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ken47 09-14-2006 03:38 PM

Poor gas mileage problem
 
I am running a 267 SBC in my new T bucket. It's a stock engine from an 81 Camaro. I'm using a new Holley 390 CFM carb (down jetted 2 sizes for 3000 ft altitude here) on a weiand 8007 dual plane manifod with a square/spreadbore carb adapter, 180 deg F thermostat, a TH-350 transmission with 2.79 rear end and 295R14 tires. I am using a stock points type distibutor (don't know the year) with a points elimination kit in it. I am using a flame thrower coil (40K volts with no balast resistance). My problem is that the gas mileage is really poor. It seems worse at low speed city driving. I thought the gas mileage would be very good with ths set up. Any suggestions as to where I should look to improve things.

poncho62 09-14-2006 03:50 PM

I think that the engine is operating out of its optimum RPM range....in other words, it is being lugged too much.

I bet a set of 3.23s or so would get better mileage and would be a lot peppier too.

JeffB 03-19-2007 12:02 AM

Carb Tune?
 
You might want to run a vacuum check,and look at a power valve change dropping to a lower size may help,a lot of times guys will change jets and get zero results.

red65mustang 03-19-2007 06:56 AM

1 Attachment(s)
I always start and do step#1..."stock engine out of a 81 Camero"= do a compression test

my step #2 is mount a manifold vacuum source vac guage in the car, test drive it to confirm approx 25Hg plus on a mild downshift with the motor warm = valves and rings are sealing

is it your size 12 foot or the motor causing the lousy mpg....vac guage will tell you while driving:
just to illustrate....
lead foot "drop the hammer" = guage will show 10Hg or less=carb is full rich (A/F 13:1) and vac advance timing is close to none and mech timing is only 24* or so
"squeeze" the pedal gradually = 15Hg-18Hg= carb is optimum A/F (15:1) ratio and vac advance is full on adding 14* or so to the 24*

squeeze the pedal and Hg still drops to 10....it's the motor

(mounted on the dash like mine...it is a "mpg police"= stay in the green zone or else!)


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