"Snap, Crackle, Pop" exhaust mutterings on 914-4, My car's Monza-style exhaust has a tendency to 'pop' and backfire mildly that perplexes me. Options

[Jezibel]

G'day fellow 914 masochists (I'm just kidding; it's all part of the joy of owning 914s!)

I have a minor problem that perplexes me (perhaps excessively). My car is a 914-4, 74 model, that has undergone an engine rebuild and conversion from FI to cabs (Weber 40-IDF dual throats). I strongly suspect that when the engine was rebuilt, its displacement was increased (with the addition of appropriate pistons/sleeves, etc.) from the original 1.8 liters to 2.0 liters (or thereabouts). There is, unfortunately, no paper trail to clue me in on this, since the previous owner left no documentation for me on the car (he passed away) or what has been done to it.

The problem in reference is the car's tendency to experience minor exhaust 'popping' and backfiring on occasion (well after after warm-up and usually when accelerating to cruise). Not at idle.

The engine was tuned at my local Porsche shop very recently, but given this glitch, I wonder as to whether 1) the car may need re-jetting...or whether the problem in reference is due to timing issues. As I said, the exhaust is an after-market 'Monza' style system with two dual-pipes in the Monza manner.


I don't know what the present idle or main jet sizes are in the dual Weber IDF 40 carbs, but I suspect that perhaps the combustion is a bit on on the lean side...a status quo that would possibly cause such symptoms as those described, from what I have gathered from others.

One other question is: If the engine has indeed been up-sized to 2 liters, would the ignition timing still be the same as for a stock 1.8 liter?

I am well aware that the best solution to this problem would be another trip to my Porsche specialist ($ka-ching-$ka-ching) to specifically address this issue but I thought I'd ask around for any ideas that might be...as Joe Biden put it..."top of mind", that may bear on something like this.

[Shivers]

An easy check is exhaust leak. All sorts of racket will come out of the back. Might sound like a tick

[NARP74]

I just went through a similar exercise. The best thing to do is go though the carbs, both of them to document what you have, no matter what someone told you was in there. You need a stable base to start otherwise you are chasing your tail like a dog getting dizzy. I had a lean pop and backfire though the air filter, usually a dirty idle circuit issue. Your could be several things. Get things documented, it's not that hard. Start with one to keep the other for reference. They can also vary side to side. I found several issues. I watched tons of vids to help me out.
And BTW, that last person that touched mine was a Porsche mechanic.

[dtmehall]

what kind of ignition system are you running? I am running a similar setup on a 1.7 with a bosch 050 distributor. i get a popping only when the timing is off.

[VaccaRabite]

Ugh! Get that wrap off your exhaust pipes. Horrid nasty stuff.

You likely have an exhaust leak somewhere. Maybe at the head, maybe a crack under all that wrap. Lets fresh air into your exhaust which mixes with the hot unburned gas and BOOM. You have your after fire pops and crackles.

Zach

[technicalninja]

(IMG:style_emoticons/default/agree.gif)

That wrap cooks the dogshit out of pipes!

Even stainless will have a MUCH shorter life.

Now, the stuff DOES work.

Really reduces the heat transfer from the pipes.

It just kills them in the process...

For exhaust pipe heat shielding I prefer a standoff SS "cover", and I'll try to add airflow between the cover and the pipe.

I would EXPECT exhaust leaks with the mummy wrap style of heat shielding.