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350 has me scratching my head
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klavinblack
Novice
Jul 16, 2010, 11:56 AM
Post #1 of 9
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350 has me scratching my head
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I have a 1958 Chevy station wagon with a 350 from a 1974 C30 truck. The
engine is a create engine that has around 22,000 miles on it. When I
start it up it runs good for about 3 minutes, than it starts to sputter
and back fire or stall when I give it any gas. I shut it off, wait about
10 minutes and start it, then it runs fine. I get it on the highway up
to 70, runs smooth with power. Head back home on city streets and it
starts to sputter again, turn it off for a few minutes, start it and
it's fine. I haven't given it a compression check to see if its a valve
or cam, but if it were, wouldn't always run badly? I have stalled 2
rebuild carbs on it (2 barrel, manual choke), new plugs, wires,
distributor cap, points, coil. The dwell was set, timing is right on at
10 degrees. What could it be?
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Loren Champlain Sr
Veteran
/ Moderator
Jul 16, 2010, 5:02 PM
Post #2 of 9
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Re: 350 has me scratching my head
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klavin; What carburetor and distributor are you using? The '74 used a Rochester and HEI distributor. If this is a feedback type carb...Or, could have a control module issue if a stock GM HEI dist....Give us as much info as you can. Choke...Electric? Should be. Is the choke pull-off working? Is the choke opening as the engine warms? So many questions. LOL.
Loren
SW Washington
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klavinblack
Novice
Jul 26, 2010, 6:03 AM
Post #3 of 9
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Re: 350 has me scratching my head
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Sorry I didn't respond sooner, but I've been away for a week. The carb is a rebuild Rochester (manual choke). The distributor is not a HEI. I replaced the points last year (maybe 400 miles on them). It also has a new coil and wires. I have a HEI distrib for it, which I think I will install. Do you think I should still give it a compression test? I started it for the first time in two weeks yesterday and it sounded fine. Little story behind the car. I bought it from my friend's brother. It originally had a 348 in it. He took it out to sell it and threw the 350 in, which is a crate engine with 21,000 miles on it. I think he may have put the older distributor in along with a generator to give it that old engine look. WHY? Thanks for responding, I hope what I told you helps.
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Jul 26, 2010, 7:27 AM
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Re: 350 has me scratching my head
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In order, Mopars went Electronic Ignition in Model year 1973, Ford in 1974 and GM in 1975. There were execptions for special duty in all.
Near sure GM went all HEI (high energy ignition or electronic=no points) starting model year 1975. I don't think changing it now with a problem with points is a good idea till that can work and the points can work fine.
What was the engine intended for or do you know in what the 21,000 miles was used in? If points as they should be you'll need at least a feeler gauge or better a dwell tack. Spec is 30 degrees - dwell.
I think all the HEIs had an aluminum or metal door and a screw (Allen headed) to adjust points if needed. A dwell tach would tell if the contact ohms are good or useless.
More on points. They sold a "UniSet" where the condenser was attached and one piece to the set which was handy.
I'll go find the feeler gauge spec now as you probably don't have a dwell tach. The bumper (nylon) must be at the top of the 8 sided cam it rides on of the points which then show the largest gap. There should be NO visable metal transfer showing which would look like a peak and valley that matched one to the other - say so if noted.
OK - by feeler the spec should be .019 and was for about all GM V8s. Err to larger if tricky to adjust as they get closer with use.
Turn crankshaft bolt if needed or click starter quickly to get bumper riding cam on top of a cam lobe.
The "Back Firing" is a clue of points all screwed up or timing way off and a bunch of other possiblilities. It's old and low miles is nice but not an assurance that it doesn't need things.
Need to know points are ok first. Get it running and worry about HEI later. I'd forget it but that is up to you.
Any more info on this engine will help. When was it last running - any info on it at all,
T
PS: Loren knows this distributor just the year when they switched from points could be different on that engine if it was for other than routinely made vehicles. If marine it could be a whole different set up and will be a pest to use in a vehicle now.......
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klavinblack
Novice
Jul 26, 2010, 7:58 AM
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Re: 350 has me scratching my head
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The engine was in a C-30 pickup. My friend bought the truck from the school department in Rhode Island. I guess the original engine pooped out and it was replaced with a new crate engine. I live in Pittsburgh now, the car was driven here from Rhode Island when I bought it 2 years ago. As for the points, I checked them with my dwell meter and they are fine. at 30. The timing is at 10 degrees. It runs like a charm most of the time. It's just sometimes when I start it I can count to 3 minutes and that's when it starts running rough. If I give it gas it either stalls or backfires. Then I shut it off, wait about 5 minutes and it's fine. Take it on the highway and I'm moving, smooth, but eventually it will run rough again. I'll stop, turn it off, wait about 5 minutes and start it, suddenly it's running great again. When it's running I can look into the carb, give it gas and see the jets working fine.
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Jul 26, 2010, 8:32 AM
Post #6 of 9
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Re: 350 has me scratching my head
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Great - that helps. I don't even see dwell meters out there anymore. Mine shows how good the contact is when the points touch. Wish I knew the ohm reading which really should be infinate or close.
