Hello all,

It's been a while since I've been on the forum because school has been busy, but I started playing with the engine and truck again just a few days ago at the start of Christmas break.

Here are the engine details: '64 225 Edelbrock 500 mechanical secondaries, Offy 4 bbl, head milled .100", big valves, ported head, erson .465" 280 deg cam, Clifford shorties. The engine is still being broken in. No exhaust yet, just out of the headers.

When I'm driving along at about 2700-3000 rpm and step on the gas in 2nd or 3rd gear, it makes a loud, noticeable, quick-repeating backfiring sound and hesistates. Can't tell if it's really a backfire or not since it's already pretty loud with no exhaust, but that's what it sound like. Could this be: preignition, spark timing issues, carb, anything else?

Thanks in advance,

Stephan

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Stephan Nelle

I dont think I would push it too hard until you get it broken in. After that I think I would check the valve clearance. It could be running rich or lean. If its rich, unburnt fuel may be igniting in the exhaust system/mufflers. If its lean it may backfire too. Its hard to believe that a 500 edelbrock power circuit would not be able to keep up with the slant six. If that is the reason (lean) you might be wise to invest in some kind of O2 sensor and gauge. If you have to go cheap, you could even use a factory type narrow band and mili-volt meter. Not as easy to use but doable if you have patience. Other things might be the vacuum advance unit or some kind of vacuum leak. Oh well, these are just the first things that come to mind. hope it helps. I'm sure others will chime in with more advice.
Good luck, enjoy!

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1979 W150 Shortbox
225, NP435, 9 1/4, Dana 44

The Good Life.

I'm going to vote for lean, what does you vacc. gauge say? If it hesitates and you have low vaccuum, and it's not in the power valve yet...it'll complain...

Sounds like it's time to do a little adjusting to the carb and see what happens.

Good Luck!

-D.Idiot

I had a bit of trouble with the Carter AFB 500 where the pump shot was a tad too much. Read those plugs, and see what's going on.
The as supplied Edelbrock jetting will be mighty close, if not right on.
Did you go with a Performer or the AVS? If the secondary air valve on an AVS opens too quickly, you will go lean, so make sure the secondary isn't opening too soon. That may be accompanied with a WOT bog if such is the case. Set up right, you should be able to leave the tree on the floor, and the transition as the secondary opens should be a smooth increase in power. If it encounters a bog, or has a sudden change in power, the mixture isn't right. It indicates the primary is too lean if the power bumps up violently as the secondary opens. With a bog, the secondary is opening too soon, or the secondary jets are too lean.
While the butterflies open mechanically, the secondary air valve is operated by air flow. It will not open more than the engine can breath, unless the counterweight on the Performer series air valve is too light, or the adjustment on the AVS air valve is set to open too soon.

You may find that the metering rod springs aren't right. What color are the springs that you are using? I'm using the stiffest springs I could find for the AFB to bring in early enrichment of the primary main circuit. The stiffer the spring, the "Deeper" the enrichment vacuum point is.
You may also notice a pop when lifting from WOT. This happens when the metering rods are pulled back down by vacuum during a transition from hard acceleration to partial throttle. It corrects itself as the metering rod springs overcome vacuum again, but a brief lean condition can occur. This is bad news for high compression engines at high engine speeds, as high speed detonation can take place and parts start taking damage.

Timing is going to be an important part of your equation. If you have a dial back timing light, you can map your timing curve and report back on what you find. To get a good map, disconnect the vacuum can and plug the ported vacuum.
You're not going to want to run a stock timing curve. You are going to want less mechanical than stock with a 4bbl, and your going to want it to "Hop out" fairly soon. Somewhere a bit above idle.
If you can get all in before 3000 rpm, that would be best. Try to limit total timing to 30 degrees or a tad less with the vacuum can unplugged. If your getting more than 30 degrees of base+mechanical timing, that may be part of your issue. Time for a governor change. At high throttle settings there shouldn't be any appreciable vacuum advance coming in, so it should not be part of the problem your describing.

It sounds like you went pretty radical with your build, so tread lightly till you get things figured out!

CJ

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Part of Tyrde-Browne Racing

If you have not changed anything on your engine before this problem started, and the weather was cold, say mid to low 30’s, carburetor icing may be your problem. If you don’t have any manifold heat to keep your fuel mixture in suspension this problem will easily develop below 40 degrees. My engine is equipped much like yours with headers. During cool & cold weather conditions it would sputter, and stumble during throttle opening, not idle with out booting it, and had rotten gas mileage. This turned out to be a carburetor icing issue.

Problem solved by manufacturing a hot spot under the carburetor heated by water from the heater loop. Check my posts from around March for how I did this.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

CJ,

Thanks! That was just the kind of information I was looking for. I'm ome for Christmas break, but when I get back I'll check out what you mentioned.

Thanks,
Stephan

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Stephan Nelle