Full throttle switch faux pas

[IROC]

I recently replaced my non-functional full throttle switch (on the '88 3.2 in my '76 911). I attempted to adjust it properly and was getting the correct continuity at full throttle.

So, last weekend I'm at Road Atlanta. About 4 or 5 laps into the first session I start hearing some loud backfires when I'm hard on the throttle. They get progressively worse until by the end of the lap the car is down on power. Any application of throttle produces backfiring.

I come off the track and can't find anything obviously wrong. Since the full throttle switch is the only thing I had messed with lately, I disconnected it, but the car still didn't run right. After it cooled off, I pulled all 6 plugs and looked at them. 5 of them looked great. One of them looked rough (#2 cylinder). The center electrode and the ground electrode had fused together into one mass. Yikes.

Luckily, I was able to run to Advance Auto and get some more plugs. I replaced them and left the full throttle switch disconnected and ran the whole rest of the weekend without incident.

My theory is that I had the full throttle switch adjusted wrong and that it was kicking in prematurely and causing a lean situation (?) that cooked that one plug? Make sense?

[DW SD]

Mike,
Have you ever measured your AFRs at wide open throttle? The computer goes to open loop once the full throttle switch is engaged. I believe this would be richer (13 to 1 ish) than in closed loop, which shoots for stoichometric or thereabouts.

Be careful, you don't want to burn up your valves. Maybe a dyno session would confirm. Steve Wong will burn a custom chip for you which should result in a safe operating condition as far as AFRs go.

You can also buy the LM-1 from innovate, which is a wideband O2 sensor. Cheap insurance, relative to the cost of your engine.
Doug

[DW SD]

I would agree that it seems like a potential lean condition.
ALSO:
Any chance you have any vacuum leaks (maybe at the joint between the two intake manifolds?
Do you have an aftermarket chip installed?

Doug

[IROC]

Doug,

Yeah, I did some dyno testing a few months ago and that's when I found that my full throttle switch was not working. I have a SteveW chip and sent Steve the dyno runs. He saw the problem right away. My AFR at wide open throttle was very rich (due to the full throttle switch problem).

My hesitation at this point is hooking the switch back up. I probably need to adjust it as best I can and then get the car back on the dyno.

I have 10.3:1 compression, but run mostly 100 octane fuel at the track. Been doing this for years without incident.

Thanks for the response.

[IROC]

Originally Posted by DW SD

I would agree that it seems like a potential lean condition.
ALSO:
Any chance you have any vacuum leaks (maybe at the joint between the two intake manifolds?
Do you have an aftermarket chip installed?

Doug

No sign of vacuum leaks, but I haven't checked the tightness of the manifold fasteners in a couple of years. Something to do...

[aquazulu2]

Originally Posted by IROC

No sign of vacuum leaks, but I haven't checked the tightness of the manifold fasteners in a couple of years. Something to do...

That is an idea..