histoy

I really need some help from our engine experts. Right now our 327 300hp will not run below 2,000 rpm. This started with the engine shuddering, bucking & jumping after I filled up at the gas station a week and a half ago. I parked the car and the engine was hardly running, but I was able to rev the engine to 3,000 and it ran OK. When I let it idle down it wouldn't run. I have a fuel pressure regulator, which I set to zero and ran the carb empty. I then set it back to normal and refilled the carb. I then revved the engine to 3,000 and it ran OK, then idled down fine. I thought maybe I had something in a needle that I had successfully flushed out. I then drove the car on the highway at 70 mph to a car show. The engine idled fine when we shut it off at the show. After the show I had the same issues as at the gas station, the engine was shuddering and bucking and wouldn't idle. I again revved the engine a few times and then it would idle, but a little rough. I drove home at 65 mph, and when I got into town it ran fine at slow speeds and idled like normal in my garage.

The following day I drove the car again and it ran rough while cold and idled a little rough. I drove it a half mile and the engine stated bucking and shuddering badly so I headed home. It wouldn't idle when I got home. It would only run at high speeds and acted like it was a carb problem. I pulled the top carb cover and removed and checked the floats, but they were fine. I installed new needles & seats and reset the floats. I ran the car again and the same thing happened... you can make it run fast, but not slow. I checked the fuel pump and I had 4 1/2# of pressure and a constant stream of gas.

Out of frustration I ordered a rebuilt carb from Chicago Corvette. The engine runs the same with the new carb (new gaskets installed). Even more frustrated I then replaced the fuel pump and installed a new inline fuel filter. I drained all the gas from the tank and replaced it with 4 gallons of fresh gas from another gas station. I blew 10# of air pressure from the pump flex line back to the tank. I could hear the air bubbling in the tank, so the line is clear. Before restarting the engine I pulled the plugs, and cleaned them, because they looked like the engine was running rich. I checked the engine compression and the cylinders read between 95 and 120. Those are the same readings that it had a year ago. I'm planning to have the engine rebuilt this winter.

I started the car this morning and it still acts the same. I checked the new pump and it has 4 1/2# of pressure, just like the one I replaced.

The engine has had points, condenser, coil, ballast resistor, and vacuum advance replaced in the last 2,000 miles, and has been running super all summer. This morning I again checked the dwell and it's at 31 degrees. I had set it at 30 earlier this summer. When I checked the timing today it was reading 4 degrees retarded at 2000 rpm with the vacuum advance plugged. I had set it at 8 degrees advanced with the vacuum advance plugged at 600 rpm earlier this year.

Is it possible that the timing chain has jumped a tooth? That's all that i can think of at this time. Please give me your thoughts...Brian

GTOguy

I would get a vacuum gauge on that engine, first off. See what kind of vacuum you have. It sounds like a huge vacuum leak, or possible ignition break-down. Could be anything. Unlikely a jumped chain. You could verify that by checking the static timing, though, and verifying #1 piston with the valvetrain and dist. rotor. If it were me, I'd be tempted to throw in a new set of points and a new condenser. Do you have power brakes? It could be a ripped booster diaphragm, and act just as you described. Intermittent bad vacuum leak, but not an issue at speed where vacuum is low. Just a thought.....

histoy

Thanks for your reply. I do have power brakes, but they are working normal, but I can plug the line and check it. I had thought about a huge vacuum leak, but felt it would be obvious. It's not burning any oil, so it appears that there isn't a vacuum leak on the lifter side of the intake manifold. I'll spray carb cleaner around the intake manifold to see if that makes a difference.

Westlotorn

Any chance this engine had a nylon timing sprocket on the camshaft? They were used for a while and were supposed to be quieter. Many failed and allowed the timing to jump around. Normally at high miles. Set your timing back to 8 adv and watch it with the light while you rev the engine through many RPM changes, I would leave the vac advance plugged at this point. It should stay the same base timing at idle and move very predictably advancing as the RPM changes. Anything odd happening would help you find the problem and move on to fixing it. The Distributor itself may have failed messing up your advance curve.

phil2302

Many possibilites here. If it happened immediately after a fill up I would be suspicious of the fuel.

Frankie the Fink

That was my first thought, I had a 'water in the fuel' flashback.....maybe Westlotorn is on to something though.

tuxnharley

It sure sounded like it at first, but then he went on to say he had drained the tank, blew out the lines, put in fresh gas, new carb, pump, filter, so..............?

tbarb

Also,

Is it possible the distributor moved a little causing the retarded settings.

phil2302

Oh yes now I see. Thanks.
Carb change and fuel line blow out would also rule out malfunction from lingering particulate.
Regarding nylon sprocket and timing chain, I have owned at least a dozen cars over the years with a 327 V8 and cant remember seeing this.

