Just like my wife my old truck always seems to have issues. The latest one is a miss and backfire through the exhaust when it's under a load [going up hill in 3rd gear only]. I cant make it do it any other time. The 292 has less than 10,000 mi on a rebuild and just started doing this out of the blue. Any thought's or inpute would be appreciated.

 

I would mess with your timing. You are either to far adavanced or too retarded. If you are relying heavaly on the vaccum advance when you hit a hill the spark advance drops off and it misses.

Try this. Remove and seal your vaccum advance line. Try accelerating. If the miss exists then I would belive your too retarded. If not try driving up the hill in third. If it still happens you may have to switch to premium.

I dont know your engine build but with a compression over 9:1, advanced spark, and highway gears this is a common problom.

I hope you figure out what is up though. Good luck.

 

Thanks flipklos. My logic trail was leading that way. The strange part though is that it only does it on on a long grade. It wont miss on short duration hills, and then only over 65mph. It starts out as a simple miss, like i'm only running on 6or7 cylinders. Then it deteriates to popping through the exhaust. Which gets worse and worse the longer I'm pulling the hill. If I pull over and idle for a minute it smooth's out like nothing ever happened.

 

What carb do you have? Is it possible you have fuel sloshing into the intake?

 

The engine is stock with the original Holley 2bbl. I rebuilt the carb plus replaced the point's, condenser, and plugs. The wires are 6 months old and I just readjusted the valves. Having done all that, if nothing else it idles smoother than my wifes new car. It just wont go up a long hill.

 

Mess with your timing first. Unless you have a "nose" bowl on the holley I dont see the flooding issue happening.

Did you make sure that the power valve was the same as the original?

 

I didnt think to check the application of the power valve. It came with the carb kit. Could that really make that much of a differance? Another thought is that maybe the vacuum advance is weak or has a very small leak. It is one of the few part's on the truck that's original.

 

The vacuum advances very often are gone, used up, creating a vacuum leak. See if your points plate rotates with vacuum applied to the line. It is super easy if dist. vacuum is connected to carb via rubber hose. Just disconnect at carb and suck. Of the last 4 or 5 distributors I've had, none had a functional vacuum advance.

The power valve for your engine possibly will be in the 8.5 range. If it is too low it will begin dumping gas too late.

I would double check your points gap and make sure it is .015. Also consider possible partial obstruction of fuel filter limiting fuel supply during the long loads. If gas tank and lines werent cleaned during build they could be cutting loose some debris.

Ted Eaton is a big fan of installing vacuum gauges in cab. So I did and it is always interesting and sometimes helpful.

 

The vaccum advance is defeninately worth checking. Though it wouldnt cause the problom you describe in my mind. I drove mine for two years before I noticed that the shaft bushings were bad. When I pulled it out I found that all that remained of the vaccum advance was the housing! Milage and power increased greatly for a $100. Just pull the distributor cap, put on a vaccum pump and see if it moves the breaker plate. Or if you dont have a vaccum pump just wrap your lips around the nipple and suck. If you get air violia! you need one of them things.

Yes, the incorrect power vavle can cause a lean miss. I still lean twords timing or octane however.

This is how a holley powervalve works........

As you apply throttle your vaccum drops so does the fuel flow out of the main jets, as less vaccum means less siginal. Holley overcomes this in two ways.

1- Accelerator pump. shoots a small squirt of fuel direcly into the airstreem richining the mixture to allow for smooth acceleration.

2-Power valve. this is held in place by vaccum so that it is shut most of the time. At a preset point of vaccum it opens allowing additional fuel to flow into the venturies enriching the mix. This will happen when you accelerate or loose vaccum due to somthing like a hill or a heavy load on the engine. This allows you to run a leaner set of jets for cruising and such then would be possible otherwise. Granting one better milage. The rule of thumb for a street driven vehicle is 1/2 the running temp idle vaccum is the power valve choice. so if you had 17in of vaccum a 8.5 powervalve would be a great choice. (For reasons only known to holleyu all power valves end in .5.) My 64 292 holds 19" at idle so I need a 9.5. I know this as the 7.5 I got in now causes a noticible bog on moderate acceleration in 3rd and 4th, and on hard acceleration in 2nd. I got the new vale in the shed but Im waiting on spring to install it.

Most rebuild kits come with a 6.5 powervalve. this is often a excessivly lean point for enrichment causing boging or even missing or pinging. I will stress again. One thing at a time. Timing is a far more likely culprit.

 

I see you narrowly beat me 46y. Touche!

 

Flipklos, I shouldnt have butted in, you were doing fine .

 

OK I can grasp the idea of the power valve causing a "lean miss", but can it also cause the backfiring through the exhaust?

 

Generaly no. That is a symptem of being rich not so much lean.
Timing, my good sir, timing.

 

If the vacuum drops low enough while under load, going up the hill, the power valve can open and cause a rich condition. So I'd say that under the right circumstances, yes it can cause backfiring.
It would explain why it happens only on long grades; it has time to fowl the plugs and begin misfiring. After idling with a good a/f ratio, it cleans itself out and is good to go.

 

Ya know back home if we had a dog that was this sick we'd just take it out back and shoot it. But for now I'll replace the power valve and see what happens. I did check the timing and it's right on the nose. Is it possible that the centrifigul advance in the distributer could be mal functioning and causing the problem?