Backfire

[des28h2]

Last year I had cam chain tensioners, SSI's and an M&K 2 in 2 out installed on my 79 SC. I now have a backfire that wasn't present before. It occurs mostly at idle after the car warms up for awhile and on deceleration, down shifts. It smells like the car is running rich as well. Any help guys?

[Flat Six]

Is this backfire through the intake or out of the exhaust? Based solely on your description of what was changed before backfire started I'm going to guess exhaust. If intake please let us know right away.

My bet -- based on limited information presented -- is either exhaust leak or possibly spark plug lead that was disturbed during install and not fully replaced.

[T77911S]

ignition timing can cause it. do you vac retard and is it connected?

your mods can too. it will usually be like pop,pop,pop,pop out the exhaust, kinda like popcorn popping. not very loud, but there. your mods can lean it out due to better breathing.

who did the tensionors? it might not hurt to recheck the cam timing if you can do it. its easy in the car.

if you suspect mixture, check the fuel pressures first, then check/set the mixture. this needs to be done if it was not done when you changed the exhaust. make sure you dont have any air leaks too.

a lot of people like the pops. it tends to bother me sometimes.

[szyzygy]

I have this issue, too. I suspect an air leak somewhere. I only get it when I'm in second going down a step hill. I dont mind the noise, I'm kinda concerned the unused gas is damaging my cat.
had to replace the cat in my mazda because I had a spark plug not working and it damaged the cat as a result.
A plug not working will also result in a noticeable drop in power, right?

[des28h2]

It's an exhaust backfire. I don't think it's running lean since the smell of the exhaust is so rich. She also doesn't seem to have to power that she should.

[Bob Kontak]

Originally Posted by T77911S

..........like pop,pop,pop,pop out the exhaust, kinda like popcorn popping. not very loud, but there. your mods can lean it out due to better breathing.

Had this, drove me nuts. The putting / slight popping at idle. Number four intake runner at head was not sealed. Found it with starting fluid or carb cleaner.

[stlrj]

Originally Posted by des28h2

It's an exhaust backfire. I don't think it's running lean since the smell of the exhaust is so rich. She also doesn't seem to have to power that she should.

Might be time for a professional tuneup which would include a mixture adjustment.

[911SauCy]

My car has the same issue, but only seems to be bad for about the first 20 minutes of driving...then the car runs smoother (less idle garbling) and backfires way less.

I'm thinking it's a tiny air leak as the engine warms it probably expands and closes the hole size, hence the limitation...?

The good thing, at over 3k, the thing pulls like a beast all the way to redline and never skips a beat

[T77911S]

cold vs hot running problems are usually fuel pressure related (WUR), but it is always a good idea to start with air leaks.

after posting my reply above, i started thinking about the timing. i have made mine pop/backfire when letting off the gas, but i cant remember if the timing was more advanced or retarded or if the vac retard was removed or not. i did so many tests that i cant remember for sure, but i do know timing can make it worse.
i am thinking my vac advance was removed giving me more advance at idle and when i let off the gas making it pop. i have not driven my car in a while but i do know it is not doing it now and my vac retard is connected.

[911SauCy]

Ok, so how does a driveway guy check his own fuel pressure on the CIS system?

I'm not incapable, but still learning about these technically advanced dinosaurs. Just made a basal shift from Chevy LS Power to Porsche.

[911SauCy]

Ok, so how does a driveway guy check his own fuel pressure on the CIS system?

I'm not incapable, but still learning about these technically advanced dinosaurs. Just made a basal shift from Chevy LS Power to Porsche.

[tobluforu]

Have you tried placing the stock muffler back on to see if it's the muffler. I find it hard to beleive that anything else could cause this since all you did was replace the muffler and tensioners. Try cranking down the muffler bolts/nuts at the flange, might get lucky.

[des28h2]

Thank you all for your advise/suggestions. I have not tried putting the stock muffler on. I no longer have it. I will start with the vacuum leaks with a good old fashioned can of carb spray and tightening exhaust bolts but I am thinking it might just be the muffler I picked. Has anyone else had a similar issue after installing an M&K 2 in 2 out? I'm also going to see if capping the one output will increase the backpressure and solve the issue. Anyone in the SE Pennsylvania area happen to have an muffler in the garage they would be willing to let be borrow to see if the popping goes away if the other tests don't yield any results.

[tobluforu]

I had an m&k, but it was on my mfi and it popped like crazy, and backfired some. Tried my hardest to get it run ok, but gave up. But you have a different year.

[Paulporsche]

911 SauCy,

You pick up a CIS fuel pressure gauge from JC Whitney, hook it up and observe.

If you do a search you will find a ton of threads on this and adjusting the WUR. Some of them are mine (LOL).

There are also some videos on Youtube of people doing this test.

[jeffs9146]

I made a pressure tester out of a left over fuel line from the top of the Fuel Dist to the WUR!

[911SauCy]

Perfect!

Originally Posted by Paulporsche

911 SauCy,

You pick up a CIS fuel pressure gauge from JC Whitney, hook it up and observe.

If you do a search you will find a ton of threads on this and adjusting the WUR. Some of them are mine (LOL).

There are also some videos on Youtube of people doing this test.

[jclotz]

I also had a backfire problem with MFI and an M&K 2 in 1 put. Much proved after going back to a stock Dansk.

[911SauCy]

Perhaps I should include that I have a Fabspeed cat delete (per-muffler) and test pipe with carbon fiber air box.

I knew going in the (lack of) exhaust could increase popping/backfire but that doesn't explain the mis-firing and gargling when warming up.

[CCM911]

Did you by any chance have a cat bypass installed? Mine began backfiring on the two-three upshift after I installed mine.