Electrical problem at higher speed

[dmallah]

Hello everyone here's a post from a good buddy who's having an unresolved issue:
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I have a R100 GS (1988), and I'm having a recurrent problem that nobody has been able to properly diagnose around here. I was wondering if someone could help...

The motorcycle runs fine until I hit 110 KM/h (about 70 MPH), but when I reach that point, I can't seem to get it to pass that limit. It acts as if the gas was being cut off almost, or running on one cylinder. Or more acurately, it feels the same way a car does when you reach the cutoff. If I let go of the throttle, it gets back to normal and is fine as long as I don't try to go past that speed. It doesn't seem to be related to RPMs, just speed.

It's only happened to me at 110, but I've lent the motorcycle to a friend who said it happened to him at 90 KM/h (55 MPH). Not sure what gear he was in compared to me. This hasn't happened to me personally.

Any input would be greatly appreciated...The bike is used and the owner has since passed away.

D

[Ken in Oklahoma]

Two possibilities come immediately to mind.

First, perhaps you have a hole in one or both carb diaphragms. The classic symptom is that the engine will run pretty well, but only up to a point. That was the case with one of my bikes. On another bike, with only one holed diaphragm, the "good cylinder" generated a vibration because it was essentially running wide open throttle and the bad cylinder was running at essentially partial throttle.

The second possibility that comes to mind is that the air vent in the gas cap is partially blocked. The bike gets enough gas because the vent is only partially blocked. But when heavy throttle is demanded there just isn't enough air passing through the vent to allow enough gas to flow out of the tank. (That symptom hasn't happened to me.)

Welcome to the forum. I expect that your good buddy will get a lot of responses regarding your problem.



Ken

[Sunbeem]

Hello Dmallah, and welcome.
I took my RS up to 110mph recently, and found a similar problem. It seems likely that having one petrol tap open didn't allow enough fuel through for higher speeds. Is your friend running on both taps?

Sunbeem.

[dmallah]

So I guess it isn't necessarily an electrical problem. Thanks to Ken and Sunbeem for the speedy replies. Those are interesting things to check. I'll wait a bit to gather more opinions, and check back with my friend to see what comes of this. He rides with both taps as far as I know. Perhaps we need to check that out next time I'm out there.

Cheers,
D

[SteveD]

If I let go of the throttle, it gets back to normal and is fine as long as I don't try to go past that speed.

Diaphragms. That's a classic symptom when the diaphragm has a split or hole.

[Duane Ausherman]

No matter what the issue is, don't run at speed with only one fuel valve open. Just don't do it. I base that on experience with hundreds of bikes with similar problems.

[ME 109]

A holed diaphragm would present problems coming up through the gears to 70mph as well.
Too low fuel level in the float bowl/s will give such symptoms.

[dmallah]

Thanks for the advice everybody, will discuss with my friend and post back.
D

[russphoto]

Also check for clogged screens on the fuel taps, or trash in the tank. Sounds like classic fuel starvation to me.

[mattcfish]

No matter what the issue is, don't run at speed with only one fuel valve open. Just don't do it. I base that on experience with hundreds of bikes with similar problems.

One exception to that.....When my bike runs out of the main fuel supply (and the next gas is farther away than it should be, because some stations still close on Sundays ) I switch off one tap and run on reserve with the other while looking at the odometer. When that reserve runs out I make note of how many miles I just covered. When I switch to reserve on the other tap I know how many more mile I can go and if I need to slow down to make it to the next station. This has saved my butt more times than I'd like to admit. The last time this happened was when I took a scenic detour through the mountains to find that a landslide (no warning signs) had blocked the road 40 miles in. I had expected to get to the next station in a little more than 40 miles. Now I had to back track and cover an addition 60 miles to get gas. I couldn't have had more than a thimble full of fuel in my float chambers when I reached the next station.