Heavy oil in throttle body after external breather

[Matty_d]

So, I pulled my air filter to do some checking on things and noticed heavy oil in throttle body after doing external breather. The throttle body was removed and cleaned when I did the external breather. I think I have blow by on the rear cylinder and not sure what is up with the front but the spark plug is heavy oily. I did a compression test and the front was right around 200 and rear was 185-190. I do not have a leakdown tester but I did apply air to both cylinders at TDC. Front held and I could not hear anything. Rear I could hear air coming through the crank case.

I am looking for some advice on where to start on trying to diagnose this all. Should I start with tearing down the rear and checking ring alignment?

Can the oil blowby from the rear go into the front on the overlap?

Thanks!

[kd]

Engine size, cams, mileage?

If you tested the cranking pressure on the front cyl first and then the rear without charging the battery or jumping it there could easily be 10# difference.  The sound of air leaking on the rear only could be telling though.  It is possible to have the rings line up and even stay there.  It happened to me on a fresh engine and on the rear cylinder.  The description was as you have stated except my leak down test showed no real difference.  Did you block off the ports to the breather when you installed the external breather and have you checked to see it is tight?  The oil will transfer between cylinders with a cam that has lots of over lap / duration.  That's why it would be good to know a little more about the build.

Here is a thread with a pic of my rings.  It could be an intake valve seal too.  If you are interested in investing in a Nub Valve tool you could save yourself a pile of money and time if it is a seal.  It is a fantastic tool and can pay for itself in one use.

[Matty_d]

I always seem to forget the build details.

107 CI
Suburban Speed pistons 4.4cc
FM Outlaw heads milled to 84.6cc
T-Man 590ps cams
10.77 comp
SE 58mm TB 5.3 injectors
S&S Stealth A/C externally vented.

The ports of the Stealth A/C are blocked off. The build has about 9k on it and have been having issues with it most of the time. Usually I get a lot of spark knock while cruising at about 75mph. 3000RPM and about 60-70KPA.

[kd]

Oily fuel can produce spark knock. Mine was belching black smoke on the dyno and wouldn't tune out.  On consultation with Tom Reiser / T-man (660SM cams) and Steve Cole (TTS tuner) they both called it as oil burning.  Tom Reiser actually said ring alignment.    Some 107 (re-bored 103) cylinder have been known to go out of round because they are thin.  Out of round cylinders can stall the natural ring rotation with the end gaps at the wide (loosest) spot.  I had my cylinders (SE 4.060 120 cu in) mapped unbelievable accurately.  The rings stalled in the loose spot even though IIRC it was only .0005 or maybe less difference.  The cylinders actually measured well within tolerance and almost perfect but that's where they stalled.

[kd]

Another thing to check if you do end up pulling the barrel is the angle of the crosshatch. If it's not correct (steep enough) it hinders ring rotation and that can make it easier for them to stall in a cylinder that is less than perfect. The rings also won't clean the oil off as well.

[misfitJason]

Not to negate that something could be wrong, but I have noticed that ethanol fuel will leave a oily yellow like funk in the throttle body over time. I just think it has to do with the gas being sprayed in there all the time. At least that's what I noticed with mine

That does not seem to be the case in your application.

[838]

Oily fuel can produce spark knock. Mine was belching black smoke on the dyno and wouldn't tune out.  On consultation with Tom Reiser / T-man (660SM cams) and Steve Cole (TTS tuner) they both called it as oil burning.  Tom Reiser actually said ring alignment.    Some 107 (re-bored 103) cylinder have been known to go out of round because they are thin.  Out of round cylinders can stall the natural ring rotation with the end gaps at the wide (loosest) spot.  I had my cylinders (SE 4.060 120 cu in) mapped unbelievable accurately.  The rings stalled in the loose spot even though IIRC it was only .0005 or maybe less difference.  The cylinders actually measured well within tolerance and almost perfect but that's where they stalled.

I had this exact same issue on my 117" bagger. Mine was most prevalent under heavy load. Required a .020" overbore to get right. I used the new super duty KB pistons. No problems since 👍

I had the detonation issued Matt-d speaks of as well. Oil in the combustion chamber.

FYI. I had no compression or leak down issues with mine when it was out of round.