Technical Rebuilt Motor Running Poorly, Need Advice

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Don’t mean to alarm you, but this is something to consider ^^^
I assume the block was hot tanked, do you know for sure that the shop replaced this plug?

The 389 rockers are non adjustable as built. You could remove the rocker nuts and substitute SBC nuts, those are crimped to hold the adjustment. OEM Pontiac nuts will not hold adjustment, they are plain nuts and that is why you must tighten to 20 ft. lbs. torque.
If you do go to Chevy nuts then with the In and Ex lifters in the base circle run the nuts down until you remove all slack at the push rod, you only want to eliminate the vertical lash. Once you have done that you can go 1/2 turn tighter, that should be sufficient preload on the lifter. Some might suggest Poly-Locs, tall nuts with an Allen set screw to hold the adjustment. If you try those your stock valve covers will not fit.

 

Yessiree, I double/triple checked all my galley plugs. Got great oil pressure up to the rockers, both sides.

I’m really beginning to think it is my rocker arms... I just spun the nuts on and torqued to spec, didn’t think that there could be any room for adjustment on these style arms. Stupid mistake on my part. I did correctly lube the cam, lifters, pushrods, etc, and use the correct break-in oil along with following proper flat tappet break in procedure.

My mechanic never checked my valve train when I had it up there, he really wanted it in and out. I get it, he’s always busy, but I don’t know anyone in the area with experience on these old boats, at least who’s willing to work with me. I’ll start looking at a hot lash adjustment on rockers, see if it makes a difference. I will test compression in the next day or two and send an update though.


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Well, I think I found my issue. I ran that compression test and cylinder 8 has no compression. And cylinder 6 has lower compression. I had someone crank the motor and I put my finger over the hole on 8 and it felt very weak. Looks like the motor’s coming out.


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Did you check valv lash on those two cylinders?
That could cause low compression, and is fixable without pulling the engine. Replace the plain nuts with a locking version and reset.

 

I did and everything checked out. One thing I totally forgot about was that my timing was off initially before I started the car for the first time unbeknownst to me. Backfired real bad on me-sounded like a gunshot. Talking to a family friend about everything and he said it’s more than likely a valve. So motor doesn’t have to come out, but I’m gonna pull the passenger head and try to find the carnage

 

these kinds of situations are where a leakdown tester, or at least a spark plug hole air adapter is really a good choice a a diagnostic tool. Put some air in there and listen for leaks when at TDC compression.

BTW, I seriously doubt that an out of time backfire damaged a valve, really unlikely.

 

Yeah I really need a leak down tester, would’ve really helped on a job like this. Not too hard to pull the head though, so I figure I’ll pull it and take it to my machine shop to be safe, especially with a bouncy vacuum reading. Pushrods look great though, cam is beautiful and so are the lifters. I can’t imagine I cracked a ring or blew a hole in the piston.

Compression on the driver’s side was a little lower than I’d like it to be, but once my cylinder 8 issues is figured out, I’m going to double check my valve train geometry and triple check my valve lash and rockers. Hopefully everything’ll be running great by then


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Can't wait to hear what you find...

 

Don’t mean to continue bumping this thread (this is the last time update from me), but I finally pulled the head. Cylinders and bores are pristine and there’s no sign of catastrophic failure from a piston or a ring anywhere. I can definitely tell which cylinders were in trouble by the valves, plus I noticed my head gasket was bent near #8 and #6. Not sure if it’s related but I’m still gonna take my head to the shop and get everything looked over. I’ll include photos down below.

Really, thank you everyone for all the suggestions and what-not. I’ve learned a lot over the past couple weeks.




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Clean piston tops on adjacent cylinders usually means bad head gasket.

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Interesting; did not know that.

 

Steam cleaned ..

 

As a former Pontiac dealership mechanic I have seen parts houses send you Chevy lifters instead of Pontiac lifters. Same diameter and look the same to the naked eye, but there is a difference in length.
Can cause issues in higher lift cam applications like bent pushrods.
It looks like you messed up the head gasket when installing your head. You should have equal color across the tops of the pistons. You have been creating steam in two of your cylinders. Did you check the deck on the block ? Did you check the heads to make sure they are flat across the gasket surface ?
Pontiac 389's are a great engine if assembled correctly.