Hello, New here

I have come accross this site many times and decided to actually register today. I see that there are many knowledgeable people on these forums and maybe you guys can help me out with an issue that has been driving me nuts for about 2 years now.

I live in Colorado at an elevation of 5000ft and I own a 1985 Monte Carlo SS with a carburated 350 engine. The problem is when the engine is warmed up it stumbles/cuts out VERY badly off of idle. However, the first minute or two of operation after a cold start up the engine runs awesome. I can not for the life of me figure out what is causing this issue. I have replaced and tuned so many things trying to figure this out it is ridiculous.

Here are the full specs:

(New = Part is less than 2 years old)
350ci block
GM cast iron cylinder heads #3947041....1969-1970...302/350......"Right angle" casting identifier, 64cc chambers
Dual plane Holley Street Dominator intake manifold
New #82651 Holley 650CFM Street HP Carburetor
New Comp Cam kit #K12-242-2, lift 0.477 int./0.480 exh., duration at 050 inch Lift: 224 int./230 exh.
(New cam was broken in with ZDDP+ oil additive as recommended by Comp Cams for Hydraulic Flat tappet lifters)
New Hydraulic Flat tappet lifters
New Comp Cams steel 1.6 Roller tip rockers
New double roller timing chain
New matched valve springs
New Hardened pushrods
New MSD Street Fire HEI distributor
New Accel Super Stock Spiral Spark Plug Wires
New AC/Delco Spark Plugs
New Proform High Volume water pump
New Summit 8in Nodular Iron harmonic balancer
New GM Fuel Pump
New Fuel Filter
New Radiator

Here are some things that I have tried:

Tried different carburetors, distributors, plug wires, spark plugs and fuel pumps with no change to the stumble.

Carburetor Tuning:
Tried several different jetting sizes, jetting currently at recommended by Holley for an elevation of 5000 ft above sea level.
Tried several different accelerator pump nozzles and cam sizes.

Timing Tuning:
Tried several different initial timing settings ranging from 0-25 degrees, currently at 12

The engine has good compression, vacuum, fuel pressure and most everything but the block and heads are new.

I have been conversing with the good fellows over at MonteCarloSS.com for a long time about this issue and we just can not figure out what is causing it.

I hope maybe you guys have some ideas to help me out.

Thanks a bunch,
Jason

This is a shot in the dark, but another place to check.
Are you certain the intake is sealing? I ask since years ago I had a edelbrock performer intake that had been passed on to me when I bought the car.
Anyways, after trying countless carbs and everything else I couldn`t get the stumble out. Finally after fooling with it a month I figured the intake was the last thing left. I pulled it and took it to my machinist, he put it on a flat mill and found it was warped pretty severely, it was so bad he whacked it with the mill .025 and it was still warped so I just threw it away and got a new one. While I ran the intake it never used a drop of oil and never smoked any, but the stumble drove me crazy. If you check the intake and find it`s okay, use a straight edge and a feeler gauge to check the head mating surface as well.

I second the intake
I had an internal leak on the intake and had same symtoms.
also my oil smelled like gas aswell.
replaced intake and stumble went away

Hmm, that is something I haven't thought about. The intake did come with the car, but the previous owner said it was new, however he wasn't the one that built the engine and has been wrong about other things. So, I don't put much faith in anything he told me. I kind of did a stupid thing when I bought this car. I bought it online and out of state without checking it out. However, the car is in good shape as expected when I bought it, I just need to figure this issue out and it would be a great performer.

Just a thought, if the intake was leaking, wouldn't my vacuum reading change from when the engine is cold to when it is hot? My vacuum stays the same whether the engine is cold or all warmed up.

Thanks,
Jason

It`s likely it wouldn`t be any different. Just as it`s funny with my intake it never used oil but I couldn`t find any leaks at the top of the intake.
While your in there, check to see what intake gaskets were used. If they are fel pro blue gaskets kind of on the thick side, it`s possible the gaskets never took a set. The standard fel pro`s are for cast iron intakes that have enough weight to crush the gasket so it seals. Aluminum intakes aren`t heavy enough to crush them so they don`t seal. I always use the paper mr gasket cheapy jobs hanging off a part stores shelf. Use these with some RTV around the coolant ports and a bead of RTV on the china walls and your set. But also make sure you have the intake checked for straightness.

What is the fuel pressure at the carb inlet with the motor running? If it's over 5 psi, pressure could be over-powering the needle and seat in the carb and blowing raw fuel into the motor. The motor might tolerate this when it's cold and needs a rich condition to run properly, but once warmed up, no longer needs the additional fuel. Just a thought.

