Carb Install Troubleshooting

[Bigcalhoun]

Gents,
Just installed a new Holley 500 2 barrel on my 302 and can't get it to fire up. Here are the details:

- truck was running fine immediately prior to install (drove it 120 miles the day before)
- setup is almost entirely stock except for Pertronix ignition and coil (can't see any reason I wouldn't have a spark after driving day before)
- I have confirmed the pump is pumping gas
- I can see the bowl is full of fuel through the tiny window near the choke
- running a clear in-line fuel filter between the pump and carb
- truck cranks and will sometimes barely turnover for a moment then cuts off
- I got it running for about a minute at one point, ran really rough and died when I touched the gas
- no unburnt fuel smell from the tail pipe
- did finally get a small amount of black condensation out of the tailpipe

I would appreciate any wisdom y'all might offer.

[zgerbic]

You did not say what else you touched when replacing the carb.

Lots of possibilities...

There can be some problems caused by working on the engine itself. Remember Tony's rule of three: fuel, ignition and compression.

Fuel. Too much or too little is a problem. Check the following:
1. Look down the carburetor and slowly open the throttle. A good stream should flow out of the two accelerator nozzles. If not, the float bowl might be too low or something clogging the accelerator circuit. Don't over do it or you will get a flooding problem. If you suspect flooding, open the carb plates fully and put a round shaft screwdriver down the opening to hold the plates open. Go eat lunch or watch a movie while the flooding gas evaporates. Don't forget to take out the screwdriver before staring.
2. Make sure you gently close the idle mixture screws and back out about 2 - 3 turns. You can adjust this later to get it leaned out.
3. Make sure the throttle adjustment screw is keeping the throttle open slightly while troubleshooting the carb.
4 With the idle mixture screws out and the throttle slightly open, will the engine at least start and idle?
5. What is happening with the choke? It should be closed to about 1/8" when cold and open as the engine heats up. If an electric choke: if you have been turning on the ignition, the choke might be open too far to richen the mixture enough to keep the engine running on first cold start.
6. Vacuum can be a fuel related problem as well. If the brake booster fitting is loose, for example, the engine could be massively lean. Check all around the carb and manifold for a loose vacuum line, poor fit between the carb, riser and manifold, or missing a gasket.
7. Many new built carbs are of less quality than those you used to get in the past and many bad stories of problems are all over the auto sites. You might need to take the top off the top or front, and look at the passages and float bowl for dirt and obstructions. When putting a new carb, or one from an unknown source, on an engine I will often open it up and make sure it is in good running order before installing.

Ignition. Did you touch something? Many seemingly carb problems are actually ignition problems. Check the following:
1. Power and disty wires are on the correct terminals of the coil.
2. All spark plug wires are tightly seated. You did not remove or move any, right?
3. Make sure you did not move the distributor or remove the cap.
4. Take the coil wire out of the distributor and set next to the manifold and make sure you have spark. Put back.
5. Make sure you are getting spark at one of the spark plug wires. Remove one and put on a spare plug and set it on the block while you crank the engine a little.
6. Make sure you have the vacuum advance/retard hoses hooked up right.
7. A bit of a long shot but when you crank the engine, the starter relay provides 12V to the ignition coil. If this is not working right, the coil will just get the 7V or less from the resistor wire while cranking. Might catch or maybe not.

Compression. Pull three plugs and see if they are wet. If really wet then the cylinder walls are probably washed down so you have low compression. If so pull the plugs and put a teaspoon of oil in each cylinder. Put back together and see if it labors a bit more on cranking and if it now starts.

Let us know what you find. If this does not get you going, provide additional symptoms.

[Bigcalhoun]

I appreciate the thorough set of recommendations. I’ll give it another go. More to follow.

[sparky72]

Are all of the unused vacuum ports plugged? If you have a large vacuum leak it will behave as you describe.

[cep62]

And if your PVC goes to the spacer under the carb make sure you have the right gasket to seal it.

[Bigcalhoun]

Thanks for the help fellas. I checked everything recommended and no joy. Slapped the old Autolite 2100 back on and her and fired right back up. I’m taking it to a guy I know with a shop who builds race cars and works on classics. He’ll get it right and I’ll quit worrying about it.

[cep62]

Please let us know what you find.

[Mach428]

Just by chance, is it possible that you’re still using the carburetor spacer? If so, check that the bottom gasket seals to the odd shape of the carburetor mounting flange of the intake. If not, there’s a HUGH vacuum leak.