Topic: Weird RPM/pull surging/hopping when cruising and slow acceleration.

[coma13]

My bike has been doing a weird thing recently. If I'm cruising at a fixed RPM (say 4K or so) or accelerating very slowly/smoothly the bike will surge rather rapidly. It is very noticeable and it feels like the bike is alternating between cruising and coasting. The RPMs surge and fall in conjunction, but only like 50-100 RPM.

This symptom disappears under moderate to hard acceleration. I can also pull in the clutch and maintain the same RPM and the surging vanishes instantly. That fact makes me think that it's a chassis/physical thing rather than an engine or carburation.

The carbs do need to be synced, they were just rebuilt, but I don't feel like that is playing a part in this problem. (I think that may have something to do with my idle rising from really low to really high at stoplights, but that's neither here nor there.)

Thank you very much for any help!

Any ideas?

[Gordon]

Riding in gear at 4K rpm and revving in neutral at 4K rpm are completely different as far as the engine and carbs are concerned.  An engine under load is behaving differently and drawing more fuel than one that's just revving.  You say the carbs need to be synchronized, so synchronize them and see if that helps.  I wouldn't be surprised if it does.  Did you do the rest of the tune-up procedures as well? 

[coma13]

Valves are adjusted but I haven't set my timing. I suppose I should just bite the bullet and buy the carb sync tool since I doubt this will be the last time I need to use it.

Another thing to note, my bike does a noticeable lurch forward if I chop the throttle and conversely a lurch back if I crack the throttle back open. I don't mean an overall weight transfer to the rear like I would expect to sustain in acceleration, but an awkward lurch at the very beginning of the weight transfer. Could these symptoms be related? The surging definitely feels like a less severe and more sped up version of the lurching I just described.

[Gordon]

How's your drive chain tension?

[coma13]

How's your drive chain tension?

That was something I was considering as it was WAY too loose. I tightened it (overtightened, actually) and took it for a ride and it was doing the same thing, just as severely as before. I loosened my chain slightly upon getting home. Haven't ridden it since then though.

[Gordon]

Pull the spark plugs and see what they look like.  How old are they?  I'm guessing at least one of them is very black, probably more. 

[ekpent]

You mentioned the carbs were recently rebuilt. Did it have those symptoms before that task ?  What size and model of bike also?

[coma13]

It's a 1977 CB750K with PD42A carbs. I am not sure if the bike did this before the carbs were rebuilt, because I didn't ride it very much before pulling them. I seem to recall not noticing it before the rebuild, though.

I'll pull the spark plugs and see what I'm looking at there. If I'm running too rich (which I expect) could that contribute to this?

[Gordon]

I'll pull the spark plugs and see what I'm looking at there. If I'm running too rich (which I expect) could that contribute to this?

It would if the plugs are fouling out. 

[coma13]

Cool. Thanks for the input! I'll check back in after I've checked out the plug situation.