[Dave C]

Hi all,
Need some help diagnosing a misfire. Went to test n tune today. The car has a skip that develops at the top of 1st and 2nd gear (powerslide). It runs fine during the burnout (6500 rpm). I leave the line and it leaves sorta lukewarm, but not missing. 60 ft fell off .05-.06 from normal. Then at about 6300 or so it starts to skip. Shifted at 6700 (still missing), ran ok until it hits 62-6300 in 2nd, where the skip started again then I shut it off.
It idles and drives arounf the pits/return road fine.

I don't think it is fuel related. No changes to the fuel system since last year and it has always been enough. It's just 1/8 mile so I don't need a huge amount of volume. It has a Holley black pump and -08 line front to rear.

I went to a tnt a few weeks ago and it did the same thing. Only got to make one pass then. Starter went bad. I thought then maybe it was jetted too lean. That day it was low 50's. Jetted 84/85 no pv's Holley 950HP Today was 75°, I still jetted up 1 size over the 50's temp. It ran fine last fall with the 84/85's in 70° weather. Even after jetting up a little and much warmer weather it still had the skip, so I think it's not too lean.

I'm thinking ignition. Setup is an MSD7AL2, MSD dist with locked timing (have tried 36° and 38°) MSD 8.8 Super Cond wires (4yrs old), MSD Blaster 3 coil, and Autolite AR135 racing plugs. The plugs were left from last year. Have 75 or so passes on them.

The only engine related change that has been made to the car over the winter was locking the timing out. It used to have 18° initial and 39° total. I locked the timing in the dist, but made no other changes. It does idle better and has a little better throttle response off idle (that doesn't matter as I leave of a two step brake.

Today I checked the plugs and they looked all the same, also replaced the MSD chips to make sure I didn't have a bad one. Only made two passes, didn't want to hurt it. Valve lash is also ok.

For those who don't know my combo: 383W stroker, Windsor Sr iron heads, solid roller cam, 13:1 comp, all the usual bolt ons. In my 69 Mustang @3025 lbs with a powerglide/5400 stall converter/5.43 gear/ 32" tall tires it normally runs 6.8x's @ 100 in the 1/8 mile.

Any input would be appreciated. Going to start with new plugs and wires this week.

Thanks,

David Cole

 

[67stang]

A high rpm miss like that always makes me think plug wires first. Let us know after you change the wires and plugs how it does. I'm assuming you've verified all plug connections are good. One other thing: check out your cap. Cracks have been known to hide themselves well and drive people crazy (i.e.:me) trying to run down something like this.

 

[Dave C]

I think I might have found it. Possible fuel related

The needle and seat on the secondary bowl doesn't seem to move as smoothly as the one on the front. I think it may be sticking and not allowing full fuel flow at wide open. I'll pick some new needle/seats up this week. For good measure I'm still going to be replacing the plugs and wires. Next test n tune is Fri. night.

Thanks for the help, I'll repost after I test it and see it if helps.

Later,

David Cole

 

[Hottarod]

When mine has done that it was related to timing and fuel octane. I changed to a 50/50 mix of pump and 112 which raised my octane a little. AFterward it didn't run too well until I increased my total timing. I guess the higher octane fuel burns longer in the chamber so it needs a bit more timing at full rev's.

Another situation related to this is if I run too much total timing with the low octane fuel. That will make mine bang out at the top of first and second just like its leaned out.

 

[Dave C]

I was running straight race gas. Phillips B-32. 110 octane. I normally have 39° timing. 13:1 compression. I backed it off to 36° to see if it made any diff. I didn't.

I'm hoping the new needle and seats int he carb will cure it.

Later,

David Cole

 

[Nastystang]

It kinda sounds like the problems I was haveing with mine last summer. It was running lean, jets were to small, jetted up and it went away.

Dont know if this helps, but you say you are running 84`s/85`s in your 950. I run 73`s/86`s in my 650 and it is a tad lean with the new free flowing heads.

Good luck