Weird Issue

So, I have an interesting issue that just started happening. Keep in mind, carbs were cleaned pretty recently and no other major issues (also just started happening out of nowhere).

The bike starts up fine, I let it warm up a bit, take it out on the road. I get on the highway, hit 6th gear (going about 70) and it's running great, sounds great the whole time. After a few seconds to a few minutes, out of nowhere, the engine pitch changes, lower pitch, deeper. And I can feel a slight loss of power. Then, really weird, every few seconds (usually 15-30) I feel the power kick back in (pitch changes back to normal) almost like I'm reving the engine for a pass, and you can feel the slight increase in revs (tach goes up about 500 revs from hovering 6000 to 6500). Dunno why this is happening, really have no clue as to where to start so I'd thought I'd ask. Does this sound familiar to anyone, anyone have these symptoms and/or know what's causing it/how to fix.

It's not a huge issue, but something's definitely not normal about it. Also, I replaced the throttle shaft seals last fall, so I don't think there's any air leaks there. And fuel mixture is pretty good, spark plugs look good/ brownish.

Thanks for the help!

 

Valves in spec?

Float levels precisely wet-set?

What did you use to sync with?

Fusebox rebuilt/replaced?

Check your plug caps for loose resistor cores.

Check your vacuum line (petcock to intake manifold) for cracks.

Check the condition of the little rubber caps on the vacuum ports; you can't tell by LOOKING. Pull them off and be sure they're still flexible. They can be as hard as a rock, and start "dancing" at certain vibration frequencies, causing an intermittent vacuum leak.

Check the tightness of the headpipe clamps at the head (don't over tighten-- loosen and re-torque to spec) and be sure none of your pipes are loose at the collector.

 

if the tach goes up 500 rpm without a corresponding speed change the clutch is slipping

 

Did running vacuum sync few months ago as well as float level set, I think it'd be odd for them to go out sync this soon but I suppose it's possible. Fuse box replaced, definitely not a prob. Valves checked same time as syncing, totally in spec. Vacuum line is new, boots are fine, caps are soft and secure, plugs are great and plug caps are in excellent condition, pipes are tight.

I was thinking it could be a clutch slip, but I don't know, less than 20k miles, and I change the oil twice a year with castrol 4t conventional motorcycle oil (20w-50 in there now).

Here's a little more info, when I really open up the throttle on it, it kind of clears its throat and I get that power back, and if I pull in the clutch some times it "burps" a tiny bit (like a minny backfire).

 

Almost sounds like a plug is fouling. Are they all the same color?

 

Yeah, none of them are black, pretty much all have a little brown patch and kinda pale on the tip. I do have extra plugs, so maybe I'll switch some out and see...

 

Also, I figured out it doesn't just happen in 6th gear on the highway. Pitch changed and same symptoms happened on regular roads after about 5-10 minutes, I so I think it happens after it's warmed up a lot.

 

ignitor unit

dave

 

Really? never thought of that, anyway to test before I buy a new one (kinda expensive), or are you 100% positive

 

you have a rubber chain

 

sticking throttle slide? do the clunk test.
stu

 

What?

Throttle's not sticking, revs jumping up without my moving the throttle at all, then back down. I did clunk test last time I took off the carbs, I guess I'll just have to take off the carbs completely and clean them thoroughly again to eliminate them as the source (or hopefully fix it).

 

there are very few ways the rpm's can go up without the bike going faster. it could be down shifting by itself then up shifting by itself.
the back tire could be shrinking then expanding. there could be spiders in the tach grabbing the needle as it goes by. the chain could be rubber and stretching and contracting at high rpm's. or your clutch could be slipping because the springs are 30 years old.
so let all the air out of your tires and get some new air, tighten the bolt on your shifter, spray raid in the tach, make sure a magnet sticks to the chain, then fix your clutch