The manual choke conversion is a nightmare area. If you can seek out the parts to make that automatic again you'll love it. It must be known NOW that if you put of OFF it's really wide open as tugging hard on a cable choke on a carb meant to be automatic choke if not exactly right will pull on cable as it can't climb the fast idle step cam unless every person who drives it know to hold the gas pedal down while setting the manual choke. It may have streched or lost adjustment - just see that it opens fully.
The stall and backfire could be spark issues or the carb over or less likely under fueling. An old trick was to tap (plastic handle or hammer) lightly right where fuel filter goes into carb and the common stuck needle valve might just snap to. Watch out if all wet with gas at a spark there is easy to make a fire.
It can run smooth for a while. Need to know if engine is fully warm or just time. It would tolerate too much fuel while cold/cool then start acting up. If totally flooding and wet that's floats, choke, needle valve usually all of which are trouble areas by age never mind miles.
Floats can sink or be heavy from being soaked in fuel. If so fuel level in bowl would be too high and always run rich - way too rich sometimes.
Gotta go soon for a while but other common crap with the age are things like ..... distributor vacuum advance, fuel pump rubber giving out, carb gaskets that hate the alcohol in much fuel used now,
vacuum leaks on vacuum trees or broken trees. Thermal sensor to pick up hot air off exhaust may be stuck to hot exhaust always which is too hot when warm. You could do without that at all just less chance of running perfectly faster from cold starts.
Can't be sure for a truck engine but the cam gear of timing chain at 21,000 miles might still be in there. Age alone if so is probably throwing chunks off of it and might jump off time and back on somehow by miricle can jump back to correct. Hard to know without looking at it if that type.
If this isn't a fuel issue but probably is the distributor shaft bushings could be worn. Low miles or not they may have had extended short runs and the oil didn't get time to keep them lubed. That will show with your dwell meter alone or moving shaft and looking at points how much more they might open if pushed that direction. By dwell meter the dwell would change by more than 5 degress - an indicator of worn distributor shaft bushings.
One more maybe for now. This may use a heat riser on exhaust manifold stuck shut. It if equipped would be a heat sensitive spring with a weight acting like a choke for exhaust. It should be freely moving - HOT - so test movement when cold. If stuck and was designed into that engine it shoots hot exhaust thru one exhaust manifold to the other really warming up intake manifold but must open and chill out as way too hot would fire incoming fuel too easy - hence backfires.
That's a bit of junk to test out and look at. If it's been running rich and for a while now spark plugs would be taking a hit and fouling up fast too.
Enough for the moment. Loren may have some ideas too as he's a senior member of the OPC (old phart club) a point where we knew this stuff new and now fighting to remember - oh just laugh,
Tom
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klavinblack
Novice
Jul 26, 2010, 8:50 AM
Post #7 of 9
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Re: 350 has me scratching my head
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Thanks a lot. I'll check out some of those things and see if I can put this to rest.
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Loren Champlain Sr
Veteran
/ Moderator
Jul 26, 2010, 5:18 PM
Post #8 of 9
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Re: 350 has me scratching my head
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klavin; Without going back and re-reading all of the posts..... Ten BTDC sounds a bit much. I'd slide it back to about 6. (with the vacuum advance disconnected from vacuum source). With the timing light hooked up, rev the engine to 2000 rpm and recheck the timing (again, without the vacuum advance) checking the centrifugal advance. Take the reading and post it. Then, reconnect the vacuum advance and take the readings at idle and 2000 rpm and post them (should be in the neighborhood of 35 degrees, total). Another thing I'd want to check is valve lash. You could very easily have a tight valve which will be worse as the engine warms. Small block Chevs are easy to adjust. With the engine running at normal operating temp, loosen the first rocker arm until you hear it clicking. Tighten just enough to where the clicking stops. Then, in 1/4 increments, tighten one full turn. Do this to each rocker arm. (3/4 turn is usually sufficient). The engine will likely falter and maybe even die as you are cranking down. If you do it slowly, you'll be fine. This shouldn't happen on an engine with 21K miles, but small block Chevs are know for the cam lobes going flat, usually on #7 or #8. You should check to make sure that the rocker arms are getting full travel on those cylinders, especially. A flat cam lobe will cause a backfire under acceleration. At temp, connect a vacuum guage to manifold vacuum. Take a reading at idle. Raise the rpm to 2500 and take another reading. The guage should be steady, as in the needle not 'bouncing', and the reading should be as great or greater than it was a idle.
With all of this said, I'm assuming (I hate that word..lol) that all of the secondary is okay (plug, wires, cap, rotor, points, condensor) and you have good coil output. Hard to check without a scope. <sigh>
Loren
SW Washington
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klavinblack
Novice
Jul 27, 2010, 6:39 AM
Post #9 of 9
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Re: 350 has me scratching my head
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Thanks Loren, I'll check all that out and hopefully solve this thing. I'm working on my kitchen at the same time, so I have to sneak out to the garage without my wife seeing me. I guess she has figured out that I'm having an affair with a 58 Chevy.
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