[450roo]

Had a cylinder running a bit lean, not excessively but more than I liked. Did all the usual things, clean and sett up the carbies, checked the timing and ignition system all ok. Then found a leak on the exhaust manifold which when fixed solved the lean cylinder problem.
I am a bit doubtful, is it really possible that a leaky manifold would cause a cylinder to run lean or is it just coincidence that the problem went away after I fixed the leaky manifold?

 

[Alan F.]

Sucking in air from any source other than through the carburetor will cause a lean condition. Your carb was doing its job without issue, the 'unmetered air' entering the system was leaning it out.

 

[doode]

Did you experience a change in idle speed after you solved the leak?

 

[Oldmechanic]

if it was a leak in the exhaust side, how is the leaning out the mixture? I could see that with an intake leak, but not exhaust

 

[WintrSol]

The black art of exhaust systems. The head pipe is like a flute or trombone, which has pressure waves of gas motion both away from and returning to the port. If you open a gap near the port, the returning waves can be diminished or shifted in time. There is overlap in the valve timing, and disturbing the timing of the returning pressure wave can force some of the exhaust back past the valve, leaning the mixture. Like a flute, these waves are frequency (rpm) dependent, and the effect can also enrich the mixture at certain rpm.

Or, it could just be coincidence that sealing the exhaust happened the same time whatever other air leak stopped.

 

[450roo]

The idle remained pretty constant but I did need to make minor adjustment to the mixture.
Off on a club run tomorrow they are usually 150 miles or more so it will be a good test to make sure the problem is solved.

 

[Alan F.]

He found a split in a rubber carb manifold, there was a photo in his other thread.

 

[450roo]

Who found a split in the intake rubber?

 

[450roo]

Back from 100 mile run, the bike ran like a Japanese watch ? pulled the plugs and both are a lovely brown color. So what ever the problem was it is fixed. Yippie!