GCD1962

That's my guess too. Look for water droplets in the carb as you give it a squirt

plaidside

You do not state what carburetor you have.
If the engine will not idle but it will run at approx. 3000 I would suspect the idle fuel restriction is plugged.
Start the engine and when it does not idle spray some carburetor cleaner into the carburetor throat. If the engine RPM flares up then it is the idle circuit.
If it is an AFB remove the top and remove the two screws on each side holding the boost venturies in place. Remove the boost venturies and look at the bottom of the thin long brass tube, at the very end is the idle fuel restrictor, I would suspect these are clogged.
Joe

tuxnharley

He stated he has the same problem both before and after a new carb, as well as many other fuel system changes......

histoy

Here's the latest guys....I appreciate everyone's input.

Two of my friends came over to assist me. We checked the brake booster and as soon as we pulled the valve there was a pop as air entered the booster, so it was OK.

We sprayed carb cleaner around the intake ports as the car was running at 2000 rpm, and there was no change to the speed. The engine continues to act like it's starving for gas below 2000 rpm, but the air smells like the carb is running rich. Maybe because of the way I'm having to pump it to keep it running. We removed the pressure regulator from the system. Still the same results. My friend started spraying carb cleaner into the carb as we tried to get it to idle down. The rpm improved, but the engine still ran rough.

Out of desperation we decided to install the original carb. We had to reuse the gaskets as I didn't have new ones. Once the old carb filled with gas the engine started running better and it idled down to normal. We checked the base timing again and we were able to adjust it to 8 degrees btdc with the vacuum line disconnected to the adv. I must have read the timing marks wrong this morning while checking it at 2000 rpm. Sorry for the bum info I gave you.

I ran the car around the neighborhood and it's running like normal. I filled the tank with BP alcohol free 91 octane gas, and drove it home. I don't know which change I made that fixed the problem I had with the old carb, but I do know that I'll be making a call tomorrow to return the rebuilt carb.

Thanks again for your input...Brian

AZDoug

That would be my suspicion, or at least something to rule out.

I had nylon sprocket do this on a car back in 1979 and the timing chain jumped several teeth, cause real poor running.

If the timing is WAY off, or if you set timing mark on the damper to TDC,a nd pull the dizzy cap, the rotor should be right at #1 or #6. if it isn't, this is a likely culprit.

Doug

Doug

Westlotorn

The first one I chased with the Nylon gear issue drove me nuts. Car ran absolutely perfect then it might backfire and ran really rough then it would run perfect for maybe a couple days and backfire and run really rough. I had a timing light on it while revving it up and down and noticed the timing could jump or hesitate and then jump. Sometimes it would jump backwards, so it would be at 20+ timing as we revved it and then be retarded. Finally the mental light came on and we pulled the timing cover and fixed it. The next one was much easier to diagnose. Common issue to find in the late 70's and 80's.

tbarb

Glad you have found something that seems to help with your problem. In post #1, you indicate the problem happened with the original carburetor so keep looking because you still may have a gremlin.

If the replacement coil is a reproduction part get a NAPA coil and carefully check all the ignition parts. Look for something simple like the ground wire inside the distributor. etc.

You have replaced so many parts, just make sure they are all up to spec and good quality so you can move forward if the problem happens again.

histoy

You're correct. I may still have a gremlin, but I'm leaning more toward bad gas at this point. The points, condenser, and coil were all installed by the Chevy dealership that had our car for about 12 years, so I'm not sure which brand they used.

tbarb

You may have a heavy float causing the spark plugs to gas foul. Once the plugs get carbon fouled then may need to be removed and cleaned/changed to fire the low speed lean mixtures. Just a FWIW..

histoy

tbarb... I did remove the plugs and clean them while I was changing the fuel pump.

Now the bad news... I've been driving the car for the last few days and it's been running fine. However, yesterday morning I started the car to take it to a show and the problem came back. It seems like the car runs out of gas after I start it up. I pump it and restart it and it surges and shutters and shuts down. I pumped it and started it again 2 or 3 times until it finally ran. After it ran OK I started to the show and it was surging off and on. I stopped at intersection and the engine died. I cranked it for awhile and it restarted and ran with some surging then OK after that. When we left the show the same things happened. Surging and dying, restarting, and finally running OK again. Very frustrating!

I'm going to drain the gas tank again and pull the sending unit and replace the fuel strainer sock and the flex line from the tank to the hard line. Those are the only things in the fuel system that I've not changed. Wish me luck!

Faslane

I would have to try points and condenser.