Backfire through carb

[cool dakota]

I have a crate engine (350), came with the car. Has about 40K on it. The problem is that is sat for 2 years. I just purchased the car from a friend and started to do a tune up. Here is the headache.

When the engine is running it idles fine. But if you rev the engine quickly there is a popping sound from the carb. It sounds like a backfire. But not as bad. If you rev it slowly, you don't hear it until you reach about 3k or so and then it starts to do its thing. Again, if you do it quickly it is more noticable.(on the demanding side)

I have done the following:

New cap, rotor, wires, plugs, fuel filter. The timing is set to 7 degrees. It also has the quadrapuke on it.

Please help, I can't figure this one out.

-cool dakota


P.S. I did a compression check, 170 all the way around.

[bobss396]

It may have an intake valve hanging up from sitting. I would add a pint of mystery oil to the crankcase and also give it a couple of doses (small amounts and keep the revs up) down the carb with it running. Run it around town easy for a few days and change the oil and filter. If it still does it, pop off the valve covers, give it a visual for things like a bent pushrod, cracked rocker arm. Give it a good valve adjustment while you got it open. I doubt that you have the distributor in 180 degrees out so this should hopefully fix your problem.

Bob

[Blue Bowtie]

I've had the same kind of problem with incorrect valve timing. It may run fine under moderate and part throttle up to about 3,500 RPM, where it will start to pop, backfire, and fall on its face.

Bob is dead-on about a hanging intake valve(s), too. That can especially be true for a used/stored engine, where old varnish and even some rust on the valve stems may be sticking some of the valves.

BTW - If you're tired of that "QuadraPuke" I'd consider trading a Holly 4160 for a complete 4MV. I finally got my garage door fixed so it will stay open by itself, and no longer need anything from Holley. (In other words, up to about 550HP, the Rochester is probably the superior carburetor.)

[nova68]

Valves are the problems just like everyone has said! might want to make sure rockers are adjusted right too? I do disagree with the rochester comment. The problem with holley is you have to get them tuned and tune them alot more. But when they are tuned they can flat out run.

[bobss396]

QJets are great if they're working well. I had this one made by Carter that was the best one I've ever had. I'm fortunate to have a good local carb shop that can do a nice job on a rebuild.

I have an expensive Holley spreadbore that is back in the box right now. That is a great carb and I've had it on big blocks, small blocks with great success. It's more suited to racing than street duty.

Bob

[alblogg]

Get some AutoRx and run through it, I have not used it yet but alot of the guys on here say it it is some good sh@!.

[450HP]

Okay, stupid question time... have you drained the gas tank and added only fresh gas back in?

Just one of those "white elephant" questions I like to ask...

Also, the carb may have quite a bit of varnish build up if it's sat for two years.

If so, the carb might need to be cleaned up or rebuilt.

Also, if the fuel line has any rubber lines in it anywhere, replace them with steel lines, or at least
all new rubber lines.

If all that checks good, then I'd start looking into what others have mentioned.

Scott