I was finally able to take the car out for a real test drive and was not very happy with the results.

Setup: 68 327/350HP Quadrajet, rebuild by me. I’ve rebuilt plenty with good results. Stock fuel pump, stock intake. Mallory duel point. Dwell 26 on primary set and 34 total. Fuel filter is a remote can type with a return line. The filter in the carb has been removed.

Senario – Came out of my housing development, everything feeling good. I enter traffic on the main street and proceed to accelerate ( not WOT) but pretty good. :) I get a muffled “POP” through the exhaust.:( If I bring it to speed gently, all goes fine. :) Anytime I give it a good dose of throttle it pops throught the exhaust. :( I have set total timing at 36 though 45 in two degree increments with no effect on the “POP”. All the advance is in by 2750 RPM. Installed a fuel pressure gauge after the pump – before carb. Pressure jumps around between 5 –7 PSI. When I rev the engine in the driveway the pressure drops to 0 and then recovers. I can rev the engine all day in the driveway and never hear the ‘POP”.

What do you think, Fuel starvation? Filter glogged, carb set wrong ( yes, I triple checked the float adjustment) Ignition, timing????????????? If you need more info tell me. I will tell all and try to not leave anything out. Help................
:confused: :confused: :confused:

 

Start with your ignition first. Make sure the vac advance is on the correct port (the port that has no vac at idle) if it's on the other one (that looks just like it) you will be setting the timimg with the vac advance fully advanced. If that's the case, if you were to load the motor as you desribed, the distributor/timing would retard excessivly - causing more exhaust noise and a spongy feeling throttle.

You may also want to make sure the coil isn't hook up backwards. On the same note, if you had a bad mechanical advance in the distributor, you would get a popping or back fire through the carb
which is not your problem

 

I've had this exact same thing happen due to faulty spark plug wires. At WOT the timing gets retarded and the spark will occur later, so the rotor in the distributor may be farther from the tab in the cap. It will try to jump to another wire or tab if one of the wires has a lot of resistance (e.g., burned a gap inside the wire somewhere).

They do go bad, though it may not be obvious. You can check them with an ohmmeter. The resistance per foot should be about the same for each wire.

-Noel

 

Thanks guys,

There is no vacuum advance on the Mallory. It is pure mechanical. I have an adjustable timing light and can watch it go through the advance stages. It seems to be correct.

The Wires are new MSDs. I'll go over them again for any burns cuts or breaks and double check the order etc..

any other ideas??

 

Are you saying that no advance occurs at light throttle / nor retard occurs when you go to WOT? This sounds rather wrong.

-Noel


[Modified by Noel Carboni, 10:13 PM 5/11/2002]

 

Noel,

The advance on the Mallory is fully mechanical it has two springs of different strengths, Plus the total mechanical advance is adjustable. I have it set up to enter the first advance @2000RPM and the second and final @2750RPM, for a total advance of 36+- a few degrees. I tune around that total for max power.

The vacuum advance on production automobiles is primarily for fuel economy at cruise speed. It is not even a factor at WOT. Check Lars article on Vacuum Advance. The only time the distributor retards timing is when the RPM is decreasing through the curve or when you go from cruise to WOT ( the vacuum advance drops out completely at WOT).

So I'm not sure if we are saying the same thing or not. Many people have reported problems with Malory on the forum. If need be, I'll go to something more Corvette specific. I've used Mallory mechanicals on other applications with no problem. This problem just has me second guessing myself.

I really think the ignition is ok and that I'm emtying the fuel bowl.

If I'm wrong on this then everyone educate me....

 

Terry,

Sorry about heading the question away from your original issue, and for my ignorance... It's just that somehow in all my years I have never come across a purely mechanical distributor. It's good to keep learning. :)

I've heard of cars that launch hard enough to pull the gas in the fuel line back from the mechanical fuel pump. In those cases what I usually hear that's done is to put an electric pump back in the tank, and quite often as well to run a regulator and return line.

I have less experience with carbs than with EFI, but isn't the amount of gas in the float bowl fairly substantial? How quickly is the problem occurring?

-Noel

 

Noel,

I can idle around fine and even cruise ok. When I pull away from traffic light or accelerate into traffic it just sorta stumbles and gives me a small "pop" or " puutt" :smash: and then my tendancy is to backoff and come to speed slowly. I'm ok again until the situation arises again. I reaal want to feel myself forced back in that seat. :D

I'm gonna start with the fuel filter and see if its clogged and also check the plug wires again. How many times has someone told us the wires are ok and then a day later they say "oh, nevermind I found a bad plug wire"? I'll then go through the basic troubleshooting list from simple to complex. I'm only home on weekends and get in too much of a hurry.

Thanks for the help. If you think of anything else to check let me know.

 

Typically, popping in the exhaust is from unburned fuel that is ignited after leaving the cylinder instead of being burned in it. The scenario you described coincides with when your accellerator pump would be adding extra fuel and I suspect that it's not all getting burned. It's possible that you're getting too much fuel but it's more likely that something else is preventing it from burning properly. A fault in the ignition, anything from a bad plug to a poor wire connection, or a valve adjusted too tight, are some possibilities. Because your scenario also involves an increase in load, I'd look at the ignition first.

 

Well, That makes sense! I just need to slow myself down and use good troubleshooting procedures. Posting my problems on the forum always helps me keep an open mind and not get locked in to throwing money at the problem.

Thanks for the input!!!!

:cheers: :cheers: :cheers:

 

Terry,

I'm not sure we are all on the same page regarding the term "pop". If you are getting a backfire through the carb and your ignition timing is correct, you may have a cam that is one tooth or more off at the timing chain. Did this popping only start recently, have you done any other work to the engine? Did it ever run the way it should?

I have a mechanical only MSD distributor too. All 39 degrees is in by about 2500rpm. My engine doesn't have enough vacuum to use a vacuum advance anyway. I start at about 25 degrees at 1500 rpm. The long duration cam permits this.

If your pop is really a bog or cough due to lack of fuel, you may need to reduce the clearance on the accelerator pump lever. Perhaps even changing the ratio by moving the linkage in one hole on the pump arm (if it has this ability).

Keep going, you will find it! :yesnod:

Chuck

 

My guess is that your accelerator pump is not working properly. Go back and recheck to make sure that everything is working properly. Make sure that all the parts are in place.

:)

 

I'm makin progress.... Got rid of the pop!!! :yesnod: :yesnod: :yesnod:

But there is still a problem. :(

I went all through the ignition. Read the plugs, checked the wires, checked firing order, set dwell again, set timing to 38 total. Went for a test drive and no change was noted. The pop was still there.

I replaced the fuel filter with one without a return line. I'm still getting 5-7 PSI on the gauge. Went for a test drive and no pop. I said ALRIGHT! :D a little more throttle DOUBLE ALRIGHT! :D Then WOT -- started out great then fuel bowl emptied and it got real quite. Coasted a little and it started right back up and ran fine al the way home. I guess I'll pickup a new fuel pump this week. Any suggestions??