4000 rpm misfire - updates

[JimW]

m10, 12:1 compression, Schrick 328, S14 crank, ported head, tii dizzy, crane ignition, (Hi6 and PS92), IE step header, 35 degrees total timing, 48 DCO/SP carbs, out of the box, on a cannon intake with 60mm air horns. running on 110 leaded fuel..

Runs like a scalded cat.... That is.... between 2000 rpm and 3900, and between 4800 rpm and 8000 it runs really well.. between 4000 and 4500, no matter the load or throttle opening it misses (even gently raising nthe rpm's in neutral it misses through that range). Under light to heavy load it really runs badly. Did not change when going between 45dcoe and 48dco/sp..

Ok. Replaced the plugs and wires, (cap and rotor are fresh and look almost untouched) the plugs were fouled badly, and the Nology Hotwires were getting old.. The miss seems somewhat less at light throttle and free revving, but it is still very much there at WOT in 2nd.. If running from 2000 rpm it pulls nice till 4000, then bucks and stumbles badly then as soon as it gets to 5k it takes off all the way to 8000..

I tried messing with the advance a bit (was at 35 degrees total, I moved it up with not much change with the exception of a better idle).

The car definitely seems rich all the way through the rev range though. (black smoke).

Update:

Replaced the coil, cap and rotor, still there. (not using a resistor style rotor, so nothing to "erupt")

Replaced the Compu-Fire with a Pertronix, no real change. Bypassd everything, used a old Bosch black coil with the pertronix, still there.. Still making some pretty good black smoke under full throttle.

When under a full load up a hill in 2nd it has trouble getting through that range.. Can't do extensive drive testing as there are no taillights in the car right now either..

Would posting a video be of any assistance

Any thoughts?

Thanks!

Jim

[Guest Anonymous]

You pretty much eliminated the ignition. So sounds like it's too rich in the midrange. What Jets? The real solution is a trip to a good dyno with a exhaust gas analyzer. You could also try a wide band O2 sensor.

[TobyB]

Go a jet leaner and see what happens.

I'm with Jeff, there's nothing left in the ignition to do it!

t

[kbmb02]

Probably lots of overlap on the cam, correct?

[allbim]

If you value your time, an O2 sensor is really worth the money.

From what I've heard, a heated O2 sensor can just be stuck up the tailpipe.

That's obviously not the best way to do it and I have never tried it. I'm sure somebody on this forum has some experience with this and is willing to tell.

[JimW]

There is going to be a good deal of overlap in the 328 cam.. I don't have the specs handy. What were you thinking of regarding the overlap?

Given that the car is running leaded fuel exclusively, would that be a problem with a wideband? I don't mine adding a bung just after the header collector (don't want to add to header itself as it's already ceramic coated)

Jim

[kbmb02]

You might do some research on reversion (though you may wish you didn't) . . . it can be caused by a number of factors including valve overlap and timing, high cylinder pressure, incomplete fuel burn, intake/exhaust system specs / tuning and more.

If it's reversion, you would likely see / feel a wet cloud over the carb with the air cleaner removed at the rpm's in question.

[Arash2002tii]

Don't know if this helps but I thought I'd throw it in in case it actually is reversion. http://www.eng-tips.com/viewthread.cfm?qid=190526&page=17

Good reading nonetheless

[aa/fd]

Just guessing but with all the changes to the ignition and two set of carbs, it might warrant a look at the fuel supply. I seem to recall Weber's are sensitive to high fuel pressure