1994 R100R rough idle

My 94 R100R suddenly started idling on one cylinder (it sounded exactly like my SR500 lol!). The left cylinder was not firing and bike would idle at ~700 RPM Rolling on the throttle and the left cylinder would start to fire and above 2000 RPM the bike runs perfectly.

I screwed in the idle stop and the got the left cylinder to fire but the idle is rough and erratic. The bike has 14K miles. Any suggestions would be greatly appreciated!

A complete baseline would be a good place to start. Valve adjustment/check, throttle sync and new spark plugs. Before you do that, check your throttle cables to make sure the ends are seated into the adjusters and there's nothing caught where the cable meets the pulley.

Spark plug wire broke/lose?

/Guenther

tune up mantra

At 14K, have you ever adjusted the valves? Start there. Valve adjustment should be done about every 15K.

It has electronic ignition and I would doubt the timing has changed. Besides, if the timing had changed it would screw up both sides not just one.

A bad spark plug wire could be the problem. I would pull the spark plug lead off and check for any signs of shorting or weird looking discoloration in the spark plug boot. Then check it with an ohmmeter. I have forgotten the values for pass or fail sorry.

If you have the valves set properly, and good spark, then it would be the carb. Problem could be anything from a bad/mis-adjusted float, to a booger in the idle jet.

With the low miles, I cannot imagine anything has worn out in the carb.

Good luck. St.

Update

I took the bike outside last night and when it was running the left exhaust pipe was glowing... too lean?

What have you found?

So, what have you found so far?

won't idle

murty said:

My 94 R100R suddenly started idling on one cylinder (it sounded exactly like my SR500 lol!). The left cylinder was not firing and bike would idle at ~700 RPM Rolling on the throttle and the left cylinder would start to fire and above 2000 RPM the bike runs perfectly.

I screwed in the idle stop and the got the left cylinder to fire but the idle is rough and erratic. The bike has 14K miles. Any suggestions would be greatly appreciated!

Click to expand...

My guess: if fiddling with the carb changes things, then it probably is the fuel delivery through that carb.
I'd inspect the float bowl, float and float needle and blow some cleaner through the idle circuit.
When the bike is running over 2000 rpm you're now on the main circuit so if its fine there then its something in the idle circuit.
If that doesn't work then,although you're not suppose to do it for very long, I'd ground out the left plug at idle to see if you have a spark.
Do a plug chop. When the bike is idling, shut off the ignition and immediately pull the left plug.
if its sooty black its getting too much gas, if its white, not enough, if the plug is wet, then its not working.
These things may help to determine if its carb or ignition and then go forward from there.

By now you've likely got it fixed but if not.

When something happens out of the blue and you haven't done anything to cause it, then look at the simple stuff first. Check that the spark plug is connected properly. If it is then take the float bowl off and have a look in the bottom to see if you have any water droplets or dirt in the bottom. Either of those will get sucked up into a jet and cause your problem. When you took the bowl off was the fuel level about 1/2 inch below the lip of the bowl. While you have the bowl off, put a container under the carb. Turn the fuel on and work the float up and down carefully to see that the needle and seat are working properly and you have a good fuel flow.

After you have checked the float bowl and needle and seat, then put the bowl back on, start the bike and see if that changed your issue. If not then you have to go deeper into the carbs and as well check the resistance on the plug wire of the affected cylinder. If you want, swap the plug wires and see if that moves the issue to the other cylinder. Then take the carbs off the mounts and then blow compressed air through all the jets and holes that you can find. You don't need a lot of pressure. At low speeds, it likely will be the idle circuit that is causing your problem and the small holes and passages are easily plugged, so concentrate on those first. It would be best if you had a carb kit, as you may have to replace the O ring on the pilot jet when you remove it. After this remount the carbs and start the bike and see if that has solved your issue. If so you are good to go, if not then you'll have to go further into the carbs.

I have worked on bikes that have had similar issues and you immediately think of the carbs but in the one case it was a bad connection where the plug wire went into the plug. Screwed them apart and clipped off 1/4 of green corrosion so that there was new cable and screwed it back together. Cured the problem. So you need to work with a process of elimination from the simple to the harder.

Update 2

Thanks for the help! Here's what I did so far:

I replaced the idle jet and idle mixture screw on the left carb. Swapped spark plugs R to L. Visually inspected the L spark plug wire. No difference, left cylinder does not fire when idling but starts firing as soon as the throttle is advanced. Seems fine above 2000 rpm.

Best,
Paul

Other things

Ok, you visually checked the spark plug wire and did not see any weird arcing or burning. Either test it with an ohm meter or, simply swap it with the other side.

Did you check the valves for proper adjustment. At 15K they should be adjusted. A tight valve might cause poor idle.

If the steps above don't solve the problem, you still have a problem in the carb. You may have a blocked by booger idle circuit. Are you sure the float is adjusted correctly?

When you put the carb back together did you make the proper primary adjustment to the idle mixture screw? I don't remember just off had what it is but you can find it in a repair manual or online.

Good luck. St.

