Backfire on decel

[baja1972]

All I have to say is I love MS, I have used OEM systems and aftermarket and this system rocks.

I have a air cooled VW 2275CC motor that I have the latest code and I started from a completely fresh tune. Before the retune I was having issues with accel, but now it runs and drives perfect except for constant backfires in the exhaust on heavy decel (off the throtle going down hill). I have messed with the decel fuel cut and it seems to make little difference between 0% and 90%. If I watch the wideband O2 on decel it will hover around 15.0 when the decel percentage is at 0%, but it still has a very large backfire. This engine does not like to run lean so I have tried to richen up the very lowest VE bins with very little success. Any ideas? And by the way MS is not controlling the timing just yet.

[loren]

I found that mine backfired (not bad, just little pops in-between the usual "burble") on decel when I had it set really lean in the decel area of the VE table. When I had it set at or near stoich, it didn't backfire.

My theory is that with it lean, there's not enough fuel to burn, but every now and then enough fuel builds up to get a pop out of it. With it set to stoich, the MS just keeps supplying fuel at an appropriate rate to keep the engine idling, so no random firing occurs.

[devastator]

You aren't using that old vacuum advance distributor are you? I had to set my VE table to 15 under decel or it would pop badly, although theory says it shouldn't, since there is not much fuel entering the exhaust. I'll play with it a little this weekend and see what happens.

[baja1972]

Still working on it. The weather has been really bad and my car has no top so it is hard to get out this time of year. I am going to try and run it a little fatter at the bottom end and see what happens.

Devastator,

What are you running for AFR at the low end on decel? Also what due you run at cruise? Mine hates to be above 14.7. Our motors are pretty close to the same except mine has no turbo (yet). As far as the distributor goes I have an aftermaket electronic with no vacuum. I am getting ready to go full MS timing control once I get off my a** and do it. Probably is looking like time to me.

[devastator]

My AFR during decel is around 15. The AFR table has 15 entered in those areas anyway. I can send you my MSQ if you want. I use MS to control the spark on my engine. I, simply, locked the 009 dizzy and hooked MS to the Pertronix ignition module I was already running. Using MS to control spark is awesome, especially with a turbo.

[baja1972]

Yeah if you culd send me your files that would be great. I have a MSD 6 box that I'm thinking of using. Now your going to make me stop working on my willys and go back to the baja. Too many projects so little time.

How about MS on a flathead 4 willys motor. I can see it now, going from 65 HP to maybe 73 HP, oh and what about a turbo (just kidding might just blow the head right off).

[devastator]

Not that my MSQ is perfect or anything, but here ya go.

[baja1972]

Nobody is perfect. I will give it a look. Thanks.