Rear cylinder running rich

[Coolbreeze]

My '56 KHK rear cylinder is running significantly richer than the front. The front spark plug is clean as a whistle; whereas, the rear plug is black with soot. There is soot on the inside of the tail pipe.
1. Does this sound like a air leak in the intake for the front cylinder? If that is the case and if the bike is tuned looking only at the front cylinder, that might explain what I am finding.
2. If there is no leak, should I tune for the richest cylinder and let the other one run lean?
3. Should I leave tuning "as is" and run a hotter plug in the rear? This might burn off the material that would otherwise accumulate on the rear plug.

[T. Cotten]

George!

A vacuum leak is not good for the motor. Please bubble-test your manifold assembly as discussed at http://virtualindian.org/11techleaktest.html

K and WR manifolds are notorious for distorted spigots, because they are so thin. They can be swaged back out and lathe-cut easily, if you have the fixture.

....Cotten

[Coolbreeze]

Thanks, Tom. I switched plugs and the front still runs clean and the rear collects soot. The soot doesn't seem oily; so, it may be due to excess fuel. Otherwise, I would suspect a failed ring. We sprayed combustible material around the front connection of the intake manifold, but it made no difference. I guess you are right: I need to do a manifold leak test. Too bad Lacon is not closer to Springfield so that I could just drop this off at your shop!

[T. Cotten]

George!

Josh Davis is your local professional. My shop cannot accomodate chassis currently, but manifolds are routine, and easily shipped. (A leak between a nipple and the cylinderhead would be tragic, however.)

It would also be prudent to test the heads' seal to the cylinders, and around the sparkplugs.

....Cotten

[Bruce T]

G’day Cotton
Wondering if you could comment on the following? I want to settle a bet…..If your manifold is leaking at the front cylinder this would result in a leaner mixture. Is it plausible however that the leak would affect the mixture in both the front and rear cylinders as air will be “sucked in” during the intake stroke of both cylinders not just the front?

[frichie68]

I don't see "compression test" mentioned. Cheap, easy to do and might answer some questions.

. . . . and you don't have to take anything apart to do it . . . .

[T. Cotten]

Bruce!

All bets are off with evil vacuum leaks; The only ones that behave the same are those that prevent the machine from running at all.

Reversion in the intake makes for a very complicated environment, indeed. Often a leak to the one cylinder will make the other run rich, as the carb is tuned to accomodate the lean cylinder.
My philosophy is to bubble-test for any and all tuning complaints, and even if aren't any yet. It eliminates the variable entirely, allowing other problems to be sorted out much more easily.

And Rich!

Bubble-testing is cheap and easy too. A compression test will only tell you if damage has already been done.

....Cotten

[Coolbreeze]

From what we have determined, after tearing down the upper end, is that the problem is not richness, but oil surplus in the combustion chamber because of a bad ring. The ring problem appears to be due to a wavy (cf, straight) cylinder wall. The bore is now straight and 0.060" over. When we get this slapped together in the next week or so, we shall know for sure if this was the problem.