RvFNG76 wrote:
Since we are on the topic of carbs. Ive been having mine hiccup (backfiire) every once in awhile usually during hard acceleration it only happens when i first get into the throttle it will pop once then never again any takers on why. Another question what "performance" upgrades has anyone done to maybe boost ponies or fuel economy?
1. Distributor timing.
2. Vacuum advance.
2.a. Worn vacuum advance springs in distributor.
2.b. Leaky, loose, or plugged vacuum advance hose.
2.c. Other vacuum leaks.
3. Pre-ignition. (Carbon build-up in cylinders.)
(This assumes it's back-firing through the carb ... not in the exhaust system.)
An old-school technique for reducing carbon build-up in cylinders is to dribble water into the carburetor when the engine is warmed up. In the hot cylinders, the water flashes into steam, knocking the carbon loose. (Just enough water to almost stall the engine but no more.)
Economy: Not sure about the big-blocks but advancing the timing a few degrees on older small-blocks results in a more complete burn.
If it starts knocking, especially on hard acceleration, you've gone too far.
My '77 B200 van, with 318 cu. in. engine, got 18-20 mpg ... if I behaved myself. (This was in the mountainous region in south-central Alaska.) Part of it was advancing the timing. The rest was driving style.
I've said it before -- you're driving a big truck, not a sports-car. Smooth and easy on the throttle makes a lot of difference.
If you have a four-barrel carb, try to stay out of the secondaries.
A four-barrel has a greater potential for fuel economy than a two-barrel, provided you avoid using the secondaries unless necessary. The primaries are usually smaller than the venturis in a two-barrel.
My '77 B200 had a Carter BBD two-barrel ... I might have been able to boost economy a bit more if I'd swapped to a four-barrel. However, it wouldn't have been enough better to justify the expense.
Our '69/70 Explorer Class A, with a 318, got 14-16 mpg ... when I behaved myself. Under-powered? No. It routinely tackled Turnagain Pass without any difficulties. I'd pick up speed (to 70-75 mph) before starting the climb and then hold steady up the long steep grade. I wouldn't have to downshift until the last quarter mile before the crest. I'd be down to 30-40 mph when I reached the crest.
On the other hand, I'd be passing small cars and light trucks before I was halfway up the grade and they were having to downshift well before me ... usually because they were forcing the throttle.
So, to repeat ... driving style makes the greater difference when it comes to fuel economy.
