45" Harley Flathead - Carbon on Valve Seats and Faces - Bent Valve

[Buster]

Rebuilt 45" flathead motor with 2,500 miles on it. Had good leak down test when built, motor has Mikuni VM34 carb and new oversize valves due to deep seats. It stopped running and had no compression. Pulled heads and had baked on carbon deposits on all valve seats and valve faces. Also, carbon on piston tops and in heads. Minor Vertical scoring in cylinder bores. One intake valve slightly bent. Could carbon buildup on valve seats be from running too rich? The bike was getting poor gas mileage, around 25 miles per gallon. Going to reduce size of main jet in carb when reassembled. Would the carbon build up between valve and seat be the cause of bent intake valve. Any help appreciated.

[tgarcia]

It's possible carbon build up one side of the seat would cause a bent valve but not real likely. You probably would have gotten a low compression symptom well before it bent.You might check and see if you have an interference problem on that valve where the combination of oversized valves and a loose guide could cause it to touch the head or something else in close proximity.

[dukekleman]

The carbon build up is a cause of an overly rich tune. I would definitely down jet the calibration wherever it need be... example: pilot jet, cut away, jet needle, needle jet and main jet.
Where the valve clearances ever checked and/or re-set since the initial rebuild and 2500 miles currently on the bike?

[Buster]

Yes, I checked valves around 200 miles and then at 1,500. They required only slight adjustment at 1,500 miles. Got new valve on order and 2 next size down main jets. No sign of valve interference anywhere. I'm also suspecting over rich condition due to poor mileage.

[T. Cotten]

Diabolical, Buster!

I wouldn't know your modern carb if it bit me, but if it was tuned to run decent to begin with, then fat must have been the only way the motor would run.

Extra fuel means not only extra carbon, but extra heat.

The extra oxygen had to come from somewhere, and that's where the fuse was lit.

...Cotten
PS: Usually a valve bent from a collision shows a nick on it, but then I've only seen bent OHV valves, which are a lot easier to stick in the guide. I bent this one myself...

S&SPIST.jpg

[dukekleman]

Was your mpg was measured while at cruising speed or general all round riding? It makes a difference when judging the calibration installed... Regardless your mpg will improve at wide open throttle with lowering your main jet 2 sizes as you mentioned your planning on doing, however it won't improve to an acceptable level where I suspect it should be improved.

In order to improve your mpg and reduce carbon build up, I would be changing the pilot jet, air screw setting, needle jet and jet needle. Those will improve mpg overall and help eliminate excessive carbon build up. Remember that excessive carbon is a sign of overly rich, overly rich is diluting your oil film in the cross hatch of your cylinders, hurting ring seal as well as ring wear and piston wear.

Carbon build up also acts like a sponge in the combustion chamber absorbing some of your fuel ratio when it enters the chamber prior to ignition. This over time allows the carbon to enhance or invite pre-ignition. In general it's starts a chain reaction of many things none of which your engine wants...

I hope this helps,
Duke