pictures of a number of variations of adapters are in a tools article on the authors website: https://bmwmotorcycletech.info/index.html
You will also need one or two thick plastic handled medium sized screwdrivers. DON’T use wood handled screwdrivers. Wood can be a lousy insulator, especially if it retains moisture. These screwdrivers are used to short the bare adapter wires to the cylinders. You will also need your wrench that fits the locknuts on the cable adjusters; and a screwdriver to adjust the mixture screw. THAT’s ALL YOU NEED!! You do NOT need vacuum devices! In fact, any vacuum ports need to either be plugged, or connected as stock.
HOW TO USE THE ADAPTORS AND THE SCREWDRIVER(s):
FIRST: Read this procedure through a couple of times before beginning.
SECOND: If you have never done this type of thing before, I suggest you get everything ready, and make a ‘mock trial run’, reading this procedure as you go, yet not actually doing the procedure.
THIRD: Understand that if the carburetors have been overhauled, that the adjustments will probably be WAY out of correctness, and you WILL have to go back and forth, as the adjustments of the idle stops and idle mixtures DO affect each other….this is the SAME situation with vacuum adjusting methods. If you have had the carburetors completely apart, use the ‘initial’ factory recommended settings for such as the idle mixture screw, idle stop screw, and have some goodly free play in the outer sheath of the throttle cable.
1. Have the adaptor tools and tools to adjust the carburetors ready. If you have a big fan, get it ready to cool the engine. You need not have one that is way big. See below on the fan. Have you check the tightness of the carburetor-to-head rubber hose clamp-bands? No vacuum leaks at carb tops? Timing and valve adjustments correct? Anything else you failed to do? float levels? do you KNOW that the jets and needle setting in your carbs is proper?
2. Be sure that the throttle cables have adequate free play…. ~1/8th inch with the throttle fully off. Be sure that when the choke [enrichener] lever on the handlebar (or air cleaner clamshell) is off, the chokes fully return to their mechanical (visual check) stops at the carburetors. Clymers manual is wrong about the clamshell lever. Choke OFF is clamshell lever HORIZONTAL. Make any preliminary carb adjustments adequate to get the engine running…if needed. Go for a ride, at least 10 miles, to fully warm up the engine and carburetors, etc., …as a fan and in your garage IS ABSOLUTELY NOT ADEQUATE “for warmup”. …..although it is sometimes done that wayto get the carburetors into a general adjustment range. I do the initial quickie preliminary adjustment in the shop without a fan, within 2 or 3 minutes of engine startup, maximum.
3. As soon as you return from your ride, turn off the engine and put the bike on the center stand immediately, trying to avoid using the side stand at all (this can cause the left float bowl to fill differently from the right bowl, and the subsequent engine idling time will be a bit lengthier to equalize the float bowl levels). Put the fan in front of the bike to keep the engine temperature reasonable. A double-ended squirrel cage fan and housing from an old junked house heater makes a really great source of cooling air, usually the lowest speed setting is about right, and may be available from a local heating supply company for NO cost!! If you do not use a fan, you will want to perform all these adjustments rather quickly….not over 5 minutes of total time, approximately, from re-starting the engine on the center stand, AFTER that 10 mile+ warmup ride.
4. With the ignition switch OFF, pull off the spark plug caps and install those adaptors you made up. TRIPLY make sure they are secure! If using wires and not spoke pieces, bend and arrange adaptor wires so they can be easily shorted to the cylinders with the screwdriver(s). On dual-plug machines, arrange so both shorting wires can be shorted at one time, to cylinder metal. This means having the wire adaptor for the lower plugs come up close to the top plug adaptor, and why the lower adaptor tool you made is longer than the top one. Do NOT have the top and lower adapters be closer than 1/4 inch to each other. Having done a mock trial synchronization well before this point, you already have them bent, at the ready, etc….riiiiight??
5. Restart the engine, blip the throttle, and then let it idle. The idle speed for 1981+ models is best at about 1050; earlier models 950-1000, except for the /2, for which I prefer about 850-900. These idle rpm figures will differ from book values. YOU CAN use 1025 for ALL models, if you wish.
