MFI Backfiring

Ron,K · 07-02-2002, 06:33 PM

I'm trying to fine tune my MFI system. I think I've gotten everything pretty close since it idles well and seems to rev well through to redline . For the first couple of minutes after cold startup, I'm getting a back fire through the intake pipes. I can eliminate it by raising the idle to about 2200 RPM. I've tried going leaner and richer on the part load adjustment and the idle adjustment, but it still backfires. Once the car gets to operating temperature, there is no problem. Is this normal or am I missing something?

Rot 911 · 07-02-2002, 07:09 PM

Hey fellow bugeye owner! Do you have the MFI thermostat hose hooked up from the left heat exchanger to the thermostat? And you really should get a Gunson CO tester to find out what your CO readings are. Guessing just doesn't get it done on the MFI.

Bobboloo · Bobboloo's Garage

This isn't uncommon. Mine does the same. Do you have a hand throttle? If you do you can bump up the idle until the motor warms up. That's what it's there for.

I imagine you could eliminate the cold backfires though if it bugs you.

Are your plugs the right heat range and gapped properly?

How about the throttle valves are they clean?

Ron,K · 08:16 PM

Kurt,

Thermostat hose is hooked up and getting good air flow. I'm reading 7-7.5% CO at idle when the car is warm. I haven't checked it while the car is cold. I thought maybe the thermostat could be adjusted to allow a little richer mixture at idle, but there is no means to adjust it other than removing discs. I'm not sure at this stage of diagnosing, that's called for yet.

I haven't checked part load CO yet. I've just leaned out the pump until it ran poorly, and added 2-4 click richer to get it running respectably. I'm not sure I can check part load with the Gunson. My understanding is that the motor has to be under load at 2400 RPM. Will the Gunson work with longer tubing?

Ron,K · 07-02-2002, 08:08 PM

Plugs are NGK BP6es gapped at .028. Throttle valves were thoroughly cleaned as part of a total rebuild. You're right, increasing RPM with the hand throttle will mask the problem, I have to believe this can be adjusted. It's become a quest at this point.