I have a 73, 2.0, with FI that has a persistent backfire on deceleration only after the engine has warmed up . I suspected an exhaust leak so I took off both heat exchangers. Both showed leaks at the head/exhaust gasket. I took both gaskets out, cleaned the head where the gasket seats and even tried filing the ends of the heat exchangers themselves so they are flat.

I reinstalled, and it still leaks I then got new gaskets and new 12mm nuts from Pelican (these are the copper colored self locking nuts). The studs seem to be in good shape and are tight. I annealed the new gaskets, carefully inserted them into the heads and reinstalled the heat exchangers. I torqued the nuts to 20 lb/ft. ,started it up and let the engine warm up and then retorqued the nuts. I figured I was good to go. Once it was warmed up, again it backfires on deceleration. WTF?

So, now what? I don't want to keep running it for too long with the exhaust backfire and I am getting damned tired of taking the heat exchangers off and reinstalling.

I am tempted to really crank on those nuts to tighten it down but I know I will snap a stud and then I will really be pissed.

Is there any high temp sealant that will work? Any other ideas?

Warrenoliver

You may have other issues causing the backfiring. Can you detect the leaking exhaust? In Jake's video, he shows how to true up the ends of the heat exchanger by using a large flat file. Just file both of the end tubes at the same time, keeping the file flat. This gives you a flat surface on both tubes so they can contact the new copper exhaust seals.

Try using some Orange High Temp silicone on both sides of the copper gaskets, wipe off any excess of course.

Also use a LARGE flat file to file both ends of the heat exchanger at one time.

Good luck!

go drive the car...get it hot.
Pull in, jakc it up and put it on stands...while its hot.
Start car on stands.
while laying under the back, hold a 2*4 flat against the tailpipe end...really tight.
If the exhaust is tight, it will stall the motor.
If it leaks....you will hear the leak get very loud while you have the 2*4 on the tailpipe.

RIch

Hmmmm........on these old FI units it maybe not anything to do with an "exhaust leak" that's causing backfiring. Based on my experience with the BMW K bike, there's a lot of backfiring due to the throttle position not being completely closed on deceleration, thus providing fuel that's not needed and which explodes in the exhaust. There's a logic in the FI that tells it to stop the fuel flow completely until below something like 1000 rpm....so if your gas pedal cable stop isn't adjusted properly or the spring isn't closing the throttle plate completely or the throttle position switch isn't properly adjusted you'll get some fuel flow on deceleration and the resultant backfiring.

Does the car still have the decel valve, and is it working?

Rich,
I know that the exhaust is leaking because I can see the carbon deposits on the exhaust gaskets so it isn't a matter of finding the leak, it is a matter of fixing it.
I tried the technique of using a very large flat file and filing both ends flat - it didn't seem to help. The only concern I have when doing the filing is that the heat exchanger jacket around the pipes is no longer tightly attached so there is some flex between the two pipes - maybe I am not getting the two pipes filed exactly parallel although, I am not sure that matters.

Thanks.

Warrenoliver

You might want to look and feel around the gasket seating surfaces in the exhaust ports of the heads.

If the leak went on for too long, the exhaust could have eroded a pit or notch in the gasket seating surface and the copper gaskets can't seal against the damage.

Isn't 20 ft lbs way too much for those nuts. I torqued mine to 12 ft lbs and the exhaust studs to 14.

20 ft-lbs is PLENTY tight. Are you sure there are no old copper gaskets stuck to the head?

What I think Rich was getting at is that you may have had an exhaust leak AND something else causing troubles. Your work on the exhaust may have corrected that part of the problem but some other cause may remain continuing to contribute to the backfiring. By doing what Rich mentioned, you can maybe rule out the exhaust as still being a problem and focus on finding an additional culprit. Or it could help you focus on a leaking exhaust if necessary.

HTH

Yes you will be, ask me how I know.

A lot of this has already been said, but...

*14-16lbs should be enough. As stated earlier, make sure that you aren't doubled up on the copper gaskets. Sometimes it's hard to see them in there.

*Make sure you tighten each nut a little bit and then move on to another one. Don't tighten them up to spec one at a time.

*Clean up the surface on the head before putting the gasket in. It's not an easy spot to clean, but doing so will help seal it up, and also let you know if you've got pitting there which a gasket won't be able to seal on its own.

*Use some Orange High Temp silicone on both sides of the copper gaskets

*Make sure you really filed your exchangers perfectly flat.

*Don't re-use the nuts or the copper gaskets.

*Try what Rich suggested and report back. You may have multiple exhaust leaks. Unless you've got a pretty big gap at the head/exhangers, your backfiring might be coming from somewhere else. Maybe a leak inside the exchangers?