Hey all,

I'm converting my 67 F100 with 390 to an edelbrock performer rpm intake and holley 1850 4barrel carb. I kitted the carburetor, and all looked well. At idle, the engine runs fine, but if I hit the throttle moderately fast, the engine acts like it's choking and has backfired through the carburetor (nice fireball). I checked my point gap and the distributor cap, and those look fine. My timing is dead on. I checked the pump shot, and ended up replacing the accelerator pump discharge nozzle because I was getting a stream on one side, but not the other, but no solution from that. It seems like there's too much fuel going through. After running for a few minutes, if I look at the primary throttle plates, they are totally soaked. My idle mixture needles are at 1.5 turns out, like the manual says for preliminary adjustment, and the float bowl levels are where they're supposed to be. I even checked that I'm using the stock acc. pump diaphragm, and it is. I'm at my wits end!
I'm thinking about throwing the holley in the garbage and getting an edelbrock performer. Any help fixing this would be greatly appreciated.

Greg

 

A good backfire will rupture the power valve on a Holley. Pull off the primary metering block and see if there is fuel in the vacuum cavity on the main body. Also try sucking on the power valve, it should hold a vacuum. If it doesn't (and you taste gas) replace the valve.

Barry

 

Barry,

He did not say if the carb is a new one or not. I thought that all newer Holley carbs came with back fire protection built in. I guess it would'nt hurt to look anyway.

I wonder if the Accl pump linkage is set up correctly. If the engine goes lean when the throttle first opens because of a lag in the pump shot it may cause the lean condition backfire.

Just some thoughts, good luck.
Lee

 

Hi Lee, haven't seen you lately. You need to come around more often!

I was assuming he had an old carb. since he kitted it. You're right, the newer ones have a check valve.

Greg, I'm with Lee on the lean out condition. You should see a steady stream of gas out the pump shot nozzle as soon as you touch the throttle. There should be no lag or sputtering, even when opening it up slowly. Things to check for are the linkage, check ball clearance in the pump housing ( I like to set it on the low side of the spec.), and make sure the check ball or needle is in the vertical passage where the nozzle screws on. Without it the gas gets siphoned up and out. Good luck.

Barry

 

Barry's right. It's a really old Holley. I did replace the power valve when kitting the carb, and was having problems before the one backfire I've had, but I'm probably going to have to check the power valve out anyways. The flooded condition was present before the backfire though. The pump shot starts right away within the first little bit of movement of the throttle, and can be seen as 2 fairly straight, smooth lines of fuel into the primary bores.

 

the check needle is in place under the accel. pump discharge nozzle.

greg

 

Hey Lee-

You are correct about the Holleys. It's either 6 or 8 years and newer Holleys have a blow-back protection check valve to keep the power valve from rupturing.

As far as the backfire- it has been my experience that when a backfire occurs through the carb, it's either because the mixture is too lean or the timing needs advanced. The way I check to see if it is going to backfire is to put the truck in gear and with your foot on the brake, slowly step on the throttle. If you feel a dead spot or a stumble at any point, it will backfire on you. Make adjustments and try again to see if the stumble got better or worse. Saves on a lot of noise.

Try adjusting the idle screws out by half a turn each- then test to see if it gets better. If it does, try another quarter turn- then test. If you get to a point where the stumble is almost gone and a turn of the mixture screws in either direction only makes it worse, try advancing your timing a couple of degrees- then test.

The best way to do it is to use a vacuum gauge. If you find yourself turning the mixture screws out by more than 2 1/2-3 complete turns, check for a vacuum leak.

Weak spark plugs and old wires will also cause a backfire. Tune-up parts are cheap. I replace everything in mine every year whether it needs it or not. I might put 6,000 miles on my truck in a year's time. It's cheap insurance.

Good luck to you-

Joe

 

[updated:LAST EDITED ON 07-Aug-02 AT 09:14 PM (EST)]After reading some other posts on related subjects, I think I'm going to check out my timing more thoroughly. My harmonic balancer marks might be incorrect... in which case, my timing is greatly retarded. I saw a post that said if you're standing in front of the radiator, taking the firewall as 12 o'clock and the radiator as 6 o'clock, the distritor vacuum advance should be pointing at about 7 o'clock, and mine is pointing at 5 o'clock.

Greg

 

Not necessarily true- depends on what position the distributor was stabbed. Mine points to 5 o'clock.

Try this if you don't have a vacuum gauge:

The carb can be adjusted but turning the mixture screws in until the engine starts to run rough, then turn them out 1/2 to 1 complete turn.

Turn the engine off. Loosen the lock bolt that holds your distributor. Start the engine and let it warm up. VERY CAREFULLY (don't stick your hand in the fan) advance the timing until the engine just starts to run rough, then turn it back until you here the RPM's start to drop off and the engine smooths out. Re-adjust your idle setting.

This should get you very close to the ideal settings. If you still have a heavy backfire or severe stumble, check again for a vacuum leak.

Joe

 

I do have a vacuum gauge, but I don't really know how to use it to tune the carb or set the timing. I get a reading of 16 inches at idle, and when I turn the idle mixture needles, the vacuum only drops to about 15 inches. How do you use a vacuum gauge to set timing and mixture?

Greg

 

forgot to mention that at idle, the vacuum reading is steady (only wavering by less that .5 ") at 15" and throttle response is a broad sweep of the reading (first low, then high I think). If I understand correctly, this means I don't have any vacuum leaks (or large ones anyways).

Greg

 

well, after further inspection, it looks like my ignition is messed up. I get intermittent spark on a couple cylinders and no spark on a couple others. Ooops.

Greg

 

Setting the carb and timing with a vacuum gauge is easy.

After you correct your ignition problems, hook up the vacuum gauge to manifold vacuum. Adjust your mixture screws until you get the highest possible vacuum. It is the same for timing- just make sure you leave the vacuum hose CONNECTED on the vacuum advance. Be sure to adjust your idle as needed.

Joe

 

my points were badly corroded. Got them replaced and the truck is running much better now. There's still a stumble off the line, but at least the trucks on the road again. I'll try to get rid of the stumble in a few days, and if I can't I'll post again.

jds - the tips on setting timing and mixture really helped. Thanks a lot.

Greg