Need Help from the forum

[cvfncrew]

I need help from the forum.

I just bought an '87 3.2 L motor with 46 IDA webers and headers to put in my '72 911. When the motor was installed, the car began to have a bad backfire when decelerating that wasn't present before the new motor installation. I have had 2 good mechanics in the Los Angeles area look at it, and both told me that I will always have the backfire because of the carbs. I know people have done this setup and theres don't sound like mine. This is a nice looking car that sounds like crap. WHAT DO I NEED TO DO TO GET RID OF THIS BACKFIRE?????
Thanks
Christian
"YELLORS"

[Wayne 962]

What is the size of the carbs, and how are they jetted?

-Wayne

[rcwaldo]

Hey there Chritian, missed you at the breakfast meeting Saturday. Saw your dad though.

Sorry I cant help with the backfire, but you have come to the right place!

Chris w/speed yellow 993

[john walker's workshop]

exhaust leaks contribute to backfiring, as do performance mufflers that basically don't muffle. most engines burble and pop on deceleration, but they have a good exhaust system to make it un-noticeable.

[cvfncrew]

The carbs are 46 webers. not sure what they are jetted at(checking).
I have tryed a few mufflers and they all pop like crazy.

does it have any thing to do with cams?

thanks
Christian

[cvfncrew]

I need your help Has anyone put a 3.2 with carbs together and if so did you have backfireing?

[MotoSook]

Like JW said, most engines will pop and burp...

My carbed motorcycle with free flowing exhaust does it on decel..and I love it!

My CIS motor with a euro-premuffler and a track pipe (no muffler) will scream then pop and sometimes just plain explodes when I let off the throttle at the track. I shoot flames out the exhaust with the track pipe on...I love it.

With my muffler on it's quiet..too quiet...I don't love it.

At Road America or any fast tracks when the fast cars are going around, and they lift to shift, it sounds like someone shot a cannon! I love it!

What kind of exhaust are you running ont he 3.2?

[jluetjen]

Also what sort of cams are in the engine? "Wilder" cams may have a tendancy to backfire more on overrun then "milder" cams.

[724doorE]

When an mfi engine begins to run rich, you get a fireworks display

[sammyg2]

Hi Cristian.
Backfiring on let off is caused by excess unburned fuel in the exhaust.
Most fuel injection systems have a way to correct this by either leaning out the engine on decell or by adding extra air (decel valve).
I'm not sure how to correct this on your engine.
Since we can identify the cause we may be able to deduce a solution:
Are your idle jets rich at all? If they are we may be able to lean them out a little to help. Try warming the engine up completely and let it idle for a few minutes them pull a plug or two. We can help with reading them, basically they should look light tan or off white. If they are dark grey or black they are too rich. Even if they don't look too bad you could try to lean out the carbs a little on the idle circuit just to see if it helps, a small change would not put your engine at risk. Just don't go too far.

Another thing that can contribute to the backfiring is inadequate backpressure. It might help to put a more restrictive exhaust on but that seems counter productive knowing your car.

Cams with high overlap can also cause unburned fuel to get into the exhaust, not much you can do about that.

I assume your valves are all adjusted right, an exhaust valve that does not close all the way will cause this problem but I doubt that is the case here. I'm still leaning towards too rich.

We may need some more information such as cam grind, static and total timing, compression, etc. Is your engine a short stroke 3.2 or a standard 3.2? You probably told me but I can't remember.

BTW for those who don't know Cristian, this car was always in the top 2 or 3 TTOD in Orange county when he had the T engine with MFI in it, it should really scream now.
It's a beautiful yellow RS clone and the pilot is quick.

[Joe Bob]

46s are pretty big for the street....how do the plugs look?

[cvfncrew]

Thank's all I will keep you posted. Any other ideas out there please let me know.
Thanks
Christian
"YELLORS"
cvfncrew@pavenet.net
Hey Sam next time I see you we will try to solve it.

[Evans, Marv]

Christian:
JW is right & Wayne is right. If your exhaust is sucking in oxygen, it will cause popping. I had a '72 911E with 40IDA Webers that kept popping on deceleration. I took it to two or three mechanics who adjusted the hell out of it with no results. Finally took it to one who looked at the venturies. It had 30mm venturies, and he put in 32mm venturies & the problem went away. Find out your ventury size & check it out.

[masraum]

Sammy2G is also right, carbed motors will pop on decel due to fuel being burned in the exhaust. The Motronic injection that is stock on the 3.2 completely cuts fuel to the engine on decel, so you wouldn't hear that on a stock motor unless you still had your foot on the gas or you had a leaky injector or some other problem causing gas to get into the system.

Exhaust leaks also cause backfiring on decel.

In my stock 3.2L car I can decel all day long with my foot completely off of the gas and never hear a sound, but as soon as I put my foot on the throttle a bit, but still decelerating I will get popping. (this is all with no muffler, a stock muffler totally kills this sound)

I don't know crud about Webers, but it sounds like you are either still on the gas, or you are getting way too much fuel to the motor on idle which would also probably count during decel, add an exhaust leak into the mix and your car could be really loud on decel. Since this is in a '72 you probably have a very open exhaust like SSI's or something, that will allow these sounds to be heard much more.

Most people like the popping and burbling, but some people don't.

[jabb]

First I would check is for exhust leaks.... once all the exhuast leaks have been ruled out I would look at mixture settings and how the carbs are set up. But u really need to be sure there are no exhuast leaks.