Boxerworks.com

Hi everyone,

I have a 1994 R100RT. Using a timing light, I found that the left cylinder fires consistently, but the right seems to fire half as often, fires very intermittently as I approach 2500 rpms and stops firing all together after 2500 rpms.

I have replaced the spark plugs with NGK BP6ES plugs which I gapped at 0.7mm before installation.

I replaced the wires with 5k ohm NGK wires and caps.

I tried swapping the wires both at the coil terminals and at the plugs. The right side still fires intermittently and the left side is fine. I tried the old spark plug wires and had the same issue, although it was actually worse with the old wires.

If it were the hall sensor or ignition module, it would probably affect both cylinders. If it were the coil, swapping the plug wires at the coil should move the problem to the other side, but its not. The only common variable is the right cylinder itself.

I would very much appreciate any input that you guys have. I've never owned an airhead, I think I'm going to love it, but right now I'm not enjoying it at all. It has 78,000 miles, but looks great and I have a stack of receipts from the previous owner who was unaware of the issue.

Welcome aboard, Mus4shiii. Sympathy attached - your unhappiness is apparent.

You say you used a timing light to check spark - what sort of light and how did you use it? I ask because I can't imagine how one could use a common 12v timing light for that purpose. It is usual to check spark by observing the plug gap while the plug is grounded on the cylinder while spinning the engine with the starter button. If you are using an inexpensive xenon strobe (like the crappy one I have) it is common to experience intermittent flashing.

Since you say that swapping the leads doesn't shift the problem to the other cylinder I'll vote for a non-spark related issue. A blocked fuel supply to the main jet in one carb will cause the symptoms you list, exactly. This happened to me recently and I was convinced it was a spark related problem for far too long. (Embarrassed emoticon here)

Best of luck getting it sorted so you can begin to really enjoy your bike.

Edit: It's not too difficult to swap carbs to confirm the above if you are reluctant to go in there just yet.

I totally agree with the carb swapping suggestion before diving inside. If you have done enough trading of electrical components to eliminate that, carb swapping is the obvious next test. I'm assuming you did all the trading one test at a time so that you have truly eliminated the coils.

I'm also assuming you have already checked the valve clearances....


If carb swapping does not switch the problem to the other side, next I'd be doing a compression check.

I did valve lash at .10 and .20 for intake and exhaust per the Clymer manual.

I also cleaned out the carbs, inspected the diaphragms and replaced both needle jets and jet needles since they were a bit worn. I then synced the carbs and set the idle mixture. I have equal vacuum pressure on my CarbTune Pro at idle, 1500 and 4000 rpms.

None of this solved my problem of bad vibration, so I bought a Mac advance timing light off eBay. It flashes consistently for the left cylinder, so I assumed I should get the same behavior from the right cylinder.

Is it possible that carburetor could have such major issues that it is causing the plug not to fire at higher rpms? Just how rich would have to be to not fire at all after 2500 rpms and what could cause that that wouldn't be apparent from general visual inspection? I might try removing the intake tube from the airbox tomorrow to see if that changes anything. Maybe there is a blockage in the airbox I can't see and all I'm getting is fuel.

Try dropping the plug gap to the low end of the range, 0.6mm.

Kurt in S.A.

Can you advise what the plug looks like on the cylinder misfiring. Do you have both petcocks open, not relying on the cross over line?

mus4shiii wrote:...

None of this solved my problem of bad vibration, so I bought a Mac advance timing light off eBay. It flashes consistently for the left cylinder, so I assumed I should get the same behavior from the right cylinder.

...

This is exactly what I get with my cheap-shit strobe on my bike. When clipped onto one plug lead it mostly works and on the other lead it mostly doesn't. I'm pretty sure that if you were to use a proper (expensive) strobe light you would find that all is good in the plug firing department.

So I'm still voting for a fuel blockage. When this happened to me a couple of months ago the symptoms were:-

The bike would start fine and idle ok after warming up.
When revved the motor would spin ok up to 2000 revs but a vibration was apparent.
When ridden it would accelerate ok up to 2500 revs but with that odd vibration followed by a definite miss at around 3000 revs, no power at all, so back to the shed ... Clearly one cylinder was not working properly.

I went through all of what you have done, swapping plug leads, coils, checking tappets, using magic words and so on.

What I found when I eventually went into the left carb was a tiny piece of rubber from a fuel line had lodged in the little chamber under the main jet (atomiser? Not sure).

It's important to note that it may not be the right cylinder in your case. You think it is because of the intermittent strobe flashing on that side but it is hard to tell when you are riding.

I had Airbears experience years back. I was stuck in a motel 1200kms from home and I did all that checking of the ignition system. Next morning I got out to 2valves place which just happened to be local.

Together we did all the ignition stuff again. No success.
In desperation I rang my local BMW mechanic who also suggested we do everything that we'd done twice. "Yep, done that! Any other ideas?"
He suggested we drop the bowls and inspect the jets for blockage.

The culprit.


Installation.


It's definitely worth dropping the jet stack out to inspect and clean. Just ensure the atomiser is correctly seated before you tighten it all back up again. Like this...


Consider a stuffed diaphragm too.

I'm leaning more towards a fuel or valve timing problem at this point. I think the timing light not firing is due to extreme vibration on the right cylinder messing with the inductive pickup.

I took the air box pipes off and was getting a little bit of fuel ignition that I could see through the back of the right carburetor. Like a mini back fire. Could this be cause only by a rich carb or would there need to be an intake valve that is improperly timed to let heat or ignited fuel back through?

I don't see the slides going up or down on either carb. Shouldn't they be moving up and down? The diaphragms look fine. No tears and they're nice and pliable.

Any tests you can think of or things you can have me check would be great. I don't have a compression, leak down tester, exhaust or ignition analyzers yet. They're all on my eBay watchlist, so mostly just equipped with hand tools, a test light, multimeter and a manometer.

Here is a video of the timing light on both cylinders. I start on the left which is fine, then move to the right. You can see that the right cuts out when I rev up the bike.

https://vimeo.com/167194561

Here is the left carb.

https://vimeo.com/167194585

Here is the right carb. Its hard to see, but at the end, the mirror gets really blurry on the right hand side. Left hand mirror is nice and stable.

https://vimeo.com/167194619

Strange, but I would have thought the slides should move up when you increase the throttle. Can you reach in with your finger...doesn't have to be while the bike is running...and easily move the slide up?

Kurt in S.A.