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Background - I've replaced the float bowl gaskets, "choke" gaskets, needle jets, jet needles, and float needles on my 1987 R80RT. She starts fine, runs fine when hot. When warm, she stumbles while cruising around 3,500 rpm in town but pulls strong under load. Also, while warm, while at a stop light, she will idle fine at ~1,000 and then die without warning or cough. doesn't do this when hot. I've adjusted the carbs - mixture and idle. Runs very smooth except as noted above.

Any ideas?

Pulls strong on the main jet, but not so below?

I think I'd clean the holes in the throat of the carb with some fishing line and follow up with a carb cleaner spray.

The black dot on the right...



...and the one behind the butterfly, from the #1 slow jet. Clean out the jet too.

Thanks for your reply, Steve. The bike pulls fine in all RPM ranges - it stumbles while cruising ~ 3,500 rpm under very light load. But if I open the throttle to accelerate, no stumble at any RPM range. It does strike me as an idle jet issue.

Sounds like its running lean at cruise, I'd raise the needles up a notch and try it.

George - would the needle location have anything to do with the bike dieing while at idle? The mode is idling fine and then it stops without warning or symptom. Starts back up easy.

A little richer on the mixture screws may help....assuming all the passages are clean.

Manfred wrote:When warm, she stumbles while cruising around 3,500 rpm in town but pulls strong under load.

I read somewhere recently a bloke had a loose dome thingee on the BING, allowing too much air...some sealant fixed the problem.

SteveD wrote:I read somewhere recently a bloke had a loose dome thingee on the BING, allowing too much air...some sealant fixed the problem.

That's most interesting. The diaphragms on both my Bings are in fine shape and the tops screwed down good. Makes one wonder...

From Motobins hints list.

The Auxiliary Carburettor itself can often become loose, so being by-passed and ineffective; the resultant air leak will also promote 'surging' at about 40 mph on a small throttle opening change the gasket every few years, and make sure the screws are really tight.

I think you'd expect some cold start problems with that though.

It gets curiouser and curiouser! Since synchronizing the carbs, she starts easy while cold (always starts easy when hot), and the stumbling/surging at 3,500 RPM is nearly gone. When I upgraded the wires to and from the diode board last year, I checked the ICU (or whatever that black box is) and made sure it was mounted well. Spark plug wires are 2 years old. I am very close to buying a Dyna coil...