what causes backfire on a Q-jet?
 

I've made a few posts about some ongoing issues with my Quadrajet converted '84.
http://forums.corvetteforum.com/c3-t...quadrajet.html
It starts up great, but I have to give it some gas in order for it to fire. It will die if I left it to idle, but if I continuously revved it would run for a while, then sounded like it was sucking air, and would try to die... if I floored it hard, it would catch again, then would backfire (flame out the carb)

The distributor has been swapped for a vacuum advance style.

it's odd because it just started this one day.
The day before it drove great, and for the last two months (which is as long as I've had it)

backfire is caused by fire in the combustion chamber while the intake valve is open

so my timing is off?
what would cause that to happen in the middle of driving?

It could be that your timing chain is failing and has slipped to a different position. In any event, your timing is way off. You can put a timing light on it, but the readings you get off the damper timing scale will likely not be correct. You can experiment a bit, since it still runs, by loosening the lock-down bolt on the distributor, getting the car started, then gently and very gradually adjusting the dizzy one way and then the other (within the range of 1/8" either direction) to see which way causes it to run better. Then, very gradually continue to turn the dizzy that same direction until it starts to run worse. Place the dizzy in the position where it runs best, then lock it down again. Now, knowing how much you changed the position of the distributor during that test can give you a clue as to what may be happening with the engine.

Or you can pull the #1 plug, pull the coil wire, take off the dist. cap put your finger over the plug hole and turn over by hand until you feel compression. Turn till the rotor lines up exactly with the where the #1 plug tower would be. Look at the timing marks and in theory the balancer and the tab should be on zero 0 degrees. If not you would know there is slippage on the chain, maybe someone installed the dist. one tooth off,.

Another vote for the timing chain jumping a few teeth. Some factory timing sets used cam sprockets with nylon teeth to help quiet things down, but you'd be amazed what some of these look like, and actually still run. Probably a good idea pulling some things apart for a better look.

Put a breaker bar on the crank bolt and lift and push on the bar.

My guess is an inch to an inch and a half of movement before the cam moves.

Nylon is good for a little over 60,000 miles.

Better Hurry

Ralph

so I should probably stop cranking it?

when my wife's Turbo Eclipse spun the timing chain it destroyed the engine... what's diffrence between that and an SBC?

who knows exactly what happened.....find TDC on the #1 cylinder and mark it on harmonic balancer and then pull the valve covers and distributor cap and hand crank the engine over and watch the valves open and close as the distributor rotor turns.....at TDC of the compression stroke both valves should be closed and the rotor should be point to #1 tower.....if it is not and the distributor is not loose, and it was running well before i go with the timing chain having slipped.

japanesse cars have catastrophic failure when this happens because the belts break.....our cars usually have crappy performance because the chains jump....however it needs to be fixed before you drive it.

Some engine's are what's called interference, or non interference designs. Many modern, higher tolerance engine designs will have open valves slamming into pistons if the timing belt/chain breaks. Dont think a stock S.B. will do this, but good to pull the timing cover for a look. Recommend picking up a new timing set, gaskets, and front crank seal before any wrenching to simplify things.

:iagree:Also,I have seen harmonic balancers slip, so best to do the above check.

this is going to be really dumb... when you say "hand crank the engine" what do you mean?

:salute: Use the crank bolt or flywheel to turn engine, not starter.

Pull a valve cover, put a wrench on the crank nut and turn the engine backwards till a valve moves.

You could do this using the timing marks to see how many degrees of slack in the chain and what's left of the teeth has.

I've seen them pretty loose.

Ralph

by hand crank i mean using a breaker bar on the harmonic balancer bolt....i have never had any luck going backwards on these bolts they always loosen.

I'll have to try this after I get my 1/2 breaker bar back from Indiana.

A bit OT, but the first Jaguar V8's had crappy plastic secondary timing chain tensioners that would break after about 35,000 miles, and on startup, would cause the secondary timing chain to jump a tooth on the cam sprocket, causing the engine to run rough. The next tooth jumped on startup would cause Mr exhaust valve to make sweet love to Mrs piston, destroying the engine! Many pissed Jag owners! And Jag wouldn't do anything about it. They did eventually upgrade their tensioners, but it was up to the owner to sort it out.
Cheers
Grant

Sounds like a massive vacuum leak to me. :toetap:

okay... ready for this?
I checked the Distributor, and its tight.

my quadrajet doesn't have an actuated choke, thanks to the DSPO, bit I love in south Texas, so it's never been an issue.
On a whim, I flipped the choke to about 1/2 closed... car runs great, no backfire at all. But if I let off the gas, it dies.
Carb problem now?

:flag::iagree::thumbs: