high altitude carb jetting mystery

gumba · 07-30-2012, 10:28 AM

Short background, 3.4 twin plug, 9.5 comp, 50 pmo's, Electromotive hpv-1 crankfire, 1 3/4" o.d. European Headers, custom muffler incorporating the cookie cutter straight pipes. Was running Bosch ws3, switched to NGK 7iex. Motor build by J.W. and raced in NorCal, moved to Santa Fe, NM (7,000 ft.) now a street car.

The problem is the plugs are black and the motor starts missing after about 1/2 hr. drive. I switched from the Bosch to the NGK's, and dropped from 60 idle to 55 idle. No difference. The other issue is the carbs don't respond to the idle screw adjustment either with the 55 or 60 idle jets. I have swapped out everything on the ignition side, crank sensor, coil packs, checked the wires, different plugs. I run dual a/f gauges showing around 13:1 idle, 12-12.5:1 under acceleration.

I run a tamer version in my bus with no issues. Stock 3.2, 46 pmo's, Electromotive hpv-1, SSI's, and sport muffler. That motor actually shows running richer than the 3.4.

Any old school high altitude mechanics around. Any suggestions?

2.70Racer · 07-30-2012, 01:32 PM

Welcome to the the land of thin air.
Russ Kelso is the Porsche guru in these parts. He is a racer and owns Performance Motor Car Investments in Corrales. Tel 505-898-6487.
He is the man. He helped me alot with my Webers on my 2.7 race car.
You didn't say what size venturies or main jets.
The NGK 7's may be a little cold, I ran NGK 6's in a similar setup.

gumba · 07-30-2012, 02:35 PM

Thanks. 180 mains, 43 venturies, 200 a/c ,130 idle air, F2 emulsion tubes. Richard at PMO suggested ngk's. I'm looking for suggestions on a few things to try. Does he have a chassis dyno?

2.70Racer · 07-30-2012, 09:14 PM

There is a dyno on Eubank just south of I-40. Folks take a handful of jets and spend a couple hours. Russ has been at this for a while, so he has a pretty good idea where to start and not waste alot of time. Give him a call, or better yet get an appointment to meet him at his shop. You will not be disappointed.

T77911S · 07-31-2012, 04:20 AM

no carb expert by any means, but from reading and looking back from when i did have carbs, if the idle stop screw is not adjusted correctly, the progression circuit can become active at idle making the idle mixture adjustment have no effect.

here is a great web site. this guy is on pelican, but i cant remember his name.

Weber Carbs Technical

gumba · 07-31-2012, 06:13 AM

The motor was rebuild and set up on a engine dyno by J.W. ,and ran like a top at sea level. A few d.e. events and some street driving. The problem I have now I believe is caused by the elevation change. Why this motor seems to be effected so much more than my 3.2 w/46 pmo's is the mystery. From what I've read going down in main jet and/or idle jet one size should put me pretty close. The fact that the motor doesn't respond to the idle screw adjustment using either 55's or 60's is what's confusing (torturing) me.

HawgRyder · HawgRyder's Garage

This is why the factory tried the fuel injection route.
The barometric sensor changes the fuel/air ratio as you go up and down the mountains.
Bob

T77911S · 07-31-2012, 09:15 AM

i rememeber having the same issue with the mixture setting. sometimes it was just dirt, but other times i think i had the throttle stops adjusted for too much idle.....looking back on it now.

gumba · 07-31-2012, 09:19 AM

The next owner can switch to efi, in the mean time I need to deal with this. Never thought I'd be moving to 7,000 ft. when we built the motor.

KTL · 09:38 AM

You've already got large main venturis and very large air correctors that are providing a lot of air to achieve a lean condition. I'd try going down on the main jets to see where that gets you.

I agree it's puzzling that the idle jet change didn't do anything for you. I had a super lean condition and lots of popping at idle one time and my switch from 55 to 60 solved it.

Also consider your e-tubes a place to change. If you go with an e-tube that has more holes in it, that will lean the mixture. I have a set of F24 e-tubes that have a bunch of holes in them and my reading material and internet info have said these tubes are for very large main jets. Makes sense- more holes = more air entrainment = balances out the large main jets?

Don't rule out your fuel too. How good is the fuel you've got avaiable to you in your area?

I think your idle is a bit too rich IMO. Best lean idle adjustment process typically puts you in the lean range (~15) at idle. But obviously its hard to fix that when your adjustments on the idle mix screws are having no effect.....