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Ziegelsteinfaust · 08-03-2022, 12:20 AM

I recently put a street Demon 750 mechanical secondary carb on my engine, and have been contrasting it to the Holley 750 VS that was on it previously.

The engine is a 1997 LT1 350 with 1994 vintage AFR 195lt heads , gmpp 4 barrel intake, Pertronix equipped distributor, 1 3/4 long tube headers, 10.7-1 compression, and a Comp 306 cam for the LT1. It is in my 1973 rcsb C10 with 4.88 gears, th700r4, and 31" tires or 285/65r17's.

The Holley starts somewhat easier, but the Demon does better once started. Due to 4 corner idle screws.

Both idle at 10-13" of vacuum. Although the Demon handles idle load slightly better in gear.

The Demon averages better in the city, but loses 4-6 mpg on the highway??? This has me lost. Since the Demon tracks at 13.5-14 afr at a steady cruise of 75 mph at 3000 rpm. With 15-18 inches of vacuum. The Holley tracked at 13-14.5 afr with 14-15 inches of vacuum. Demon highway mpg 13-15, and the Holley typically tracked 18-20.

If I hit a hill my vacuum goes to 10-12 inches, and my afr to 15-16. I went up the Cajon pass twice today to test again. As I made changes since my vacation to Bishop. I put my 9.5 power valve in which is the highest I had. If it opens up I go pig rich or 11.5 afr. The Holley with nearly the same jet sizes didn't go as lean, but would lean out some. I plan to put the Holley on tomorrow, and take the 14 fwy to see family Thursday.

Neither combo pings audibly or looses power on the hills. Timing has been the same 32* total from 12* initial. With 8* available on the vacuum canister.

My thoughts are my combo is to much for a carb to adapt to between the lopey cam that makes 50% more vacuum at speed vs idle combined with the 3000rpm everywhere I drive. I hit the mechanical limits of the toys I have.

Or does anyone have anymore options I should try like getting a 10.5 power valve, and taking a jet or two out of the primaries?

08-03-2022, 12:20 AM	#1
Ziegelsteinfaust Registered User Join Date: Dec 2012 Location: Temple City Posts: 3,561	My tuning skills have hit a wall. I recently put a street Demon 750 mechanical secondary carb on my engine, and have been contrasting it to the Holley 750 VS that was on it previously. The engine is a 1997 LT1 350 with 1994 vintage AFR 195lt heads , gmpp 4 barrel intake, Pertronix equipped distributor, 1 3/4 long tube headers, 10.7-1 compression, and a Comp 306 cam for the LT1. It is in my 1973 rcsb C10 with 4.88 gears, th700r4, and 31" tires or 285/65r17's. The Holley starts somewhat easier, but the Demon does better once started. Due to 4 corner idle screws. Both idle at 10-13" of vacuum. Although the Demon handles idle load slightly better in gear. The Demon averages better in the city, but loses 4-6 mpg on the highway??? This has me lost. Since the Demon tracks at 13.5-14 afr at a steady cruise of 75 mph at 3000 rpm. With 15-18 inches of vacuum. The Holley tracked at 13-14.5 afr with 14-15 inches of vacuum. Demon highway mpg 13-15, and the Holley typically tracked 18-20. If I hit a hill my vacuum goes to 10-12 inches, and my afr to 15-16. I went up the Cajon pass twice today to test again. As I made changes since my vacation to Bishop. I put my 9.5 power valve in which is the highest I had. If it opens up I go pig rich or 11.5 afr. The Holley with nearly the same jet sizes didn't go as lean, but would lean out some. I plan to put the Holley on tomorrow, and take the 14 fwy to see family Thursday. Neither combo pings audibly or looses power on the hills. Timing has been the same 32* total from 12* initial. With 8* available on the vacuum canister. My thoughts are my combo is to much for a carb to adapt to between the lopey cam that makes 50% more vacuum at speed vs idle combined with the 3000rpm everywhere I drive. I hit the mechanical limits of the toys I have. Or does anyone have anymore options I should try like getting a 10.5 power valve, and taking a jet or two out of the primaries?