I have a problem that no amount of swearing or drinking seems to help

[Aaron]

I have lurked on this site for a while getting tips and advice, THANK YOU to the guys that run it!! My problem is with a 1980 cb750f that has about 10,000 miles and was sitting for many years. It pops while at steady acceleration. It starts doing it about 3000 to 3500 rpms. I have checked valve clearance, cam chain tension, ignition system (replaced caps, wires, coils, plugs)  cleaned out the carbs meticulously, comp. check was about 140 across the board, and checked for leakage in intake manifolds with no change in popping. The popping occurs in either #1 or #2 cylinder, always. My gut is saying that I have a sticky valve but I just don't know anymore!

[TOOLS1]

Sounds like the "air cut valves'" They can be plugged with small pieces of rubber cut from an old inner tube. Just put "air cut valve" into the search feature at the top of the home page.
TOOLS

[Aaron]

Sorry should've said, I plugged the air cut valves and bumped up the jetting. I am running stock exhaust (i think) but repacked the baffles lighter than what was. I have been driving it and doing plug chops to make sure I'm not running too lean. Also, I did synch the carbs...

[Hoosier Daddy]

Sound like you have eliminated a vacuum leak, Do the vacuum pistons move smoothly in their bores of the carb hats?

[Aaron]

Yes, when I had the carbs apart I also took the tops apart and physically cleaned out everything. After I had them together I checked with my finger to make sure everything was moving freely.

[Lucky 1]

Forget that "sticky valve" ignorance.

The valves wiil go up and down no matter what.
The cam turns- they go down(open).
The 400 lbs valve spring pressure makes them return to the closed position.


Did you replace the points and condensers?
Or does it have electronic ignition.
Did you clean the idle jet with a #10 guitar string?
Can you see daylight through the idle jet??

If you did not replace them, that will cause the problem.

You cannot reset old used points.

[Aaron]

The bike has CDI. I removed all the jets and passage plugs multiple times and inspected/cleaned them. While i had them out I ran air and carb cleaner to check to make sure there were no obstructions in the passages. Obviously there is something I'm missing here with my problem so I give thanks for your insight!! I know its cylinder 1 or 2 that's the problem so I've spent more time on those carbs and still havent been able to see it

[Johnnylevi]

You've probably already done this, but have you fine tuned the air/fuel mixture screws? mine was doing that awhile back and i spent more time than i'd like to admit adjusting the air/fuel mixture screws and now there are no pops eva.

[Aaron]

Yeah, I have but you saying that got me thinking, when your bike was popping how did you adjust to to fix it?  Meaning, did you turn the adjustment out further? I ask cause I have idled and listened and adjusted accordingly but I remember the last time I did it, the problem cylinders didn't seem to react to the adjustment like the others did.....I wonder if something is stil clogged with those passages....I've had those carbs off so many times cleaning and checking with no effect, but there's something that I missed. Side note, the adjustment screw tips and the o rings were in good shape when I cleaned out the passages last time

[TOOLS1]

Since it is not the aircut valves, and it is doing it on 1 and 4, it is your ignition system. Start with the coil, and then the CDI unit.
TOOLS

[Aaron]

My problem cylinders are #1 and/or #2. The popping has been coming out of that muffler only. Originally I thought my problem was number 2, the exhaust pipe didn't heat up the same as the rest (was cooler). However, lately it has been heating up better but something is still popping...