Backfiring - should I be concerned?

[Si|v3r72]

Ever since I put the new exhaust system on (no resonator w/ flowmaster), every time I let off the gas the car backfires, sometimes loudly. If I'm careful not to stay on the clutch too long, I can accell through the gears with very little backfiring, but it's almost impossible to avoid when decelerating.

What worries me is I've heard that backfiring can hurt your carbs and engine. A buddy of mine blew a cylinder in his 99 Ram pickup when it backfired. He showed me the piston today and the side of it was completely gone - SCARY! Am I putting my engine/carbs at risk by running the car like this?

Anyway, I've been contemplating adding a resonator anyway. As is, the car is a tad too loud and I don't need to give the police a reason to pull me over. That, and I'm the only car I ever hear backfiring down the road.

[2ManyZs]

It depends on how intense a backfire it is. Is it just a little popping or is it M-80 loud?

Does your car still have the smog pump and emissions "garbage" on it? The anti-backfire valve may need to be replaced. It's the big can behind the front carb that attaches the hose from the smog pump to the exhaust tubes and the intake tube.

Have you checked the mixture since you put on the exhaust? You may have to check your plugs and see if it has leaned the mixture out since you opened up the exhaust. Also, if you have a plug wire or plug going bad it could cause the backfiring. If you haven't yet, you could try a fresh set of plugs and then check the mixture after you drive it a bit.

[Si|v3r72]

2ManyZ's, give someone else a chance to answer please. .

j/k bro .

The backfiring isn't really as loud as an M-80, but it is as loud as a firecracker. And yes the car still has all the smog equipment . As far as mixture goes, I did a "ZTherapy" tune of my carbs 2 weeks ago - I thought it went well but it was my virgin attempt. I was hoping to lean out my mixture enough to improve my mileage but it didn't really go up much. I figured on looking at the plugs (new NGK last fall) in another tank or so.

Good ideas tho - I'll look into them. Danka.

[2ManyZs]

Looks like I need to race the Big Brown Truck and block him for a while.

If you have all the smog equipment, the anti-backfire valve is the first place to look.

Here's a quote from an old service manual:

This valve is controlled by intake manifold vacuum and is used to prevent exhaust system backfire during deceleration. At that time the mixture in the intake is too rich to burn and ignites when it is combined with injected air (from the smog pump) in the exhaust manifold.

The anti-backfire valve is used to provide a supply of air into the intake manifold thereby making the air mixture leaner to prevent backfire.

If the valve is not working properly, the fuel mixture will go through the combustion chambers without being ignited, mixing with fresh air and at high temps backfiring will result.

[Si|v3r72]

Originally posted by 2ManyZs

Looks like I need to race the Big Brown Truck and block him for a while.

R O F L (inside joke).

[Daniel]

Sometimes worn plug wires can pass power across each other through induction.It's always best to seperate the wires as best you can.Also they can leak power to ground as when laying on the valve cover,this would cause weak spark=unburnt fuel=backfire.Check for good wire seat in the dist. cap. as well as wear in the cap and corrosion in the cap where the plug wires go.I once had that happen and found the screw tip on a spark plugs where the cable goes was almost falling off! Daniel

[Royce]

All good advise but I have one more thing to add to 2manyZs and Daniels suggestions.

You say this started just after installing a new exhaust system!? The worse kinds of backfires are through the carbs, lean condition. Sounds like you may have an exhaust leak that is allowing some air to get in the system and help unburnt fuel to burn. You might want to start at the exhaust manifold and work your way back making sure everything is tight and that there is no leaks or cracks. I would look closely at any unions or welds but there could be a crack along the pipe someplace too. If you have a leak over time you would see black carbon build up in that area.

[2ManyZs]

Tom, can you legally remove the smog equipment in DE on a car that is over 25? If you can legally do it, or if you don't have smog checks, that would be the simplest solution to the problem. Not only that, but your car will probably end up running a lot better. Might want to check into it if you haven't already.

[Si|v3r72]

2ManyZ's, I looked into it when I bought the car (failed emissions the first time and barely passed the second time). In Delaware anything past 1968 has to pass a single speed emissions test, and anything past 1980 has to pass a two speed emissions test. So no go on the exemption. What I don't know is if the cutoff changes as the years go by (which would make sense - but often things don't make sense in Delaware). IF the exemption date moves as time moves, I'd be exempt in four years. THEN I could think about removing the smog stuff.

I really think my backfiring problem is due to removing the resonator and replacing the muffler with the Flowmaster. With the old exhaust system, I had zero backfiring. I've looked at the plugs and plug wires and they all look good. The plugs are new and the wires appear to be new. Haven't had a chance to look at the anti-backfire valve, but I tend to think it's ok since I had no probs before rebuilding the exhaust.

Regardless, I'm seriously leaning towards installing a resonator ($40), if not only to help with backfiring, but to quiet the exhaust. I had no idea how throaty at low rpm's and how racey at 4000 rpm a Z could be.

[Royce]

I just remembered a friend who had a similar problem with a stock VW bug back in high school days. He removed the stock pipes and put on a dual "quiet pak" muffler setup with two outlets that was very free flowing. He would backfire quite a bit. So he changed to a single "quite pak" muffler with a single outlet and that seemed to solve the problem. By the way the dual setup worked fine on his friends but that was modified substantially (did 13 ETs on the 1/4 mile). I wonder if it isn't just too free flowing for your stock motor? Anyone else ever seen this happen on a Z?

I think you may be on the right track. Let us all know how it turns out!

[2ManyZs]

I must be geezing. I drove by the testing station that sits beside 495 outside Wilmington 5 days a week for 3 months and didn't dawn on me till now. I need a vacation.

BTW, instead of a resonator you could use a "race muffler" like I just got from Summit Racing. It's a bullet shaped straight thru so it wouldn't take up much room, and it was only 32 bucks. It's made by Dynomax, but not sure how small a pipe it fits, I think the smallest was 2 or 2 1/4. Resonator isn't much different than a muffler anyway, just a few less baffles.

[Zvoiture]

Yeah, I just got new exhaust too--all straight thru and a test pipe. I get the pop-pop-pop-pop on decel too. Not too loud most of the time, but if I come off an offramp and drop it into 3rd, it gets a bit more 'pronounced'. I always thought it was just unused fuel burning in the exhaust system too....nothing too dangerous. I have heard with a straight-thru system you can actually get some flame out the butt at night!!!!

I know this is the tail end of the thread and will probably never get answered, but if unused fuel igniting in the exhaust system is 'backfiring', what is ignition through the intake system called? You know, from messed up timing or other things? The old 'flame out the carb. I always thought that was backfiring. That is the truly dangerous kind, right?

Another question....The old 'shut off the ignition and turn it back on' backfiring, where is that igniting at?

steve77