Now I'm REALLY confused (MFI)

[Jay Zollinger]

I've owned my '73 S for just over a year. Although it was running fine (it NEVER backfired through the exhaust), I discovered that the RPM transducer/speed switch was not working as I worked my way through CMA. I took the switch out and ran it for almost two months without the switch even installed. This had no effect on the car (still no backfiring). I found a rebuilt switch from a very reliable source and--POP--it now backfires--a lot. It regularly pops when upshifting and can backfire badly when accelerating out of a corner after coming in to it at higher RPMs. It seems to be slightly better when I am very careful about keeping my foot off the accelerator, but it still spits unburned fuel out the exhaust even then. I had always understood that these were the symptoms of a bad RPM transducer, but the good speed switch seems to have had the opposite effect on my car. Any ideas? (I know the speed switch is good because the car "hunts" as it is supposed to when the micro switch is depressed with the idle held at 2K RPM).

[Jay Zollinger]

Anyone?

[Paul Thomas]

If it ran good before, unplug that sucker and dont look back. It is outdated emissions crap anyway. Hanz at H&R Fuel Injection in NY, one of the handfull of people left around that can custom make space cams, takes the stop solenoid off and puts on an aluminum block-off in its place. I unhooled mine years ago.

Paul

[tobluforu]

Old thread, bringing it back. Last year after not having the micro switch hooked up because the transducer was broken I decided to get that part of the puzzle fixed. Since then my car back fires just like the org post in this thread. if I unhook it, it does not back fire. What the heck?

[304065]

It's still dumping raw fuel out the exhaust valve. It's not just a question of emissions-- all that extra unburned fuel on the overrrun (when the pump is still putting out a rich mixture and yet you've closed the throttle plate) has to go somewhere. It's washing the oil film off your cylinder walls and rings and accelerating wear everywhere it goes-- unleaded gasoline is a pretty poor lubricant.

Probably what is happening is the action of the speed switch is leaning the mixture out now to the point where it actually is close enough to stochiometric to light off with the available heat in your exhaust.

So. . . here's how I would go about fixing it:

1) What is your Idle AFR? Part-throttle Cruise? WOT under load?

Backfiring occurs when you have a combustion event that's either too late or too early, so it happens when a valve is still open. This is caused by the charge burning either way too fast or way too slow, which, absent your timing being messed up, is a function of AFR. So knowing which end you're starting from before thinking about the action of the solenoid makes a good starting point.

2) Are you absolutely, positively certain that you have the right speed switch and that the cut-in points are correct? With the throttle held above idle, say at 3000 RPM, go ahead and engage the speed switch microswitch on the crossbar. What precisely happens to your RPM? We know what the books says happens, for diagnostic purposes we need to know what happens to your tach needle. I would even take a short video if possible.

3) Is the microswitch adjusted correctly?

[tobluforu]

Switch is only a year old and was bought new, oem-took forever to get. It oscillates when I press it with engine kept at 3000 rpm, but not sure the rpms. To be honest, I think my car is running lean now. Just checked the plugs after a long run yesterday and they are not right. Last year when I had my lm1 hooked up I had it set pretty damn good, but it would still back fire at times. Sometimes if I get on it in second and switch to third, it pops in between. Timing is spot on, but up until last year when I did everything by the book, before I timed by ear for 17 years and I swear it ran better. I think there are too many variables with these old cars, different cams, space cams, pistons, that setting everything stock is a good starting point, but I feel one has to deviate. I’m running 7 heat range ngk plugs and they might be too hot for the way I drive, I’m going to put some 8’s in that have. My tb’s are org, so I’m sure that has something to do with it. I think it’s just strange that running without it plugged in causes it to run better, and I had it like this for 17 years. Granted the plugs didn’t look that great, but it ran like a bat out of hell.
There are numerous post in regards to this, and lots say that backfires in the exhaust in regards to mfi is the micro switch being broken. Then there are the others that say it’s the micro switch and to unplug it.
I will say this, I get much better gas mileage with it hooked up.

[jclotz]

I've been chasing this same issue too. Newly rebuilt engine (2.7 with euro pistons, dc30 cams, rs pump cam, 34 mm intake). I have a dash afr so I have the mixture dialed in well (14 at idle, 13.5 under load). It would consistently backfire when I shift, or open the throttle after engine braking. When watching afr, it would go very lean (>16) during engine braking when the solenoid was activating. This lean is what caused the backfire. Unplugged the switch and problem solved. Now it runs at 14.7 afr during engine braking and all backfire gone. Unplugging the switch (or just backing off the screw) didn't affect normal operating afr.

[Flieger]

It is supposed to be lean when the switch is activated. It is supposed to be so lean that nothing is burning. The idea is that it shuts off the fuel and the engine is just pumping air.

The switch solved the backfiring for me, assuming that I let off the throttle quick enough/fully enough to close the switch.

[jclotz]

Sure, clearly. But maybe my SSI get too hot during that lean, causing backfire when the fuel comes back on?

[tobluforu]

I think the main issue is that one is not driving a car where you are either on the gas or off unless you are racing. When driving on the street you are always feathering it to some extent, therefore you get pops here and there. I guess when you have it disconnected you are dumping pretty much raw fuel into the system which cools combustion, then when you get on it you burn it off. I just find it interesting that half have it hooked up with no back fires, when before they did, and visa versa.

[RSTarga]

Have you checked your Points and dwell ? The symptoms are similar. If it was the switch it normally backfires with throttle off, or when you get back on the throttle after decel.Is it adjusted properly?
Make sure and take your foot completely off the accelerator. Try the screw 1/4 turn in more.
Start with basic adjustment + 1/4 turn. If that doesn't work, check your points.

[jclotz]

It may be that when the MFI is set-up on the rich side, you get backfires with the solenoid disconnected, and when its set to the lean side, you get backfires with the solenoid connected. I know I'm on the lean side. When first setting-up my MFI I was way rich, and it backfired like crazy with the solenoid off or on.

[tobluforu]

Points are new and spot on as well as timing, no prob with screw making contact with micro switch. Dwell was set last year, but it's really not a concern with cdi.