I have occasional carb backfiring. I can hear it under the hood, the carb backfires into the air cleaner.

What can cause this? It happens mainly at part throttle, if I accelerating wide open, no back fire. However, during full throttle, it does seem to hesitate/miss as I get closer to about 3000 (maybe when the secondaries are opening?)

It's a 327/350 engine, but has a carberator off of a '65 chevelle 396. Could the jets maybe be too big? Too rich? The exhaust pipes are quite dark inside, plugs seem a bit darker than what I would expect too.

Maybe since that carb is from a 396, they had larger jets in it then should be.

It's a holley carb, can anyone tell me what size jets I should have in it?

For ignition, I have a pertroniz unit in it, with new coil, good plug wires, etc.

Any help is greatly appreciated.

 

kyo -
Backfiring up through the carb is an indication of a lean condition - not rich. This assumes, of course, that you don't have a bad valve, broken valvespring, bad timing chain, or grossly incorrect ignition timing...

 

I guess if I had a bad valve, timing chain, etc., I would know, right? I would see/hear other things happening, right?

I know you hate the pertronix units, and I would like to run points, but I don't really have the time to set them correctly, nor the skill.

What do you suggest I do?


[Modified by kyosho, 11:54 AM 8/13/2002]

 

What should my initial timing be at? i.e., I disconnect the advance tube from the distributor, hook up my timing light, and rotate the distributor? What should this be at?

The distributor was recently rebuilt, with the advance mechanism replaced, so I know all that stuff works good.

 

kyo -
I'd start with the obvious and easy stuff:

Check and verify your timing and total timing - make sure you're getting 36 degrees of total mechanical advance at elevated rpm (with vac disconnected). Make sure this total setting is giving you an initial advance at idle of about 12-18 degrees.

Then, make sure your are getting a full 12 volts to your PerTronix unit. If you are using the stock coil wire (wire that on a stock system runs to the "+" side of the coil), you are running through a resistor wire, and you're not getting 12 volts. The PerTronix requires a full 12 volts to fire correctly, and will misfire if you hook up the stock wiring.

Obviously, if you have a broken valvespring, burnt valve, or other serious mechanical issues, you will have other symptoms, such as rough idle, engine shaking under load, and erratic exhaust tone. Verify that none of these conditions exist.

Once you've verified & adjusted these paramaters, and the backfiring through the carb still exists, try stepping up your primary jet size 2 sizes. If this does not cure the problem at this point, you have something other than a carb issue still going on...

 

With a timing light, how can I check the total timing? All I know hwo to do is point the thing at the harmonic balancer and get the initial reading there. Do you 'estimate' it base don how far the light moves when you hit the carb linkage?

You say initial advance of 12-18 degrees. This is 6-8 on the flywheel, and the other 6-10 is coming from the mechanical advance of the distributor? Is this correct?

I think I am using the stock wire to the + side of my pertronix unit. That could be a problem, but I need to double check it.

Lars - Thank you. You are an invaluable resource on this forum.

 

kyo -
Here is an extract out of an article I did on setting up the timing. It should give you answers to most of your questions. Please contact me if you need assistance, as setting the total timing is one of the most mis-understood concepts of Corvette (and general automotive) tuning.

How to Set the Timing (non-HEI)
When you think about it, setting the timing at idle speed makes no sense at all: You don’t operate your car at idle, and timing changes as the rpm changes. Fact is, the timing spec at idle speed is provided as a simple way for most people to set the timing, and is not a good procedure for optimum performance.
Small and Big block Chevys (and most other GM performance V8 engines) perform best when the total timing (full centrifugal advance plus the initial timing setting with vacuum advance disconnected) is all in by 2,500 – 2,800 rpm and is set to 36 – 38 degrees. If you have an adjustable timing light, this is very easy to check. If you don’t, you need to scribe a 36-degree mark on your harmonic balancer. Here’s how:
Measure the circumference of your harmonic balancer using a sewing tape measure (or other flexible tape measure). Get it as accurate as you can. Take this measurement and divide by 10. The number you get is the distance to 36 degrees. Measure this distance CLOCKWISE (as seen from the front of the engine) from your existing harmonic balancer timing mark and place a clear mark on the balancer.
Tighten the distributor hold-down bolt tight enough to keep the distributor secure, but loose enough that you can still turn the distributor. Disconnect the vacuum advance. If you have the stock distributor advanceaprings installed, remove one of the springs (this will make the centrifugal advance come in quicker and at a lower, safer rpm). Start the engine. If you’re using an adjustable timing light, set the light to 36 degrees advanced. Now rev the engine while observing the timing marks with the light. The timing advance should come in fairly quickly, and the timing will “peg out” below 3000 rpm. Bring the rpm up to where the timing does not advance any further, and rotate the distributor to set the timing at this point to 36 degrees. With an adjustable light set at 36 degrees, align the stock timing marks with “0” when the timing is “pegged out.” With the non-adjustable light, align your new 36-degree mark with “0.” Rev the engine a little to make sure the timing will not advance any further. Now drop it down to idle and hook up the vacuum advance. Adjust your idle speed and idle mixture for the new timing setting. Shut it down.
Now do a road test.
The 36-degree advance curve is optimum for performance, but may require premium fuel. Lug the car around, and punch the throttle at low rpm while listening for detonation (“engine knock”). If you’re getting any audible knock, you MUST retard the timing. Retard the timing in 2-degree increments until engine knock stops. Engine knock will seriously damage engine components if not corrected. If you get no knock, you may see slightly improved performance at 38 degrees total timing. This is particularly true if you’re running at high altitude.
Your timing is now set for best possible performance. Make note of the new setting at idle, and use this for your future tune-up work.

 

Also -
You can check out my article under the post "Vacuum Advance Advice" posted here in this section - this will give you some tuning info and specs so you can check out the operation/specification of your vacuum advance control unit to make sure it's not producing an over-advanced condition.

 

You might want to check the power valve. Backfiring usually means doom for them. Not one of Holley's better ideas. Save the Wave.

 

sounds like a sticky valve, my old vette engine did this but it was very intermittant

 

Well, before I read Lars post on setting the timing, I talked to another person and did about the same thing. Measured the circumference, and made a 36 degree mark on the balancer. I hooked up my timing light, reved the engine, and lo and behold, the timing did not move off the idel setting. At higer RPM, I still had about 8 degrees.

I took off the cap and rotor, and I discovered that the plate that the weights sit on was froze to the distributor shaft. They are supposed to rotate independently. Some wd-40, and working it a bit, and I finally got it freed up, and rotating. I also discovered that my weights were pretty chewed up. So, I went ahead and bought a new curve kit, put the middle springs on, and made sure that I could rotate that plate and have the weights move, and the springs return them to their central position.

Needless to say, the car runs much better now that it has some mechanical advance!!!

One problem though, my vacumn canister is into my intake manifold, how can I correct this to get more timing?

Thanx for the help guys.