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

[Steve Slocombe]

Dear Buster, the VLs had three compression rings and no oil rings, and I've just taken out the front oil rings on two bikes with those modern pistons. There is also various advice on piston clearances, with Harley saying one thou for the T-slot pistons, causing disaster if you're using different ones. With those modern pistons I'm leaving about 6 thou on the front and five on the rear, as no-one today will follow the Harley 1000 mile break-in period.

[dukekleman]

The rear plug on left is significantly darker.

Buster,
The difference in measured compression, 7 psi, is not an issue at hand.
The front spark plug is close to as it should, the rear is very rich. The spark plug is the viewing component that allows you to observe Combustion temperature, Ignition timing, Oil control, fuel delivery. From the photo you provided, I can't verify all those but as mentioned, the front plug looks close.

So then, the question remains...Why so different between the two cylinders?
Here's what I would look into:
A) I would verify ignition point gap settings for both cylinders and then measure the KV voltage at the high tension leads of each cylinder.
B) You also posted a photo showing the flow dynamics of the front cylinder intake valve port to combustion chamber. There appears to be heavy valve shrouding...This will greatly affect the wet flow dynamics. Aggravated by a factory camshaft which has low lift . Low lift is affected the most by the shrouding.
I know first hand from wet flow testing cylinder heads what the fuel and air will do in this situation. In short the vaporization efficiency is reduced in this situation.
So then I wonder is the rear cylinder shrouded in the same manner? Same valve stem protrusion, same seat angle diameter dimensions, etc.
C) Lastly I would measure the running temperature of each cylinder at the base of each spark plug where it contacts the cylinder head. I use Thermo couples and a meter. You use a infra red temperature gun to give you a general idea but almost impossible when riding down the road. This information will tell you (us) a lot as well.

Unless we measure and test, we can only assume and guess...

I hope this helps,
Duke Kleman

[Buster]

Thanks all. I have just removed front oil control ring, as Steve said and which I have considered doing for some time. I have not checked point gap on both cylinders, will do this and monitor heat at heads with infare red meter I have. I am currently running one heat range colder plug on front cylinder. Will have running today and start testing. I'll get back in a few days.

[T. Cotten]

Somebody please explain to me,..

How the heat range of a sparkplug is supposed to correct the combustion temperature.

Thanks again in advance,

....Cotten

[T. Cotten]

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.

So you are saying, Steve,..

Retaining all that hot oil keeps them cooler than pushing it out the exhaust?

Total loss Indians didn't have the problem.

These are air-cooled engines!

....Cotten
PS: I have contended for years that the more prevalent heat signatures commonly found on the front assembly of American v-twins, even those with oil in the top end, are a result of receiving more air than the rear.

[Buster]

Removed front oil control ring and rode for 4-5 miles. Still tuning carb. Pics show rear plug on left and front on right. Pics show front plug turned in two different ions. Front plug much darker at thread pre meter and starting to get color in one side. Other side is light.

checked heat on cylinders with indeed gun

metal base of plug.
Front. 438 degrees
Rear 362 degrees
head at plug base
Front. 426 degrees
Rear. 372 degrees
exh port
Front. 350 degrees
Rear. 379 degrees
cylinder base at barrell
Front. 190 degrees
Rear. 161 degrees
int port
Front. 382 degrees
Rear. 346 degrees
header at spigot
Front. 225 degrees
Rear. 182 degrees

Points gap
Front cylinder. 0.022”
Rear cylinder. 0.021”

checkin compression tomorrow. Steve, since I remove the front cylinder oil control ring can I expect to see a lower compression on the front cylinder. With all three rings it had 57 lbs compression. Thanks.

[BigLakeBob]

As Duke said in #17 A about spark wires, can you switch the wires front to rear and see if that changes things?

[dukekleman]

Buster,

Are both cylinders Re-sleeved?

I notice a few things from the photos and the data you took.
However, my comments I'll state are based off not knowing how many miles are on these spark plugs when last photo'd and if they were new from your previous photos in this post...

So here goes:

A) There are signs oil control issues on these spark plugs.
Major signs? Well that depends on the time they have spent in the combustion chamber prior to your photos.

B) Based off the spark plug photos and the temperature's you've listed, the engines spark advance is too retarded. It needs to be advanced more. Keep in mind when I say this from a computer but...I'd guess at least 4 degrees.

I would still like to know the measurement of KV voltage to each cylinder. It tells a definitive story, however I understand that every one does have that tooling to measure it.

I hope this helps,
Duke Kleman

[Buster]

Both cylinders are sleeved. The plugs are not same as the other photos. I have several sets I have been media blasting to do plug checks. None have over 10 miles on them. Go to figure out how to test KV from coil. I have run with two different coils, switched plug wires front to rear, and changed both plug wires. This is a 12 volt system. In the oil control, the dark perimeter of the front plug came immediately after I removed front cylinder oil control ring. I will try advancing spark. This would put advance at abou 34 degrees. Thanks for all the help.