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?

Re: My tuning skills have hit a wall.
 

My guess are the main air bleeds are different sizes. Try matching them to the Holley and see what happens. Next time you have the power valve out check and see if there is a removable brass plug with a hole in it. This is what's used to adjust the power valves flow rate. Vacuum just sets the opening and not the actual flow of the circuit. Demons are notoriously rich.

Re: My tuning skills have hit a wall.
 

Thanks. I will look this afternoon

Re: My tuning skills have hit a wall.
 

If it's going rich under load, maybe go down on the PV number to open later. Might try a little more timing as well, like 3+ degrees. Put the Holley back on it with maybe a 6.5 PV. That cam will want a little more timing as long as your running 93 octane...

Re: My tuning skills have hit a wall.
 

It goes lean under load like climbing a hill. Due to my engines set up. I get the choice of lean or rich. Unless I buy a expensive carb, and the items to tune it. I think I need a 7.5 or 8.5 PV to keep it going safer for awhile. The 6.5 is a hair to low for my combo.

Diving deep into the weeds last night I am convinced my air bleeds on both carbs are wrong. With the Holley being less wrong on the freeway atleast. As I don't get any progressive richening I get to much progressive leaning of my afr. So I tuned my afr for 3000 rpm where I my driving rpm is.

My timing is pretty maxed out already for this engine, and 89 octane. I did extensive tuning with 91, and had little in the way of positive results. More hp/tq yes, but only slightly. Better mpg no. I used a 1/4 mile app for testing, and results were margin of error levels. The cam bleeds off enough pressure for 89 octane, and is good even in 105* weather. Hell it will run 87 good to about 75*, and about 85* when part throttle detonation starts its return. I was in Reno area with it picking up a built model B engine for my dad's friend. When the station only had 87, and I was on fumes. When I was forced to test it out. Luckily my afr was 12-13 at that elevation so I had cushion. Till I got to the next town. Further testing was at home where I could drain out the 87 if it didn't work.

The cam doesn't need more timing. It's pretty much where power roles over, and my starter has no issues. I have been tuning the truck bi-weekly for several months. Even if it was only a small very small adjustment.

Re: My tuning skills have hit a wall.
 

I read your comments, and checked. So I feel as you do it is the Air bleeds, and I am at where there is no more. Other then fine tuning a slightly more idealistic balance.

What I need to do is fuel inject the motor to where I have full range tuning. Not that can not be done with a carb, but not for that money I would be required to spend.

There's a guy in the City of Lights. Perris ca who has some parts I should just go buy. Namely a couple of manifolds

Re: My tuning skills have hit a wall.
 

Lean under a load could be a float that is too low

Re: My tuning skills have hit a wall.
 

If the air bleeds are not the screw in type you can convert them. Same with the fuel passages in the metering block. Pull the pressed in bleed out and tap them. Then you can buy cheap brass set screws and play all you like. Pin drill bits are pricey. I usually chuck the set screws in a drill and just touch the bit to it. Goes quick. Also the grease off your nose is a good lube for the bits.

Re: My tuning skills have hit a wall.
 

Upper middle of sight glasses

Re: My tuning skills have hit a wall.
 

I have decided to go to the dark side, and just go with fuel injection. Next week I am going to go get more LT1 items to help with the conversion. Since I want to drive the truck everywhere it makes more sense unfortunately. Plus tow or haul stuff around safer.

Lastly I am not much of a machinist, and with my hand injury I have to play it safe for quite awhile.

Re: My tuning skills have hit a wall.
 

I've considered the same after tuning my carb with a AFR meter. It's pretty much dialed in but I do see quite a variation in the AFR depending on load and engine speed. At WOT it's solid. I'm thinking that if the EFI system continuously adjusts the AFR then I would achieve optimum mileage and still get best power.

Re: My tuning skills have hit a wall.
 

Terry, I agree with you but technically FI goes lean and rich constantly so it never is actually perfect. If your AFRs are moving around then your air bleeds are not correct or mis sized. Sometimes they MAB and metering block package need increased as a pair. Same curve but less responsive. There are guys out there that can get a holley about as good as FI.

Re: My tuning skills have hit a wall.
 

I'm running a Edelbrock AVS2 currently. I may have mis-spoke about them moving around. As I drive thru town lightly accelerating the AFR changes. Steady hwy speed not so much. I still have a lot to learn. When I started, backing in the garage stunk. now almost nothing. I'm just under 12 at WOT, idles at low 13 with cruise high 14 to mid 15. Any leaner on the cruise and it surges really bad and stumbles on light acceleration. The engine is a vortec head 350 and the cam specs are 215/223 at .050 on a 108 LSA. I had the truck chassis dyno tuned at one time with a Holley carb on it and it made best power at just under a AFR of 12 to 1, so I tried to match that.

Re: My tuning skills have hit a wall.
 

You are right but possibly a little rich under most conditions. Probably caused by timing.

My point on the varying AFR was that the computer constantly goes rich lean to find perfect burn. It's rich as much is it is lean, constantly. Cruise at low throttle should probably be around 15:1. Rich is more combo dependant. You are on the right track.

I reread that and I think a AVS is a carter carb. I don't know the circuits on those specifically even though i do have one. Used carb that I swapped on but broke a flywheel starter ring just afterwards so very little time playing with it.