I did use the thicker gaskets when when I put my intake on and I used RTV to seal the corners. I think I will pop the intake off and check a few things. Also I will use the the thin paper gaskets this time. BTW, if I do end up replacing the intake, what intake would you guys suggest for my setup? I want to stay dual plane for good torque.

The fuel pump is new and it is the regular GM mechanical fuel pump with the larger can and return line. I took a fuel pressure reading from the carb side of the fuel filter recently, but I am not sure how accurate it was as I got some funky readings. I got a max of 8lb of pressure when I held higher RPMs, but at idle , about 1000rpm, the gauge was all over the place. Here is a video of my gauge when I did that...

Fuel Pressure Video

Thanks,
Jason

So, is 8 lb of fuel pressure too much? The manual for my carb does say the recommended fuel pressure is 5-7 psi.

Thanks,
Jason

Some Holley carbs can handle 9 psi fuel pressure, some can`t. You could lower yours to about 5 psi and see if any change occurs. However if you feel like it`s flooding here`s one way to check. With the engine at idle, remove the air cleaner and shine a light in the carbs throat, if you see fuel dripping or pouring out of the boosters either the floats too high or the fuel pressure is too high. If you see no drips at all then shut the engine down, if excessive white smoke starts to come out of the carb it`s flooding out of the idle circuits.

Ok, could I just get an adjustable inline regulator like this one...

http://www.summitracing.com/parts/TRD-2329/

Or, does it have to be one of the more expensive ones with the return line?

Thanks,
Jason

I`ve used the inline ones before without issue. It controlled fuel pressure well, the upside was easy to set dial pressure. I`ve also used the dual line or return line style, what I didn`t like about it was having to break the lock nut then set the pressure and relock, other than that it worked fine. Upside for the return line style is it`s rebuildable.

OK, I think I will go ahead and try the inline one and also one of those small more accurate liquid fuel gauges that I can screw into my fuel rail. That should be a fairly accurate way to be sure I can get at 5 psi. It couldn't hurt and it is cheap, so I 'll give it a go.

I'll let you guys know how it turns out.

Thanks,
Jason

What is your vacuum at idle. You said it stumbles and then goes. Do you see any black smoke when it finally goes. Your fuel pressure is a little high but I dont think it will cause your issue. When you changed the squirter how big did you go. Did you change to the hollow screw for the squirters. Do you have a vacuum advance on the distributor, if so have you tried it with it disconnected. Have you checked the adjustment on the pump arm. If you change the pump cam you usually need to adjust the pump arm. It should be adjusted to the point where there is just no play in the arm and there needs to be at least .015 travel still available at WOT so nothing gets bent.
At 5000 feet a carb that was designed to run at sea level is not going to work without some adjustments. You may be going the wrong way with your adjustments. You may be making it to rich. If you can get ahold of a wide band A/F ratio meter it may help you identify the issue. I would shhot for ~13:1 to start with.
Not sure which way you have been going but the thinner air will require smaller jets.

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Chet

There is good advice here. A couple areas I'd check. Several were talking about the intake manifold and I'm assuming the area that attaches to the heads. I've had a car that stumbled and like you changed everything imaginable. I had a leak between the carb and intake manifold. Like the other person, the surface wasn't flush. I noticed gas leaking out onto the manifold but didn't notice any vacuum leaks. I used a thicker gasket and it worked. My other problem was I was simply running too big of a carb. 650 CFM might be OK for your engine. I had a 650 Holley DP and it was just too much regardless of squirter changes, jetting etc. I dropped a 600 CFM on and the stumble stopped immediately. The engine even pulled harder.

My vacuum at idle is 8lb, which is equal to 13lb at sea level. It does stumble then go, but has a lot less power than the first couple minutes of operation. I never see any kind of smoke out the exhaust. I can't remember off hand how big I went on the squirter, but I tried several sizes up to not needing the hollow screw. I have also tested with and without the vacuum advance with no change to the stumble. I have also tried all colors of pump cams on two different carbs and there was no change to the stumble. I am pretty confident that the fuel/air ratio is good as I got the carb jetted as suggested by Holley for my elevation and my spark plugs are always that nice tan color, never white or black.

I do have a very thin and old carb gasket so I might need to check that. I am running a 650 carb now coming down from a 770 carb that was on the engine when I got it. If I remember right I think that 650cfm was recommended by Holley's carb calculator on their site for a 350.

So, I guess my plan of action right now is to get a regulator, check/replace carb gasket, and maybe pull the intake and check for straightness if needed.

Man, if I could just get this engine to run all the time like it does for the first couple minutes, I would be one happy camper.

Thanks,
Jason