I had my 1.8 L Jet motor with Bursch exhaust running well with immediate start up, smooth idle, and strong running up to redline. Removed the Bursch and replaced it with a tuned header with megaphone exhaust and now it's so lean that it won't run below 2000 rpm, and won't idle. It starts on the key with no pedal but dies immediately. If I work the pedal when cranking, I can get it to run, but only at higher rpm. With the Bursch I had it set to run with AFR of around 10-12 at higher rpm, and around 15 at idle. Now it's 15-16 at high rpm and it hits the max reading of 19 on the gauge as it drops from 2000 rpm and stalls.

Ignition and timing have been set and verified, and I have once again been trying to make sure that I have no vacuum leaks. Bursch had a fitting for EGR, and I had that hooked up when it was running well, but new exhaust has no EGR fitting so I have blinded it off for no flow. Tried it open to atmosphere but that didn't help or make things any worse. AAR and Decel valve both still hooked up. Tried disconnecting and plugging vacuum lines to EGR and Decel valves with no change. I'd like to remove them but I'm unsure of how best to plug them off so I don't create any new vacuum leaks. Could use some advice there.

Before I go back into the AFM box to start re-adjusting things, I'd like to know if others have had similar problems when switching to a free flow performance exhaust. Should I have expected to have to re-adjust the AFM settings to richen the mixture because of the more free flowing exhaust, or do you think I have a new problem that I need to find and address first.

All comments/suggestions welcome.

Have you ruled out an exhaust leak at the heads?

I checked mine using a $5 doctor’s stethoscope I got from Walmart with the end part cut off. Stick the tube up next to the exhaust studs. If there’s a leak it will sound like you’re standing in a wind tunnel.

I might have a minor exhaust leak at one of the heads, but that would only give me an inaccurate AFR reading and not cause the engine to run so poorly. It might mean that it's not actually running as lean as the AFR is indicating.

You're probably lean because the engines making more power and needs more fuel.

Make a component change and it goes rich, you're making less power.
Make a component change and it goes lean, you're making more power.

Still trying to tune engine with new exhaust on it. I have my wide band O2 sensor hooked up and I'm getting good idle at AFR of around 12-14 and good midrange in the 10-11.5 range. But the AFR drops to 9.0 at wide open throttle and the engine bogs down because it's way too rich. Tried adjusting the "static" and "dynamic" settings in the AFM box, and no matter what I do, nothing seems to affect the WOT mixture of 9.0. Doesn't the TPS send a WOT signal to the ECU, and could I disconnect that wire so the AFM flap signal governs the WOT mixture rather than the signal from the TPS? Any thoughts from the L Jet people out there? I'm tuning at an elevation of 7,500 ft above mean sea level and that's not helping either.

That's too rich across the board.
Adjust the wiper for 12.5 at WOT. You can adjust the idle mixture with the bypass screw in the pocket on top of the flapper box.

No matter what I adjust, the WOT mixture sits right on 9.0. I can lean out static and dynamic adjustments and the WOT mixture stubbornly stays at 9.0! Bypass screw is suspect, as it turns but does not seem to "trim" the idle mixture as it should.

Have you checked your fuel pressure?

Perhaps regulator is failing and pressure too high?

I'll do that in the morning.

Another half day dinking with the AFM and I now see that changing this exhaust has put the L Jet just beyond its adjustment limitations. When I adjust the dynamic setting (the big gear that controls the tension to the wiper spring) to lean out the mixture at high rpm, my idle and mid-range go too lean. To fix that, I richen the static adjustment (the position of the wiper on the spring loaded arm), and this makes the spring on the wiper arm a little weaker, richening the top end mixture. After walking through this pattern of adjustments and trimming idle mixture with the bypass screw along the way, I reached a point where the static and dynamic adjustments were just too far away from each other and to eliminate a mid-range lean miss, I had to reduce the dynamic wiper arm spring tension, and by doing that, I lost the high rpm mixture as it went rich as the wiper arm went beyond the optimal position. So I'm thinking of physically limiting the maximum amount of wiper arm travel so the high end mixture will be optimized rather than going rich While maintaining a good idle and strong mid-range. It's either this or back to the old exhaust.