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- Shovelhead low compression after rebuild
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11-26-2019 #1
Shovelhead low compression after rebuild
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?
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11-26-2019 #2
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.
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11-26-2019 #3
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...
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11-26-2019 #4
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.
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11-26-2019 #5Senior Member
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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
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11-26-2019 #6Senior Member
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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.
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11-26-2019 #7Senior Member
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Post a pic of your plugs...............
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11-26-2019 #8Senior Member
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Jim
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11-26-2019 #9
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11-26-2019 #10Senior Member
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https://www.jockeyjournal.com/forum/...d.php?t=114178
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11-26-2019 #11
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.
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11-26-2019 #12
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11-26-2019 #13
Here's a few pics:
Front piston, and the rear one for comparison.
A view of the head, the exhaust port and the valve.
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11-26-2019 #14
And the plugs:
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11-27-2019 #15Senior Member
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How does the hone pattern look .. ??
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11-27-2019 #16
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11-27-2019 #17Senior Member
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Would you say that the crosshatch is near 45* or much flatter ... ?
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11-27-2019 #18
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11-27-2019 #19Senior Member
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11-27-2019 #20Senior Member
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