New motor stumble from idle/WOT below 3K RPM

[marshall_mosty]

Guys,
I've put together a new motor and am having issues with a off idle stumble/miss.

It will idle fine, but when you matt the gas in neutral it will hesitate and then "catch up". Same goes with throttle application in gear.

I go for a dyno tune on Friday, but want to know if I should hold off until I can correct this, or if it's something they can "tune out". Here is what is new:

new bottom end (331, 10.5:1 Forged Pistons, Zero Balance)
injectors (24lb to 30lb)
MAF (75mm-24lb calibration to 80mm-30lb calibration)
T-body (65mm to 75mm)
Bigger air filter for the new MAF
New plugs, cap, rotor
New fuel filter

Timing is at 14-degrees BTDC
Fuel pressure is "in range" at approx 40psi with vacuum line pulled. Anything over 35psi is "good enough". I've run as high as 50psi on smaller injectors.

The previous combo had a "no name" chip in the computer that I pulled since I'm having a chip burned to drop the power levels.

Once the car is over 3K, it runs like a scalded ape. I'm just concerned with throttle response, in general.

All the cylinders are firing, but #4 (passenger rear) and #5 (driver's front) are "colder" than the others. I don't know if this is a Ford thing, or part of the problem...

Ideas?

[GlennCMC70]

Move the #4 and #5 injectors to see if the "cooler" temps move w/ them. If so, likely time for new injectors or have them cleaned.

[Casey_SS]

It sounds like you've made some significant changes (injector size, MAF, & TB) which probably require PCM adjustments but don't have the new chip yet? Is that right? Are you having a custom chip burned at the dyno or are you doing the mail order thing?

[marshall_mosty]

Glenn,
The injectors are Ford Motorsport units which are brand new out of the box. I've already swapped them around once and the cooler temps remain.

Casey,
The chip will be dyno tuned. No mail order "stuff". I was "scary fast" over 3K when it cleaned up, so I'm expecting some significant tweaking will be required.

[edrock96GT]

Maybe do a compression check. I had a bent valve give me similar symptoms. It stumbled down low, but cleaned up around 3k. Mine idled like crap though which yours isn't doing, but still something easy to check.

[Fbody383]

All the cylinders are firing, but #4 (passenger rear) and #5 (driver's front) are "colder" than the others. I don't know if this is a Ford thing, or part of the problem... Is that just an effect of scavenging efficiencies of the manifold? Just reminds me of either a Roush or Saleen article about individual cylinder CRs to try and optimize some of that. Would be interesting to know what you find.

[cjlmlml]

try putting in smaller jets..

[marshall_mosty]

Can I have your left over ones to see if I can cram them in the injectors??

I called the dyno shop and they aren't concerned. They said "bring it"... Appointment is at 11am tomorrow morning, so I should be good by early afternoon.

[way2neary]

Marsall,

I suspect the issue ralates to your MAF calibration being out of sync with your new chip. Only way to fix that is on a dyno and watch the AFR.

Good luck tomorrow!

[marshall_mosty]

I will let you guys know how it goes. I'm going to get a "raw" baseline and then have them flatten out the curves to my AI numbers.

[marshall_mosty]

Had a good day at the dyno. After 10 pulls trying to figure out why the MAF voltage was pegging at idle ended up being a bad jumper harness from Granatelli... Oh well. Fixed that and we were off and running. Ended up tuning to 381/395 before checking to see if SAE Smoothing 5 was selected. Of course, they are running their dyno on STD so it looks better for their customers. As soon as it was "reset", that number dropped to 347/359. We started pulling timing from the high points and ended up at 316/335 for my cert pulls. Water temps were 180/210/210, so it was representative of what I'm going to run on track. Also, it was pull #22-24 for the cert, so the motor and driveline were WELL warmed up.

So, my new minimum weight is 3015. I'm expecting 2 45lb elevator plates to be added for ECR.

[Casey_SS]

Great results Marshall! Glad to hear you got the stumbling issue sorted out...gotta love that Granatelli quality ;) If it's any consolation, they made and heavily marketed F-body MAFs back in the day which were notoriously problematic as well...I always thought they just hated Chevy's and were trying to help out the Mustang street racing crowd. Guess they're more of an equal-opportunity bunch than I gave 'em credit for.

Mmmm....3015 race weight. I bet that car is just a hoot to drive....

[RichardP]

a bad jumper harness from Granatelli...


Hello!?! If you bought Granatelli parts you deserved to have problems. You don't have any other Granatelli parts on there, do you? Their suspension parts aren't as strong as the tie rod ends you made...

Glad you got it worked out, though. Looking forward to seeing it on track. :-)


Richard P.

[way2neary]

Marshall,

Glad to see you're up and running! Jeesh, it would be fun to have two "tunes" so you can utilize all that HP on open track days! Who-hoo!

All kidding aside, looking forward to seeing you back on track. My car ran well last weekend. Just had one weird rocker arm failure, but I've got a whole box of spares now!

[michaelmosty]

You don't have any other Granatelli parts on there, do you? Their suspension parts aren't as strong as the tie rod ends you made...


Richard P.
Hahahaha, that is the quote of the year!!!!!

[marshall_mosty]

All my MM parts are badged Granatelli... :)
The new tierods are never going to die... 4130N and the same diameter as MM aluminum ones...