looking for some better mpg/setup advice needed

[mike mc]

first, a little info 1999 single cab s10 3600lb.
1985 West coast engines stage 1 350 bored .30 over, stock pistons,  stock heads,  204/214-272-284 melling cam .420 lift 112 separation
edelbrock rpm manifold,  hei with full upgrades,15 initial,19 mechanical, 16 vacuum.  short kit headers, 2.25" to flowmaster scavenger y, 3"out to dynomax bullet.  patc 700r4 trans 1800 lockup stall converter. 4.10 gears.295-50-15 tires 27" tall.
the carb is a 17088041 800 cfm. this carb is an oddball, it is almost identical to a 17085213. I think the jets are .76, the primary rods are marked 61s, I am not sure what size they really are, I need to open it back up the apt is set at 1.25 off bottom it has a j hanger and EK secondary rods the power piston spring is stock, probably old. the piston changes position when the vacuum drops to 7 hg. I am getting 15 to 16 mpg but I could use a little better mpg. considering I drive 80 miles a day. any rod jet hanger or spring recommendation would be appreciated.

[Cliff Ruggles]

Sounds like a big block Chevy motorhome carb.  They used a very generous fuel curve, most had 75 main jets teamed up with full tapered .060" primary metering rods with .036" tips.

Use the APT system to fine tune part throttle A/F for best fuel economy, and 73 or 74 main jets would be PLENTY for what you are doing.....Cliff

[mike mc]

I also forgot to mention the idle air screws are turned out 5.25 turns. I think I need to drill them out to .086. also do you thing the power piston spring is allowing the piston to move at the right hg of vacuum. I also run a wideband o2. cruise is 13.5, at full throttle it goes to 10.0.  also at cruise my vacuum is 14 to 16 hg.

[429bbf]

what does that have for compression . im thinking 8.0 someone correct me if im wrong. i have gotten 4mpg better on a bbford by raising the comp. to appox 9.5 . i dont think you can burn the gas properly. im shooting for 10.2 on the next one will see how that does. should know in a few months . i run all qjets on my ford and get excellent milage and power. fwiw

[mike mc]

it is probably 8.0, I know I would get better mpg if I would swap for a set of vortec heads and d dish pistons. but right now the motor is still under warranty, and I still want to dial the carb in as close as I can get it.

[Cliff Ruggles]

Head castings play a HUGE role in efficiency.  There were quite a few offerings for SBC's over the years of production.  Many are "door stops" having crappy combustion chambers and very restrictive ports.

A good set of "modern" heads and higher compression will certainly help, as will tight quench. Most engine builds are WAY too low for compression with WAY too much quench in them, so they are turds for power production and suck tons of fuel for "normal" driving.

The RPM intake and RV cam bring nothing to the table for mild engine builds and stock compression.  Unless the block was decked/squared, tight quench, etc, it's probably less than 8 to 1 compression if 76 cc heads were used on it.

Anyhow, for the carb, the PP will be down for all "normal" driving with high vacuum, so use the APT system to fine tune part throttle A/F for best efficiency/fuel economy.  The amount of timing added by the vacuum unit is a big player here as well, and low compression engines require more timing or at least like a lot of timing to be most efficient.

For sure I would make the APT accessible without taking the top off the carb, and verify the hanger arms are level and exactly even.  Smaller main jets for the low compression 350, and put one of our kits in the carb to get it up to par for this new fuel......Cliff

[mike mc]

I don't know specifically what head is on the motor but west coast engines stated the comp rate was 9 to 1. hp dynoed at 310 at the crank. 360 torque. I have a feeling the primary hanger arms may be bent. the reason I say that is I adjusted the apt all the way down which should have leaned the mix out . it did not. I would think with a .75 jet and 61s rod you should be able to lean the mix out so much that it shouldn't run. I am at a point where if the temp outside is cool "60 degrees" it will scream with no bogs. and get around 15 mpg. if it is hot "95 +" it usually will bog late in the acceleration, right before it goes into the next gear. I would think that would indicate running a little rich.

[Cliff Ruggles]

I agree, something amiss if you can't lean it out with the APT system using those parts.

At least 95 percent of the carbs we get in here for rebuilding have bent hanger arms for the primary metering rods.  It's AMAZING that if anyone got to the carb in it's life for rebuilding, that they bent the arms taking it apart or putting it back together, but it happens almost every time......Cliff