Another lean on one cylinder question - Harley VL

[Buster]

My 1931 VL suddenly started having lean condition with no color on sparkplug on one cylinder 1,500 miles after rebuild. Still runs great, but compression on lean cylinder is now 35 lbs and leak down at 20 lbs pressure is about 25%. Almost all leak down is at intake valve, very little at exhaust. I have done 15 lbs pressure test with Snoop and/or soapy water on intake manifold, intake nipples, head gasket, and flange between carb and manifold. No leaks. Found leaks on both heads at bosses where sparkplug plug gaskets seal. had bosses milled and no leakage now. Removed both primer cups and plugged holes. Intake valve is only leak I can find, and it is significant. Getting ready to tear back down again. Would an intake leak cause the cylinder to run lean. I have been told that a leaking intake valve would not affect the lean condition and only a leaking exhaust valve would make cylinder run lean. Is this correct or if not, anybody have any other ideas so I will know what to look for? Thanks.

[T. Cotten]

Just to make it complicated, Buster,...

No two vacuum leaks behave the same (except those that prevent the motor from running at all!)

And reading plugs is futile with modern 'anti-deposit' additive fuels: Any "color" at all is over-rich.

"Would an intake leak cause the cylinder to run lean. " What else?

"I have been told that a leaking intake valve would not affect the lean condition and only a leaking exhaust valve would make cylinder run lean."

Spoken like a true arm-chair engineer. Tell him I said so.

....Cotten

[dukekleman]

Buster,
Absolutely, positively with certainty a leaking intake valve will cause the cylinder to run lean.
Question is, why is it not sealing properly after 1500 miles on the new rebuild?
Have you checked the clearance at the tappet?
Has this clearance been checked during service intervals after the rebuild?
If there are any oil control issues in that cylinder or if it ran to rich, it's possible that excessive carbon is causing the leak down issue.

Either way, it shouldn't a worn valve guide, valve or valve seat issue this early on after rebuild.
This means lack of clearance at the tappet or carbon is most likely the cause of your leaking at the valve seat.

Duke

[T. Cotten]

...Still runs great, but ..

But .. ..

...I have done 15 lbs pressure test with Snoop and/or soapy water on intake manifold, intake nipples, head gasket, and flange between carb and manifold. No leaks..

The bubble-test is performed with the carb removed; Please review http://virtualindian.org/11techleaktest.html.

Testing the manifold-to-carb interface ain't so easy,

...Cotten

[Steve Slocombe]

Dear Buster, is it front ot rear cylinder? Any problems are usually on the hotter running front cylinder.

[Buster]

It's the front cylinder. I'm guessing it is carbon maybe since there was no problem before, I will report back with findings after disassembly. Thanks for info on intake valve leak. Going to road run next week, so might be a couple of weeks on report back.

[Buster]

I know its been a while but still haven't found problem. Fixed leaking valve problem. Pulled front cylinder and found middle ring landing was pinching middle ring on one side of piston. I'm guessing this was a heat problem. I clearance the narrowed area of the landing with file and reinstalled the ring. I know I will have to replace the piston and rings. Checked timing with degree wheel and found timing set at 35 degrees. I installed new mark on flywheel at 30 degrees and have it timed there now. Put back together and front cylinder still lean. I found a hard carbon build up about a 1/3 size of top of piston on valve side of piston. Other side of front piston and rear piston are clean. compression test cold shows 50 lbs on rear and 35lbs on front dry. I can add oil to front piston and it comes up to 50 lbs. I know I have a ring problem. Leak down test shows much air leakage into crankcase. Could the 15lbs lower compression on the front piston be causing my lean problem? Would leakage thru the rings cause a lean problem? Thanks for any help.

[T. Cotten]

...Could the 15lbs lower compression on the front piston be causing my lean problem?... Thanks for any help.

A lean problem could certainly cause the lower compression, Buster.

...Would leakage thru the rings cause a lean problem?

Hand-dressed ring land damage makes it a candidate.

But something else already got that hot.

....Cotten

[Buster]

Pulled head and cylinder. Baked on oil on top of one piston. Looks like vacuum is pulling oil past rings on one side of cylinder to top of cylinder due to damaged rings and landing. Cotten, as always thanks for your help. To clarify your comment "A lean problem could certainly cause the lower compression", can the air from crankcase sucked in to cylinder due to bad rings cause a lean condition similar to a leaking vale or nipple? Evidenced by baked on oil on top piston a substantial amount of oil is being sucked from crankcase into combustion chamber.

[T. Cotten]

...can the air from crankcase sucked in to cylinder due to bad rings cause a lean condition similar to a leaking vale or nipple?..

There's almost always some leakage, Buster...

They are never absolute, especially on a Flatty, but a lot more getting to one combustion chamber than the other is undoubtedly going to be an issue.

I hoped somebody else would chime in sooner!

....Cotten
PS: It's hard to figure where the fuse was lit after the bomb went off.

[dukekleman]

Buster,
Bad piston rings, or bad valves or tightly adjusted valves all have the same affect which will cause it to run lean on that particular cylinder when talking carburetor motorcycles.

All components comprised within the combustion chamber create the pump that makes fuel delivery possible to that particular cylinder. I teach it as "everything connects to everything" ...

My findings as a 35 year HD technician is that rarely ( pretty much, almost never) is one thing the cause. Multiple things are wrong and need to be addressed for the system to work.

Please ask all questions as there are no dumb questions...

Duke Kleman

[Buster]

Thanks so much Duke. Having to get cylinder bored to next oversize and new piston and rings. Will have parts in a week and reassemble. I'll report back.

[dukekleman]

Buster,
Be sure to determine the cause of the piston and piston ring failure. If left undetermined, odds are it'll re-surface and be back to the existing scenario.
Do you have a way of posting good images of the damaged piston, piston rings and cylinder bore?

[Buster]

No way to post image at this time but working on it. Believe piston damage was due to laefe amount of air being sucked through pitted boss on head where sparkplug gasket seals. I had pitted bosses milled and stopped that problem.

[Buster]

image0.jpeg

Having problems with photos but got this one in. oil coming past rings near where ring landing where damage was. After after only 6-7 miles, upper area of piston has oil baked on hard and wet oil at lower area and wet oil running down to at head gasket. Also small areas of wet oil in head combustion chamber. Cylinder bore has damage and is has approx 0.005" too much clearance at area of ring landing damage. Getting bored 0.020" over with 0.020 over piston. Got to order piston Monday. I'll report back.