I am getting a back fire on deceleration. I can replicate the back fire fairly reliably, and avoid it as well. I am hoping this may help provide some hints as to where to look.

The car is a 1.8 running 40idf carbs, with an unknown spec carb cam. It has been running great for nearly the last two years until this started. Ignition is a SVDA dizzy with a pertronix.

I have cleaned all plugs. They were all in nice shape, without carbon build up.

I have tightened all intake and exhaust nuts/bolts. I have examined the exhaust for leaks or holes, and have found none.

Valves adjusted about 1,000 miles ago.

If I take my foot off the gas, and leave the car in gear allowing it to slow, I will get a back fire almost every time.

If I take my foot off the gas, put the car into neutral and use the brakes to slow the car, I will not get a back fire.

Assuming that there are no leaks, and the the timing is good, where should I look? How do I tell where the problem may be.

sounds like an exhaust leak...

I thought so too, but I have tightened everything up, both when cold and when hot.

Similar problem here. I keep tightening 3 & 4 but they wont stay. Is it OK to torque them when hot?

I'm thinking locking nuts, like "Stage8"

Timing? Sometimes you can have an exhaust leak and not be able to tell right away. If you look under the car while the engine is running at idle and have someone block the exhaust with a rag or piece of cardboard or something you might be able to find something. It isn't always easily identified.

If it were running differently, but its not at all. That's why I mentioned the dizzy and the electronics. No Dwell or points to deal with.

All one really needs is a stethoscope or a length of hose to find where an exhaust is leaking. Having a lift is a plus

Backfires on decel can also be lean idle jets, or possibly a clogged one.

I'm not sure if the 1.8 deceleration valve is plumbed the same as on the 1.7, but when I got my car it would consistently backfire when I let off the throttle above 3000rpm and stayed in gear. Likewise, if I simply went into neutral and braked, it didn't backfire.

The problem on my car was that some idiot (scotty boy?) had unplugged the front-most vacuum hose of the deceleration valve, that plugs into the plenum. Then left that hose sitting disconnected on top of the engine, and plugged the port on the plenum. Once I reconnected the hose the problem went away.

I'm sure your hose is probably still connected, but it's possible you might have a leak around that hose.

Edit: scratch that! Nevermind, just noticed you said you're running carbs.

Joe's running carbs. It's more than likely exhaust related ... it has a history of exhaust leaks


Bring it to shop class tonight

take a small piece of 2*4 and after the car is running, lay on the floor and hold the piece of wood flat on the end of the tailpipe.
Seal the tailpipe so nothing can escape.

If you have a leak, you will have a much easier time finding it using this method.

rich

I'll try that Rich. Thanks for the tip.

Shop was cancelled till next week. Did you get the message?

No. I didn't make it anyhow ... stayed at the shop to crate up a motor