My Airheads have had 2 instances of rough running as a result of resistor caps failing.
The symptoms were different each time.
1st time, the left cylinder stopped firing while stopped at a looong traffic light, I first checked that there wasn’t any water or junk in the float bowl, then swapped plug wires. Viola, the cap had failed.
2nd time, there were intermittent “misses” when warming up, then the bike would run fine.
After a bit of trouble shooting, I changed the caps and all was good after that.
Or it could be...
Another time, it was a small bit of the float bowl gasket that was pinched off that blocked the jet.
Hope it helps.
Bill

Update 3

I swapped plug leads, no change - still not idling on L cylinder. I guess next step to check valve clearance, then go deeper into the L carb?

Thanks,
Paul

still not idling on L cylinder

Click to expand...

When hot/cold/always? With or without enricher engaged?

When starting cold an the motor is idling, does the left exhaust manifold warm up?

Verify that the enrichers arms on both carbs move about the same when you turn the enricher lever on the handlebars.

Next I would clean the idle jet.

/Guenther

Hello,

Yes hot or cold, it only idles on the Right cylinder. I replaced the idle jet and idle mixture screw.

Thanks for your help!

Best,
Paul

Any small twist on the throttle overrides the small amount of mixture from the idle system. If the motor runs fine besides idle I think there is something 'stuck' in the idle system.

The air intake for the idle system is a small opening at the main air intake of the carb. I would blow compressed air into this hole. Also into the hole of the idle jet and the hole of idle mixture screw.

/Guenther

murty said:

Hello,

Yes hot or cold, it only idles on the Right cylinder. I replaced the idle jet and idle mixture screw.

Thanks for your help!

Best,
Paul

Click to expand...

Hi Paul,

Some observations, suggestions follow, but trying not to repeat other suggestions.
1. Problem is isolated to the left side, so anything shared by the left and right is not the problem
2. Engine runs okay at 2000+ RPM implies the idle and/or mid-range could be your problem.
3. Water in the idle or main jet on the left can cause this problem. If you remove the main jet and see a blob on the jet or in the well, it's water. Remove.
4. Jet needle & needle jet control mid-range. Newer needles are aluminum and the slots have been know to fail (break or become enlarged) so the needle is not where it belongs. Verify the slot the needle is in is correct and the condition of the needle slots.
5. Remote possibility: Needle jet cracked.
6. Main jet O-rings broken, weak, failing
7. Choke hanging up only on the left can cause this problem. Check if the choke cable adjuster is loose, a frayed cable, broken return spring.
8. Water in the left float bowl?
9. Remote possibility: Left slide is stuck and not going down all the way. Remove air tube to verify the slide is all the way down and moves easily.

Non-carburetion.
1. Spark plug wire shorting
2. Spark plug cap shorting
3. Spark plug wire has intermittent break in wire
3. Fouled spark plug on left (check color-if dark, or wet, that's a problem)
4. Corrosion on left spark plug wire where it connects into the coil tower.
6. Remote Possibility: Low resistance (1000 Ohm) spark plug wire used on left. Should be 5000 Ohm.


I hope this helps.

Best.
Brook Reams.

I had the same problem with my 92 RT. Did a bunch of trouble shooting as stated above. What I found out was the IGNITION MODULE unit was at fault. Bought a new on at EuroMotorElectric and all was fine. Remember that there is a heat sink past to use between the module and the aluminum to help cool the unit. Good luck!
https://www.euromotoelectrics.com/Ignition-Module-BMW-R-12-14-2-325-284-p/ignmod-r284hd.htm

Problem solved

Final solution: I had both carbs rebuilt by the Bing Agency, synched them with a carbmaster instrument. Running great!! Thanks to all who helped.

Thank you for the followup

Thank you so much for the follow up. Despite all the aggravation, you now have a well tuned bike and a lot more knowledge under your belt.

Happy riding! Cheery bye St.

One more thing to check

I replaced my stock coil years ago (in 2013) with an aftermarket unit. In the interim, I had considerable amount of work performed on my 1991 R100GS, including a carb rebuild by a factory trained and excellent mechanic. Picked up the bike, she was running beautifully (for 20 miles.) Then I went to one cylinder. By holding the revs high at stoplights, I was able to keep it running, and once underway, she'd start hitting on both cylinders.

I was so convinced it was a carb problem, since that was the last engine components disturbed. Called the technician (self employed in his own shop) who drove 50 miles to my home to double check his work. I'd convinced him it had to be carbs. After a quick carb teardown, and blowing carb cleaner thru the passageways, he determined that all passageways were open, floats were set correctly, diaphragms ok. So, not a carb issue. We decided I'd trailer the bike back to his shop later.

In the interim, I started doing some plug wire swapping ...... right cyl to left, left to right. Doing so caused the problem to move from one cylinder to another. I'm thinking "how is this possible .......... the airheads use a lost spark ignition ...... when one plug fires, so does the other"

Long story short, the problem was with the aftermarket coil. A short phone call to Rick Jones (Motorrad Elektrik), and I was informed that this type of coil failure was possible. Ordered new coil form Rick, installed it, problem solved.

This may be rare, but it happened to me.

That's my story, and I'm sticking to it.