carb backfire edelbrock 1406

sagacious · #1 (**permalink**) 06-07-2004, 02:47 PM

i have a 73 nova with a 350 stock engine, i've replaced the lifters with performance type, and added a double roller timing chain that is advanced 4 deg. the ignition is aftermarket HEI with 55,000V coil, and MSD ignition module. the intake is an edelbrock air gap rpm and the carb is edelbrock #1406 (600cfm). flowtech headers and 2.5'' dual exhaust straight back, 4 aluminum radiator and 160 thermo.. the engine runs great and smooth until i hit higher rpms.. then the carb starts to backfire a couple times and you can tell hp drops a little...
what could be causing it to backfire? the car seems to run better the more the timing is advanced. and it is advanced all the way right now, but the problem is still there. i've noticed a small difference when the car is running hotter... it seems to get "better" than it was.. but never goes away. i just fixed the ac in the ac and i've seen the temp hit 210 at idle for a while and it'll run 190 when cruising. that seems a little outrageous to me and too hot.. but like it said i've noticed it's not AS bad when its hotter... or it could just be me. it's a low compression 8:5, and without the ac on the car stays 160-170 temp. could too much fuel be going into the secondaries? too little air? too large of jets for the stock engine? any advice is appreciated.. thanks a lot.

xlr8or · #2 (**permalink**) 06-07-2004, 03:41 PM

Something has to be igniting the fuel in the intake or the ignited charge is being allow back through the intake by an open valve. You say it only happens at high RPM, Are you maybe getting some valve float? How old are the heads? How much lift and overlap? Valve Springs? Valve adjusted a little to tight and not sealing?? Just some things to ponder.

sharpe 1 · #3 (**permalink**) 06-08-2004, 12:21 PM

You might check for a lean condition in the fuel system. Lean causes backfire.

sharpe 1 · #4 (**permalink**) 06-08-2004, 12:23 PM

Also a lean running engine will overheat.

wilf leek · #5 (**permalink**) 06-08-2004, 02:59 PM

Gotta be valve float.

or.... do you have a rev limiting chip in that MSD? What revs?

sagacious · #6 (**permalink**) 06-08-2004, 03:48 PM

"Something has to be igniting the fuel in the intake or the ignited charge is being allow back through the intake by an open valve. You say it only happens at high RPM, Are you maybe getting some valve float? How old are the heads? How much lift and overlap? Valve Springs? Valve adjusted a little to tight and not sealing?? Just some things to ponder."

lean condition? rev limiter?
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i've questioned the valve float, the heads are as old as the engine, since 1973. they were recently taken off and fel pro valve guide seals were installed. the engine backfired like this at high rpm a short while before the lifters went out, so i know the valves arent adjusted a little too tight. valve springs are stock, could they have just all of a sudden wore out? as far as a rev limiter, i cant find any information on the MSD HEI upgrade module having any kind of rev limiter.. it's an onboard chip for HEI's. as far as the lean condition, i checked all the plugs and they do have that tan/lean color to them, none of them are black.
maybe the 2% lean stock edelbrock #1406 needs to be re calibrated? seems strange the engine would need more fuel though... i run the same carb on a milder performance 350(345hp), crane energizer cam @.450 lift and 9:5 compression.. that engine seems to be getting sufficient fuel and doesnt backfire, but the ignition on that car is just stock HEI with no upgrades other than plug wires. i know that all in all i might have to suck it up and realize the engine is 30 some years old and it could just have a worn cam lobe or weak valve springs. but im trying EVERYTHING i can think of before i resort to that conclusion. btw, the car never blows smoke in any condition and the oil pressure has always been no less than accurate according to the books for a low volume low pressure oil pump. just out of wild curiosity though... the drivers side header is smashed in pretty good on one of the tubes, could that possibly contribute? thanks for all the advice.

Steve R · #7 (**permalink**) 06-08-2004, 06:53 PM

It sounds like the carb is lean at that area of throttle opening.
You should purchase their tuning kit that has the various rods and jets. Take the car to a shop that has a dyno and an air/fuel ratio analyzer. have them tune the carb through the various throttle positions and loads.
That MAY get this carb to work. Personally, I don't think it is a very good design. The secondaries lack of squirters and their counterweighted "air valve" design doesn't do very well under aggressive throttling.
If there is someone out there who loves these carbs and knows how to get them to work as well as a Holley, I would love to hear from them.
Steve