I've got a 12A with a Mikuni recently installed and I'm getting a backfire when I shut the car off. I've tried adjusting the timing and the fuel pressure but it still backfires. Anybody have any ideas?

 

What's your exhuast setup? Did you get it pre-jetted for your engine? What's your setup; stock port or street port? What have you set ignition timing too? How does it run, do you have good top end power, or does it bog out or miss? Same with the bottem end...

Stock fuel pump?
Fuel Pressure wise you should be running 4.5psi.



Matt

 

Stock port, RB exhaust, Timing exact, fuel pressure ?, no guage but I keep adjusting up and down, Holley Blue F/P

 

do you get backfires at any other time? Like when letting off the gas after a hard accel?

What do you mean your timing is exact, is it at stock setting?

 

Yes, stock, and no backfires at other times, just occasional rumbles from the exhaust.

 

Get a guage and check out what the fuel pressure is at. You can pick up a cheap one at your local store and at least get an idea.

One question i didn't ask is when the last time the carb was rebuilt because if it hasen't been rebuilt in a while fuel could leak through the bowl and down into the chambers after being shut down... leak in the exhuast chamber, hit the exhuast and go BOOM. I've seen that before in a few old carbs. Which also happened with my old nikki carb.

Try running Racing Beat's timing:
L 2* BTC
T 19* ATC

this is done at idle

matt

 

I give that timing a try and I'm getting a F/P guage tomorrow. As far as I know the carb hasn't been touched in at least 4-5 years. That could be a problem I'll have to fix.

 

4-5 years... yeah I think it's rebuild time... now only if I knew where to get a rebuild kit for the mikuni's

I'm running racing beat's street port settings and I've tried other settings and this is the best I've run so far

matt

 

it might be running a little rich

 

I agree. Sounds like it may be running rich. What plugs are you using?

 

I had this issue once when I was using my mikuni way back when. It turned out to be a issue with a couple pin-hole leaks near the rear muffler. It would backfire sometimes when letting off the gas. But mainly, it would backfire one loud one after shutting it off. Usually it only happened if you didn't let it idle for a little bit before turning it off.

 

OH, and the remedy, tac the pin holes shut, no more backfire.

 

I think I do have an exhaust leak at the flange by thr muffler. As soon as I get the guage set up today I'll know my pressure. I'd like to get a a/f mix guage but they're expensive.