Thread: New topend rebuild but can't get rid of the backfire

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Recently rebuilt the top end of a small block chevy 350. I am having a problem with the engine backfiring through the carb when the secondaries open on the carb. I was using an edelbrock 600. I tried everything from tuning the carb with different metering rods, jets, springs but nothing got rid of the backfire. I played with the timing thinking i needed to advance more but that didn't solve the problem either. Being completely lost and having nothing to blame i bought a new holley 600 carb thinking it was the edelbrock giving me this issue. I still have the same problem. The engine fires up great, idles fantastic, revs with alot of response and can cruise down the road fine. Once i go Wide open throttle it backfires with a flame right through the air filter. The total timing is right on 35 degrees. I didn't change anything with the holley yet. What else on the engine can i check that would cause this to backfire? I can go out and change the wires, one got the coating melted a little and is now taped, would that cause this? The truck has hydraulic lifters and roller rockers.When i installed the roller rockers and started to tighten the nuts , i went slow till i felt a slight drag on the pushrod then went a 1/4 turn past that. Is that not enough preload ? This is my first 350 engine rebuild. Thank you

You've got your valves too tight. Tech (Techinspector1) posted a very, very good article that gave a sure fire adjustment method for SBC lifters (he likely wrote it) - http://www.crankshaftcoalition.com/w...stment_SBC/BBC

its not going to back fire with a 1/4 pre load on the lifter i set hyd lifters like that for 30 years but can be abit loud. have you checked that you have true TDC damper like a OEM damper ring can slip. timming curve is wrong or sticky ? you said it pops under a load ? bad plug .. lean pop ? plugs wire . cap ,rotor. cam could be going south many other things as well as .check engine with a vacuum reading

Just to be safe i will readjust the valves using the link you posted. Speaking of plugs, would the plug gaps be a factor ?

Pat's right, an extra 1/8 to 1/4 turn is not going to cause your backfire, but I'd readjust them to be sure they're right. I wouldn't think plug gap would be an issue if they're close to 0.035" (or as recommended by your ignition system if aftermarket?). A burned, taped plug wire? Replace it, but again it's likely not causing your backfire.
Pat mentioned checking TDC against the marks on your damper and your timing mark indicator on the block/front cover. If you need it there's a pretty slick way to check it without any risk of having something sticking in a plug hole to get bound up - link to a previous post Need help with ignition timing
When you say

Once i go Wide open throttle it backfires with a flame right through the air filter.

is that a single backfire as you go to WOT, or multiple pops as you're at WOT? What ignition system are you running?

It continues to backfire if i hold it open going down the road but if i just go wot just sitting in the garage its one flame backfire and then i let go of the throttle and change my pants. I have a basic hei distributur also

Sounds like a timing problem, but Pat mentioned lean pop which is a possibility, too. Have you pulled your plugs to check color? Have you gone through your distributor to ensure that it's not a simple crossfire/tracking issue? After eliminating basic ignition questions I'd verify TDC to eliminate that from your list.

How can i tell if the distributur has a simple crossfire issue. Will there be markings on the inside of the cap ?

Im making a check list from everyones suggestions and will do everything this week. Ill share the results by no later than this saturday. I will continue to watch this page as the week continues just in case anyone has any more thoughts.

Originally Posted by mikesaik

How can i tell if the distributur has a simple crossfire issue. Will there be markings on the inside of the cap ?

Mike,
Sometimes you can see carbon tracks inside the cap between electrodes. If there's any question about the rotor & cap I would change them as a normal tune up step chasing a misfire/backfire. Same with your plugs. With the move to offshore manufacture by many of the "major" names there have been too many cases of bad plugs out of the box. Everyone has their favorites, but I now use NGK exclusively.

Find a cap and rotor with a high dielectric strength. Cheap caps and rotors will allow arcing between terminals causing crossfire. Stick with a major brand, Mallory, MSD, etc. and you should be good to go.

I had a minute to pull off the cap on the distributor and snap some pictures. Does anything look out of the ordinary?

It looks okay, but if it was me, I would get another cap just to rule out a cap issue. It's not one of those caps that re-direct the wires is it?

What do you mean by redirecting the wires ? If i was to upgrade and get new wires and a distributor with cap, what would be a wise choice as far as best major brand name without going broke.

As Lynn said the cap looks ok, but hair line cracks are hard to detect at times and is the rotor made of thin plastic? If I were going to change the dizzy I would pick the best one I could afford within my budget and IMHO stay away from anything from Accell. As I said earlier I would look toward Mallory or MSD as both make a quality street unit that won't break the bank.