Missing through midrange 2800-3200rpm, where to start?
 

Car is 2.7 K-Jetronic, runs fine at higher rpms, just misses and stutters through 28-3200rpms when on the gas. Where to start? Clogged fuel injectors?, bad plug or wires?, vaccum leak ...any other suggestions?

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Shakenbake (Chris Baker)
1974 911 Coupe
'91 "A" stock champion WV
'90 SCCA FTD
'99 SCCA 3x "A" stock winner

Start with the easy stuff, pop the top in the dark and see if you have a light show under there.

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Robert Stoll
83 SC
83 944

Chris,

Vacuum leaks at cylinder head or the six rubber boots from the intake runners to the air box ... comes to mind, first, but you may want to look at the spark with a timing light at that rpm range, just to be sure! Be sure to check all of your sparkplug wires for good, solid 3-4000 Ohm readings end-to-end with a digital multimeter!
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Warren Hall
1973 911S Targa

[This message has been edited by Early_S_Man (edited 02-02-2001).]

I'm getting ready to test my wires and ends, is there any reason the ohm meter needs to be digital?

No, analog meter will work fine.

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Warren Hall
1973 911S Targa

Chris, I am having similar mysteries. I am wondering if a plug wire is shorting to the stainless steel braid. That would make it less visible or audible. So, I'm gonna change wires.

Anyway, here goes: I'd make sure the valves are in adjustment. Maybe they are. Poorly adjusted valves will cause surging, particularly at lower speeds. Surging, not missing.

Missing is likely ignition. Ignition is a good suspect in any poor running. If you don't have steel-wrapped spark plug wires, then run your engine in the dark and look for sparks. You'll hear the snap, too. But make sure your ignition system is working fine, including timing, before you go on to fuels stuff. Dwell, if you have points. advance mechanisms.

If that's not it, we'll look at the more complicated and less likely fuel issues.

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'83 SC

Don't know if this was the cause but I cleaned out the airbox which had some oil blown in it from the vent tube. After driving around the sputtering stopped altogether. All plug wires checked out fine. No vacuum leaks either. I'm thinking the airbox was the culprit...what do you think Warren...?

BTW, thanks for the advice.

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Shakenbake (Chris Baker)
1974 911 Coupe
'91 "A" stock champion WV
'90 SCCA FTD
'99 SCCA 3x "A" stock winner

Well, I'm not Warren, but I have suspicions about these air boxes. I can't imagine air does not cavitate, and this may explain the subtle surging I am experiencing and may impact idle hunt problems. Dirty air flow sensor plates can cause roughness, so it makes sense that oil in this area might cause missing and stuttering. I'll try it myself.

But again, my usual method is to first make sure the ignition system is perfect.

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'83 SC