Exhaust Leak or Timing (or both) Cause for Backfiring and Poor Running?

[lev16gt]

I've been chasing a backfire and poor running condition. Today I smoke tested the exhaust and found the SSI-to-head flange at cylinder #6 is leaking excessively. There were also very slight leaks at both SSI-to-muffler flanges and at the slip/clamp-on muffler extension/tip.

The exhaust leaks will be addressed regardless, but would the large leak where the heat exchanger flange meets the head cause backfiring/popping in the exhaust at idle and low speed/low load (I suspect yes), and/or cutting out under load between 2000-3000 RPM (this I'm unsure of)?

Backstory: Got the car a few months ago with virtually every vacuum leak a CIS car can have. The car backfired and ran poorly when I got it. After considering my options, I decided to convert to EFI and remove all the CIS components. When I finished the EFI conversion the car fired right up, but the same issues were there. I have quadruple checked for vacuum leaks on the intake side, did a leak down test, checked the head studs, checked the air injection plugs, adjusted the valves, new cap, rotor, plugs, tested resistance on the plug wires, checked the wiring on the pre-existing MSD Streetfire and Pertronix, cleaned and checked grounds, checked alternator and voltage regulator, checked timing*. I'm sure I'm forgetting other things I've checked.

Pertinent info:
1976 911s 2.7
SSI thick flange heat exchangers
m&k muffler
MSD Streetfire
Pertronix ignitor
EFI conversion w/fuel only (no ignition control)
Megasquirt MS2 ecu
14point7 wideband o2

*I noticed my distributor shaft has excessive vertical play so I contacted a distributor shop today who is [seemingly] knowledgeable with Porsches and come to find out he's done a few EFI conversions on 911s with the Megasquirt ecu, and works extensively with Porsche distributors (rebuilding and recurving). After describing the distributor shaft play and my symptoms he asked where the timing currently is and if the vacuum line was connected from the distributor to the throttle body. I indicated the vacuum line is connected and timing is at factory specs, 5 degrees ATDC at idle and seems to advance properly. He immediately said I'm way off where it needs to be and should advance the timing to 14-16 degrees BTDC at idle and disconnect the vacuum line and cap it. Any thoughts on this? I have not read or heard this about the initial timing setting anywhere in my months of reading about EFI conversions. My last 911 was an '84 Carrera (Motronic) and everything else I've owned has been newer, so I've not had to mess with timing via distributor adjustments on much of anything.

My plan today was to smoke test the exhaust and then start advancing the timing incrementally to see if it made a difference, but stopped at the smoke test once I found the massive leak.

I know there are many variables at play here, but any insight on the exhaust leak or the timing recommendation would be greatly appreciated. Thank you!

[Funracer]

A few quick things come to mind re smoke test as I have done several in the last year.

First is get a good smoke machine that puts out a lot of smoke (ie, don't use a cigar) Sounds like you have that already. After its hooked up to whatever inlet you want to use, let the smoke machine run for a long time. My definition of that is at least 5-10 minutes.

Does the 76 have a brake booster? If so check the long vacuum line that runs all the way up front to the booster in the frunk.

Lastly, try smoke testing the engine after a drive. Things change with rising temps and you may find leaks that were not apparent when the system is cold.

Mine is currently leaking smoke in between the left side cam chain box and the head. Took me forever to find it.


Good luck

[lev16gt]

Good idea to check after the car is warmed up; I’ll do that. I let the smoke machine run for 5-10 minutes and there was so much smoke coming from the cyl 6 heat exchanger flange that it was hard to see if it was coming from anywhere else. The smoke hose was put in the tailpipe which was sealed with a rubber glove and tape.

Any one have a recommendation on gaskets and hardware for the heat exchangers? I just replaced the heat exchanger to muffler flange gaskets, so I’m surprised they were even slightly leaking.

[KMoore68]

I have recently completed a similar conversion on my 77 911 S Targa. I have a fuel only EFI ITB set up running.

A few thoughts:
- I don't think an exhaust leak will cause the problems you indicated.
- If you are getting backfires and poor running, it sounds like your tune and/or timing.

