Rough Running R100

I have been trying to figure out an intermittent problem on my '83 R100RT. After pulling a moderate grade, the engine starts to run rough almost like it is running on only one cylinder. It takes a couple of minutes to start with the higher load. After about two minutes of decreased load, i.e. reaching the top of the grade, it goes away and all is back to normal. While it running rough, I can still maintain speed, and accelerate though power is down slightly. Vibration is synched with engine speed. Pulling in the clutch causes the frequency to drop and get a bit rougher running at idle.

Same symptoms with wet or dry weather, modest temperatures (60s), and different tanks of fuel. Valve adjustment is right on.

Any thoughts? Ignition module, coils, spark plug wires? I'd rather come up with a strategy than just start replacing components.

Thank you

My limited knowledge tells me to

Take a look at your sparkplugs right after you experience the condition. If one of them shows a lean burn you my have a float level or carb issue. Or if one is black or wet it might be an ignition issue or lead to a to rich carb issue although not likely.
Always start with the basic and inexpensive stuff first.
Zeff

I had somthing like that happening. Turned out I had put regular fuel in and not Premium 93 octane. Hope its just that easy

Check carb diaphrams you may have a small hole starting

rough running sounds like fuel issue

clean everything! pull the jets, the petcock filters, blow carb cleaner into all the little openings followed by compressed air.

Changes only when going uphill? I'm going with float levels first..worn out spark plug wires second.

Let us know what you find out.

I vote cracked diaphragm, sticky beancan advance

Some folks have reported problems with coils...bottom line...check it ALL out!

JRunner said:

I had somthing like that happening. Turned out I had put regular fuel in and not Premium 93 octane. Hope its just that easy

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I've had no problem in my 81 R100RT running regular (87 on the east coast). I don't think that's the problem. I agree with the float setting. You may just be running out of gas.

Thank you for all the suggestions. I guess I have my work cut out for me...

I had not even considered the carbs.

I also think that it's the carbs:
*Dirty or worn needle valve
*Small hole in diaphragm
*Floats need adjusting or replacing

In second place is somewhere in the ignition:
*Wires
*Coils

Hope you find it quickly

83014 said:

I've had no problem in my 81 R100RT running regular (87 on the east coast). I don't think that's the problem. I agree with the float setting. You may just be running out of gas.

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Yes, all USA BMW Airheads since '81 designed for regular gas.

richardak said:

I had not even considered the carbs.

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Ignition system is WAY more bulletproof than carbs.

Diaphragms are 2-year items.

Check the float level. I would bet one is too low or the fuel isn't getting to the bowl enough under the load (going uphill uses more fuel) You are running out of gas on one cylinder. Of course, I could be wrong...

This problem screams "inadequate fuel delivery".

It could be:

Petcock
Itty bitty in-line filter
Partially clogged main jet
float level
leaking diaphragm

I guess I didn't think fuel since it runs fine for much of the way up the hills. Seemed somehow related to temperature. Wouldn't inadequate fuel show up shortly after heading up the grade? It takes several minutes before it starts then continues even after heading downhill for several minutes.

Like a dirty fuel filter, it may take a few minutes to drain the bowls. I suspect that if it's breaking up under load, as you are experiencing, it's not ignition. If it were ign related it would break up sooner - maybe. As most of the posters have said, I'd start with carbs.

PGlaves said:

This problem screams "inadequate fuel delivery".

It could be:

Petcock
Itty bitty in-line filter
Partially clogged main jet
float level
leaking diaphragm

Click to expand...

If it were mine, I'd start in Paul's order. When was the last time the petcocks were removed, examined and cleaned? You might be surprised at the accumulation of tank lining and other grit on the screens. I'd clean the petcock(s) first, and clean the main jets whether they need it or not (and if you haven't cleaned them at all, they need it). Both of these steps are relatively easy. Come back here for help if you need it. The float levels and diaphragm theories can be addressed later if the first two don't get the job done.

Probably should look at the fuel lines and especially the cross-over tube, too. It's harder to believe that both petcocks were causing trouble. One petcock should be enough to keep the engine running fine. The original post mentions that it "almost like it is running on only one cylinder". If one petcock was OK and the cross-over tube was clear, then the bike should run well.

I would agree with some of the earlier comments regarding inadequate fuel supply. I once had a truck that would sputter and bog on long hills. The problem was a clogged fuel filter. Under high demand the filter could not pass enough fuel.

As was stated, it may take a while to drain the bowls. The problem persists on the way down because now the bowls have to refill.

This does not mean the petcock screens are clogged. There could be other problems that could cause the same symptoms. Check the easiest item first and one by one eliminate them.

I would also agree that one petcock should supply enough fuel to keep the engine running. I normally run on only one side.

I will pass on something that happened to me. It is not likely that this is your problem but you never know. The engine quit on me one day. I had traveled about 6 miles and it sputtered and died. I thought it was ignition but it was not ignition problem. I opened the other petcock and the engine ran fine. When I got home I looked at the petcock that I had been running on, thinking it may be clogged or had some other problem. I found nothing. But, when I left the house that day, it looks like I did not have the lever in the proper position. It was partially on, not fully in the detent. Thus, I eventually drained the bowls. I think the internals of this petcock are wearing and I no longer hear that distinct click.

Maybe the internals of your petcock are worn and you are not able to get that lever in the proper position to deliver enough fuel.