400 small block Chevy problem

Vacuum gauge
> My son and I just finished up installing an 400 sbc engine in his > '79 El > Camino. Has a mild cam and an Edelbrock alum. intake. (Performer). > When it's cold, it runs fine. After the engine completely warms up, > it > starts to miss and run very rough. The HEI dist. was working fine > when we > took it off the old 305, and it now has new plugs, cap and wires. > Spark > seems very strong. I'm thinking we have a manifold leak that shows > itself > after the temp. has allowed the leak to open. Am I on the right > track > here? My son was very careful with the manifold installation and I > watched > him. Used silicone permatex between the gasket and manifold, and > Copper > Coat between the gasket and head. Put a bead of the silicone at the > front > and rear instead of the gaskets that always pinch out anyhow. How > can I > determine if it's leaking? I've heard of spraying carb. cleaner at > the > manifold to head junction and see if the miss clears out. Any help > would be > appreciated. We need to get this thing fixed. > > Garrett Fulton > > > > >
Reply to
Jim Smith
Garrett,
Is it running rich? Cold motors like it rich, hot motors hate it. If the mixture is set incorrectly it'll run fine when cold but start running like buttcrack as it warms up.
Is the choke opening fully as it heats up?
Doc
> > > > >
Reply to
"Doc"
Adjust the lifters AFTER the Engine Has warmed up !!
> My son and I just finished up installing an 400 sbc engine in his '79 El > Camino. Has a mild cam and an Edelbrock alum. intake. (Performer). > When it's cold, it runs fine. After the engine completely warms up, it > starts to miss and run very rough. The HEI dist. was working fine when we > took it off the old 305, and it now has new plugs, cap and wires. Spark > seems very strong. I'm thinking we have a manifold leak that shows itself > after the temp. has allowed the leak to open. Am I on the right track > here? My son was very careful with the manifold installation and I watched > him. Used silicone permatex between the gasket and manifold, and Copper > Coat between the gasket and head. Put a bead of the silicone at the front > and rear instead of the gaskets that always pinch out anyhow. How can I > determine if it's leaking? I've heard of spraying carb. cleaner at the > manifold to head junction and see if the miss clears out. Any help would be > appreciated. We need to get this thing fixed. > > Garrett Fulton > > > > >
Reply to
Santa
My son and I just finished up installing an 400 sbc engine in his '79 El
Camino. Has a mild cam and an Edelbrock alum. intake. (Performer).
When it's cold, it runs fine. After the engine completely warms up, it
starts to miss and run very rough. The HEI dist. was working fine when we
took it off the old 305, and it now has new plugs, cap and wires. Spark
seems very strong. I'm thinking we have a manifold leak that shows itself
after the temp. has allowed the leak to open. Am I on the right track
here? My son was very careful with the manifold installation and I watched
him. Used silicone permatex between the gasket and manifold, and Copper
Coat between the gasket and head. Put a bead of the silicone at the front
and rear instead of the gaskets that always pinch out anyhow. How can I
determine if it's leaking? I've heard of spraying carb. cleaner at the
manifold to head junction and see if the miss clears out. Any help would be
appreciated. We need to get this thing fixed.
Garrett Fulton
Reply to
gfulton
If the carburetor is for a 305 cubic inch it may be running fine when cold as the choke is on. Bit as it warms up and the choke is off the carburetor is probably way to lean for a 400 cubic inch motor. The jetting may be your problem. Drive it long enough to get the motor up to temperature. Then pull the plugs and read them. I bet their bone white, too lean. A lean condition will act like a vacuum leak.
Brian
Reply to
el Diablo
The choke is opening all the way. The carb. ran fine on the 305. I'm pretty sure this is a fuel thing, not spark, just from the way it's acting.
Garrett
Reply to
gfulton
There are a number of issues to look into. Timing might be too far advanced, carb needs tweaking are the two most likely. Get thee to a vacuum gauge and post the reading at idle. Change up the timing 4 or so degrees each way and post those too, along with any change. Are you sure you have the vacuum advance hooked to the right port on the carb/ Which carb is it, btw? If it's the stock carb off the El Trucko, how many vacuum lines do you\did you have going to it?
Reply to
John Alt
Man, if he didn't rejet that badboy when it was swapped off the 305 (I'm guessing stock Q-jet, 750 cfm but jetted WAY down for the baby 8) it's a wonder he hasn't burnt right though a piston yet!
Doc
Reply to
"Doc"
Thanks to everyone for the advice. I'm sure the vacuum lines are connected right. We've set the timing at factory spec. and ran it both ways after the problem occurred, with no change. We do have a set of jets for the Q-jet that are one step richer. I can't start on the thing until Tues. morn. If I get it sorted out, I'll let everyone know and thanks again for taking the time.
Garrett
Reply to
Garrett Fulton
Was it ever re-jetted from the original 305 configuration? If the answer is no, yer' gunna have to go more than 1 step up bud...................
Doc
> > > > >
Reply to
"Doc"
--------------------------------- Misses at idle,or part throttle? If it's part throttle I would tend to agree with others here about jets--although you will need to totally tune the carb.for the new engine,ie. primary metering rods,power piston,secondary rods,throttle pump adj., and float level (check the specs for a stock 400 and go from there.) However if you are saying that it starts missing at idle as well even after setting the idle screws,I would check to see if slowly closing the choke by hand makes it worse or better,and go from there. If it is a carb problem I might even get one off a 400 (they're $16.95 at our local self-serve) and try that. Let us know, Best....Brian O.
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Reply to
Brian Orion

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