View Full Version : Chevy 350 4 bolt main, running good at low RPMs and bad when accelerating
09-28-2009, 10:20 PM
I have a 1978 chevy with a 350 4 bolt main, it has a summint intake, holly double pumper, headers, and thats about it, the motor starts right up, has no problem idiling, it is a 4 speed manuel with a granny low, 1st gear it is fine, goes to 5000 rpms if you want it to, 2nd gear goes good untill about 3100 rpms, then it sounds as if the passinger side of the motor completly shuts off, it sputters and choaks it self, then you shift into thrid, the driving gear, and it is fine at about 2500-3000 rpms, then if you try to accelerate it studders and does the same thing, i just changed the oil, and the fuel filter, as i said it start right up and has no trouble under no motor strain, the oil pressure is good upper 30's and things like that, so i dont know maybe if my timing is off, or i have some bad compression issues, it started doing it out of no where one day, and now it wont go back to normal, i was thinking maybe the timing some how got thrown off, but i dont know, Thanks for the help and sorry for posting in wrong area, i just need help
09-29-2009, 12:09 AM
Check float level in carb. Prolly running it out fuel. Doesnt sound like timing, but its easy enough to check it anyway.
09-29-2009, 08:49 AM
I would check the cam!The new oils that are on the market today have had the zink removed from them and that is very bad for flat tappet cams and will ware the lobes off.All of the newer motors have roller cams.
09-29-2009, 09:19 AM
I had a coil go bad and do that. Also could be the module in the distributor. I take it you checked fuel pressure to rule out a bad fuel pump.
09-29-2009, 10:34 AM
As a mechanic and an owner of one of these trucks for years I have ran into this problem several times. This could be a couple of things. One is a fuel problem. As srt20 mentioned it could be you are running out of fuel at high rpms. You will need to check that the floats are set correctly and if they are does the fuel pump have enough pressure to provide adequate amounts of fuel at the higher rpm. Float level is easy to check on a holly. All you need to do is remove the side plugs on the side of fuel bowls. If fuel trickles out when they are removed they are set correct. Fuel pressure is a little more complicated. You will need to hook a fuel pressure gage to the fuel line and check to see if you have enough pressure while driving it. I do not remember what the pressure is suppose to be exactly you will need to look it up. However having said all that, the usual reason for this is an ignition problem.
There are three things, in the ignition system, that can cause this. The first thing is high resistance in the spark plug wires. If it has been more than 30K miles since you change your wires change them. If they are below 30K you can check them by using an ohm meter to check resistance. Should be less than 6 ohms per foot. If they are good or you replace them and you have the same problem the next thing would be the ignition module. This is most likely the culprit but there is no real way to check a module to see if it is bad other that change it out. I still keep a known good module in my tool box so I do not have to buy a new one to check it. They are not very expensive thou (around 15-20 dollars) so if you do not have a known good one and have to buy one you are not out much. The last thing it could be is the pick-up coil. If you do not have any experience changing out one of these this could be difficult for you but if changing the wires or the module out does not fix the problem this is most likely the problem. Hope this helps.
09-30-2009, 08:31 PM
well all that is correct stevis,, except a pick up coil will typically make a large boom through the exhaust on deceleration as it dies then turns back on intermittentely.. typically they will just die,, not have a half pulse,, its just a coil after all..fuel pressure should be in the range of 6 to 7 psi for that app.. its a mechanical pump bein that old and the pressure i highly doubt will dump off under rpm being mechanical,,unless you have a restriction in the line before the pump...race in snow might be on to somthin there... ive seen lobes wear right off,, will run at a idle but run like crap when under load... you can pull the valve covers and make sure all the rockers are moving full travel to check for cam lobe wear.being your having a issue while under load i would for sure check..back to float level,,i dont think that will change by itself,so i would proly scratch that idea. worth checking the power valve in the front fuel bowl,, if the diaphram breaks it will cause issues like that...
hope this helps
09-30-2009, 09:36 PM
I think if he rounded a cam lobe the engine would be ticking like a time bomb. The lobe will not just round out over night. Its takes time and makes noise as it wears.
10-01-2009, 12:12 AM
it would actually make less noise as it wears as there is no lobe to push the lifter...if it cant move up and down it cant make noise... also if the exhaust has a rumble or even worse a exhaust leak you would never hear the rocker clacking...ive personally experienced this in a 69 bird with a 400...the symptom is there,,, if it will rev to the moon with no load you have to relate the load to the issue....
10-01-2009, 12:25 AM
I also have experienced this. The first thing that happends is the rocker arm goes out of adjustment and clacks. I don't know how old you are but I was racing small block chevys back in 1970.
