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.
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?
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?
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