Problems with rebuilt 390
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Senior User
Joined: May 2012
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From: Taunton, MA
How would i determine if that was the case? I believe they are stock pushrods and stock spec lifters but i'd like to check anyway.
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Senior User
Joined: May 2012
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From: Taunton, MA
Ok, I understand the part about the heads and oil gallery, but how different are t he bolts supposed to be? Would you possibly have. Picture to help me see what you mean? Thanks, Alex
Fleet Mechanic
Joined: Aug 2009
Posts: 1,771
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From: Western Washington
Hi Alex, the only thing I know to try would be to put in shorter push-rods. When a motor is rebuilt sometimes the head and block get shaved and a stock length push-rod is too long. Take one of yours to a machine shop and haven them get you a few that are 0.060" shorter and see if they help.
Here's a picture comparing the oiling hole bolt to the regular rocker shaft bolt: http://image.hotrod.com/f/web/953663...online16_l.jpg
Sam
Here's a picture comparing the oiling hole bolt to the regular rocker shaft bolt: http://image.hotrod.com/f/web/953663...online16_l.jpg
Sam
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Senior User
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From: Taunton, MA
I just checked the bolts and they are all in the right place, and the block hasn't been decked so thats out. When it runs the backfire is mostly intermittent, almost like something is moving?
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Senior User
Joined: May 2012
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From: Taunton, MA
Ok, the test results are in and i think we have a winner:
cylinder 1- 90
cylinder 2- 80
cylinder 3- 85
cylinder 4- 90
cylinder 5- 85
cylinder 6- 90
cylinder 7- 90
cylinder 8- 67
On top of that i can not rocke the rocker arms at all on that cylinder, like the lifters will not pump down and collapse.
cylinder 1- 90
cylinder 2- 80
cylinder 3- 85
cylinder 4- 90
cylinder 5- 85
cylinder 6- 90
cylinder 7- 90
cylinder 8- 67
On top of that i can not rocke the rocker arms at all on that cylinder, like the lifters will not pump down and collapse.
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Senior User
Joined: May 2012
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From: Taunton, MA
Well there may be other issues at hand here. I disconnected the plug wire for the #8 cylinder and the popping did not stop so im at a loss again. Is there anything else that could cause this? A timing chain being off? My new carb being junk? Im an idiot? 
Posting Guru
Joined: Mar 2006
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we're out of my area here-
but i think all of those compression numbers are too low for a brand-new engine.
you may be onto something with the timing chain idea.
but i think all of those compression numbers are too low for a brand-new engine.
you may be onto something with the timing chain idea.
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Logistics Pro
Joined: Mar 2005
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From: tujunga, calif
Trying to help but over the interwebs without a hands on everything is almost a guess. Did you try removing one plug wire at a time to see what cylinder is popping? Who put the plug wires on?, maybe they are on the cap clockwise rather then counter or the other way around. But if it ran ok a year ago , and the only change you changed was the carb, thats still a possibility..
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Senior User
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From: Taunton, MA
Yeah, I know it hard to diagnose stuff this way, I'm just annoyed that ite being so difficult. I have checked the plug wires and they are in the right places, and I started doing the one plug at a time thing but haven't done them all. Could a failing ignition box do this? I just remembered I borrowed the nos one I had before to convert another truck and put a used one back on this truck. Thats the only other part thats different it didn't run great before, but that was a timing issue and it wasn't backfiring like this. Thanks again, Alex
Logistics Pro
Joined: Mar 2005
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From: tujunga, calif
Yeah, I know it hard to diagnose stuff this way, I'm just annoyed that ite being so difficult. I have checked the plug wires and they are in the right places, and I started doing the one plug at a time thing but haven't done them all. Could a failing ignition box do this? I just remembered I borrowed the nos one I had before to convert another truck and put a used one back on this truck. Thats the only other part thats different it didn't run great before, but that was a timing issue and it wasn't backfiring like this. Thanks again, Alex
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Senior User
Joined: May 2012
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From: Taunton, MA
Well now we are on a weather hold until the snow quits but im getting a new box to test once i can get the battery back (i have to share the battery and ignition box with the plow truck for now) Heres hoping its the box and the two trucks just have different wiring that wont share the same box.
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Senior User
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From: Taunton, MA
Well we're back at this with no improvement. I checked the timing chain to see if the kid that did the work years ago messed up, but the chain looks new and the marks line up right. While at it I measured the stroke, about 3 3/4 so its a 390 for sure. Could a crack in the manifold cause this? We're not entirely sure whether it is good or not so if it is cracked could that make it run this way?
Logistics Pro
Joined: Mar 2005
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From: tujunga, calif
Another stab in the dark but are you sure the carb base and the manifold spacer are compatible and there isn't an air leak around the pvc port or somewhere else. You should be able to spray some WD40 around the carb and and manifold while it's running and see if there is a change, same if you think the manifold may be cracked.
Fleet Mechanic
Joined: Aug 2009
Posts: 1,771
Likes: 9
From: Western Washington
Hi Alex, I was re-reading your first post and you said you set the distributor to zero degrees? It needs at least 8, maybe 12 degrees of (initial) advance to run well at all, and vacuum advance is pretty critical.
A real bad vacuum leak could cause issues, but that sounds like a bad leak, something like the brake booster.
Also, when checking for vacuum leaks WD40 works, but carb cleaner works better (it's a bit more flammable)
Sam
A real bad vacuum leak could cause issues, but that sounds like a bad leak, something like the brake booster.
Also, when checking for vacuum leaks WD40 works, but carb cleaner works better (it's a bit more flammable)
Sam