10-01-2009, 12:57 AM
im a youngin,, wasnt born till 82'...lol... age aside.. your absolutely right,, it will clack in the begining but will for sure lighten up quickly.. its like hittin the lobe on a lathe,,it will go fast when the process of cutting begins... im a mechanic for a living,, and ive been around engines since i could talk.. i love me a ole small block.. thats my stompin grounds.. my last build which i run now in my maro is 350 based,, just stroked it to get the 383, a blower and some juice and she goes...the cost of the custom pistons is what killed me..the rest fell into place..
back on topic though,,,,
while on the coil subject,,, i think we need more info.. is it hei?? or external coil?? what module if running any??? old school should be a 4 pin.. <font color="ff0000"></font><font color="ff0000"></font><font color="ff0000"></font><font color="ff0000"></font> maybe your even running a points distributor????
if its a points distributor i might have an idea on that also..the gm points distributors are natorious for eating housing bushings... the points cam will wobble under load and will cause the engine to pop and fart bad...make sure your dwell is correct also..
10-01-2009, 05:09 AM
You can also check what kind of vacuum (12 is a good reading) you are getting and is it routed right I mean coming off the right port/manafold on your carb for timing advancement. also old school trick hold a piece of paper at tail pipe you will see if lob is worn down,if i remember right it should get sucked back towards pipe depending on which valve is not opening correctly and if your removing valve covers you will also see which lob is worn.I am also leaning toward the Holley float level good luck keep us posted
10-01-2009, 12:37 PM
Check the wires going to the module (inside the distributor). As the vacumn advance moves the distributor plate back and forth the wires work harden and eventually break making an intermitant curcuit. Repaired abunch of them when I worked on street cars.
10-01-2009, 12:53 PM
One last thing. If you replace the ignition module or any ignition part, make sure it's a quality part. I suggest Napa or GM. We have had cheaper modules bad right out of the box or only last a week. Good luck and let us know what the problem was.
10-01-2009, 04:19 PM
I am leaning towards a fuel issue. Sledman411 said it is only one bank that seems dead and it runs fine under 3000. If it was a cam issue it would be both banks. I wonder if he has a plugged fuel jet on the secondary on the passanger side. Which would explain why it runs fine at idle and low rpms bucause it is only using the front two holes, then when he needs the power and the secondary opens up, the passanger side hole only has air and no fuel and the drives side has both. I would start buy taking the carberator apart and seeing if their is anything plugging up a barrel. It is a cheap thing to start with and all you are out is time.
10-01-2009, 08:08 PM
i think if it was a fuel issue you would see it in first gear also... he said he can rev it over 5000 rpm with no issue in first,,( where load is at its least) from what he is saying,,the more load thats produced the lower in the rpm range it acts up... you make a decent point on the secondary jet,,, but then again we need more info.. is he actually getting into it enough to open the secondaries???? or is it doing this off the primary's?
a cam issue will not make both banks go dead,,,nor will it make a whole single bank die,,if it did,,the cam aint spinnin.. it will mearly dump of the cylinder the lobe is worn on... when a lobe wears it still opens and closes,,,just a crap load less, which will let it run decent with no load on it.. start throwing load on it and whammo buck buck pop pop..
diagnosis is near impossible with out actually touching, feeling, seeing.. we are still lacking vital info on distributor type and such..
10-01-2009, 09:38 PM
Yes it could still be a fuel issue. What happens is on an idle even a bad fuel pump could still pump enough fuel to fill the float bowl. Now with a full float bowl the engine will rev and start to use the fuel in the float bowl rapidly. This could be why he gets the revs in first gear. After that the level of fuel in the bowl is down and the fuel pump can't replace the amount of fuel being used. Years ago, if a person complained there car would not go over 50, or 40, the first think we looked at was the fuel pump.
the kid next door switched a couple plug wires
10-02-2009, 12:41 AM
yes sir,,, excellent point.. would need to run a fuel pressure gauge into the cab,,or to the windshield and see if there is a pressure drop while this is going on..
10-06-2009, 10:18 PM
Well tonight i finnally got at it and took the carb off its a holly double pumper, but any ways, i took it off and the jets closest to the cab were completely clogged, so i cleaned then and did all that happy stuff, so i was hopeing that this would clear it up, my distributer is a interal coil set up, i cant think of the name off hand, but anyways i drove it and like i said i can take it to 5 grand in first, then second it start to sputter and miss fire, then third gear is the same thing, its really pissing me off, but i think i might have to pull the valve covers off and check to see if all are moving right, i have a leaky exhuast manifold so i could maybe be missing a knock or something like that, iff its the cam that would be nice, ive been looking for a reason to throw a mutha thumper comp cam in there, but i dont know how bad i want to spend the money, the truck has the 68-79 350 thats for the truck and high preformance app, 4 bolt main. i just want to get this baby figured out so i can start taking the motor to 4 grand with out sounding like its going to blow a connecting rod through the block
10-07-2009, 09:59 AM
o.k., if your jets were plugged you probably ran the motor lean, befor I go spending money on a cam I would do a leak down test or a compression check and see if you get good compression on all of the cylinders. You might have burned an exhaust valve or melted down a piston. So step one is do a compression check then go from their.
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