6. Using an insulated screwdriver (or two), first short one plug adaptor wire (both on one cylinder if dual-plugged) for about 2 or 3 seconds, then switch to the other, noticing the engine rpm (AND SOUND!) during each shorting. Clear any engine loading up by blipping the throttle if you need to, then repeat the checks. If the cylinders are not of equal sound during shorting, adjust one throttle stop screw until the cylinders are equal. If the idle speed is not near correct [with NO shorting], make equal adjustments on the idle stops until it is correct.
7. Adjust the mixture screws for fastest idle. This is almost always the point JUST BARELY BEFORE further screwing INwards results in a noticeable (very hear-able) rpm drop. See notes on altitude changes effects and adjustments.
8. Recheck for cylinder equality again…the shorting method.
9. It might take one or three more rounds until the idle speed, equality, and mixture adjustments are all correct. In the future, very little or only minor adjustments will be necessary. Congratulations, you are nearly done! Don’t let the engine overheat. If not using a fan, and you’ve been at this for more than 5 or 6 minutes…go for a short ride, then repeat the process. Blip the throttle during the adjustment process now and then, to help the engine avoid loading up, etc. All the prior adjustments are done at idle rpm, throttle OFF. Once you are unable to make improvements in balance, idle rpm, and idle mixture, you are DONE with that part of the procedure.
10. Now it is time to adjust the throttle cable lengths for equality of actuation of the throttle levers on the carburetors, and at the same time ensure some free play. Increase engine speed to to 1500 rpm. DISregard advice from anyone that says this is to be done at much higher rpm. What you are doing here is making a small handlebar throttle setting that will open the butterfly in the CV carbs a small amount, moving the lower edge of the butterfly somewhat away from the critical idle passageway holes at the bottom of the carburetor. For the slide carburetors, you do the same thing, you make a small handlebar throttle setting, but for them, you are moving the slide vertically a tiny amount, moving an edge of the slide away from the idle passageway holes. Lock the throttle at that position you have selected, or if no locking device installed (the BMW throttle friction screw under the throttle works just fine, if you have it), hold the throttle. Repeat the shorting side to side method. This time, what you are looking for is a bit different: You want to wait….for the engine speed drop upon shorting the cylinder, to as low as it will go in ~three seconds …especially with CV carbs….and to LISTEN for equality, just as before. You will adjust the throttle cables outer sheaths. You want to end up, with the throttle closed, with free play in the outer sheath of the throttle cables of about .080 inch or so. By eyeball, let us say roughly nearly half of a throttle cable end ferrel height.
You may have to play with the cables a bit to get this. So, you are adjusting the throttle cables themselves for equality at 1500 rpm during spark plug shorting back and forth, and then you rotate the throttle off, this may require holding the throttle off, and check that you do still have a reasonable amount of free play at the cables. I do that by pulling up slightly on the outer sheaths….see what free play I have.
It is probably a good idea to keep the throttle cable adjustment locknut a little bit snug when doing these adjustments, loosening a TAD, if really necessary. This helps keep the cable adjuster from moving during final tightening, AND keeps the threads upward loaded, as there is often a little bit of play in those threads. Do not let the engine stay at these higher rev’s for more than a minute, without a big fan or going for a ride to cool the engine.
11. The final check is exactly the same as #10, but you do NOT let the engine settle with 3 seconds to a final low value. INSTEAD, you do the back and forth shorting and taking note of the speeds, each within about a second. This may or may not require a very small adjustment in cable length. Again, this is at 1500 rpm. I also like to do a check at 2500 rpm.
12. Once you make the final tightening of the cable locknuts, at what you think is your final synch adjustment, you want to double-check the synch/balance, just in case the tightening of the locknut changed things, or you accidentally rotated the cable itself and it is under a slight strain. Lock the cables adjustment locking nuts. As a finish up, rotate the steering from side to side, the rpm’s must NOT change when at IDLE, and preferably not when at a higher speed.
