Carb Hic-up

[billpedalino]

I rode my knuckle for about 20 miles on Sunday and all seemed ok until the it experienced an occasional pop-back through the carburetor. I was loafing along nice and easy at about 2,100 RPM and it popped back through the carburetor and I lost power until it caught up with itself and then ran fine again. I turned around and headed home real easy and it did it once more. I made it home and let it cool down a bit and restarted it. I noticed that when in neutral, when I rev it a bit, it sometimes pops back when transitioning from low to mid rpm.
I have not checked the plugs yet so I don't know if it's running generally lean but I doubt it as I'm careful to run a little dirty until the motor breaks in a bit. I will check the plugs after work today though. I'll also change the condenser. I've also checked all of the ignition wire connections and they were fine.
This feels like carburetor. I'm running an M74B temporarily in lieu of the correct M35 because I wanted to shake out any other possible motor anomalies before installing the permanent carburetor. I had carefully rebuilt the M74B and I had faith in it as it ran fine (until now).
So, I've got some troubleshooting to do - Any other suggestions??
Thanks

[T. Cotten]

For any tuning problem, Bill,

I start at the manifold.
If this machine is using PEEK seals, please snug them up.
If not, please bubble-test again.

Plug reading seems futile with today's fuels, since additives, such as injector cleaners, will clean a plug white even in perfect tune.

Other than that, please only change one variable at a time, particularly with the electricals.

....Cotten

[Steve Little]

Hi Bill. This may not be the issue with your carb, but is easy to check. The spring that holds the emulsion tube in place is prone to rusting away where it sits in the bowl nut. Water collects in the steel nut and on occasion the bottom section of the spring will rust away and allow the emulsion tube to drop down. The bike will still start but will have intermittent loss of power and back firing as the tube jiggles up and down in the recess. For quick and easy check, remove air cleaner cover and see if the height of the tube looks right. This might not be your problem but I had a similar issue recently.
Regards Steve

[billpedalino]

I always use PEEK seals. I did bubble test the installation before installing the carb and it was completely sealed at 20 psi, so I don't think its a manifold seal, but I'll give them a snug-up as you suggested.

Plugs do not show any conclusive information, as the oil pump leaks into the lower end and the motor wet sumps when first started until the oil is scavenged out, after which it runs clean. So the plugs show a little oil burning which obscures any meaningful information - but they are don't appear to be running white which would still show up after a 20 mile long ride.

The elimination process will continue this weekend if I can find some time.

For any tuning problem, Bill,

I start at the manifold.
If this machine is using PEEK seals, please snug them up.
If not, please bubble-test again.

Plug reading seems futile with today's fuels, since additives, such as injector cleaners, will clean a plug white even in perfect tune.

Other than that, please only change one variable at a time, particularly with the electricals.

....Cotten

[billpedalino]

Hi Steve. The carb was completely rebuilt (by me) with only a couple of hundred miles on it and the nozzle spring is brand new, but thank you for the suggestion.

[T. Cotten]

Bill!

All seals need retorques after a few heat cycles.
If you are using mass-produced PEEK seals, you will find they will require repeated retorques, as they are made to brass specifications, which was intentionally thin to allow it to 'spring'. PEEK is compliant, not springy, and should be cut to different dimensions for the best results, and re-useability.

The seam between mainfold and carb cannot be bubble-tested, so I suggest that the flanges be arbitrarily dressed to "flat". You may be able to diagnose a leak there, or at the throttleshaft, with an un-lit propane torch while the machine is idling.

If that still comes up negative, then we should address conditions within the carburetor, such as a loose venturi, bore wear, etc.

Good luck!

....Cotten
PS: 20 psi is overkill, as absolute vacuum is equal to only 14.7 psi.

[Bruce T]

Gday Bill, I had a similar problem to what you have described on my 46FL. I found on the front head that the seal between the spark plug washer and head was poor and leaked. Start your knuckle up and throw on some soapy water. you'll soon see if there is an issue. I fixed the leak by refacing the spark plug seat and the bike ran perfectly after that. Cheers BT

[harleytoprock]

Bill, It sounds to me your a little lean. Richen it up a few clicks and see if that helps.