Starting your Airhead motorcycle, including in cold weather

Starting Your Airhead Motorcycle in Cold Weather

It was in November of 1971 that BMW recommended opening the throttle some during cold starts. This can be a necessity depending on the bike, the temperature (does not have to be really quite cold), & how the carburetors and cables are adjusted. Later, BMW dropped the recommendation of slightly opening the throttle. The truth is that opening the throttle a bit …after the engine begins to start …may be quite helpful. For many Airheads, a bit of throttle movement, both up and down some, is helpful even in mild weather startups (seldom if the engine was still warm or hot).

Cold weather starting is nearly always done, & properly so, by using 100% full choke and manipulating the throttle a bit during cranking, as the engine begins to start. Have the clutch lever at the handlebars pulled-in during the cranking to reduce loading of the starter motor by the transmission with its cold thick oil. Helps the battery too.

Many manuals, including the factory Owners Manuals will say to not touch the throttle. In my experience, that is wrong. I have found that most Airheads require some throttle manipulation upon starting in cold weather, and often in mild weather.

As soon as the engine is running, reduce the amount of choke as soon as you can, yet if you need to, and you likely will, keep ‘some’ choke on, until you have smooth running, including when riding. Too quick a reduction may result in the engine stalling/dying and needing a restart. Typically the choke lever is returned to ~half-way within a minute. Even in the coldest weather, the choke lever should be returnable within a very few minutes to the half-way position, and not long afterwards to full off …or, nearly so. For very cold weather, try to keep the rpm between 1200-1500 during non-moving time until some decent warmup is had.

Never blip the throttle to high rpm when starting, this is particularly bad with a cold engine …and wear will be high. In some situations you can break rings or collapse an oil filter with a quite cold engine. Generally, you can start an engine & take off modestly, using quite moderate rpm, after 30 seconds to 2 minutes of high idle rpm (1200-1500), if the temperature is down to as low as 40°F or so. I suggest using modest throttle when taking off, and not going over 4500 rpm, preferably not over 4000, until the engine is warmed some, which takes a couple of minutes. Don’t use too low an RPM for cruising though.

Warm the engine a bit longer the colder it is. An engine at 0°F may seem to require 5-10 minutes of warm-up, but you can start the engine, let it run for a minute or two, then take off gently, using light throttle & quite moderate RPM, until the engine is warmed. Be cautious when first moving off, if the temperature is fairly cold ….because not only is the engine affected, but so is the transmission and rear drive, even such as wheel bearings, etc. Taking off in quite cold weather can result in oil spewing out the top oil breather/inlet of the rear drive ….worse if speed is increased too soon before the rear drive warms up. Remember that wear increases the colder the mechanicals are at startup.

If consistently starting in cold weather, you should change to a Winter grade of oil, & the grade to select is in a chart in your owners booklet, but I have the very latest BMW chart in my website, about half-way down this article:
https://bmwmotorcycletech.info/viscosity.htm

For Airheads with the choke lever mounted on the air-cleaner clam-shells (pre-1981), the lever is down for choke on, & the lever points to the rear for choke off. Some Clymer’s manuals are wrong, and have that backwards!

When the engine is bitterly cold it is best to warm the engine before starting. It is better to have the entire motorcycle above freezing; in, say, a heated garage. Wear on the engine, transmission, rear drive …and other parts …is vastly increased during extreme cold-starting. But, warming the motorcycle is hardly always practical.

There have been times, in extremely cold weather (considerably below freezing), when I have warmed an engine slowly, overnight. Lots of ways of doing that. I have even thrown a big insulated blanket over the bike and used a 150 watt lamp on the floor under it. One must be exceedingly careful about such heating…fire hazard, gasoline fumes ….etc.

Most of you will not start nor drive in quite cold let alone quite extremely cold temperatures, but some will, particularly many sidecarists, who tend to love Winter driving, even in snow.