13. Check that the throttle levers on the carburetors WILL both go to their full-on mechanical (visual look-see) stops at full handlebar throttle setting. If they won’t, it is likely that you have too much free cable play. You might even have the throttle assembly at the bars IMproperly assembled. It is possible to have the throttle cables with an excessive number of zip ties to the motorcycle frame, etc….and the cables have to be routed properly. NOTE that those Airheads that have ONE cable at the throttle at the handlebars, have a T adapter under the tank. The input side of that adapter is adjustable to set the TOP THROTTLE CABLE FREE PLAY. With the throttle rotated FULLY OFF, and HELD THERE, the free play on that cable should be a bit under 1/8″, with handlebars moved full left and full right. If you have too much free play you can NOT adjust the carburetor throttle cables correctly. If you have too little, the free play may disappear during turning the bars, and increase the rpm. I ALWAYS check the top cable adjustment on these bikes FIRST, before starting synchronization in the slightest.
It is NOT as critical that the carburetor throttle levers both hit their mechanical throttle stops at the same time,,,,it is MORE important that the levers both lift off the carburetor idle stops at the same time …which is taken care of by your adjustments of the cable lengths.
WHEN you release the throttle at the bars (unscrew any friction devices), the outer sheaths of the throttle cables at the carburetors and at the bars throttle, must have some free play.
At this point your carburetors should be closely synchronized!!!
14. Carefully done, this method can produce better actual results than vacuum gauges, which are not really responding as much to actual power developed. Really well-synchronized carburetors will show a particularly smooth response when the throttle is opened slowly, engine in neutral, and also when using small throttle openings in trying for a smooth takeoff from a stop. The vibration levels at all rpm’s will be decreased. Parking lot manoevers are much better controlled, engine vibration on the road is decreased, the engine starts more easily, etc.
If you have a dual plugged machine, you will probably NOT get a good rpm drop if you try this procedure and short only the top plugs. Some have tried shorting the bottom plugs to ground during all of the adjustments, doing the cylinder-to-cylinder shorting on the top plugs, only. They have claimed good success. I personally have not had such great success when trying that. However, it is worth a try, it may be that some models take well to it.
In the future, the cables are the most likely parts to need adjusting, and even then the adjustment will be very small. Although the Bing CV carbs tend to compensate fairly well for altitude changes, the engine is still affected by altitude, particularly, as far as idle rpm goes. Expect the idle rpm to DEcrease at high altitudes, unless you compensated for that (SOME compensation IS possible)…….>>>
That completes the procedure. However, in some instances, particularly when the altitude varies considerably from where adjusted and where ridden, there is some, perhaps as much as 250 rpm or so, difference in idle speed at sea level versus perhaps 7000 feet (pick a number). You may well find that you can eliminate a fair amount of this variation…quite commonly seen BTW; by adjusting, EQUALLY, the mixture screws a small amount, NOT OVER 1/4th turn, and usually about a scant 1/8th turn, outwards or inwards, depending on whether you had done the adjustment procedure at the higher altitude or lower altitude. The theory is that if you are tuning AT a lower altitude, and ride to higher altitudes, that the air thins, causing mixture enrichment at the higher altitude, so you would adjust a tiny amount leaner at the lower altitude, which means screwing the Bing CV carb idle mixture screw INwards a tiny amount, again, not over 1/4th turn, and commonly not quite 1/8th turn. The reverse also holds true, if tuning at a higher altitude, one might consider a tiny amount OUTwards. NOTE: the CV carb and slide carbs are REVERSED in mixture screw effect. …that is, on the slide (NOT CV) carbs, the mixtures screw is screwed OUTWARDS to lean the idle mixture.
How often do you synch the carbs? If your cables are in good condition, this likely should be done at every 5000 miles valve adjustment check time. Usually only a very slight adjustment is required.
***The left carburetor throttle cable is easy to put a kink into it, by not being cautious when you check your oil dipstick. This will cause added friction and cause the inner cable to wear, eventually fraying it, and if you do not inspect closely, you will find a snapped cable someday. YOU DO carry cables?
***BE SURE to keep the cable barrel end parts at the carburetor levers well lubricated. I put a drop of grease or moly lube or oil on those barrels after EVERY bike wash. The barrels MUST ALSO be free to move in the levers at the carbs.
How often do you replace or clean spark plugs? If you have a spark plug cleaner, you can use it every 10000, and replace the plugs at 30000+ (on today’s unleaded gasoline). …but cleaning the plugs is no longer the requirement it was with fully leaded gasoline’s….I recommend you just replace the spark plugs every 15k to 45k. This interval may be too long if you have plug fouling from a worn engine, etc. NOTE: Do NOT adjust spark plug gap after the spark plug has had ANY use in the engine!!!….you may weaken the ground electrode metal which has a different crystalline structure after it is used. It may break, and cause damage to the engine. BMW even had a service bulletin on this. It is a very rare occurence, but why take the chance? After all, the engine will run fine on gaps well over the maximum BMW specifies.
