Cone Shovel, Low Compression Weirdness

[Slamgears]

So I bought a 74 inch shovelhead a few months back, random dude on cycledope. According to him it had good compression, good crosshatching in the bores when checked with a boroscope.

Finally got it in. Static timed, have spark at plugs, can get the plugs wet so I have fuel as well. Spark, fuel,timed correctly and no boom so I checked compression naturally, and I get 30psi dead nuts, exact same on both cylinders.
Even went out and rented another tester to double check.
Also checked all pushrods and adjusted within spec as well.

Weird part is, it doesnt seem to bleed off quickly like its leaking, and I tend to believe the dude I bought it from when he said it had compression because nothing seemed too sketchy and I've gotten got on parts before.

Any ideas for why the compression would be that low and the same in both cylinders? I would think maybe a stuck valve from sitting for so long, but the pushrods seem to move alright, but I cant think of anything else other than I got fucked and got a shit motor.

[JBinNC]

I just did a top end on a shovel that had 30psi and 50psi on the cylinders cold. It had a few problems, foremost, the rings were so worn out the first and second rings were sharp on both sides.

Jim

[Dragstews]

Don't forget to pull the timing plug when you do this test.

[Tattooo]

And have the throttle wide open....

[Slamgears]

Throttle was wide open but I surely did not pull the timing plug...

[Dragstews]

Not a compression tester ..... It's a leak-down tester....

With the timing plug out, yo're going to hear big time air coming from that hole....
..... Bettca .....

[Slamgears]

Yeah I realized on second glance. Is it worth a leakdown at this point with compression that low or should I just dig into it?

[Slamgears]

Also should I just go ahead and do a top end, or just rings and check everything else and replace what's broken or worn?

Trying to do this as cheaply as possible due to already spending a bunch of cash to build the bike and have motor issues. But it's also not worth saving money to have issues in a couple days/weeks/months because I didnt just do a top end.

[JBinNC]

Leak check with compression that low is kind of a waste of time, because you need to take it apart anyway, and then you can visually inspect and measure. Top ends on old shovels is expensive because of the number of components in need of replacement, and the hours involved, of course.

Jim

[Dragstews]

Yeah I realized on second glance. Is it worth a leakdown at this point with compression that low or should I just dig into it?

Yup.... It needs to come apart ....
But if ya wanted to hear the air going past the rings just to confirm that's the problem and not the valves leaking...

[Sky]

The wrong combo of new parts can create super low comp
Crank, cylinder, piston....

"Redneck leakdown"
Pull the pushrods, apply compressed air, listen for the leak

[TriNortchopz]

Hmmm. wonder if there have been compression release valves installed in the heads and they are stuck open - coulda happened...did you look for anything like that?

[farmall]

Trying to do this as cheaply as possible due to already spending a bunch of cash to build the bike and have motor issues.

Cheap and Shovel don't go together. Better to do it right even if the engine has to sit between money infusions. Bandaid fixes don't tend to last.

I'd empty the cam chest and inspect lobes, tappet bores, rollers etc. Inspecting is free while failure is expensive. Inspect the bottom end, rod bearings etc too to know what you have. Don't be surprised if it needs a total overhaul. It's an old air cooled engine. That comes with the territory.

Don't buy any parts until you know everything needing work you can plan efficiently.

[Slamgears]

Ended up coming across a new top end from a trustworthy local dude for a decent price, so I snatched it up. In the process of pulling it apart and inspecting now, have the front head off and by the looks of it, the rings were no good.

Haven't had time to get much further as of yet, and it's 8 with a windchill of -8 outside today so I haven't gotten back outside to mess with it yet.

This will be my first time doing a top end so i'm spending the time not working on it reading the service manual and researching