Small Bore Carb vs. Big Bore Carb

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Brandon Atwell

Brandon Atwell

Well-Known Member
Joined
Mar 11, 2012
Messages
1,202
What are the advantages/ disadvantages of either? I've always been under the impression that bigger was better, but is that not the case? Is their a "perfect" medium to look for in bore size?
 
Brandon Atwell said:
What are the advantages/ disadvantages of either? I've always been under the impression that bigger was better, but is that not the case? Is their a "perfect" medium to look for in bore size?
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Brandon

Years ago, many the 21 engines had bores of around .190 thos. up to .260 thos, today they come with around .320 to .375 thos. I ran a picco .21, with a .430 bore in my .21 seducer, with a .085 spray bar, it was an extremely fast boat,won a lot of races because of the extreme speed it developed with the OS carb and the bigger spray bar. In my F boat I currently running a .110 thos. spray bar and carb is .580.

JM2CW

Moby
 
Yes I got a custom .550 bore carby with a .118 spraybar which I use on my 80.

Big bore slow air velocity, small bore fast velocity. They both have pros and cons
 
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Large ID, straight through bore carburetors, that have no vacuum signal to draw the engine's necessary fuel, can be made to work extremely well when there is adequate tuned pipe pressure available. Tuned pipes with inadequate volumes, which result in short, large ID stingers & low internal pipe pressures, will not work in this application. This applies to both nitro & gas type engines! The carburetors shown are mounted to both nitro & gas engine types. They are 100% maintenance free & have extended working part lives. The carburetors shown range in size from .625" bore to .725" bore & all are straight through designs.

Jim Allen
 
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Would having a crank case pressured system help to equalize the pressure or have a more constant pressure needed to help a big carb with not a lot of velocity
 
Kevin,

You would find the crankcase pressure to be to high at low throttle. If you were running the engine at WOT, this could be made to work. I have not tested crankcase pressure with an automatic fuel metering device built into the carburetor; similar to the one shown in the photo. Notice how small the "V" shaped slot cut into the inside tube has become, as a result of the outside tubes rotation, which is fastened to the barrels rotation. Tuned pipe pressures vary with the engine's RPM & it doesn't make any difference, on the chamber, where the pressure fitting is located.

Jim Allen
 
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okay so if you put some sort of metering devise on it so that you could cut back the high pressure then you could tune it to help the low end sustain enough pressure that it will not choke the engine down with excess fuel, once this is done then you could copy it into a fitting with a reduced hole sizes or something of that nature. this would give you a more constant fuel pressure all the time not up an down depending on the rpms varying. Is this correct.
 
Kevin,

I wouldn't have a clue as to how to make some sort of miniature metering device that would be capable of bleeding off some of the high crankcase pressure at the right time for low throttle operation without effecting the engine's performance. Maybe a two needle carburetor such as the OS MAX 9B could be made to work in this application. However, you really want the pressure that is pushing the fuel to go up & down with the engine's RPM's. Also the fact that it is going up & down makes the use of an air bleed type carburetor possible. The photo shows the same size bore carburetor without the metering circuit in the barrel. This carburetor uses a needle controlled air bleed circuit which begins to open at half throttle. After 7* of rotation the circuit is completely open & now the air needle valve's setting determines how much additional air is coming in as the barrel closes. There is no obstruction of any kind in the carburetor's bore.

JA
 

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