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ITB race car doing something strange


markraspi

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Hello to all of you.. Here`s a diagnostic puzzle. My 1970 2002 ITB race car is doing something new and quite different (for this car). For the first few laps of any given track session, be it qualifier, race, etc. , the engine exhibits a high speed, high rev misfire. Usually starts around 6000 rpm right to 7000 or wherever I shift. Does not do it in second gear. Only third and fourth.

The strange part is the fact that this completely goes away after about two laps as if nothing happened. I can go another 14 or 15 laps right to the finish with the car running perfectly.

Initially thought of the ignition system. gave it a new cap and rotor. Checked the plugs...reset the gap...etc.

heres what I`ve got...

Stock engine

Stock distributor

Luminition Ignition

Bosch Blue coil

Stock wires

NGK Iridium plugs BPR7IEX (rec. by ngk tech guy)

Weber 32/36 carb (set up for track use)

Only recent change is the addition of a K&N air filter. I was running without one but agreed to try one as a favor to my engine builder. Tried with AND without but saw no difference in lap times. Actually had my fastest time WITh the filter.

Looking for suggestions......Thanks......

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Certainly sounds ignition related. Coil going bad? Spark plug wire ends coming loose from connector? Coil wire not fully inserted? Plugs tight in head? Plug wires rubbed thru from contact with fenderwell?

Intermittant, high RPM misses are a beast. Please keep us posted.

Jim Gerock

 

Riviera 69 2002 built 5/30/69 "Oscar"

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Hmm, I would try with normal plugs, whatever the tech says.

Try NGK BP7ES and see if it does anything.

Also some component in the Lumition could be going bad. Electronic components can become very heat sensitive when aging.

Tommy

Racing is Life - everything before and after is just waiting!

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Had a similar problem with an ITB rabbit, high speed miss, changed everything, but could not repeat the condition unless we were at race speed. Turns out the master kill switch went bad, changed it problem went away. It took two years to find this silly problem and we lost a ton of races. Change this part yearly, 25$ insurance.

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Put a fuel pressure gauge on it, too, just for s&g.

I agree, it sounds ignition, except that it heals itself...

my street car has a fuel line that's too small

(note: 1/4" brake line is not a good fuel line substitute!)

and leans out after about 6k at WOT.

If you have consistent pressure, that'll rule that out,

but you need to confirm that you DO have good pressure at high rpms.

Careful with the plumbing.

hth.

t

"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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