I cant get my new engine to run smooth. Seems untunable.

[SquareRoot]

What brand of intake gaskets? Edelbrock?

 

[AyWoSch Motors]

What brand of intake gaskets? Edelbrock?

Felpro.

 

[SquareRoot]

Felpro.

Ok good. I had brand new gaskets (provided with my Edelbrock EFI kit) delaminate causing similar issues. It caused a leak between the intake runners. Unnoticeable until I pulled the intake. Felpro is good.

 

[Ricko1966]

Well it's a manual, and it has brand new intake gaskets. Could be the vacuum lines. Will check that..

Just because it has intake gaskets doesn't mean they are the correct ones,and doesn't mean they are installed correctly. No bashing, we all make mistakes.

 

[Ricko1966]

I just read your post better, if its slow to come down from a Rev it's getting air from somewhere other than the throttle plates.

 

[AyWoSch Motors]

I just read your post better, if its slow to come down from a Rev it's getting air from somewhere other than the throttle plates.

Maybe there big black cardboard looking spreadbore gasket that goes under the carb? That was used and a bit old. Like 1984 orginal old. Looked fine, but you never know been I guess

 

[Grit dog]

Following. Symptoms sound exactly like the crusty 258 original 6 banger in my CJ5. (Minus the great power part, lol).
It too, after warmup will idle too high. And it makes this almost constant “odd pitter patter” sputter sound from the exhaust. Only when idling no load. Runs like a top when you’re on it.
I’ve never tried to really diagnose it. If I turn the idle down enough when it’s warm, it’s too low when cold.
It’ll run from here to your house in NM and back, slowly, if I wanted it to. Just weird behavior.
No obvious vac leaks. Tune up is fresh (15 years and about 1000 miles ago, lol). Carb rebuilt 10 years ago. And I recall timing was right. Checked it a long time ago.
Good luck I’m very interested in what you find.

 

[Matt69olds]

Make sure the weights in the distributor are free to move. If they stick in the advance position, it will idle fast.

Is the secondary throttle hanging open a little? Not the air door on the top, but the lower one. I worked at a Cadillac dealership years ago, 90 year old guy brought in his 70 Fleetwood for something, don’t remember what it was. The kid working on it had to drive the car, he came back from the test drive with the engine idling fast. Turns out the kid driving it was much more aggressive with the throttle, the secondary throttle blades stuck in the carbon, hanging the throttle open a little.

 

[85 Squarebody76]

Really sounds like a large Vac leak.. How much vac does it have while running? Should have at least 15 inches. I just fought a similar prob with my '85 305 and found the base plate gasket was bad... could not get over 11 to 12.5 inches at idle. replaced the base plate gasket and it jumped to 15.5 and solve the idle problem..

 

[Grumpy]

If your aluminum intake is used, you may have a pin hole underneath the intake through the exhaust crossover. The air sucked into the intake is UNDER everything and won't be seen. The engine WILL consume a small amount of oil at first - and only at higher RPM.
A similar condition is the intake gaskets not matching underneath and a small amount of air is getting sucked in from below. This MAY or MAY NOT suck oil into the combustion chamber.

Review your spark plugs again and note if there is oil on any of them.

A note for intake gaskets: both Edelbrock and Fel Pro will include in their installation instructions a reminder to verify fitment on the cylinder head AND intake. Remove the carb and verify smooth port transition from the intake to the cylinder heads using a piece of coat hanger - it can also help noting if the gaskets are where they should be.

Have you re-used any vacuum lines? I have seen the rubber on them split lengthways , not just across.

Cap off your lines to the brake booster, the transmission vacuum modulator..... everything except the distributor advance. Recreate the conditions on the stand.

Make only one change at a time and write everything down; it is notable how we fail to remember small details while diagnosing a condition.

 

[rich weyand]

Let me guess: You have it on ported vacuum, right?
If so, that's why it won't idle worth a crap.
Make sure the vacuum line is connected to manifold vacuum. That's where the engine is telling you what it wants.

Ignition Timing for First-Generation GM V-8 Engines

I pulled all my ignition timing rants from multiple forums together into one post for another forum, so I thought I would post it here as well. Ignition timing, including base timing, total timing, mechanical advance, and vacuum advance, which can be either ported vacuum or manifold vacuum...

www.gmsquarebody.com

 

[Cyrillious]

Hello. I need some help, I'm going crazy trying to tune this engine.

