1980 CB750k Idle Issues

[UT1980CB750K]

I own a 1980 CB750K w/ Vetter with 34,000 miles that I have been working on for far longer than I care to admit. (10+ yrs)
(I also own an identical motorcycle with around 23,000 miles.)

Without getting into too much detail, my problem is this: I cannot get this thing to idle. I have rebuilt/replaced the carbs twice, checked and rechecked valve clearance (.004 - .005) and run compression tests. Compression numbers are a bit low, but all my tests have been done when the bike is cold. The compression numbers are slightly higher on the 23k engine.

While syncing the carbs with a mercury sync tool, I noticed that the 34k engine draws half as much mercury as the 23k engine. This tells me that my intake "suction" through the carburetors into the engine may be why I am having so much trouble running it at low RPMs. What do I need to do to remedy this problem? Do I need to tear it down and check the rings and seals + cylinder and piston tolerances? Any advice would be greatly appreciated.


On a side note, I am also looking into raising the gearing so that I can travel at high speeds at a lower RPM. What size sprockets (front and rear) would I need to accomplish this? How small can i go on the rear sprocket?

[ArjaiC]

Just a guess. Valve timing might be slightly off.

[TOOLS1]

Are the compression readings within 10% on each cylinder? 34K is not that many miles on one of these engines. Nearly every idle problem can be traced to plugged idle jets, or a vacuum leak. Have you taken the temperature of the exhaust pipes near the head? This will tell you if one or more cylinders are not running correctly. Another thing to check is the ignition timing, and advance.
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[shinyribs]

TOOLS1 wrote

 Nearly every idle problem can be traced to plugged idle jets, or a vacuum leak.

This ^^^ 100%


Also, don't worry about the rpms at speed too much. These bikes like to rev and are happy as a clam sitting at 6000-6500 rpm all day long. Gearing them to lower the revs often just has you opening the throttle further to compensate for the lost power, which ends up to be you using more fuel and actually working the engine harder. Unless you are needing to cruise at 80+ mph, then you might need to re- gear. Just a thought.

[TOOLS1]

What he said ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TOOLS

[UT1980CB750K]

How do I adjust valve timing?

[TOOLS1]

Unless you have had the engine apart, and put it back together with the cams out of time, there shouls not be any need to adjust valve timing.
TOOLS

[UT1980CB750K]

No vacuum leaks (all air-box and intake boots are new, clamps tight, and a WD-40 leak test was negative.)


About 15 years ago I paid someone to to tear it down and replace all the gaskets and seals. The bike ran fine for 100 miles or so but started to lose its tune. I then mothballed it for 10 years and managed to get it running again after it was in storage. I started having engine-racing issues and other poor performance problems. I pulled and cleaned\rebuilt the carbs (on both bikes), triple-checked every part down to the air-shut off diaphragms. The 23k bike purrs like a kitten (OK, Tiger) and has no issues. I (think) I even swapped carburetors and I STILL can't get the 34k bike dialed in. The lack of vacuum while syncing the carbs is the only anomaly I have been unable to diagnose.

Notes:
Could an exhaust leak cause these problems?
Run sync test with air filter removed?
Did the shop I paid to replace all the gaskets and seals miss something?
All exhaust headers are hot after running the bike.

[TOOLS1]

An exhaust leak will not cause these problems. The bike ran fine when it left the mechanic, so I doubt he missed anything. You say it started racing, that is a vacuum leak. Just because the WD-40 test did not indicate a vacuum leak does not mean it does not have one. How about the little O-rings and washers on the fuel/pilot screws, are they good? Again, was the compression within 10% between cylinders? Have you checked ignition timing?
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[Rodgil]

I changed the rear sprocket from 46 to 40 on my 82 CB750F. Front stayed at 18. Highway revs dropped from around 5,000 down to 4,200. 4th gear is now what 5th used to be. 1st gear is now what 2nd used to be, which can be a nuisance only in very slow traffic, but once moving, the bike is much more relaxing to ride. Use the search box to type in "gearing" or "sprockets" for other opinions.