R100 stumble just off idle

[pmdave]

OK, if this has already been covered, slap my hand and send me to the right string.

My R100t has had a little glitch for about the last 10 years. Everything works fine, valves set, carbs balanced, good compression, pulls cleanly at WFO etc. etc. But just off idle the engine stumbles. Not enough to be a big problem, just an annoyance that I don't want to live with any longer.

From idle, if you just tweak on a little throttle as you ease out the clutch, it almost stalls for a second until the revs pick up. If you rev up the engine a few RPMS before de-clutching, it pulls smoothly.

Does this symptom remind anyone of a common problem--say throttle shaft O rings worn, or needle worn?

pmdave :dunno

[James.A]

Does this symptom remind anyone of a common problem--say throttle shaft O rings worn, or needle worn?



Well Dave, I'd say any or all the above, and maybe diaphragms to boot. How many miles on the bike? Carb service history? Have they held old fuel for more than a few months? Are the slides absolutely free in their bores? How do the spark plugs look? I'd consider a good cleaning and some new parts. Matt Parkhouse wrote in one of his columns that the main needle and jet could be worn sufficiently to de-grade performance at 40K. I would think that those parts would bear more on fuel economy and performance thru the RPM range.

BTW, what is WFO?

[TZOLK]

wide frickin open

[pmdave]

The carbs have been rebuilt before, but the bike has many hard miles on it pulling a sidecar, and during the time it was owned by my son got infrequent attention. I guess I already know the answer: complete rebuild of both carbs, including all the little difficult things like the shaft O rings.

I was hoping to get some shade tree advice based on the symptoms, so I would know where to look more closely.

pmdave :wave

[jdiaz]

Aren't there two microscopic holes in the bottom of the carburetor bore, adjacent to the butterfly edge when closed, specifically intended to help this off-idle situation? The CV's on my R65 have them. Maybe they are plugged.

[lkchris]

From the Bing manual:

"Engine Misfire (0 to 1/8 throttle) The mixture in this throttle range is controlled by the idle jet and mixture control adjusting screw. Misfiring in this range is a result of a 'too lean' air/fuel mixture that is corrected by resetting the mixture screw or changing the idle jet to the next larger size, and matching the screw setting to the larger jet."

or

"Low Power Upon Acceleration This condition, if a fault of the carburetor, would be an excessively rich air/fuel mixture. The various stages of throttle opening are similar [to above] except the components would be selected to obtain a leaner mixture."

http://www.bingcarburetor.com/manual.html