Backfiring RESOLVED

[spidernut]

My son recently purchased a 77 Spider which is stock excluding a Crane Fireball electronic ignition. It has plenty of power and runs great apart from one significant thing - it backfires during shifting when the rpms drop rapidly. This happens only above approximately 2,300 rpm. It has also happened twice when turning off the car.

I checked the timing at it is at 0 BTC (per the shop manual)
I set the fuel mixture, checked it for backfiring (which it did), then leaned out the fuel mixture with no change
All vacuum lines are connected and appear to be leak free

I read a thread that said the culprit could be the fast idle electrovalve. Is there a way to test this valve? Any other suggestions regarding what could be causing the backfiring?

[GeorgeT]

Possibly the valve timing may be off by a cog or two.

[Frog2Spider]

It ready sounds like it's running to rich or has a vacuum leak.

I'd suggest checking the spark plugs. As you likely know, black is to rich, white is to lean. Ya want to see a nice brownish grey.
Check for a clean air cleaner and that the choke cable is properly adjusted, so the choke completely opens.

You've checked that the vacuum lines are all connected. Check that they are all connected properly, and that the lines themselves are not cracked or somehow damaged.

if all the above is good. Start re-adjusting the timing, 1 degree at a time and test driving between adjustments.

Let us know when the problem gets resolved and what it turned out to be.

[So Cal Mark]

popping in the exhaust is different from backfiring. A backfire comes out of the carb, popping in the exhaust is unburnt fuel in the exhaust system. A straight -through muffler can also cause popping.

[RoyBatty]

Re: Marks post above.
If it's firing out the carb, you have a lean condition or early timing or some combination of faults that is causing the fuel/air charge in the intake manifold to get lit off.
Firing out the exhaust is excess unburned fuel going past the exhaust valve and getting lit of in the exhaust system.
Once we know which we are dealing with we can move forward from there.

[JerryAebi]

Possibly the valve timing may be off by a cog or two.

I agree with George T, That's what was wrong with mine, after I went through changing jets, to replacing with new carb, and all along I was one tooth off my cam pully. fixed that, no more back fire. Good Luck.

[spidernut]

It is backfiring out the exhaust. There are no issues at the carb as far as I can tell. The motor runs smoothly, pulls strong, doesn't smoke, doesn't miss, etc. This is also more than a popping out the exhaust - it is more like a handgun being shot. If I could be behind the car at night, I'd bet it is shooting out a flame.

I checked a few things and found that the micro switch on the clutch pedal needs to be adjusted. It is not breaking the electrovalve ground circuit when the clutch pedal is released. In reading through a Haynes manual and the Fiat shop manual, the fast idle circuit is controlled by the electrovalve which opens the flap in the carb to reduce the rich fuel condition which can cause backfiring. If the fast idle is holding the flap open, it would make sense that the fuel mixture would have to be set very rich to compensate for the additional air which could explain the backfiring.

I'll be tackling that circuit and the fast idle settings today to see what I find. My current belief is that this is the culprit. We'll find out shortly.

[spidernut]

Thus far:

- Fast idle solenoid works but needed to be adjusted - FIXED
- Vacuum lines - O.K.
- Electrovalve switch - FUNCTIONS properly
- Microswitch on the clutch pedal - DOES NOT FUNCTION
- Fuel mixture - O.K.

I removed the microswitch on the clutch pedal. When the clutch is released, the ground circuit is supposed to be broken. When the clutch pedal is depressed, the ground circuit is supposed to be completed thus activating the electrovalve and increasing the RPMs and opening the throttle plate to add more air during deceleration. When I used a jump wire on the ground circuit at the clutch, the wire to the ground was dead. The wire going to the electrovalve is a good circuit.

I used a different ground and can jump the circuit activating the electrovalve and fast idle solenoid so the circuit functions with the idle increasing appropriately. I then removed the microswitch which looks like a brake light switch and it works backwards. When the clutch is released it completes the circuit and when it is depressed it deactivates the circuit.

So, I apparently need this switch which I cannot seem to find anywhere. Living in the People's Communist Republic of Califgomorrah, I cannot remove the emissions equipment so I am forced to make it work. Suggestions other than moving out of California? I spent some time at a local parts store and can't find a standard brake light switch that will work.

[majicwrench]

So the switch is closed in one position, and open in the other?? I would say the switch works. Perhaps your interpetation of the wiring is not correct.

Even without the electrovalve working, methinks it would not give a BANG like you describe, if everything else was OK. But by all means get the fast idle feature working.

[TStark]

Manual Shmanual, 0 Base timing seems too late.

10BTDC at least.

[spidernut]

Thanks to Midwest-Bayless, I purchased a NOS diverter valve. I installed it, found two vacuum lines to the valve that were reversed, fired it up and it runs smooth as can be. It'll be nice to take it for a road test tomorrow!

[RoyBatty]

Hi John,
The technical detail monster in me is stirring.
With regards to the two vacuum lines,
Is one of them ported vacuum that would be used to to pull the valve closed during anything but off throttle conditions, and the other straight manifold vacuum, used to open the valve for off throttle, deceleration mode?

[spidernut]

Tom...It was a two-fold problem.

1. The diaphragm in the diverter valve was leaking and it took an extreme amount of vacuum to move the internal piston. The amount of vacuum required was far beyond anything the motor could provide. That necessitated replacing the diverter valve. With the new valve, a normal amount of vacuum will operate the internal piston with ease.

2. The second problem was exactly what you mentioned. Had the original diverter valve been in good condition, the improper routing of the vacuum lines would have still resulted in the same backfiring issue. In fact, it would have a amplified it. I found the mis-routing when I installed the new valve and double checked the connections. I missed this issue when I initially traced the lines due to the unclear vacuum diagram that I have.

Now I can change the oil and get the emissions testing done!

[124JOE]

can you post you diagram on here?

[spidernut]

PM me your e-mail and I can send it to you. It is a PDF file and won't post on this forum.