Shovelhead low compression after rebuild

[Nenad]

Did a top-end rebuild last year, of a '71 Shovelhead with S&S 84" stroker kit, solid lifters and Andrews B cam (same configuration since '79). New guides and seats, new S&S pistons, 0.020". Also replaced the old Stromberg with a Mkuni HSR42.

Other than the front cylinder consistently showing 10 PSI lower than the rear from day one (didn't bother me too much as I was still running in the engine), it ran like a dream for months, until the front exhaust pushrod adjuster snapped (in service for decades) and the carb blew out of the flimsy rubber flange. Took off the head for inspection – no damage. Reinstalled everything, with new S&S lifters and pushrods and a machined aluminium manifold pressed on the carb.

Since then, the front plug has been fouling regularly, lasting anything from three weeks to ten minutes. Sooty black, not oily. Checked the compression a few day ago, down to nearly nothing (but unreliable Chinese tester, only one available here) with pushrods removed and a single kick from piston bottom position. No sound of air escaping anywhere. Squirting in a bit of oil did nothing. Off with the head, again. Gasket is OK, the valves, guides and seats are fine, but the head and piston are thick with carbon deposits. Removed the rear head for comparison and no deposits whatsoever.

Haven't removed the cylinder yet, not until I'm sure that the culprit isn't the head, but the bore still shows nice cross-hatching, no scores anywhere.

Any suggestions, please?

[farmall]

Did you leak check the valves by pouring petrol or solvent in the ports?

I'd pull the cylinder while it's down this far to be sure. It's either valves, gasket seal or piston/rings. Compression has nowhere else to leak.

[Nenad]

Yes, poured petrol in the head yesterday, level stayed the same for hours. Still half full today, I assume the rest evaporated.

I do suspect the rings, but unless they're in pieces a bit of oil would've improved the compression briefly, surely? And if they are, wouldn't that be obvious in the bore? Going down to the garage now to remove the cylinder. With lots of clean rags underneath to catch any loose bits...

I had a car aluminium head once, with a tiny crack that was invisible and closed until the engine reached work temperature. Still invisible with a borescope, but showing coolant bubbling in...

[Nenad]

Cylinder and piston off. Piston looks almost brand new, other than the deposits on top and very faint, shallow (can't feel them with my nail) vertical marks front and back, matching the lower half of the cylinder. But that's, as far as I'm concerned, normal.

Quite a bit of oil collected in the wrist pin holes, though. Never seen that before.

I'm stumped.

[JBinNC]

A broken adjuster screw can be a symptom of a valve sticking, or valve float. Either way, the valve train decouples and that hammers the screw.

With as much carbon as you describe, I can't believe the exhaust seat is not carboned up, whether it holds fluid or not. You need to pull the valves and check stem to guide clearance, and whether the exhaust valve stem and valve seat are carboned up.

That said, I'm thinking the rings are sticking because of the excess oil. That will give your low one kick compression number.

Incidentally, if kicking, you need to kick the motor through until the compression gauge stops going up, throttle wide open of course.

It is possible that the cylinder is sooty because in remounting the carb, you may have unknowingly cured a vacuum leak that the carb was masking by being tuned rich.

Lastly, weak ignition will make a cylinder appear rich (poor combustion) when in fact it is not.

Jim

[TriNortchopz]

Checked the compression a few day ago, down to nearly nothing (but unreliable Chinese tester, only one available here) with pushrods removed and a single kick from piston bottom position.

Wouldn't you want to have the pushrods in place to measure the compression under actual operating conditions? I'm thinking no pushrods means no valve movement, so nothing in and nothing out.

[Tattooo]

Post a pic of your plugs...............

[JBinNC]

Wouldn't you want to have the pushrods in place to measure the compression under actual operating conditions? I'm thinking no pushrods means no valve movement, so nothing in and nothing out.

Good catch. I missed that little nugget of information. Yeah, that was NOT a compression test. I need more coffee.

Jim

[Skjoll]

... it ran like a dream for months, until the front exhaust pushrod adjuster snapped (in service for decades) and the carb blew out of the flimsy rubber flange...

Quick question, how are you keeping your carb attached to the the intake manifold?

[farmall]

Quick question, how are you keeping your carb attached to the the intake manifold?

Good point, and a secure carb brace is required. There are some slick ones in this thread:

https://www.jockeyjournal.com/forum/showthread.php?t=114178

[Nenad]

A broken adjuster screw can be a symptom of a valve sticking, or valve float. Either way, the valve train decouples and that hammers the screw.Jim

That's what I thought too, when it happened. But I verified that everything was working, i.e. moving freely and within specs, both the rocker arm and the valve. I figured that, with the angle of that particular adjuster, and after nearly four decades, it was caused by metal fatigue. (An engineer by schooling, I never liked the angle - too much side force.)

With as much carbon as you describe, I can't believe the exhaust seat is not carboned up, whether it holds fluid or not. You need to pull the valves and check stem to guide clearance, and whether the exhaust valve stem and valve seat are carboned up.Jim

Yes, there's carbon build-up on both sides of the valve. I'll post all pics in a while.

Incidentally, if kicking, you need to kick the motor through until the compression gauge stops going up, throttle wide open of course.Jim

I do/did that, of course. Same readings. But then tried without pushrods, with both valves hopefully closed, and starting from the bottom of the stroke. One kick only, just to see if there would be any difference, and possibly hear air escaping.

It is possible that the cylinder is sooty because in remounting the carb, you may have unknowingly cured a vacuum leak that the carb was masking by being tuned rich.Jim

I used the carb "out of the box" and, after a week of trials decided that no tuning was required. The bike ran well across the register, the plugs had the right chocolate colour. I also check for manifold leaks each time I make a change there, spraying carb/brake cleaner all around.

Lastly, weak ignition will make a cylinder appear rich (poor combustion) when in fact it is not.Jim

I covered that too. And replaced the leads, points and condenser just in case. I've also played with swapping plugs and leads, and even crossing the leads to check the coil. Still, it's always the front plug that fouls.

[Nenad]

Quick question, how are you keeping your carb attached to the the intake manifold?

With the aluminium adapter, pressed on to the carb and screwed on to the manifold.

[Nenad]

Here's a few pics:

Front piston, and the rear one for comparison.

A view of the head, the exhaust port and the valve.

[Nenad]

And the plugs:

[Dragstews]

How does the hone pattern look .. ??