Can difference in comp ratio between front & rear cylinder heads effect plug color?

[Buster]

I posted several months back about a lean plug on front cylinder of my 1931 VL. Got many good answers on pressure testing, etc. Found many leaks. Replaced both in take nipples, faced leaking sparkplug washer bosses at heads, fixed leaking head bolt, verified valve guide clearance, verified no valve leakage, fixed incorrect timing, tried different coil and plug wires, tried different carb, and repaired scared flange surfaces at manifold / carb connection. No leaking head gaskets. I appreciate all the answers I received. After months of chasing this lean running front cylinder, I have now noticed that the front cylinder head has had extensive decking over the years. I believe the front head is now significantly higher compression then rear head. I read that raising compression raises combustion chamber temps making the cylinder leaner and requiring the amount of fuel to be richened. To test and see if this is my problem, would the motor run without the head gasket leaking if I installed the head with two head gaskets as a test to temporally lower compression ration before I attempt to buy better heads? Is it possible this significant difference in compression ratios is the cause of my lean running cylinder.

[T. Cotten]

I posted several months back about a lean plug on front cylinder of my 1931 VL. Got many good answers on pressure testing, etc. Found many leaks. Replaced both in take nipples, faced leaking sparkplug washer bosses at heads, fixed leaking head bolt, verified valve guide clearance, verified no valve leakage, fixed incorrect timing, tried different coil and plug wires, tried different carb, and repaired scared flange surfaces at manifold / carb connection. No leaking head gaskets. I appreciate all the answers I received. After months of chasing this lean running front cylinder, I have now noticed that the front cylinder head has had extensive decking over the years. I believe the front head is now significantly higher compression then rear head. I read that raising compression raises combustion chamber temps making the cylinder leaner and requiring the amount of fuel to be richened. To test and see if this is my problem, would the motor run without the head gasket leaking if I installed the head with two head gaskets as a test to temporally lower compression ration before I attempt to buy better heads? Is it possible this significant difference in compression ratios is the cause of my lean running cylinder.

Probably not, Buster.

....Cotten
PS: (Just my humble opinion, based upon my personal experiences which are totally anecdotal.)

PPS: Almost forgot: Plug reading has been futile since "Intake Valve Deposit" inhibitors have been Federally mandated many years ago. They should be white.

(You can always add Marvel to feel better.)

[Buster]

The rear plug on left is significantly darker.

[Steve Slocombe]

Dear Buster, that's standard for the VLs. The front cylinder runs notably hotter than the rear because the flywheels turn clockwise seen from the timing side and splash oil preferentially into the rear cylinder. Take out the front piston oil ring, if fitted, and an extra couple of thousandths clearance in the bore. The front cylinder will still run hotter but not so much. You can run different heat front and rear spark plugs if you wish, but a pale front and dark rear plug is normal.

[dukekleman]

Buster,

Before we converse on the spark plug visuals, I like to ask if you've measured kicking compression with a quality compression test tool?
If so, can you please list the results from the test performed... front cylinder, rear cylinder, number of kicks per cylinder.

Thank you
Duke

[Buster]

Duke, the kicking compression cold is 50 lbs on rear and 57 lbs on the front. This is cold with four kicks and throttle wide open. I have tried additional kicks and the compression does not increase on either cylinder. I am running modern ULH pistons with all three rings and one piece oil rings. Thanks!

[Buster]

I need to add this. This is same motor discussed in "Another Lean on One Cylinder Question - Harley VL" thread. This post noted that the front piston landing had deformed, and middle ring was locked up. I just had front cylinder bored .020" over to clean up damaged bore and installed new cylinder and rings. Cylinder is sleeved and is currently at 0.020 over STD. The current leak down test on the front cylinder reflects at 25 lbs pressure, the cylinder holds 24 lbs pressure. I am not opposed to removing front cylinder oil ring if needed, as proposed by Steve.

[T. Cotten]

Dear Buster, that's standard for the VLs. The front cylinder runs notably hotter than the rear because the flywheels turn clockwise seen from the timing side and splash oil preferentially into the rear cylinder. ...

How is that any different from ULs and FLs, Steve?

(Or even Chiefs, for that matter...)

Thanks in advance,

....Cotten

[Buster]

Just pulled head and same repeating deposit on piston adjoining. This cleans off with brake cleaner and I believe to be gas stain. First pic shows original spot and second pic shows wiped down area and thickest area of deposit. Is this preignition.

[T. Cotten]

Just pulled head and same repeating deposit on piston adjoining. This cleans off with brake cleaner and I believe to be gas stain. First pic shows original spot and second pic shows wiped down area and thickest area of deposit. Is this preignition.

Pre-ignition is "knocking", Buster,...

I don't think you've got there yet.

Raw gas has to come from somewhere.

Is it just the pic, or is your intake valve pretty "sunk"?

....Cotten

[Buster]

My intake valve is pretty sunk. I have relieved around top of seat to expose valve face at area above top of seat.

[T. Cotten]

My intake valve is pretty sunk. I have relieved around top of seat to expose valve face at area above top of seat.

It would seem, Buster,..

Fuel is coming out of solution in the air flow: "puddling".
Here water is coming out of solution as it passes over an obstruction causing pressure drops:

Puddling.jpg

Somehow, it appears you are getting clouds of raw fuel in just one intake tract.

....Cotten

[Buster]

Cotten, I agree. Could this amount of unburnt fuel be causing the lean condition.

[T. Cotten]

I can only conjure, Buster!

You are at the mercy of your own powers of observation and patience.

There's got to be more clues than just a sunken valve.

....Cotten

[Steve Slocombe]

Dear Tom, the JD and VL front cylinders run hot for the same reason given earlier. The later bikes have recirculating oil systems and run cooler, maybe even a tad hotter on the rear because of poorer cooling air flow. Harley reversed the con rods and took out the cylinder baffles in 1940, but I believe the 1936-39 OHV bikes still ran OK.