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Author Topic:   rochester ideas!
Dirt Full Roller

Total posts: 46
posted December 26, 2002 10:42 PM
look for the casting 7042838,this has the large bore.if your allowed to modify it,take a dremel,and very (carefully) smooth the casting flash off the middle of venturis.(it's best to use a sanding bit,and not a boring it by about 2mm,and smooth with a light sand paper.if you have a glass beading cabinet,it's nice to take them,and give them a beaded finish.use it from a good distance so your not eating up the metal to the booster,this is very important,if you have a drill bit set.find a bit that fits into the holes in the tubes.once you find that bit,go one size up.and open 3 outer holes(no more),and then the bottom hole on each.if you want even more cfm,take vacuum bridge out with a die grinder to the point where it meets with normal bore surface.i don't mean the vent side near the fuel bowl section.the reason i posted this,is for those budget drivers who are looking for an extra boost.getting rid of the vacuum bridge allows better airflow,as does opening the venturi must be careful not to take out to much,unless you're running on a real long track.the booster is mainly for throttle is wise to leave the sprayers alone.jetting will always depend on altitude,and what type of motor,and so on.these are a few tips,but each situation is different,so use caution.don't go crazy on those things,they are hard to find.have fun!

Dirt Roller

Total posts: 20
posted December 27, 2002 07:18 PM
Are you sure of that number? It doesn't cross to anything I can find. Is it a Marine Carb maybe?

Dirt Freak

Total posts: 408
posted December 27, 2002 08:30 PM
The 838 carb is an EXTREMELY rare find. It has the same 1 3/8" dia venturi as many many other castings we've found over the years. The one mentioned above happens to be from a 1972 model year. The 4-5 years spanning that era (69-74) are probably the easiest to find 138's, 118's, and other such big bore bodies.
All of them will have the same diameter throttle bores (1.687") as do almost all the commonly found 2G designs.

In addition to the few mod's mentioned above, there are dozens of other things we do to improve the fuel curve, idle circuit responsiveness, and top end power of these carbs. Many of which cannot easily be done without special tooling.
Simply removing the parting line in the venturi and the vent "scoop" will gain maybe 25cfm alone on the flow bench. Hardly enough to show on a dyno let alone a stopwatch on the track.
The trick to overall performance gains is in the entire fuel management package throughout the rpm range, i.e, how the carb meters fuel at all times, and how well it is mixing it with the incoming air.

Hope that helps.


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