Oscillating revs

I'm trying to diagnose what I suspect is a carb problem.

The bike is extremely difficult to start. Cold it will only start on full choke and with the throttle fully open. When warm, it still needs the throttle fully open and takes a good few turn-overs to finally pick up and go.

When cold, it will idle fairly well, but as it warms up, the problems begin!

The revs climb quite quickly to around 4-6k and begin oscillating! The revs will drop about by about 1,000 rpm, then pick up and rev high again, then drop, then rise. This is with no adjustment to the bike in any way (no throttle, no idle knob adjustment, no choke). Adjust the idle knob to try bring the revs down to a decent idle sort of works ... the revs do drop, but the bike almost always stalls out. If it doesn't stall out, touching the throttle sends the revs way back up where they hang out quite happily. Once or twice when restarting the bike after a low-rev-stall-out, the bike backfired.

The bike had bad vacuum leaks before, but I replaced the carb -> engine boots and the carb spray test doesn't show up any leaks now.

Any ideas what the problem may be? Dirty carbs? Poorly syncronized carbs? Poor air-fuel screw adjustment in one or both carbs?

Thanks!

False air and carbs badly out of synch are the two things I know of that can cause these sympthoms. If the leak is done, check synch.

What is "false air"? Would one of the simple manometer type vacuum sync's described elsewhere on the forum work?

False air = unwanted air, or a vacuum leak. Check your boots again, make sure your idle adjustment screws aren't leaking by. Take your time when you do the leak check and don't over use the carb cleaner or whatever you are using to snoop the leak. Once you have the leaks fixed, synch those carbs. An out of synch set of carbs can make the revs oscillate and cause the engine to make a growling sound and make your timing chain sound loose and rattly.

Vacuum caps on? ed

Hmmm ... methinks false air AND out of sync carbs.

I checked the vacuum caps and one was missing :blink: Made a temporary one from a piece of tubing bent over.

Now the bike runs very well when cold. Starts with just a touch of the starter and has good throttle response But as it warms up, the revs become erratic. Not oscillating anymore, but it will rev high until the idle adjust knob is turned out. It will then drop back down to ~1500rpm, sit for a bit and then drop to a stall. When idling like this, a quick blip of the throttle causes it to bog down and sometimes stall out. Slowly increasing the throttle will get the revs up, but they hang around 3000rpm.

Do carb sync issues get worse as the engine warms up?

Timing advancer might be sticking. Could lube pivot points.

Good Fortune!

Yes. Sounds like you need to also set your mixture. I synch mine and set the mixture with the synch tubes still on and then re-synch them if necessary. I keep doing that until everything is settled.
If you truly have no more leaks and the throttle cable and petcock, and and and is working correctly and all of the planets align etc, your bike should be responsive and smooth.

Balanced the carbs but the problem persisted.

One key symptom I noticed was that if the petcock was set to prime for a long time, gas would begin dripping from the carb overflow tube. *queue aha! moment* Stuck float needle!

Opened the float bowls and sure enough one had a stuck float needle. Lots of rust collecting in the bottom of the float bowl too, so I stripped them down completely and gave them a thorough cleaning.

Reassembling and the bike now starts on the first try warm or cold and idles like a purring tiger

One thing I did notice is a fair amount of mineral-like white deposits around some of the internal carb fittings. It was actually some of these deposits which were responsible for the stuck needle. Any ideas what it may be and how to prevent it forming?

Thanks all