64mgb

iTrader: (1)

I know there are a lot of posts about carbs, and there are a lot of posts about backfiring. I've read quite a few but have not been able to find the answer to my problem. Here's the situation. I rebuilt my carb ('79) because the car had been running poorly and wouldn't idle. Now it runs really well, but backfires when I get on it pretty hard then let off the accelerator quickly. If I let off easy, no backfire. This happens when sitting still and revving the engine or when running it happens every time I shift if I let off the accelerator quckly when shifting.

The timing looks good...just slightly advanced from the marks for both leading and trailing. Plugs, wires, cap, and rotor are all fairly new. A mechanic friend suggested maybe I have the idle mixture too rich and it's pulling too much fuel through the transition slot when the throttle is closed. So I adjusted the mixture screw in until it started to stumble then backed it out until it ran smooth. Still backfires. So I adjusted it out several turns. Still backfires. (Once I'm done tuning the blown '85 I'll move the LM-1 to this one to set it correctly)

I'm pretty sure all hoses have been reconnected correctly, and can't find any vacuum leaks.

The engine is completely stock...still has the rats nest, air pump, everything. Any ideas?

Thanks,
Rich

Midwest 7's

iTrader: (4)

your probably running a little rich, nothing to worry about.
all my cars i have tunned act the same way.

64mgb

iTrader: (1)

Yea, but mine has never done this...I don't like it. It's not a small pop, if i get on it hard and let off hard, it's a big bang. Besides being obnoxious, it can't be doing anything in the exhaust system any good.

Rich

THansenite

Haha, I do that in mine on purpose. But yea, it sounds like it is running a bit rich. You could try playing around with the AFM screw and see if that helps.

trochoid

iTrader: (6)

Check your trailing ignition.

edwin-82/rx7

iTrader: (1)

does your exhaust have any leaks? mine did it until i put on the rb streetport system (i had a crappy fabbed-up exhaust that leaked like a bitch) oh and i took off the emissions and that helped too.

Paradox

iTrader: (1)

^ ya check your exhaust, even a very very small leak can cause massive havok with backfire. i originally had a small leak around my header, which then from heat warped it even more into a huge leak. i had to get the flange milled, put it back on after that and the car hardly ever backfires (besides the regular rotary pop pop pop decelerating from a hard run).

64mgb

iTrader: (1)

I was afraid someone would suggest the exhaust...it DOES sound different than it used to. But I hate the thought of having to replace the exhaust...it's still original!

Rich

edwin-82/rx7

iTrader: (1)

rb streetport system here you come! the first thing to go on my old exhaust was the connecting pipe from the cats to the muffler... don't know if thats consistant with the sa setup tho?

Paradox

iTrader: (1)

time to get under the car and start checking for leaks.

boyee

iTrader: (3)

seafoam will help you find leaks =]

Dan_s_young

iTrader: (1)

It wouldn't hurt to look for a small exhaust leak, seafoam would be great for that as already suggested. But this statement you made makes me think you should still be looking for a vacuum leak. If you have a vacuum gauge stick it on your brake booster intake manifold line (MAKE SURE TO HOOK IT BACK UP AFTER!!!), just to ensure you are getting the proper 17-18" HG. My car did similar things for the longest time and the problem turned out to be a bad intake manifold gasket... At what rpm is your car running with idling? Does the car stumble when you snap the throttle???

iceblue

Heat exchanger for me. Put header blocked the mani off removed the heat cat and no more probs

64mgb

iTrader: (1)

Thanks Dan_s_young - I'll hook up the vacuum gauge and check that out. The car idles great now, at about 800 rpm. No stumble when snapping the throttle...just a bang through the exhaust.

I don't understand the reason that an exhaust leak would cause a backfire...can someone explain it to me?

Thanks,
Rich

MosesX605

Hrm...

My '79 did this as well during hard acceleration runs at the autoX course. When I let off to brake there would sometimes be a backfire like a rifle shot. When I changed my exhaust to the RB header it went away, so I'm going with exhaust leak as well.

edwin-82/rx7

iTrader: (1)

i think i can explain the whole leak causing backfire thing, or i can at least try...

has to do with extra air entering the exhaust path early in the system, while there is still unburnt fuel thats very hot in the pipes. pounding the accel then letting off hard would lead to a lot of unburnt fuel coming out the exhaust, right? this mixes with the air and BANG! you think someone is shooting at you.

with my fabbed exhaust, i never got it to seal well at the connection right below the passenger's feet. i got a small pop at idle sometimes, and it tended to backfire quite a bit.

the exhaust i had before that (broken off right before the muffler) had a small pop at idle but no backfires. perhaps that was a result of the cat doing its job, or the gasses were cooler because they were further back from the origin.

someone please correct me if i'm wrong!

get under there and tell us what you find!

64mgb

iTrader: (1)

Yea, it sounds like maybe the muffler itself has a hole in it. I'll have to take the heat shields off to be sure though. A sad day...another piece of originality goes away...sigh.

Rich

gsterror

Eh, the Racing beat looks almost identical to stock. No one'll ever know. And you can garuntee that no one on here will hate you for putting a RB on

64mgb

iTrader: (1)

I found the problem! In the Haynes manual, in Chapter 3 (Fuel, exhaust, and emissions control systems) in section 34 it describes the "Deceleration control system". It says: "The deceleration control system is operated by solenoids and valves, and it's main function is to prevent backfiring when the engine is decelerated and immediately after the car is shut off. This system comprises an anti-afterburn solenoid valve, a dash pot and dash pot delay valve, and a coasting valve."

In my case, I had to adjust the dash pot. I still have a small amount of backfiring when I get on it hard and then shift, but it is far better than it was, and does not backfire at all when just revving the engine. I think there is still a little adjustment in the dash pot, so I'm hopeful that I can get rid of the backfiring completely.

If you look this up in the Haynes manual, be aware that the pictures for this say they are pointing at the dash pot, but in reality they are pointing at the vacuum break diaphragm on the choke mechanism. I've attached a picture of the dash pot.

Rich