HI guys, i am trying to properly jet my holley. I followed Lars' paper on how to tune a holley, and having trouble reading plugs. i have a 750 mech. double pumper with stock 71 primary and 81 secondaries. I have been running the car about 2 miles and got it up to 60 for my spark plug check after i put in new ones. Just to see what rich plugs looked like, i put in 79s in the front and compared them to the 71s which i suspected to be lean. They dont look too different but i know 79 must be rich. Ill add some pics of plugs with 71, 73, and 79 jets- let me know what you guys think. thanks

 

You are following the correct procedure up to a certain point. You need to take the car to an open stretch or drag strip and put new plugs in and stand on it at WOT to 160mph not 60mph Then you will notice some differences

 

I am trying to jet the primarys right now- lars said moderate acceleration and take it around the block...wouldnt WOT show me secondaries? Thats not what im trying to get right now.

 

79s

http://i955.photobucket.com/albums/a...s/DSC08829.jpg

73s

http://i955.photobucket.com/albums/a...s/DSC08827.jpg

71s

http://i955.photobucket.com/albums/a...s/DSC08817.jpg

 

Do what Lars says

 

I think you might have an old obsolete paper if I'm recommending "reading the plugs." A few points of interest:

As you know, I do quite a bit of dyno testing and tuning on engines that I build, so I have access to quite a bit of real test data and tuning observations. With modern pump gas, there is no practical way of "reading the plugs." The modern fuel blends will not run light grey/white at optimum A/F ratio: They run very dark. I've had engines on the dyno running pump gas at the perfect A/F ratio at WOT for multiple high-rpm pulls, and the plugs will look dark grey or lightly black sooted, with the exhaust ports completely black. When I change over to race gas with the same carb jetting, the plugs will go light grey and the exhaust ports turn white.

I don't believe there is any practical way of "reading the plugs" to optimize A/F ratio with today's pump gas. You really need a wideband A/F gauge to do it right.

Lars

 

Thanks for the clear up lars, i think i found it in an old thread...What about the power valve selection? in the paper i read it said pick about 2 inches below the vacuum reading in drive? My reading in drive was about 13inches and theres only a 6.5 inch in there now. I've also heard alot of people say cut that number in half which would give me a 6.5. Do you still go with this method of choosing 2 below that would give me like a 10.5? thanks

 

I'm with Motorhead, you can't change the jets and expect to see a huge difference in AFR at cruise. At cruise it's more on the idle mixture screws the IFRs and the IABs.

If this is a serious motor, buy an LM2 or have a shop set your carb up on a dyno.

If it's not. Tune the idle mixture screws with a vacuum guage and leave the stock jetting alone unless you have problems with it.

 

Don't screw with the PV unless you're having problems with it. I went to a 4.5 because I only have 7" of idle vacuum. After scratching my head for a year without a wideband, I eventually bought a wideband and went back to the stock 6.5. Good bye lean stumble.

 

The PV selection is part of your finesse tuning. As long as the PV has a rating that is keeping it closed at cruise, it will have no effect on cruise mixture. As enkeivette states, even the primary jetting has very little effect on cruise mixture: at cruise, the carb runs primarily on the transition circuit, which is controlled by the IFR and the low speed air bleeds.

I state to run the PV at about 2" below the vacuum reading obtained at idle in "drive," whereas many people (including Holley) state to take the reading in "drive" and divide by 2. I have found that many performance engines need the power enrichment to come in a little earlier to avoid a lean tip-in stumble, as enkeivette states. Having the PV open a little sooner will not cause a performance problem, other than some black smoke if it comes in too early. Having it open late will cause a lean stumble, just as enkeivette experienced. Once you have it opening early enough, you can always drop the number down and find the opening point that your engine/car combo needs to avoid the stumble going into the power circuit - that's part of the tuning process.

For your application, leave the 6.5 in there unless you're having a lean stumble problem going to WOT. Then, get a wideband A/F setup to tune the transition circuit, which is what your car is using at cruise.

Lars

 

I have recently put a BG mighty demon 650 mech 2nd on my 302 with a lunati 540 lift cam and afr 195s heads.Honestly I have been having hell with it.My motor goes to poping when rev to about 4500 and more.Also if I just hold the throttle about 3-3500 for constant it does the same.I have a gear to gear timing set up and a hei conversion distributor with a 6 AL box.Guys I have kids and am a mechanic in Austin struggling with cost of living and very limited funds.My motor has been built by me and ebay.I am getting very discouraged and need some help with this.Checked valve springs and timing just dang carb is kicking my butt.Thought demon was awesome when I was 25 all ever heard about now I am almost 50 and wanna throw out the door.Please give me some suggestions for a poor guy trying to have some fun and take my kids to the track.Thanks everyone for everything I have read so far

 

Not sure what you're running the 750 on....but you'll get it right. It's about one of the most universal carbs out there.

I've got a 50 year old one on a mild 302" Mustang. I rebuilt it and started with 69 primaries and the 6.5" PV. I'm at 67's now which pull fantastic and clean on plugs...but I installed an 8.5 PV which almost took 100% care of the light tip in under part throttle. I'm going to try a 10.5 and see how it does...there's still a hair of a tip in issue...but it's very close.

This same carb has been on everything from this mild 302...through 327's, 350's, 383's, 400's, 427, 454's and probably some others with just minor changes.


JIM.

 

Welcome to the Forum, but this is a dead 12 year old thread that doesn't have anything to do with your 650 Demon carb.You will get much better response if you start a new thread topic defining your problem in the title and submitting it as a new thread.