Getting ready to pull my hair out

[buckeye1049]

Hi All, I've posted many times about my 1969 Porsche 912 and everyone has been very helpful...But I'm like one craigslist ad away from driving another 5.0 mustang....Arrrgh!
I have had Dellorto drla36's that pop and backfire up through the carbs for the last 1000 or so miles. I've rebuilt the carbs and now it seems worse with loading up at idle and hesitation on acceleration after stop light idle load up....The engine has about 2500 miles on it now and I have not adjusted the valves. It runs great on accleration but anytime my throttle load plateau's there is a "pop" up through the carbs, mostly cyinder #2 but i happens on others as well. I've moved around the timing, messed with the idle mixer screws at the base of the carbs but nothing helps...Can anyone talk me off the edge? Or does anyone want a Sand Beige 69?

Thank you,
Toby Maxwell
Maumee, Ohio

[Jaems]

You probably have a leak. It can be where the exhaust muffler connects or where the carbs or manifold connect. The bases and the flanges must be square and flat with single new gaskets.

[JT912]

Jaems can testify that a loose muffler clamp can cause quite a shocking backfire (through the muffler though). He's followed me a couple times when this was happening to me. New clamps solved that problem. Backfiring through the carbs can be just as Jaems suggested.
If you haven't posted this over at www.912bbs.org, you might. It is a site dedicated to 912s.
Good luck.

[buckeye1049]

Really, Since assembling the engine I have had a crude bundle of snakes collector with a bullet muffler and now am running a Bursch system but I never put clamps on the J-tubes where they connect to the muffler, the fit seemed tight enough as it was.....I'll clamp them tomorrow. I feel sure that I have a good seal at the intake and carb bases.


Thanks All,

Toby

[JT912]

I have a stock setup and thought the same thing. J-tubes seemed to fit tight into the muffler w/o clamps. Tried the clamp thing and my backfire though the exhaust went away. I don't think this is the answer if you are having backfire through the carbs though. That can be a valve thing or base plate of the carb being warped.

[Aldo]

Toby- you need to be sure the valves are adjusted correctly before any other adjustments are made. Carb adjustments and timing settings depend on this.
Regards- Alan

[Bob Ashlock]

Make sure you don't have an intake air leak around the base of the carbs or the intake manifold-to-cylinder intake port. I had a similar situation (seemed most specific to one cylinder) that drove me crazy. When I finally pulled the engine and shroud, I found the center stud that holds the manifold down was stripped in the head and was not keeping enough tension on the manifold to make a good gasket seal. The gasket had actually sucked part-way into the intake port, allowing raw air to enter and creating a lean condition whenever I would level-off on the throttle. You can successfully diagnose intake leaks by spraying carb cleaner around the base of the carbs, but it is difficult to do the same for the intake manifold-to-head interface because it's under the shroud.

[buckeye1049]

Thank you all,

The valve adjustment was not even close on cylinder #2 (the one that popped)..
It was too tight. I adjusted the valves and it runs much better, not perfect but alot better. I parked it for a few days and road my Trek to work to cool off.


Thank you

Toby Maxwell
Maumee, Ohio

[lep]

I originally had Solex carbs which I could never get to stop popping via any adjustments. Solex are wonderful carbs when they are tweaked, however, I transitioned to Webers during the last rebuild and have never experienced popping since.