I know it is difficult to analyze from afar, but I need some opinions. My 56 Chevy starts right up, but will not run below 1200 RPM. It is a 350, Holley dual feed vacuum secondary, Mallory unilite, Accel Supercoil. The fuel pressure is a solid 6 pounds. It ran fine yesterday. This morning it started and stalled out. I tried many times, and it starts right away but will not keep running below 1200 RPM. When the engine quits the tach drops to zero immediately like it has lost a signal. I am thinking the module in the unitlie is going bad. The distributor has been in there for years with no trouble, but fuel does not appear to be the issue. The car is dead in a hotel parking lot, so I need to get going on it quickly. Any ideas? Thanks.
Always good to help out a fellow Czech... Do you have a stock distributor and coil that could be dropped in it? I always liked to have a set up I could use to diagnose odd problems like yours. It does sound more electrical than gas related, I could be wrong. Bob
I would check to make sure the PCV hose, or power brake hose if it has it, aren't off rotted or broken or that a plug on the manifold or carb didn't blow off.
Thanks for the responses. I did get a full tank of gas at a no-name station last night a few minutes before I parked the car. If it is bad gas, I may be able to get some gas line and run off a can to test it. I will check vacuum as well. It is possible something has come loose or rotted out. It just seems odd that all was well yesterday.
It may be just coincidental that it was running well until you got gas, but that is the first place I would look. Get a glass jar and pull the fuel line and pump some gas into the jar. Let it sit, and if there is water it will settle to the bottom of the jar. If so, a heavy dose of some "Dry Gas" product will get it going again. If that fails, next guess is vacuum leak. If you have power brakes the boosters can fail internally and create a hell of a leak. You can pull that hose and plug it for a test. You can also shoot some starting fluid around the intake area while the car is running and if it speeds up you have found your leak. But be careful as the ether can explode or flame up if you have an ignition leak somewhere. Don
I think the first thing I would do is crank the idle mixture screws out and shoot some carb cleaner into the holes, then reinstall the needles. See if it runs then. I think the larger likelyhood that it is a fueling problem and not an ignition problem just because most electronic ignitions are either a go or no go kinda deal.
take the front bowl off of the carburator, clean it out, clean the needle and seat and check the power valve.
I am leaning towards a vacuum leak since it runs above 1200 rpm. At least that is the way I took the post...
Plugged idle circuit in the carb. Try this, rev it up and then immediately put a shop rag with both hands over the carb. This will block all or most of the air from going through the carb, creating a large vacuum in the fuel circuit pulling whatever is in the way through. I have done this before and it will pull a small piece of crud through the idle circuit and sometimes fix the problem.
You guys may be on to something. I didn't even notice he has a Holley carb. Those things HATE dirt, even a little speck will shut them down. My Son spent many a date pulling the bowls off of the Holley he ran on his Olds convertible because the car would start breaking up. He would find a speck so small you almost couldn't see it inside and once he cleaned it out the car ran good again. If you are going to run a Holley, or any carb for that matter, a good water separating fuel filter before the pump really helps. Don
Thanks for all the replies guys. I will be trying to get it running within the next couple of hours. I will report back with the results.
mine acted the same way. had gunk in one of the jets in the carb, once that was good it ran fine. and it was an all of a sudden thing too.
Rado se stalo. Go with the fuel angle first, I didn't realize you just filled it up... with suspect gas. I like BJR's suggestion of choking the crud through the carb, it works sometimes. It was almost a never fail on the old Rochester 2GC. If you do find water in the gas and can't find dry gas, a pint of denatured alcohol will work. Bob
No luck. I tried running off a can of fresh gas, blowing out the idle circuit with carb cleaner, checking the float level, cleaning up all connections at the coil, forcing a high vacuum by covering the carb, checking vacuum lines, etc. The car starts and will usually run at 2200 RPM or more. I ran it for 20 minutes at 2200 tonight, and then it quit. It might run for a minute, or 20. When it quits, the tach drops like it was disconnected even though the engine is still spinning. I disconnected the tach to rule it out, but no dice. I just don't get why it will start and run at high RPMs, but not less than 2000 and sometimes not start at all. Sometimes when it quits it detonates a lot, almost spinning the engine backwards. I am still thinking ignition, but I need to research this some more. Thanks for the help.
Does it run with your foot on the gas and die as soon as you let off? If it were a vacuum leak it would cause it to run lean, which after twenty minutes of running lean it would be very hot and cause it to diesel when shut off. What do the plugs look like?
Yes, it runs if I hold the revs above 2000. As soon as I let off enough that the revs drop under 2000 it dies. It dies so quick that quickly pressing on the gas pedal has no effect. The engine is not getting hot (about 190), so I don't think it is running lean. Tomorrow I will measure the coil and the Unilite module. Luckily I have a ohm meter with me.
Did it backfire before it quit? Any black smoke out the exhaust? When I read Holley, I think of a blown power valve. Start the engine and have some one put their fingers over the bowl vent tubes and see if the idle smooths out. If it does, the power valve is ruptured dumping fuel inside the carb.
If it isn't lean and the carb is clean check to make sure you do not have a leaking diaphragm in the power valve. BobM beat me . . .
Stick another distributor in it like an HEI and will probably be fixed...Unilites are nothing but trouble.
I measured the coil and unilite module this morning. Coil checked out fine, but the module was reading 4 volts. It needs to be 2, so I was thinking the module was blown. It had been trouble free for years, but the amount of warnings Mallory gives about blowing it up by looking at it sideways always worried me. Anyway, a GM HEI will not fit due to the firewall, and finding a Unilite module on the weekend is impossible, so I found a point distributor on Cragslist. After installing it the Chevy started and idled first try. Looks like I am going traditional, as I am not swapping the Malory back in. It is refreshing to be able to troubleshoot rather than guess about electronics. I have some tuning to do, since I don't have a dwell meter handy, but the car idles and runs. I may have blown the power valve with all the weekend BS, so I need to look at that. A big thank you for everyone that replied. I really appreciate it. Hopefully I can return the favor.
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