runaway throttle!

[Steveo123]

Okay guys;

Here's my latest issue, which I believe has to do with the carburetors and/or throttle on my 66 CB77. Once the bike is warmed up, and I proceed up through the gears, the bike will often literally just rocket up the RPM's like I've floored the throttle. The bike does it on its own, and the only way I've found to address it is to just turn the bike off. And when I say the RPM's take off, I mean it; the thing goes nuts! I can then just restart the engine, and things settle down until the next time, which could literally be 30 seconds later or 5 minutes later. I can't seem to pin down when/why it happens, but it occurs at gear changes.

I've only been noodling around the neighborhood so far, so it's always been relatively slow-speed/lower gear moments on empty streets. However, obviously this issue makes the bike unsafe to ride in any sort of practical way.

I have done the basic, initial carb set-up per the owner's manual, and the jets/carb internals are all new. I cleaned the carb bodies thoroughly before reassembly, but I'm not sure I adjusted the floats well; not sure if this would cause or contribute to this issue, but thought I'd mention it. Also, at the moment, I don't have the air filters on, but I recall this issue happening with the filters (which are the original filter bodies with UNI green foam instead of the rotted paper) installed. The throttle cable is older, but appears to work responsively (i.e. isn't sticky or slow). That said, I do have stuttering of the throttle response when first turning the bike over/when it's cold.

Any thoughts or suggestions as to how to address this problem, and more fundamentally, what the heck the problem actually is (!) would be great!

Thanks in advance!

[DJM]

I had just this problem with my CL77 a while back, the problem was nothing to do with the carbs but related to ignition timing, having the wrong advancer unit fitted (actually a CA unit) and also the said advancer being in less than perfect condition and thus giving around ten degree more advance than it should have done. Wore the brakes out trying to hold it back but it wasn't really safe to ride in town traffic!

Fitting a 'new' advancer of the correct type had the bike running better than it ever had before. Ticks over like a traction engine now and pulls like a train from tickover right up to full revs, no coughs, splutters, flat spots or spit backs, still backfires a bit on the over-run though!

Too involved to post full details here but I wrote a piece for the VJMC magazine on the subject which I can E-mail to anyone interested.

[Steveo123]

YES! Thank you for your response. I now remember this issue arose after adjusting the points plate to get the timingmark to hit closer to the F Mark! R u suggesting that the points need replacement?

Or can I just adjust the plate to retard the timing a bit and avoid the run away throttle?

Thanks!

[DJM]

I'm guessing that you adjusted the points plate to get the engine firing at the F mark on full advance, IE you were using a strobe light?

I think that the problem isn't with the points but with the advancer.

IF you have the same problem I had the advancer is advancing the spark too much and at the wrong engine speed, only solution is to change the advancer.

With a CA or CB it's an engine out job but it can be done on a CL with the engine in place.

Give me your E-mail address and I'll send you a copy of the article I wrote to see how closely it matches your problem.

[Nick]

You sure you didn't set the static timing to the full advance timing mark? The static timing mark is right next to the "T".

[Steveo123]

I'm going to revisit timing, in light of the issues mentioned here. Thanks for the additional thoughts!