76 F1, No throttle above 2500

[Bojangles]

Long time lurker, first time poster. So much so that I already have a good idea of the levels of snark I'll see in the responses lol, but here goes.

Rescued an abandoned 1976 CB750F build to bring it back to its former glory in time before any major mods were done. Bike had not been sitting long and was mostly intact apart from a swiss cheesed tank, although the carbs are 7A carbs which i believe came on earlier models (?). PO recently had carbs rebuilt to factory settings (verified needle setting myself) and replaced ignition coils, starter, clutch, and flywheel. I replaced the tank, petcock, wiring harness, both right and left hand controls, fuse box, and a few other odds and ends. Everything functions properly

Finally got it wired properly and running, it would rev well into the top of the rpm range. Then, took it for a 1st gear lap around the parking lot, suddenly it does not want to rev past about 2500 whether in neutral or in gear. When i let off the throttle, the bike immediately dies.

And before anyone asks, yes it has pods on it as I'm deciding whether to either repair or replace the stock airbox


Any advice is welcome!

 

[dirtdigger]

Pods are not an issue on regular carbs only issues on cv type carbs, you just need to jet correctly. You should see what main jets are in it so you know what direction you need to go. My guess would be you sucked some garbage into the jets, pull the bowls see whats in there then pull the jets.

What have you done to the points? Verify timing? Point gap? dwell? Verify ignition advance is working correct and is free? How is your compression? What do the plugs look like? Have spark on all cylinders? Valve clearances? timing chain adjusted?

 

[Bojangles]

Pods are not an issue on regular carbs only issues on cv type carbs, you just need to jet correctly. You should see what main jets are in it so you know what direction you need to go. My guess would be you sucked some garbage into the jets, pull the bowls see whats in there then pull the jets.

What have you done to the points? Verify timing? Point gap? dwell? Verify ignition advance is working correct and is free? How is your compression? What do the plugs look like? Have spark on all cylinders? Valve clearances? timing chain adjusted?

Since my last post I've pulled the carbs (previously bench synced), floats are good and went through the jets with a guitar string to clean out, all good there. Cam tensioner adjusted, ignition advance cleaned and lubed, replaced the points, gapped and timed, replaced spark plugs, adjusted valve clearances (were way too tight), spark on all cylinders. Fired right up and sounded great for about 4 seconds but same issue followed. Also failed to notice something embarrassingly obvious; while it was idling, i took a laser thermometer to the exhaust, cylinders 3&4 are cold. Both are getting spark and briefly ran the bike with the valve tappets off to verify the cam was still working. All good there. Now I'm even more stumped

 

[Bojangles]

Pulled the tank, made sure coils were connected correctly, fired up on all 4 cylinders. But cylinders 1&2 pipes got to about 200 degrees quickly, while 3&4 were in the low 100's

 

[Bojangles]

Compression 150/100/150/160. Adjusted carbs using a 1/16 drill bit, previously had used 1/8. Seems to be running much better, albeit a little rich. Carb sync next. Still a little hanging throttle probably due to throttle cables being too long