Need help with severe backfire on deceleration

[guards73/2.0]

Here's my car:
1973 2.0L (fresh topend rebuild)
40 idf's
009 ( coming off for 205A) compufire

So far I've tried replacing the intake manifold gaskets ( had EFI ones) to rule out vacuum leaks and messing with timing

any suggestions? It detonates LOUD when down shifting, and coming off the throttle slowly with the car in gear. Gets worse when it warms up. Lousy power, too. BTW, I know 009's suck, and mine's coming off, but there are plently of 914's
with 009's that don't backfire like this, so I'd like to get this under control before I start pulling the dizzy.

[John Rogers]

Valve clearance has gone away, especially if the heads were put on recently and the valves have not been adjusted since. Since the valves may get tighter as the engine heats up (if you have alloy push rods), the valves are probably not shutting very well. Good luck.

[alex9s]

I've had a similar problem. It turned out to be my heat exchangers were loose or not seated down and there was an exhaust leak!! While the car was running i pushed up hard on each heat exchanger and the left one was leaking as i could tell by sound. Tightened it and the problem was solved, until it came loose again. Also they say that if your throttle switch is not adjusted right(20 even clicks) it will "pop" out the tail pipe, similar to a backfire. The throttle switch might not be a really bad backfire though. Of course John could be right too with the valves. I'd check the heat exchangers and hope to get lucky

Alex

[Joe Bob]

Yep...my vote...exhaust leak....

[guards73/2.0]

No throttle switch, I have carbs. I'll check my heat exchangers, though... I like that idea ( I also seem to recall having difficulty getting a good torque on them when I installed them recently, car seems kinda louder than usual, also).

Originally posted by alex9s:
I've had a similar problem. It turned out to be my heat exchangers were loose or not seated down and there was an exhaust leak!! While the car was running i pushed up hard on each heat exchanger and the left one was leaking as i could tell by sound. Tightened it and the problem was solved, until it came loose again. Also they say that if your throttle switch is not adjusted right(20 even clicks) it will "pop" out the tail pipe, similar to a backfire. The throttle switch might not be a really bad backfire though. Of course John could be right too with the valves. I'd check the heat exchangers and hope to get lucky

Alex

[guards73/2.0]

First, I'd like to thanks all you guys for taking a crack at this. Next, I'd like to know if there is anyone out there who can describe just how an exhaust leak can cause a backfire. I'm just one of those idiots who
like to know just HOW this works to learn from my mistakes....So here's my question: How would an EXHAUST leak cause a backfire, which I've always understood as originating INSIDE the combustion chanber?

[John Rogers]

Exhaust leaks at the head connection could cause the backfire especially if the cam is more radical than stock. What happens is the exhaust valve is closing and the intake is opening and the vacuun sucks air in from the exhaust leak and creates a really lean connection and when the mixture can actually start to detonate. You can hear the leaks if you listen under the car near the heads. Since you said power was down, I still think it is the valve adjustment that is off. Good luck.

[guards73/2.0]

Originally posted by john rogers:
Exhaust leaks at the head connection could cause the backfire especially if the cam is more radical than stock. What happens is the exhaust valve is closing and the intake is opening and the vacuun sucks air in from the exhaust leak and creates a really lean connection and when the mixture can actually start to detonate. You can hear the leaks if you listen under the car near the heads. Since you said power was down, I still think it is the valve adjustment that is off. Good luck.

Thanks! I just got the car back from a factory trained Porsche and VW mechanic who adjusted the
valves. After having a guy who didn't know know what he was doing assemble my valve train incorrectly and then try to adjust them, I asked a reputable parts dealer if he knew of anyone "qualified to adjust the valves on a 914/4" and he recommended a guy who has his own German specialty repair shop
now, but worked as a mechanic at both the local Porsche and VW dealerships during the 70's and has done literally 100's of such valve adjustments, and he found a real mess in my car and wound up having to shim a couple of the rocker stands to get the thing
to have adjustment within the limits of the adjusters. For the first time, the valves SOUND right! Very quiet. So, hopefully, they're right now for at least a "seat-in" period. I live in MN, and good Porsche mechanics who can and will work on 914's are expensive and hard to find. Very few 914's being driven on the street up here, LOTS of parts cars in people's yards, though.
QUESTION: IF I indeed have an exhaust leak at the Heat exchanger-to-head gasket ( I have the SS exchangers), aside from the obvious approach of tightening the nuts down, how would you fix that?