If you start any engine at ~40°F (4.4°C) or below ….be cautious …don’t excessively rev the engine when cold, it is hard on the pistons, camshaft lobes, oil filter element & many other places. Most engine wear occurs within the first minutes during startup. This is true at any temperature, but very especially when the engine and oil is cold. If the motorcycle has not been started in several months, use a kickstarter if you have one, to rotate the engine a few times; then start the engine with the starter motor; and have the clutch lever pulled back. Be cautious about kickstarters ….early versions were weak, and best to be used only for emergencies if the starter motor system failed. Under severe conditions, especially if the engine has not been started in months or years, it would be best to drain the oil and filter, install a fresh filter, and install heated oil, preferably into a reasonably heated engine. That heated oil should be installed not only into the crankcase, but the valve gear covers should be removed and some of the oil put down both pushrod tubes in both cylinders to protect the camshaft lobes, etc.

Aluminum expands & shrinks fast with temperature. Tolerances get taken up. Metal can rub against metal. When it is cold, be extra cautious! If you are guessing that ZDDP in the oil (almost all motorcycle oils have some) is an especially good idea in cold weather, you’d be correct.

For regular use in relatively cold temperatures I suggest using a premium quality motorcycle part-synthetic multi-grade oil of lower viscosity range and containing a goodly dose of ZDDP. A winter grade oil that contains a decent amount of ZDDP is not just for starting ease and some ultimate surfaces protection; but, for over-all quicker lubrication, and the fact that premium oils stick to the metal surfaces better during storage. You need oily surfaces during even the first engine rotation during starting-up. Since the oil temperature is not likely going to be reaching into the upper range, use of thinner oils is appropriate. Be sure to see the BMW chart in the viscosity article.

Full synthetic oils (or, perhaps, part-synthetic, such as Golden Spectro 4) are good choices for quite cold weather.

The latest BMW factory recommendations, in chart form, is located inside the following linked article: https://bmwmotorcycletech.info/viscosity.htm. I suggest you consider the air temperature after you start the bike, and during and when ending your trip. You may have to make a compromise in oil viscosity. Oil compounding has changed! ….that chart has changed! Be sure to read the comments!

Here are two things not usually mentioned anyplace: It is better to err on the side of a thinner oil, than a thicker one, if you are beginning the ride in quite cold temperatures, and continuing later in warmer temperatures. Oil flow, which is much easier with thinner oils, is more important, particularly with modern oils, than having a higher viscosity oil, when you start out on a cold day. I wouldn’t worry about the bearings, etc., holding up with a thinner oil. You may want to read my oil essay, because many have the wrong idea about oil thickness, oil film strength, etc.: https://bmwmotorcycletech.info/oilessay.htm
In some areas of the world multigrade oils are not easily available. You will have to make compromises. Be cautious! Do not use too thick an oil.

For normal temperatures, from ~40°F to as hot as it gets anyplace in the world, I suggest you use a quality 20W50, such as a top-grade semi-synthetic like Golden Spectro 4; or Spectro’s full synthetic. You do not need a full-synthetic, although it might come close to paying for itself with the longer drain intervals that are entirely OK if you don’t do many quite short rides. Short rides (under 20 miles) do not allow condensation products in the oil to be burned-off; plus, there are more such condensation products, and they mix with sulfurs, etc., in petroleum oils, and the acids formed are not good for the engine parts. Full synthetics are better in short ride situations.

Your Airhead will start more easily with the proper grade of oil, if the starter motor is in good condition, has a good battery… and the clutch lever at the handlebars is pulled-in during starting. The battery, cables, connections, starter and starter solenoid must be in good condition. You could take voltage drop measurements to be sure of proper system operation. Do a formal battery load test yearly or even every 6 months, to avoid bad battery surprises. Anything else marginal, such as in the ignition or having valves with too little clearance, can cause poor starting in cold weather.

Helpful ideas for seriously cold weather riding:

Spark plug gaps can be broadened to .032″ maximum; ignition coils and towers inside and outside checked & cleaned; wires & spark plug caps cleaned & checked. Proper ignition points gap if you have points. I suggest 1000 ohm early style caps or 1000 ohm NGK spark plug caps on points models. You can not use 1000 ohm caps on the 1981+ electronics models, they must have the 5000 ohm caps. Do not use resistor spark plugs on any BMW Airhead.

