TBI backfire through throttle body

[Freeborn]

I have a '78 2F freshly rebuilt with fuel injection from Affordable Fuel Injection. Specifically my TBI kit includes their coil and ECM-controlled distributor. 99% of the time it runs amazing.

-BUT-

Every now and then (and there's no real pattern), when I start to accelerate from an idle, it hesitates and backfires through the throttle body. Not like a BANG, but more like a cough. Whenever this happens, I've noticed that my tach jumps from 700-ish up to 2000+. Thoughts on what might be going on? I've checked the throttle position sensor, and it seems fine. I just don't know how to diagnose if it's an input to the ECM that's causing the issue or a failure in the ignition system after the ECM.

 

[Downey]

I have no idea what distributor AFI provides, but wonder if it uses an "ignition module" in it's system ??? Also wondering if you are somehow burning off an occasional puff of excess gas due to fuel pressure being too high (doesn't seem probable though) ???

 

[Freeborn]

I have no idea what distributor AFI provides, but wonder if it uses an "ignition module" in it's system ??? Also wondering if you are somehow burning off an occasional puff of excess gas due to fuel pressure being too high (doesn't seem probable though) ???

I think the dizzy is my next stop. I thought about it being a fuel supply issue at first just because it feels almost exactly like vapor lock. Given the tach behavior, though, it seems like it has to either be the dizzy or the ECM. Hard to imagine the ECM behaving seemingly random. The no-name dizzy on the other hand...

 

[pjohnson]

Does it occur with slow acceleration or quick?
Did you call AFI?

 

[Downey]

Somehow seems like a sudden flood of gas that kicks the RPM way up, which would then seem like the electrical are in fact working ???

 

[Freeborn]

Somehow seems like a sudden flood of gas that kicks the RPM way up, which would then seem like the electrical are in fact working ???

That would make sense if the engine actually revved up. In this case it’s just the tach that jumps. Engine actually drops in RPM - almost stops.

 

[Freeborn]

Generally with slow acceleration just under 1000 rpm. I haven’t talked with AFI since I realized the tach was jumping. I’ll shoot Jesse an email again.

 

[Freeborn]

Generally with slow acceleration just under 1000 rpm. I haven’t talked with AFI since I realized the tach was jumping. I’ll shoot Jesse an email again.

 

[pjohnson]

I would check wiring connections at the distributor. It does sound like a dizzy issue to me.

 

[Downey]

It also sounds to me like a dizzy/ignition module problem, but another thing is the Vehicle Speed Sensor works with the 02 sensor to control idle functions below 1,000 rpm, so problem could stem from this area as well. I believe AFI and Howell program out the VSS, plus since the owner of Stealth Conversions passed away, his brother is no longer making the 2 pulse VSS's available, so you could be up a creek if this is the problem. Guys who discover they really want a VSS now have to put in a $$$ Dakota Digital dash to get the VSS.

 

[SteveH]

I have exactly the same AFI setup and have never experienced a backfire/cough condition. I would check all wiring endpoints and grounds, in case you have an intermittent electrical connection somewhere.

 

[Freeborn]

It also sounds to me like a dizzy/ignition module problem, but another thing is the Vehicle Speed Sensor works with the 02 sensor to control idle functions below 1,000 rpm, so problem could stem from this area as well. I believe AFI and Howell program out the VSS, plus since the owner of Stealth Conversions passed away, his brother is no longer making the 2 pulse VSS's available, so you could be up a creek if this is the problem. Guys who discover they really want a VSS now have to put in a $$$ Dakota Digital dash to get the VSS.

Luckily I don’t have the vss. ...one less thing

 

[Freeborn]

Update: I got a response from AFI...

ME: It seems like my issue may be coming from the ECM-controlled dizzy. Is it possible that the module that controls my timing is defective? Is this anything you've ever heard of?

THEM: In my experience yes. The modules are usually the things that go bad. I have forwarded your email to Norm, owner and engineer. I would wait to hear his response before any action is taken but that seems like the likely culprit.

...more to come

 

[Freeborn]

Documenting for the next guy here...

From Norm at AFI:
First check your timing and maybe set it so that it is about 4-6 degrees advanced from where it is designated to be. As long as you do not have any spark knock that will be OK with the advanced timing. It is also very possible that either the pickup coil in the distributor is bad and/or the ignition module is bad. We have had more issues with the module than the pickup but lately have witnessed a bad batch of pickup coils also. One other thing is it is possible that the air gap on the pickup coil to the reluctor wheel needs to be adjusted tighter. You can check that, as it is adjustable, give it about .006” clearance on the closest tooth of the reluctor to the pickup. If that is already close and set and you are still having issues we can go through some other things.

——-
I set the timing to the bb on the flywheel ... so 1 degree more advanced than he specified, but easy to line up and no knock. As far as I can tell it didn’t actually change much about the timing after idle. Hasn’t acted up since, but it hasn’t been driven nearly enough to say for certain if anything has changed.

[still] more to come...