Problems with rebuilt 390
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From: Taunton, MA
Problems with rebuilt 390
Ok, so about 15 years ago my dad's friend rebuilt this engine as his project for a class he was taking. Although he is not sure, my dad thinks it has a billet crank and a cam for tractor-ish torque. As far as i know the engine was never run after the rebuild until about a year ago when i finally put a distributor in it and fired it up, but only for a few seconds, but it ran ok considering its lack of timing adjustment and the crappy carburetor. Then it has pretty much sat until last weekend when i got a new carb on it and went to try and dial in the timing, but i can not get it to idle under 2000 (measured by ear, tach broke) without dying and it runs terribly rough. The distributor is set to 0 degrees (crank on tdc and distributor pointing to #1) and the vacuum advance hose removed. Does anybody have any idea whats wrong?
Heres a vid of it running if it helps-
Heres a vid of it running if it helps-
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From: Mddl A MexCans
That thing has some issues. I'd run it long enough to get it slightly warm and wait 10 min. Pull the plugs and look at them for fowling. Toss any losers.
1. Run a compression test (disconnect coil to dist cap lead, open carb throttle 100%) If that checks out ok 10-15% diff between holes.
2. put plugs in it, cap the trans vac line at engine, tape VC vents with duct-tape spray every vacuum leak source with heavy a soap solution, (if you dont know them, spray the engine with a mist) using a low pressure air source (canned air, gauged compressor line) feed 5-10lbs pressure to the crank case via intake port source. Look for bubbles.
Setting that long and cranking it up, my guess is the rings broke immediately.
1. Run a compression test (disconnect coil to dist cap lead, open carb throttle 100%) If that checks out ok 10-15% diff between holes.
2. put plugs in it, cap the trans vac line at engine, tape VC vents with duct-tape spray every vacuum leak source with heavy a soap solution, (if you dont know them, spray the engine with a mist) using a low pressure air source (canned air, gauged compressor line) feed 5-10lbs pressure to the crank case via intake port source. Look for bubbles.
Setting that long and cranking it up, my guess is the rings broke immediately.
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From: Taunton, MA
Ok, the pcv port was open, but plugging it did not change anything.
The engine pops constantly through the intake and when it stalls trying to idle i will sometimes get a flame out the intake. All the vac ports are plugged, there is no noticeable hissing to indicate a vac leak, though ill check that, and the compression feels even when the motor cranks (ill get numbers once i find my big plug adapter. At this point i think its the top end from a botched rebuild, unless i find that redmanbob's theory about the rings turns out to be true.
For the record, i did not just fire this engine up, i made and adapter and primed the oil system, put a scope down the cylinders to check for rusty walls (found nothing serious) turned it by ratchet with mystery oil in all cylinders, and once assembled cranked it several times with the starter incase anything else was stiff and then fired it for the first time. Thanks for all the help!
The engine pops constantly through the intake and when it stalls trying to idle i will sometimes get a flame out the intake. All the vac ports are plugged, there is no noticeable hissing to indicate a vac leak, though ill check that, and the compression feels even when the motor cranks (ill get numbers once i find my big plug adapter. At this point i think its the top end from a botched rebuild, unless i find that redmanbob's theory about the rings turns out to be true. For the record, i did not just fire this engine up, i made and adapter and primed the oil system, put a scope down the cylinders to check for rusty walls (found nothing serious) turned it by ratchet with mystery oil in all cylinders, and once assembled cranked it several times with the starter incase anything else was stiff and then fired it for the first time. Thanks for all the help!
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redmanbob's on the right track with the compression test...but i'm going to go one step farther and recommend a leakdown test.
if its backfiring through the carb, you either have serious timing issues, or sticky valves.
double-check the firing order, maybe you'll get lucky. also- you may be able to narrow the problem down by pulling one spark plug wire at a time.
if its backfiring through the carb, you either have serious timing issues, or sticky valves.
double-check the firing order, maybe you'll get lucky. also- you may be able to narrow the problem down by pulling one spark plug wire at a time.
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From: Taunton, MA
Ive triple checked the firing order and had the timing all the way in both directions to the point that it would not run and the backfiring persisted. I would absolutely agree with valves though, i never was totally happy with how the engine was stored before i started on it so rusty stems could be very possible.
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Ive triple checked the firing order and had the timing all the way in both directions to the point that it would not run and the backfiring persisted. I would absolutely agree with valves though, i never was totally happy with how the engine was stored before i started on it so rusty stems could be very possible.
i ran a gallon or two of weedeater gas (2-stroke oil/gas mix) through my truck. it smoked a bit, but it quieted down some.
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From: tujunga, calif
Ive triple checked the firing order and had the timing all the way in both directions to the point that it would not run and the backfiring persisted. I would absolutely agree with valves though, i never was totally happy with how the engine was stored before i started on it so rusty stems could be very possible.
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From: Taunton, MA
Yes I did, but no matter where it's timed the engine runs the same. Im doing a compression test tomorrow to see what I've got, but the constant intake backfire makes me think the valves are sticking as was suggested earlier.
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From: Mddl A MexCans
PULL THE VC'S
If you think they are sticking, pull the valve covers and watch them (the intakes) If the valve/s are sticking open the pushrod will be quite free to do the jig. If a lifter/s are the issue they will all stay bound in union.
If the pushrod length, or rocker valve train wasn't setup right, it could be allowing a valve/s to remain open.
If a lifter isnt bleeding down at all, ... all of which can be fished out without intake removal. But without thr removal of the front clip "I might shoot myself before finishing"
lot to look at.
I forgot the rubber cap for the PCV vac hose aback of the carb plate on my 66 once and it wouldn't stay running for crap and backfired through carb when throttling down.
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From: Taunton, MA
well no progess yet, been too busy after work. tomorrow ill try to do the compression check and pull the covers to see whats up and report back, as long as nothing else comes up. thanks again for all the help!
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From: Taunton, MA
Well i ran out of time for a compression test, but i did pull the valve covers and it did appear that the valves were not closing all the way all the time when i cranked the motor over. I ran the engine and some times it ran fine for a few seconds, then a valve or two would stick and it would backfire and try to stall from the interruption or air through the carb. Just for fun i pulled the rocker shaft and put a magnet down the hole to see if the lifers were tight, but they were all just fine. Then i pushed down the valve springs to see if the valves were stuck, but that did not appear to be the case either, though i cant be sure since i did not push them all the way. Could there be something in the lifter valley that is moving around and intermittently jamming the lifters?