I recently put a new 350 in my 86 GMC and now it runs and drives, but I can't seem to tune the engine right.

It's a L98 350 out of a 85 TPI Corvette.
I have an Edelbrock performer dual plane intake and right now a Rochester Quadrajet, but I had an Edelbrock AVS650 on it before and couldn't make that work either. Brand new fuel pump, fuel filter, brand new 65k 4pin HEI, with 10.5mm HP plug wires and new plugs.

When I had the engine on the run stand it ran quite well so it appeared, but after I got it in the truck, it didn't.
I messed with it and tinkered with it, but couldn't get it quite right. It had a weird pitter patter lopsided exhaust sound. And it smelled like gas out one exhaust pipe ( dual exhaust).
After telling my boss (the PO), he said that he had put mis matched jet sizes in that edelbrock to make it work on the old 305 because it had a weak cylinder on one side.
So I took that off and put a Rochester on that was tuned already for a 350 and I knew ran well, thinking that would solve the issue. Nope. It runs a little better now, it will stay running, choke works, and it will idle, but it's not happy.
It will only idle at about 1600-1800 RPM, any less and it wants to die. It has the same odd sounding pitter patter lopsided sound out the back. It also does an odd slight little rev at random while its idling.
I took the carburetor apart, and cleaned it and put it back together, and same thing. I messed with air/fuel a bit until it sounded a little better, then I had my friend who is really good at tuning come over and mess with it. He got it to run a little smoother, but still not great. Can't get it to idle down at all. And after we tuned it to sound a little better, when you rev it, it stays revving higher for a second after you let off, then it comes down slowly.

Also checked the timing. The way its running right now at such a high idle, it wont give an accurate reading. It says it's about 16-18 degrees BTDC with the advance unplugged, but that's also at about 16-1800 rpm. When I had it on the run stand and kt was running smooth. I had set it to about 10 degrees and it sounded great. I'd assume that hasnt changed as far as base time goes.
But we did mess with it anyhow too, it seems to like more advance, did smooth out a little the more advanced it, so I reset it a little higher.

We took it for a ride after all that. The thing freakin flys. It runs great when you're driving it, doesnt bog, sounds smooth, just goes. Way better than it used to, its almost scary fast now. Likes getting romped on, did a nice burnout with a locker, very easy.
.... but, as soon as we get back and park it, it's running like crap again.
Super high idle, cant lower it, choppy exhaust sound, weird little surging thing.
I have no clue why, both me and my friend agreed its the most fickle engine we've ever tried to tune. Just work run smooth no matter what we do.

Anyone have any ideas, please help. Thank you.

I had a similar issue with my 350 after I rebuilt it hard to start and idling too high approximately 1300rpm. I had to get electricity to the electric choke and that helped with starting. I had to go left then right gradually with the high idle screw (left facing the engine bay) and then the low idle screw (opposite side) while listening to the engine purr down to approximately 750rpm. I had a vacuum gauge while doing this. Went from 13"Hg to a more comfortable 17"Hg.

 

[AyWoSch Motors]

Let me guess: You have it on ported vacuum, right?
If so, that's why it won't idle worth a crap.
Make sure the vacuum line is connected to manifold vacuum. That's where the engine is telling you what it wants.

Ignition Timing for First-Generation GM V-8 Engines

I pulled all my ignition timing rants from multiple forums together into one post for another forum, so I thought I would post it here as well. Ignition timing, including base timing, total timing, mechanical advance, and vacuum advance, which can be either ported vacuum or manifold vacuum...

www.gmsquarebody.com

All my vacuum lines I have are running out if the carburetor. There is no manifold vacuum port on my intake.

 

[Ricko1966]

Let me guess: You have it on ported vacuum, right?
If so, that's why it won't idle worth a crap.
Make sure the vacuum line is connected to manifold vacuum. That's where the engine is telling you what it wants.

Ignition Timing for First-Generation GM V-8 Engines

I pulled all my ignition timing rants from multiple forums together into one post for another forum, so I thought I would post it here as well. Ignition timing, including base timing, total timing, mechanical advance, and vacuum advance, which can be either ported vacuum or manifold vacuum...

www.gmsquarebody.com

Yep,this one drives me nuts. I have a 1985 c with a different engine,different,heads,a different carb and manifold and aftermarket distributor. Where do the factory timing marks line up?

 

[potent rodent]

sure sounds like a vac leak to me check your base plate gasket were it mount to intake check your throttle linkage make sure its not binding up