[Mackpipes]

I have a miss at 5500 rpm at full throttle. I put in new plugs (with correct gap), new wires, cap and rotor. The rebuild has 2k miles on it. I just put in new crank and flywheel sensors, as well as a head temp sensor. The motor pulls hard up to 5500 and then stumbles slightly and then continues to pull up to 7000. I should add that I rebuilt the motor with Raceware rod bolts and head studs as well as Assco sport valve springs and Ti retainers. This symptom is intermitint. The miss happens every 3rd or 4th pull to redline. Where should I start looking?

[matt de maria]

Aside from guessing that maybe the air vane is sticking, did you set the gap for the flywheel sensor? The fallback position would be backpinning the computer to scope the injector pulse and primary waveform to coil.
Since (I assume it is pulling strongly after stumble) it is not flattening out from 5.5k to 7k, I don't think it would be fuel supply/pressure related. Any valve float problems would be consistant and repeatable.

[Marc A]

Ted:

1) put the stock chip back in and see if the problem goes away.

2) make sure all plug wires are well connected.

3) Disconnect the O2 sensor and get the car hot and see of the problem disappears.

4) If all else fails borrow my extra AFM to see if your's is messed up.

[Mackpipes]

Matt,
I set the gap for the sensors at .08mm per factory spec. How do I check if the injectors are pulsing correctly, in laymans terms.

Marc,
Plug wires are good.
I'll try the oem chip.
I'll let youknow if I need the AFM. Where does it attach to, or plug into?

[Marc A]

Originally Posted by Mackpipes

I'll let youknow if I need the AFM. Where does it attach to, or plug into?

Its that big aluminum box in the middle of the engine thats connected to the air cleaner on one side and the throttle housing on the other . . . .

[APKhaos]

I agree with Mark that the AFM is a suspect.

I've seen the carbon resistance layer worn through along the arc that the contact point travels. Usually happens around the point where the engine spends most of its time - somewhere between 4000 and 6000.

There's a good test in Bentley that involves pulling the AFM connector and connecting an ohmmeter across the contacts. As you move the vane, the change in resistance should be smooth and linear. The hallmark of the problem is a glitch in the resistance curve.

Had one of these at SP during the last race weekend. It was a 944, which uses the same barn door AFM the mid 80's 911. I generally don't like tinkering with the contact adjustment because I'm seen then go from a dead spot to completely wrecked while trying to 'fix' things. That said, I managed to get this guy going [without the dead spot] for the race. At best it is a temporary fix. If the AFM test shows a dead spot, ask our resident parts sourcing expert, Marc.

BTW, Ted......regular running to 7000 will definitely reduce the life of your rebuild. Its great for racing, but racers will trade off more frequent rebuilds for the chance gain an advantage - eg running from T2 through T3 at SP without having to take 4th then downshift to 3rd for T3.

[Mackpipes]

Tony,
I agree with you about motor longevity. I don't take it to redline very often. Yesterday I was doing a shake down run with the new Steve Wong chip. He programmed in a 7k rpm red line. According to the dino (pre-rebuild) my peak power came at 5600 rpm. The only benifit to staying in a gear past 5600 would be to pass. I'm going to try the stock chip today. If the problem still exists, I'll test the AFM. How detrimental is this to the motor? Should I be running it regularly while this condition exists? Granted, I won't be doing redline runs unless I'm testing to see if I remedied the problem. I really appreciate yours, Matt's and Marc's help on this. VIR looms near, gotta get this fixed.

[Mackpipes]

Its that big aluminum box in the middle of the engine thats connected to the air cleaner on one side and the throttle housing on the other . . . .

Before all of those who have read this post think I'm a total slipknot. I was thinking Marc was talking about an exhaust gas analyzer. I'm sure some are wondering how I can rebuild my motor and not know what an AFM is. Although this might explain why the damn thing don't run good.

[BobNovas]

I'd check for a nut loose behind the wheel.

[BW911]

Smackpipes writes:

I don't take it to redline very often.

Right, and RL bought a 993 because he won't do anything to it.




Bill