“I have the old valves/valve seats/etc., I need LEADED gas…what do I do??”: WELL, you are advised to review the extensive write-ups done by OAK, since there were two differing problems…then decide on a valve job …or…additives. You may want to refer to my article, dated entitled “Valve Seat Recession–A Definitive Reply” on this website, or a more extensive article on the authors website:
https://bmwmotorcycletech.info/valves.htm
The so-called autoparts stores additives for valve problems do NOT contain lead (TEL)…unless they are for off-road use only…and even then, I am not sure they contain lead, unless they specifically say so, perhaps something like ‘contains tetraethyl lead compound’. It is ONLY that form of lead that REALLY helps; and it does not help the 1981+ valve problems. ValveTect may or may not be helpful. I have not had good results from additives, except lead. For those who WANT to add lead to their gas (this is illegal), only the tiniest amount of leaded GASOLINE is required for full protection. Also, some of the protection from a tank of leaded, or leaded treated as follows, lasts more than just that tank… likely two or three tanks.
The best and by far cheapest method of obtaining leaded gas (It is illegal to do what I am describing for on-road travel…take that for what it is worth…you are NOT supposed to use leaded gasoline for road-going vehicles in the United States) is to take a 2 to 5 gallon can to your local airport (preferably a small airport for mostly private planes). Purchase leaded gasoline meant for aircraft. It may help to tell them something like this [since they are supposed to not sell you any….but the line- boys often do not know or care….]: “”I am building an ultralight plane, want to test out the converted Volkswagen engine”” (or, some such story). This fuel use to come in 3 basic grades: 80/87 octane, typically called ’80’, and it contained a fair amount of lead. SOMEtimes UNleaded CAR gasoline is sold, it is identified. YOU DO NOT want that. These days, mostly the only aircraft gasoline is the 100LL type. Use about a cupful, every one or two m/c tankfulls. You might see plain 100 aircraft leaded fuel. That works even better…has more TEL.
If you use any of the LEADED aviation gasoline’s, you need 1/4th to 1/2 cup per every few motorcycle tankfulls at the absolute minimum, and a cup per tank gives near maximum help. Use of these fuels in these concentrations will NOT be likely to cause you to change your spark plugs every 10,000 miles, “just like in ‘the old days”. DO NOT operate your non-racing motorcycle on pure aviation gasoline, nor, a high percentage of same. It will NOT run correctly.
In particular, very high percentage use of aviation fuel is LIKELY to cause your bike to be hard-starting due to the fact that aviation fuel stays the same, season to season, and the Reed Vapor Pressure specification is rather low in any case, AND, there are other reasons due to the formulations. One cannot have vapor lock ‘up there’ you know. AGAIN, there are some other qualities that make it unsuitable for high % usage. There is NO problem, except the legality, with using the small amounts recommended here. There is military fuel that has an even greater octane that is quite rare now, it was 130/145 octane. I well remember using it at 14.5:1 compression ratio in my ‘street’ BSA Gold Star, when I could not afford the alcohol I normally ran….and I was getting the aviation mil grade for nothing…a long story.
Aviation fuel at an airport is roughly at least 50% more than the cost of car gas, per gallon, which makes aviation fuel, as I described its usage, very cheap for use in stabilizing the pre-1981 valves, since a couple gallons will last you a VERY long time….and it is highly stabilized against long term storage deterioration.Common road fuels begin to deteriorate immediately, not so av-gas. Av-gas, used in small amounts as described, is GREAT for those home gardening motors and such, that have the valve seats that require leaded fuel. Aviation gasoline can be stored for YEARS without deterioration. Additional information about carburetors can be found at the author’s website noted below.
Leaded fuel has NO GOOD EFFECT on BMW valve seats as originally installed from 1981-1984. The only fix is updated valve seats, and installation of those is VERY MUCH BEST DONE by a BMW Airhead Specialist.
Snowbum
Rev. date: 02/19/2021
https://bmwmotorcycletech.info/index.html
