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

[AyWoSch Motors]

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.

 

[AyWoSch Motors]

My thoughts were a couple things... maybe a vacuum leak, but I have no idea where. Everything is new.
Maybe the carburetor has something wrong with it that I didnt know.
Maybe the timing is way out now for some reason.

My friend thought that maybe because that engine was TPI, and was designed with fuel injection in mind that maybe it cant handle a carburetor. Or maybe the Cam duration is way different than what were used to.
Or maybe the stock rottenchester, with a high flowing performer intake, and a potentially high flowing, higher compression heads on the L98, just aren't getting along.

Those were our only thoughts if that helps at all

 

[Ricko1966]

Your going to have a rough time timing it with the idle that high.The weights are kicking in. The cam isn't to much injected cams are typically really mild,high compression again isn't going to affect your qjet. My first thoughts are spray carb cleaner and look for intake leaks but since it ran good on the stand,my first guess is you pulled the cap and wires for install and you have 2 wires crossed now you have everything adjusted up to make the poor thing try to idle on 6 cylinders. 18436572.

 

[AyWoSch Motors]

Your going to have a rough time timing it with the idle that high.The weights are kicking in. The cam isn't to much injected cams are typically really mild,high compression again isn't going to affect your qjet. My first thoughts are spray carb cleaner and look for intake leaks but since it ran good on the stand,my first guess is you pulled the cap and wires for install and you have 2 wires crossed now you have everything adjusted up to make the poor thing try to idle on 6 cylinders. 18436572.

I sprayed carb clean around it, but I realized my carb clean is no flammable. And I'm out of starting fluid. Will try that.

I've checked and triple checked the firing order, its definatly right. I know the firing order. I'd know if it was running on 6. Definitely running on 8, just not well

 

[Originalthor]

Its amazing on how many things aren't flammable now adays. I use my blue or yellow plumbing torch for vacuum leaks. Works Pretty good too. Easy way to find intake leaks also that are on inside.

 

[CalSgt]

Is it possible you stabbed the distributer a tooth or two off after installing in the vehicle?

 

[scrap--metal]

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.

Where'd the Qjet come from? If your carb change and fiddling are having incremental improvements, it seems like your on the right track (no mention of misfire, backfire, smoke, or stink). Any chance the carb's not as fresh as you remember it to be?

I'm sure you have another one in your fleet that you could swap around. We call it the "Chevy shuffle" at my house.

 

[Radiohead]

Or maybe needs a carb spacer - to let the throttle blades close completely?

If not a vacuum leak in the normal places, how about the power booster and it's assorted plumbing? Steel lines can crack, rubber is notorious.

Spitballing here...

 

[AyWoSch Motors]

Is it possible you stabbed the distributer a tooth or two off after installing in the vehicle?

I left the distributor in when I installed it. Same as run stand.

 

[AyWoSch Motors]

Where'd the Qjet come from? If your carb change and fiddling are having incremental improvements, it seems like your on the right track (no mention of misfire, backfire, smoke, or stink). Any chance the carb's not as fresh as you remember it to be?

I'm sure you have another one in your fleet that you could swap around. We call it the "Chevy shuffle" at my house.

Off my friends 84 service truck, ran great before, but he put a Holley on.

Believe it or not, only my van and my Vettecart have Qjets. Ones super hard to get to, and the other is set up perfect for the 283. Everything else is either 2bbl Rochester, TBI, or Vortec.
Only other running carb I have is a 950cfm carter racing carb that runs super rich. Got about a mile or 2 per gallon when I ran it on the 454.
I have 6 other Q-jets in the cabinet, but I've never run them, would want to go threw them first.

 

[AyWoSch Motors]

Or maybe needs a carb spacer - to let the throttle blades close completely?

If not a vacuum leak in the normal places, how about the power booster and it's assorted plumbing? Steel lines can crack, rubber is notorious.

Spitballing here...

Good idea. I could cap off all the vacuum ports temporarily to see if that helps.

My friend also had the thought about carb spacer, but for the idea that it might atomize the fuel better.

 

[SirRobyn0]

@AyWoSch Motors If this was me I'd start with the basics. I know it ran good on the stand, but who knows why something changed! Not wanting to idle below 1,500 make me think massive vacuum leak. Does that manifold have a EGR? is the booster leaking off the vacuum? Hook a gauge up to it. Not that you'll find a leak with gauge but if it's ridiculously low at 1,500 at least you'll know to looking in the right direction.

If there is any question about the internals of the engine do a comp test. You can learn a lot from that even though it's a pain to do, at least if it comes back with good results you know your good there.

You don't by chance have the brake booster hooked up to the manifold and the big port on the back of the Q-jet wide open or visa versa. I'm just trying to spark some ideas.

If all else fails you could always post some video of it running, maybe one of us will catch something in the vid.

 

[AyWoSch Motors]

@AyWoSch Motors If this was me I'd start with the basics. I know it ran good on the stand, but who knows why something changed! Not wanting to idle below 1,500 make me think massive vacuum leak. Does that manifold have a EGR? is the booster leaking off the vacuum? Hook a gauge up to it. Not that you'll find a leak with gauge but if it's ridiculously low at 1,500 at least you'll know to looking in the right direction.

If there is any question about the internals of the engine do a comp test. You can learn a lot from that even though it's a pain to do, at least if it comes back with good results you know your good there.

You don't by chance have the brake booster hooked up to the manifold and the big port on the back of the Q-jet wide open or visa versa. I'm just trying to spark some ideas.

If all else fails you could always post some video of it running, maybe one of us will catch something in the vid.

Good thoughts.
I was thinking vacuum leak too, but nothing jumps out.
Does an EGR port with block off plate. Maybe that gasket went bad. Booster is hooked to big port at back of carb. There is no port in manifold that I remember.

I do have a compression tester, but it's such a pain to do all 8, but I probably should. Good idea to check plugs too. I suspect compression is good though.

If I can get it to run and warm up where it won't die if I leave it, I'll get a video of it.

 

[SquareRoot]

Vacuum leak. Intake manifold gaskets, carb, vacuum advance pot, vacuum modulator on the trans (automatic) or associated lines /hoses, brake booster.

 

[AyWoSch Motors]

Vacuum leak. Intake manifold gaskets, carb, vacuum advance pot, vacuum modulator on the trans (automatic) or associated lines /hoses, brake booster.

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