If he's seriously thinking about buying one of the Holley carbs, I'd go to Holley and get their advice. That's their job, and why they make the big bux.

In general, a smaller carb won't give significantly better gas mileage than a bigger one. In normal driving your engine might be using between 50 and 250 CFM. The amount of air the engine can take in is limited by the throttle opening. The 650CFM carb will only use more fuel than the 500CFM carb when that last 150CFM of air is going through it.

There are other differences. For example, an engine uses energy sucking air through the intake, and pumping it out the exhaust. A small carburetor running wide open will have smoother airflow than a big carb with the throttle plate almost closed, which means slightly lower pumping loss.

Having the carb tuned properly for the engine is a lot more important than the size of the carb, for mileage, responsiveness, and power. Actually, your brother might be perfectly happy with the carb he's got if it were set up by somebody who really knows what he's doing.

 

I don't doubt you are right, but he has had it to several shops, plus many friends prior to the shops, and still the same problems persist. Either no one in this area can set up a holley, or they are junk. 25 yrs ago i had holleys on several projects. I converted to rochesters and carters and never could have been happier. Possibly just the holley 650, it never performed well in any application for me. The edelbroch (sp) was very highly recommended in off-road applications, beating the pants off the other contenders. This did not seem like a marketing story for the carb manufacturers.

 

I have been running the new holley off road truck avenger carb for about six months and have had no trouble with it. I have wheeled it hard a couple of times and only had it stall once. Much happier with it than the MC I was running before. They may have gotten it right this time. Is the s10 a street or off road truck?

 

One of the biggest keys to running a Holley off road is dropping the float level, and also limiting drop.

 

To me it's hard to believe that a carb that's reasonably close can't be made to run well on a given engine. On the other hand, not many people know how and have access to the right jets, metering rods, air tubes and stuff. And some carbs don't have much adjustability in their design.

If you can't get it running right, there's not much reason to keep fiddling with it.

 

There are plenty of CFM calculators on the web to see if you are running the right carb, here's a link to one:
http://www.corvetteactioncenter.com/tech/tools/carb.html

 

i had a 650...ran like..not well......so iwent down to a 465 holley, extended vent tubes, lowered the float level a lot(only when 4x4ing though)runs excellent now...

is your 650 a vacum secondary or a double pumper....?
double pumpers deliver way too much fuel for 4wheelin..
another thing you can check is the power valve....if the number is to high ( 7 to 10 ) it wont work well at low speed.... the higher the number the sooner the valve opens...my 650 had a 7.5 which ment it was open at cruising speed..bad for economy..they should be about 5.5 for 4x4....it will just run out of legs at the top end...i lost 1000 rpm

 

It has vacuem secondarys and is used for serious 4wheeling. As far as any of the other questions, I have never worked on the thing, however, I could ask him. As said, that may be his problem. He has brought it to a shop that specializes in speed machines, they may not understand the differences required of a 4x4 carb. I will talk to him and suggest he try different tuning.

I am sorry, but I am kind of carburator illiterate. Would anyone like to explain or point me to a site where I could learn a little about tuning this thing? I have changed jets, (simple in a holley), & metering rods, but have never heard of a power valve. At least without looking at a rebuild schematic. Also, how do you limit float drop? I know how to adjust float levels, but that is about it. Actually, I have rebuilt quite a few carbs, but I always just set them up per the specs. I never really understood how they work.

 

you cant miss the power valve ...when you take the metering block off , its smack bang in the middle of it,about 15mm in diametre......here's a link to look at ...have fun

http://www.mortec.com/carbtip1.htm