everytime i shut my car off i rev it to about 3000 rpm and turn it off. my mechanic told me to, to get all the extra gas out to make for an easier start next time. but about 3 seconds after that, there is always a big backfire. my mechanic just fixed the timing, and its running well, not rich...


i also have a problem with the idle. sometimes it idlesat 1000, sometimes at 1200 and sometimes it fluctuates between 200-700 rpm. and sometimes when it fluctuates low, it is running perfectly fine (air flow meter), like not a bar rich or lean. and sometimes it goes all the way lean, with out a single lit bar, like its not sensing and gas.

 

As far as the fluctuating idle goes, I'd start by looking for vacuum leaks.

 

Yep, sounds like you drive an RX-7




About the backfiring when shutting off, mine will sometimes do it, just depends how hot the exhaust is at the time. When you rev it and shut it off, it does get all the extra fuel out of the engine. But guess where it goes? It gets dumped into the exhaust. If the exhaust is hot enough when you shut it off, it'll ignite the gas and you'll get a loud backfire. Sometimes is will be a VERY loud backfire. I'd watch out if you have a stock muffler though. One time when I did it the muffler exploded. The entire seam down the passenger side of the muffler was ripped apart by it.


I agree with slashdawg on the idle, check for vacuum leaks.

 

lol it was a cold morning yesterday where i live and i was revving it a bit because it was going to stall and BANG i jumped a mile it was a huge backfire, Nothing wrong with the exhaust though i checked. but christ did it make me jump lol

 

check ignition modules.

 

Bah!
"Gets all the fuel out of the engine".
That's silly.
The increased rpm and then letting off the throttle only provides outragious amounts of manifold vacuum to the idle circuit. The idle circuit feeds the carb just below the throttle plates (why I said manifold vacuum).
You're simply sucking fuel into the engine no matter what.
Best thing to do is let it idle and then shut it off. Under these circumstances you will leave the least amount of fuel in the intake...not that this should even really be a concern!

Regarding your fluxuating idle, I'm more inclined to think it's linkage related. Sounds like either there's binding, or someone's had the carb opened up and didn't properly reseat the throttle plates.
The reason I think it's mechanical and not vacuum is because vacuum leaks tend to remain relatively consistant.

Running that low and off the idle circuit, the ratio gauge means very little.

 

I was told by my friend never to do that when I shut the car off. I didn't question him because he's been working on them for years. He told me to just let it sit and idle a little then shut it off.

 

Reseat the throttle plates? Can you explain? I don't understand.

Good point. I never thought of it that way before.

 

Well often when someone mucks about with a carb, they'll decide they're gonna try to remove the plates for whatever reason.
If they are not tightened in a particular mannor, the throttle sticks and won't always close.
More often, it's a matter of the linkage spacers not having been reinstalled in the correct order (or even some missing) that can cause the same problem.
The shaft needs to have a tiny bit of in & out play in it when it's partially open so that the plates seat themselves, while the linkage bits need to be tight on the shaft.

 

Do a search for the "shutter valve"