Valve & valve seat condition (& valve clearances especially), compression pressure & ignition timing must be good. Tight valves, or a worn top end, particularly with low compression, will make starting difficult …or even impossible.

The engine must spin over relatively fast to get a good start in cold weather. Kick starting may be especially difficult and should be avoided. Kickstarting is very hard on the transmission parts associated with the kickstarting mechanism, this is particularly so on the earliest 5 speed transmissions.

The incoming mixture will be less homogenized. A very fuel-rich mixture is needed in cold weather; which is obtained on the CV carburetor models by using the choke lever, which is actually a lever to turn on the Enrichener. Be sure the Enrichener is properly assembled and adjusted. There have been a considerable number of Bing CV carburetors in which the enricheners are wrongly assembled, even some where the Bing factory mis-marked the enrichener shaft dot. Photos of both correct and incorrect assembly are on my website. Carburetor choke (enrichener) levers that do not reach full-on stops (or nearly) may not give full enrichment. If the enrichener circuits, etc., on the Bing CV carburetors are not assembled correctly, or the gasket is leaking, or the jet in the side of the bottom of the float bowl is clogged, or the dip-pipe cracked, starting may be very difficult. The pure slide (non-CV) carburetors get their starting mixture enriched by over-filling the carburetor by means of a ‘tickler’. That enrichment sometimes must be repeated.

If the gasoline is old, or summer-grade, starting can be difficult. Starting may be nearly impossible if you are using aviation fuel, as its volatility is much less than road fuel.

Spark plugs must be clean & dry at the electrode end. If spark plugs are carbone’d up, or otherwise fouled, they must be cleaned or replaced, & the reason for such carboning, if excessive, fixed.

If your Bing CV carburetor’d bike starts, & then seems to die after maybe 10 to 20 seconds, even if holding a small amount of throttle, inspect the carburetor bowl contents, & especially check the bowl gasket that it is intact and good around the enrichener well area …..yes, that gasket needs to be good in order for the enrichener to work properly….and be sure to inspect the enrichener’s little jet in the bottom of the corner well of the bowl and check its side-hole inlet at the bottom of the same bowl well. Failure to have fuel properly flow from the bowl into that corner well area will cause this particular 10-20 second problem, & a clogged or partially clogged jet is commonly found. If the bike does not want to start at all check that the corner jet well has fuel in it. No water is allowable to be in the bowl. A rare occurrence is a split brass tube that dips into that corner bowl area, this can come from water freezing inside it. It is possible for the corner jet to completely clog. If so, cold-starting will be very difficult.

Other jets can plug-up with crud, causing starting & running problems. In some particularly neglected motorcycles, even the bottom of the tank & petcocks could have frozen water in them. Be sure the petcocks allow enough fuel to flow into the carburetor bowl.

If you have a GS, it probably has no oil thermostat at the canister area. The BMW factory recommends that in quite cold weather you cover the oil radiator. BMW sold covers for this. You can make one. Over-cooled oil that never gets even close to 200°F will not lubricate correctly, and will gather contaminants faster.

Dual-plugging, done correctly, will make starting far easier.

Watch out for water in the float bowls & fuel tank; water condenses in the fuel tank overnight; this is particularly so in areas where you see dew collecting; and very particularly so if the fuel level is not quite high. This water will rot out the tank bottom! Clean your tanks. I do it yearly. In some areas, you may have to do it more often. If you have aftermarket paper fuel filters, be sure they really will flow sufficient fuel.

Many street tires are very slippery when quite cold, so be cautious about your tires when riding off. Drum brakes in cold weather can be grabby (especially if damp or in high humidity overnight) for the initial first use.

Keep in mind that the colder it is the more the engine is affected …but so is nearly every other part of the motorcycle. Even something like a seat covering or a plastic tail-light lens is more brittle ….and if its screws had been even slightly over-tightened, a crack could occur.

Revisions:
09/24/2017: All previous revisions, including the one from 2003 incorporated, and article copied to airheads.org, and edited specifically for this website.
01/30/2021: Minor clarifications and cleanup.

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