Secondary jet sizing

[rblong]

How do you tell when you have large enough jets in your secondary without going back and forth to the dyno? When I had mine dynoed a while back they said I was running lean at WOL. I upsized the pri and sec from 76/83 to 79/86 and found it too rich at low end so I had to go back to the 76 on pri, but left the 86s in on sec. Low end now back great but I feel I could feed more gas on the sec side - it just acts like it could use more. I have also changed my pipes to less restrictive. Any good tricks?

I have Street Demon 750, fairly stock 428 (I think it has a one step up cam)with edelbrock performer intake.

Bruce

[Tongue Pirate]

YOu would have to hook it up to some meters to get an accurate reading but as a rule of thumb I believe that your Carb should come stock with jets that match the maximum flow of your carburetor. Meaning at maximum flow the engine should be pulling as much fuel as the venturiis will give.

Someone correct me if I'm wrong here, it's been a while

[Hotfingrs]

You can write a book on this subject. Basically you're on the idle circuit until 3,000 rpm. If you were lean under 3000 then open up the idle screws a turn. There should 6 jet sizes between the pri and sec. I recently bought an LM-1 air/fuel meter and have been playing with this. I was lean until 3000 rpm, and I mean 22-1 lean, and as soon as I got on the main circuit I was in the 13-1 range. I talked to Don Gould at 4secondsflat and he said to open up the idle screws. I did this and got the A/F I was looking for and problem solved. IF you're not using an A/F meter everything is just guess work. I had tuned my idle for max vacuum, which was the old school way, and ended up waaaaaay to lean.

[2KWIK4U]

Hey Jack,

How bout some details on the a/f meter. I'm in the market for one. Main problem is that I don't have a bung in my headers or sidepipes and can't locate a meter that doesn't need one, if one even exists. Thanks

Brad W.

[Hotfingrs]

Brad,
You're in luck, cause you can buy the optional tailpipe clamp, which I did, and just hook it on the end of the pipe. Also good for using it on different cars. You can monitor up to 6 different events with the optional aux box. Right now I'm monitoring A/F Rpm and Egt.

You can read all about the LM-1 here http://www.innovatemotorsports.com/

I bought mine from Wayne Presley at http://www.verycoolparts.com/

I got the LM-1, the aux box and the tailpipe clamp. This has to be the best spent money in while on a tool for me.

[Dennis Kelley]

You can run a jet spilt of up to 10. BG carb came delivered with an an 8 step from primary to secondary. 650 dp bg comes with 70 primary and 78 secondary. BG said not to go more that 10 jet sizes different from primary to secondary. I run 72 primary and 81 secondary with no problems. I think that the 750 comes with 72 primary and 80 secondary. Put more jet in it on the secondary side. Just don't go over the 10 jet split from primary to secondary. Take it to a dyno with a wide band or do what the other have suggested and see where it is at WOT and jet accordingly.

I think the 6 jet split is for Holley carbs. My air fuel runs very good with this jet split. Just my opinion

Good luck

later dennis

[Hotfingrs]

My Mighty Demon came with a 6 jet split.....

[rblong]

Thanks everyone. Sounds like I am at a point I need to hit the dyno, or find a fuel/air ratio meter.

Bruce

[Dennis Kelley]

I checked the video I received with my carb on the speed demon and it said up to 11 jet split. Stock jetting for the 750 demon is 76 primary and 83 secondary. I don't know about the mighty demon. One Full throttle run on the dyno with another backup run with the jetting shouldn't take or cost that much. try and 87 andd see if it gets better.

later

dennis .

[rblong]

Yea, dyno run about $65 around here, so off I go!!