Your comment about needing 14-16 BTDC for idle seems way off from the published specs. I do believe it should be about 5 BTDC at idle. I would work on confirming with a timing light that your timing at idle is correct.

What about the tune, are you running TunerStudio? Can you confirm the AFRs?

[lev16gt]

Originally Posted by KMoore68

I have recently completed a similar conversion on my 77 911 S Targa. I have a fuel only EFI ITB set up running.

A few thoughts:
- I don't think an exhaust leak will cause the problems you indicated.
- If you are getting backfires and poor running, it sounds like your tune and/or timing.

Your comment about needing 14-16 BTDC for idle seems way off from the published specs. I do believe it should be about 5 BTDC at idle. I would work on confirming with a timing light that your timing at idle is correct.

What about the tune, are you running TunerStudio? Can you confirm the AFRs?

Published specs for a '76 911s 2.7 are 5 ATDC, which is what my Innova timing light shows (the later SC cars are 5 BTDC). However, the jury is still out on how accurate the Innova light is since I have an MSD and Innova says their lights won't work correctly with MSD. I have a report from the previous owner's shop indicating they just checked timing in May and it was at factory spec but it doesn't say what it was actually set to. I too think 14-16 BTDC seems too far advanced and is a bit ambitious, and agree with you that around 5 BTDC could help.

I'm running TunerStudio but the logs have been so inconsistent that it's hard to pinpoint anything. Although tempting, I would prefer not to try to tune around any known or possible issues until things are fixed or ruled out as non-issues.

[lev16gt]

Slight update: I moved the fuel pressure regulator vacuum hose from the bottom port on the front (facing front of car) of the throttle body, to the bottom port on the rear of the throttle body. The rear lower port is where the distributor vacuum hose was so I capped the vacuum pod on the distributor. The backfiring is completely gone now. The car runs differently than it has the entire time... it runs horribly, but very different than before.

It now has a slight hunting idle and will bog down and almost die when you give it throttle at idle, like it's sucking for air when the throttle body first opens. I fiddled with the idle screw which basically does nothing, but it seemed to get a little better so i took it for a drive. It ran horribly including bogging down at/from idle when giving it gas from a stop, so much it died on me once, and also cutting out at low RPM under load (up to 3k rpm). It does get a little better above 3000 RPM, but still not great. But no backfiring like it has been doing since I got the car with CIS and after the EFI conversion... so weird.

I need to get ahold of a timing light known to work with MSD so I can confirm where it's at, and adjust if needed. Then fix the exhaust leak(s) since that needs done regardless, and go ahead and rebuild distributor.

The saga continues...

[Showdown]

1.) have you checked your timing to confirm it’s correct. Use a timing light. Any one should work.

2.) your tune likely needs to be adjusted. Check the logs to see what’s happening.

You have efi which means you have data, stop guessing and start working the problem logically. Plenty of folks here can help if you post some screenshots of logs.


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[lev16gt]

Originally Posted by Showdown

1.) have you checked your timing to confirm it’s correct. Use a timing light. Any one should work.

2.) your tune likely needs to be adjusted. Check the logs to see what’s happening.

You have efi which means you have data, stop guessing and start working the problem logically. Plenty of folks here can help if you post some screenshots of logs.


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Thanks, Showdown. I'm just trying to check things off the list, as I've only owned the car a few months and there are so many variables in play. I would hate to try to tune around things only to find out later that something needs addressed, then retune, etc.

I checked timing and it was correct, but will check again. I just wasn't sure since Innova says their lights don't work with with MSD.

Below is a view of a log from last night, which is representative of all the logs I took last night, other than vacuum going to 98.3 kpa in the other logs.

[Showdown]

I’d recommend taking a good long log- from start up, showing idle and driving and post that to the msextra forums where you might have more traction with specifics on the tune and what’s going on.


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[lev16gt]

Originally Posted by Showdown

I’d recommend taking a good long log- from start up, showing idle and driving and post that to the msextra forums where you might have more traction with specifics on the tune and what’s going on.


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I have plenty of logs, ranging from just a few minutes to 20+ minutes. I take one every time I drive it. I’ve been meaning to post over on the msextra forums to see what’s what, so I’ll definitely do that. Thanks for the reminder!