Topic: 1972 CB750 K2

[LTS70]

Was able to buy my dream bike but getting it to run right has been a nightmare. Has always ran rough since I bought it. Had the carburetors gone through in Utah where I bought it before I went to pick it up. They also installed new intake boots, velocity boots and a airbox that was not cut into. They also had a company clean and line the gas tank and installed new petcock and gas lines. I was hoping to ride it home to Idaho. It seemed to run good but as soon I got on the freeway it fell on its face.

Trailered it home. Ran rich and new plugs were black. Coil wires were in bad shape so I got a new dyna electronic ignition kit with new coils and wires. Seemed to run better at first like before but went right back to running rough. It even started back firing and was difficult to even get started at this point.

Did a compression test and it seemed low averaging around 80 on all Four. Had a freind that works at a bike shop check the valve lash. He said the intakes had two that were lose and two that were good. The exhaust all 4 were too tight. He reset things and it would start great and sounded great.

Thought I had it wiped. Took it for a ride and it started out great but top end was sluggish. Then slight miss back to running like crap again. Need to borrow comprehension tester again to check again, mine won’t fit the thread size.

Any suggestions?

[LTS70]

.

[scottly]

 Ran rich and new plugs were black.

Check the plugs. Are they black and dry, or black and wet? Once fouled, they tend to stay fouled, even after fixing the carburetor problem that caused fouling in the first place. Either replace them or burn the black off with a propane torch.
Also, did you set the ignition timing with a strobe, or just static timing?

[LTS70]

New plugs with each major change. Even went a step hotter. Black and dry has been the normal. Exhaust always seemed rich untill after valve adjustment then the exhaust looked great but still went back to running like crap.

It’s like if it sits for awhile it will run good at first then as it warms up goes yo crap. One of reasons I did coils and ignition.

[scottly]

If the plugs are black, there is too much fuel or too little air. You have a carburetor problem. First, make sure you are using the choke properly: it should only be needed for cold starts, and should be open once the bike is warmed up.

[jlh3rd]

fuel cap ?

[LTS70]

Choke has been used only for cold starting.

Fuel cap seems to seal well, I will double check the gasket.

[scottly]

Fuel cap would not cause the engine to run rich. If the vent in the cap was plugged, the engine would slowly "run out of gas", as air couldn't get in to replace the fuel.

[M 750K6]

Are you certain it's fuel, not oil on the plugs?

[Stev-o]

Had the carburetors gone through in Utah...

Did they install aftermarket carb kits?  You need to confirm you have genuine brass Keihin parts or you could be chasing your tail tring to tune it.

[willbird]

Fuel cap would not cause the engine to run rich. If the vent in the cap was plugged, the engine would slowly "run out of gas", as air couldn't get in to replace the fuel.

I put an infamous four into one cap on my K2 and I have to burp it. I had soaked my OEM one in EvapoRust without realizing how much stuff it had going on inside there.

Bill

[LTS70]

Carbs were ultra sonic cleaned and new orings an d bowel gaskets used. No new brass parts.

[Stev-o]

Carbs were ultra sonic cleaned and new orings an d bowel gaskets used. No new brass parts.

Ok, but are they genuine Keihin? 

Never can tell who's been in there and done who knows what on a 50 yo bike!