i'm about to replace a 750 Holley vac. sec. with same from Barry Grant. The factory jets are 5 sizes larger than what I am using now.
74 vs 79. Anyone know if it will need that much bigger jetting? I hate to start rich with a new motor but tech says to do it.

400c.i./auto

 

Don't compare jet sizes with different carbs. Holley made numerous 750 carbs and they all came with different jet sizes. All factory carbs are jetted for sea level. To start in a ballpark range you should go down 1 jet size for every 2000 feet in elevation. After that you adjust the jets by reading the plugs. Each engine demands a different jet setting so just because someone else may have 79/84 jets in their 750 doesn't mean that's what your engine will need.

My 383 had an 850 on it and ran best with 87 primary and 95 secondary.

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Stephen's racing page

87 IROC-Z Pro ET Bracket Race Car
383 stroker (carbed) with double hump cast iron heads and pump gas
454 Big Block buildup now in progress for the 2001 racing season

Best results before the 383 blew up
Best ET on a time slip: 11.857 altitude corrected to 11.163
Best MPH on a time slip: 117.87 altitude corrected to 126.10
Altitude corrected rear wheel HP based on power to weight ratio: 476.5
Best 60 foot: 1.662

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!
Member of the Calgary Drag Racing Association

87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley

 

if you have a regular Holley 750 on your motor now and you are dead set on useing a Demon carb then go with one that is smaller. The Demon carbs are actually larger than they are rated. I personaly have not had any good luck with a Demon carb. We put one on a 11.1 454 that was pushing 585hp with an out of the box Holley annular discharge 850 double pumper. We then put the 850 Demon on it and we could not get it to idle at all. Above idle it would run good but it just would not idle. We called Bary Grant and they told us that it needed some air bleeds and we should ship them the carb back so they could install them (for and extra charge of course) and also informed us that the 850 flowed closer to 910cfm. Then he informed us that we should go with the 750 carb. After this we talked to a few other people that had Demon carbs and they had the same problems. We then boxed up the Demon and sent it back and but the Holley 850 back on and life was good again.

 

Thanks for the input guys. I'll repost after I put it on to tell you how it worked.