Backfire Options

[mike_the_man]

Hi all,

I was working on my teener this afternoon, setting the timing. I got everything set and it seemed to run pretty good. I let the car cool off, and started it again, and it was backfiring out of the exhaust. It backfires all the time, at idle or when revved, but it only seems to backfire when the car is cool. Adjusting the mixture screw on the CPU seems to help a bit, but not enough.

The car is a 74 2.0L with fuel injection. I have replaced the CHT and the MPS holds a vaccuum, and I have recalibrated it.(I'm pretty sure I calibrated it right) Any thoughts? Backfiring out of the exhaust means lean, right?

Thanks,

[Andyrew]

How long since last valve adjustments?

Thats my only thought..

andrew

[mike_the_man]

Last year, but I've only driven the car about 3 times since then. I'm going to re-adjust them tomorrow though, just to be on the safe side. Any other thoughts?

[SirAndy]

Last year, but I've only driven the car about 3 times since then. I'm going to re-adjust them tomorrow though, just to be on the safe side. Any other thoughts?

exhaust leak? (IMG:style_emoticons/default/confused24.gif)

[nebreitling]

i don't know sh-t, but i've delt w/ a few backfires:

has the car sat a long time, only now to start running (and backfiring)?

does it backfire only on decel? or all the time?

follow the exhaust all the way back to the heads to check for leaks. check the torque on the nuts of the exhaust heads -- make sure that is a good seal (although an exhaust leak @ the head has a pretty particular sound: you know it when you hear it).

did you put the CHT in soundly w/ the little washer -- i.e. no leak there?

when my TPS was wildly out of adjustment i had a few backfires. worth a quick check.

try enrichening the mixture by putting a potentiomenter in series w/ the cht. add up to a few hundred Ohms of resistence, see if that helps to clear it up.

of course, adjust those valves.

check for leaks EVERYWHERE in the intake and exhaust.

are you positive that your timing marks are good -- i.e. that the car is timed correctly? got points?

[mike_the_man]

Thanks, I'll check for exhaust leaks today, and I'm going to try and add a potentiometer as well. I'm 99% sure that the car is timed properly, and I just put in a Pertronix, so points aren't an issue.

Which way do you turn the mixture knob on the ECU to richen the mixture, I'm forgetting right now?

The car has sat since about October. The washer was installed with the CHT, and the TPS is adjusted.

The car backfires all the time when cold, even during idle. Adjsuting the mixture on the ECU helps, but doesn't totally fix it. The fact that it seems to run better when warm woud lead me to believe that it has something to do with the CHT.

Thanks,

[McMark]

Did you set the timing with the vacuum hoses removed from the distributor?

[nebreitling]

yeah, definitely plug those hoses. it SHOULD only affect the timing a few degrees, but you never know...

turn the mixture knob on the ecu to the right to enrichen. left to lean. of course, this affects the mixture only at idle.

i just reread your original post -- you mentioned that you had recalibrated the MPS. hmmm... did you use an inductance meter? regardless, that's the first thing i'd look at. swap it out with a known good one and see if that clears up the problem. you may need a re-re-calibration...

[mike_the_man]

I did use an inductance meter to set the MPS, as per Brad Anders' instructions. Also, I pulled and plugged the hoses to the distributor when setting the timing. Whats the general thought, does backfiring out the exhaust mean I'm running lean or rich?

Thanks,

[Bleyseng]

What settings did you use?


Geoff

[nebreitling]

lean, probably lean. although a way rich condition will also cause a backfire (or, if it is in the exhaust, more aptly called an "afterfire" -- a true "backfire" is up through the intake but i have no axe to grind w/ terminology so let's continue)

basically, you have unburnt gas in the exhaust. this could be because you are running significantly lean, thus not igniting consistently, thus letting some unburnt gas into the exhaust where it is ignited. or maybe you've got so much damn gas going through that it builds up in the exhaust where it is ignited. either scenerio sucks for your car, and will make it hate you.

these are the most common conditions i know of, please correct or add to this list:

1. exhaust leak (O2 getting into the exhaust, usually on decel)

2. you are running way lean (above). check typical stuff -- make sure you have good fuel pressure, a good cht connection, mps, injectors, blah blah

3. f-cked up ignition firing (whereas you are not firing consistently, thus building up unburnt fuel in the exhaust where it is later ignited. check for good spark on all 4 cyl. did you pull the dizzy when you installed the pertonix? double check the orientation of the distributor drive shaft. make sure your distributor cap is good. make sure the plug wires are in the correct places. hell, make sure that your plug wires are good.)

4. way rich. exhaust smells gassy.

5. ? others?


this assumes that your timing and valves are set right.

does the car make good power?

nathan

[Bleyseng]

start with the basic stuff
Retime the car right
Make sure the firing order is right with the spark plugs/wires
check for vacuum leaks/hoses off and mis placed

Geoff

[mike_the_man]

Thanks guys, I think it's time to start from scratch and look at the basic stuff, alright. I tried putting a potentiometer on the CHT, and it didn't seem to make much of a difference. Increasing resistance should make the car run richer, correct? When I cranked the pot all the way up (an extra 5K) the car would start to stumble and not want to idle, but small changes made little difference. This would lead me to believe that the car is already running rich.

As for the MPS, I set it as per Brad's page: 0 in. Hg - 1.39 H, 4 in. Hg- 1.18 H, 15 in. Hg - 0.71 H, although the numbers were slightly adjusted for my alltitude. I'm fairly certain that it is correct.

I did pull the dizzy to install the Pertronix, and I was careful to put it in the same, but I will re-check everything.

Thanks,