[Solved] R90/6 right side no fire?
76 R90/6, about 90k miles. I rebuilt the carbs, including floats, new points/plugs/coils/condenser/wires. Ran pretty good after all that but I was still dialing in the carbs. Went for a ride to warm up the bike and it ran fine, pulled strong. At about the furthest point from home, in the middle of nowhere (of course) it started to misfire/backfire a little, especially when starting from a dead stop. That got progressively worse, to the point that I could only get it to run fairly smooth at about 2800 rpm. I got it home that way, spittin' and snortin' (about 10 miles). Left jug was warm but not nearly as hot at the right side (I could touch the left head, not the right). Checked the left float and adjusted a little - thought it might be overloading. No joy. New plugs, in case one might be fouled, no joy. Valve adjustment is good. The right side seems to be fine, but the left is definitely not firing right. Touching the top of the piston with a rod brings back a wet gas sample on the left, nothing from the right. One other point, but I don't see how it could be an issue, as the right side is good - I replaced the battery with a smaller footprint, less CCA Optima AGM. Any thoughts would be appreciated. Thanks as always for your technical expertise!
I am sure it has nothing to do with the battery. I would be looking at the coils or the plug wires. Maybe try switching the plug wires from side to side and see if the trouble switches sides too. "wet gas sample on the left" suggests un-burned air/fuel. I would guess spark or compression in that order. A compression check and/or cylinder leak down test would be informative as well. Technical expertise?, let's not jump to conclusions.
You mentioned new floats. I experienced an almost inexplicable problem with a new float I purchased from EME (white plastic). I have heard of two other potential cases on other forums. The floats seems just fine and well adjusted with the bowl removed, but somehow binds or misbehaves with the bowl in place, flooding or starving the carb. Just thought I would throw it out there. I replaced my floats with ones from Bing and issues were resolved.
Thanks James and David! I tried James' suggestion of swapping coil wire destinations and the problem stayed on the left side. I then did a compression test and got 130 on the left (bad) side, 120 on the left (good?) side. Not great, but I'm thinking it's not the problem. Dropped the float bowls again and the left is definitely loaded up. Seems to shut off when I move the float manually. They are new floats from Bing, white ones. I set them using the 'parallel at 45 degrees' method, but maybe there's another/better way? I took the carbs off to see if there was something obvious that I had missed and I did notice that the 'bad' barb air mixture screw seats significantly higher than the good one. Picture attached. Not sure how that would happen - both have o-rings and are of the same length. I turned them in until they just touched, I believe. Thoughts? I have a new set of Mikuni's in the box waiting to jump in if I can't get the old Bings fixed. I'd like to keep it as 'original' as possible, but I also want to ride! Thanks again.
Compression test - (you said left and left, sure just a typo) Did you do it with the carbs removed? It’s best. Truest readings.
Float setting - not sure what the 45 degrees would be. Basically, the molding seam on the side of the float (easy to eyeball) needs to be parallel with the bottom of the carb (bowl removed) when the fuel flow just shuts off.
Screws - yes, something looks wrong there. Those are idle mixture screws, and really have little affect at mid to high rpm’s, however, as out of whack as they are, I am unsure. Are the screws new? Need to take those back out and investigate. Look for something preventing them from seating in all the way. If my memory is correct, the head of the screw should be below the surrounding barrel. Is there an old o-ring stuck down in the bore?
I should say that my carbs are a few years newer, so I don’t know if yours are exactly the same. Surely the two idle mixture screws should be the same.
I tried James' suggestion of swapping coil wire destinations and the problem stayed on the left side.
1. But then, did you move the spark plug ? Spark plugs have a huge impact, and if you had some bad fuel even new plugs can be fouled very quickly.
2. The Idle Mixture screws should be set 1-1/2 turns out and left alone after that. As David said, yours look wonky?
3. Have you inspected the diaphragms for holes/ rips ?
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With compression testing, what I am looking for is similarity. As an example, I used to own an R100/7 with nearly 160,000 miles on it. I was having trouble getting the carbs in sync. The compression test indicated 120psi on the right side, but only 90psi on the left. No wonder the thing wouldn't balance out. I would not worry about your 10 pound deviation.
I was able to get the 'proud' mixture screw to behave - I think there was a rough spot in the threads and I didn't want to screw up the seat. Just a little more pressure worked it out. I'm going to put the carbs back on and do the float adjustment with them functioning. I will hopefully report all good in the next update! Thanks guys.
I'm surprised, but not 'shocked' that I identified the wrong side in the title of this thread! It's actually the left side that is having the problem. Getting older has it challenges, hey? Anyway, latest update shows that adjusting the floats with the carbs installed did require adjustment but made no difference in the outcome. (Please tell me that floats from Bing are not position-sensitive!) I'm getting fuel, as evidenced by wet sparkplugs (new). I've done everything I can think of electrically and recommended by others (thanks!) and will now install the new Mikunis to see if they make a difference. After that, I guess it's pull the heads, which I was sort of planning on for the winter project, but the weather keeps teasing me with "sunny and warm" forecasts here in Chesapeake, VA. As to the ongoing discussion about the BofD and the future of ABC, I'll bet Oak is spinning!
The floats are same for right and left. Sure wish we could put our heads together in person. The Bings should run the bike just great. The fact that this ailment started mid-ride (and hasn’t quit) means that something failed, it would seem. Not just a poor adjustment, or anything like that. Something went south. Diaphragms can go south. Have you looked there? Are the “chokes” working ok? From your description trying to get the bike home, it almost sounds like the enrichment (choke) was on, or partially, on one side. Just guessing.
• Your title says "Right side no fire", but I see nowhere in this entire thread where you physically pulled the plugs and actually visually witnessed that the spark plug does or does not have a blue flame. Instead you seem to be gauging this entire action by the heat of the cylinder head. While that's a great BASIC tool to know which cylinder is acting up, it is NOT a problem diagnosis tool.
After that, I guess it's pull the heads...
• And what will that do for you ? Pulling the heads will let you look at the rings and valves, which from the compression test we already know are both passable. You reported the valve adjustment is correct, so we can assume the seats are not working their way out or anything such as that.
Excuse me if I've missed it.
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@4949 David- I did check the diaphragm (new) and it looks fine. I think your 'choke' suggestion makes sense and I'll pull that today. Thanks again.
Well, I'm out of options - spark is good and fat (not exactly 'blue', but certainly strong) and enricher/choke also looks good. I'm going to pull the heads and have them refreshed. Thanks all for your input.
Take an old spark plug (that you know is good) and rip the L-shaped side electrode completely off. Plug that into the plug cap on the cool running side. (You'll also need to have a plug in the cap on the "good" cylinder.) Lay both plugs on the cyl head and spin the engine with the electric starter. Does the test plug still show a spark ??
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