fuel mileage

[muleystruck]

Hello, all. I have an 86 Scottsdale with a 4.3 v6 and the my6 trans. The po had replaced the original carb, Im assuming a qjet. with an Edelbrock. I'm not impressed with the carb as fuel mileage is bad. I know  these trucks aren't the best for mpg but only 10 mpg isn't cutting it. I don't drive the truck hard as I'm trying to get the best mpg I can. I will admit I'm not very experienced adjusting carbs. I didn't get anything with the new edelbrock (info,manual,) so I borrowed a book from my bro. He has a v8 Vega with the Edelbrock. What carb was actually on this truck from the factory? I know it has 3 73 ratio rear gear. This truck is very clean, only 2 small rust spots(cab corners).Appears it was well taken care of by the original owner. After he passed on other members of his family used the truck and didn't respect it. I appreciate any info from yall. Thanks   Muley

[Irish_Alley]

what size carb?

[hatzie]

It should have a Rochester M4MED quadrajet.   The best I was ever able to wring out of a 4.3 in an 86 was 13mpg.  The 305 in my 85 got 15mpg.  The 4.3 doesn't have enough grunt to take advantage of the overdrive in the MY6.

[Stewart G Griffin]

i think the key to all this ( i have a 4.3 too) is to find out what size the idle tubes, main jets and metering rods were that originally came with the carb on this engine.  i am going to try to get the bottom of it after i get the engine in. 

So, i don't think the problem is quadrajet or even edlebrock.   The main issue i think, is calibration.

The cfm for this application may have even been 800cfm (pretty sure, not 100% positive though).  This particular carb for an 86 4.3 had the dual capacity accelerator pump and unless the truck was originally a california truck, the carb should be otherwise non-computerized.  i'm pretty sure that if you leave the dual pump unplugged, the carb will just act like a normal single accel pump.

Right now, i am using a jeg's reman quadrajet # 15805.  This has .073 main jets and -50M metering rods.  The best mpg i could get was 16.59, but  the engine was hosed from the beginning.

Every now and then i see NOS GM 4.3 carbs on ebay.

i am not familiar with the MY6 trans.  What is the top ratio?

[zieg85]

MY6 is the OD trans with .73 as high

[Rattler12]

Wouldn't an 800 cfm carb be about twice as much as would be needed for a 262 cu in 6 ?  I'm running a 750 on a 454....?

[Stewart G Griffin]

Wouldn't an 800 cfm carb be about twice as much as would be needed for a 262 cu in 6 ?  I'm running a 750 on a 454....?

You would think so, but i think all 85-86 trucks got the 800cfm version irregardless of engine size-----maybe to save money?