rear cylinder fouling plugs

[russrudder]

I have a 1976 XLH that is fouling the rear plug. the front plug looks great. I have checked the timing and set it by ear adjusted the push rods and tried to adjust the adjustable jet in the bendix carb. I'm not sure if the jet in the carb is adjusted right. the plug wires are only a year old with about 4000 miles on them. The bike runs good for about 10 miles or so then it starts to back fire and loses power. pull the plugs and the front looks good white tip no carbon and the back is black with carbon. I recently rebuilt the top end and bored it .040 over and haven't got it to run very good ever since. the compression it 140psi in both cylinders. could it be a weak coil or the carb or what?

[hdbikedoc]

I would shoot time to make sure its bang on. check accelerator  outlet and make sure its shooting to the centre of manifold,if not drop bowl and turn to aim. switch plug wires front to back for a test and I would try and report

[garyajaz]

wet black (oil) or sooty black (gas)?
'since you have but one carb doubt that is it.
maybe oil ring on that cyl or guides/valves.
i had bad oil fouling on rear plug.  everything checked out fine.
turned out front guide wallowed out.  oil went across intake manafold and fouled out other cylinder.
in other words dont rule anything out.
\p.s. that last one was definetly a head scratcher.

[tomfiii]

If gas fouled it points to a intake leak in front cylinder.

[eglideic]

 One plug is fine, one is bad.
Carb? I doubt it.
I would do a compression check and see what you have.

[tomfiii]

One thing, try swapping plug wires front to rear,rear to front ,just another thing to check.

[Lew]

Strictly a guess..might do a re-torque.  Could be expansion after warming up is allowing oil to migrate into the cylinder/combustion chamber.

[Hillside Motorcycle]

Is this a points-triggered system?
Scott

[dsvracer]

I would think it is a ring or piston to wall clearance problem. It worked fine before the boring and now has a problem with the rear cylinder.  dsv

[wavlovr1]

R U sure the black plug is not oil from a bad valve stem seal?

[Hillside Motorcycle]

Is this a points-triggered system?
Scott

Points??

[saltcaveminer]

i see 272 people read this and only a few replied.now i can be dead wrong but the problem started after the top end job?seems like as the heat builds in the engine the problem starts(popping etc.)i suggest a compression check of both cylinders when hot and compare.also you really cant time an engine by ear.you need to strobe at a minimum of 2200rpm with a hot engine.hope this helps.Salty 

[wreck74]

If the front plug is white it's running too lean, check for intake leak.

[Boe Cole]

If the plug is white, I'd be concerned about it being too lean (intake leak?) - should be tan.

To me the rear plug if its oily/fouled and you just had the jugs bored and os pistons installed, I'd be looking for a piston ring (the middle scraper ring) being installed upside down or an intake valve seal leaking or improperly installed.