Been reading around on the forums and from what I can gather, the car is running too rich?

After the car has been warmed up and driving for a while, upon decel I get a pop-pop-pop-pop from the exhaust. ONCE in a while upon shifting I get it too, but rarely, just inbetween shifts when the gas pedal is let off.

Exhaust leak of some sort? Or do I need to look for something more complex?

My main concern - will this harm the car in any way shape or form?

Also has Bursch exhaust on it.

No.

Sounds like you have a exhaust leak ...
Andy

SirAndy, is this harmful in any way to the engine? That is my main concern.

A real backfire can damage your exhaust system. The occasional pop should be fine.

Is this FI or carbs?
Andy

FI. 1.7. Bursch Exhaust.

I haven't heard any actual BOOM backfires, it's like a crackling pop upon decel.

Could vacuum leaks cause this? I cannot find any leaks in the exhaust thus far.

Well, the car seems to run a bit better now. I thought it had stayed on, it came off once already. This time I lobbed part of the end off so it would stay. Gonna get more vacuum hose tomorrow.

Didn't get to take it on a test drive since it's late - could anyone tell me what that goes to? What piece to what piece? Looks like the air box to next to where the oil goes in, and there's a small elbow - which is partially cracked.

Go here for vacuum routings

http://www.pelicanparts.com/914/technical_...4_17FI_diag.htm

Copied this for you..

Adjusting the air bleed screw has no effect on idle speed (possibly accompanied by light backfiring during overrun, especially when the engine is cold)
Idle mixture too lean
Set the CO level to factory specs by using a quality gas analyzer. If a gas analyzer is not available, use the "trial-and-error" method of adjusting the ECU knob one click clockwise at a time, re-adjust the air bleed screw to set the idle and repeat if necessary.

from here....

http://members.rennlist.com/pbanders/DJetP...htm#Drivability

I had the same type of problem that I blamed on an exhaust leak at the heads. After removing and reinstalling the heat exchangers three times, I realized that it was not caused by an exhaust leak. I actually found the solution in the Haynes manual (I think) of all places! Turns out that I had turned the ECU knob all the way counterclockwise (too lean?) and the manual said that could cause popping on deceleration. I turned it back to the midpoint and adjusted my idle with the idle screw and the popping went away - no exhaust leak for me.

YMMV

Good luck

Andy,
Wouldn't an exhaust leak be audible at any engine speed, i.e. whenever there was exhaust coming out the exhaust port?

i had a nasty exhaust leak recently when I fit up some new (to me) SSIs and it was VERY noticaeble, even at idle.

to the OP -- an exhaust leak is not a good thing. The very gracious retired Porsche mechanic helping me fix my leak (Rory) informed me that the exhaust, when properly sealed against the manifold, transmits heat away from the engine.

Exhaust Leak = bad seal = heat in places it shouldn't be = bad for any engine, and especially our air cooled IVs.

IF it goes to the plenum then to the oil fill, might be a PCV line. Take pics of both ends.

Do the simple thing first turn the ecu knob clockwise, as posted twice above. Then if that does not work then tear your car apart and play whack a mole..

Will do this tonight when I get home, you guys rock, thank you!

Hi guys,

I have not responded to this in a VERY long time. I thought I had it all sorted out. I have not had a huge chance to drive the 914 a lot this year but one of my BIGGEST projects was sorting out all my vacuum leaks.

I figured "Well hell... while I am working on stuff and rewiring this thing, I may as well.."

So here's the thing... after I had my harness incident last year I got all my wiring issues sorted out, the harness was a success. The car ran exactly like it did before the harness went up. I was happy.

It was not until I replaced the vacuum lines one by one that the car started to run worse and worse.

Well, I have not yet checked for leaks at the intake manifold or anything like that, however, I did go through all the vacuum lines and replaced all of them. The car seemed to run better but it actually spits on decel even WORSE than it did before. I find it odd because it SMELLS rich. I found that what I was smelling "like gas" was not a fuel leak (believe me I hunted all over the place looking for one) but came actually from the car after running a minute or two. The car is just running rich.

If that's not bad enough... if the car idles long enough, say a minute or two while in traffic, it may just die and not want to start back up for a few seconds. I can literally go to start it again, give it gas and it's like it just flat out ran out of fuel or got so rich it floods itself and dies. Cranking and giving it gas and after 10-20 seconds she'll fire back up.

Weird thing is... if I keep my foot ever so slightly on the throttle, a hair above idle, it'll never die, not once.

- Still popping on decel
- Now it idles horribly after fixing vacuum leaks
- Checked all my vacuum routing and all is well

So this leads me to believe maybe either I have leaks elsewhere (exhaust system) that cause the popping on decel and that maybe something is set wrong in the injection system.

I have not yet checked the adjustment setting, I need to do that. Yes, I realize, I forgot to do it. So I will indeed check that.

I did take the muffler out and installed new gaskets. I have now gone through 3 sets of copper gaskets for the heat exchangers and it would appear the last time I had them off I had no leaks, it looked sealed to me, no weird soot and no soot at the head either, so that made me feel good that I had the surfaces flat and working. However, leads me to believe either the muffler leaks, or that there are issues elsewhere.

It smells like it is running stupid rich after doing all of this.. I took it up for autocross and it just wreaked of gas after a run or two. After running long enough on the way home and on the way over though it also was a bit odd because every now and then it'd just feel like it would cut out and then go again. I found it really really bizarre. If I'd put my foot down, give it gas and accelerate hard for a second or two then maintain throttle again it would stop, then 30 seconds later it'd start doing it again. Has me baffled.

Checked to see if I could adjust the mixture, perhaps the 70 isn't adjustable, mine doesn't have a knob.

Pinched around and used starter fluid to hunt down odd vacuum leaks and found none.

I figured I'd check my timing. Oddly... if I pinch the lower hose on the vacuum advance the idle corrects itself and the car runs smooth. Perhaps I have set my timing wrong and it's been off this whole time?

does it seem sluggish off the line (low rpms)? if so, you might try advancing the timing a bit.

i had similar symptoms after removing my dizzy to check the trigger points. popping from the exhaust on decel was much better after i advanced the timing. had to do it by ear (advance until ping, then back off a bit) as i still can't find the timing mark on the cooling fan. still have some pops though... plenty exhaust leaks on mine!

since you seem to have tracked down and fixed some vaccum leaks, probably a good idea to go through the general tune up procedures in the shop manual again. you're probably getting quite good at that stuff by now!

Good point. It's probably safe to say that the car probably wasn't going to tune up right when it was in vacuum leak shape. Perhaps it'll run better. I need to get a dwell meter so I can check the dwell as well, that is one thing I did not do.