What would a larger carb bore do?

Tell us what you do to get the best performance from your engines.

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NAES
Posts: 627
Joined: Sun Apr 01, 2001 12:28 am
Location: USA

What would a larger carb bore do?

Post by NAES »

You know if you put a 4barrel on a 350 chevy you get a bunch more air and fuel into the motor and a bigger bang.

Does the same apply to 2stroke model aircraft engines? I've seen perry carb swapps before.

Thanks, NAES

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BigCountry
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Post by BigCountry »

I can't say across the board but I know in the case of my 25fx's I've put a carb from one of my 32sx's from the old Class C days on it and it make a very noticeable difference. I'm no engine guru but from what I understand it's the same theory...

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Cajun
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Location: USA

Post by Cajun »

Take a look at the ST.45. It has a .90 size carb with a huge venturi. It makes bunches of power and yet idles good.

My experience with retrofitting a larger bore carb on small engines has been they will make more power, but at the expense of idle and transition problems. But, if you are not interested in the engine idling, go for it.

The only other problem is if you go to far the fuel draw suffers and you will need more muffler pressure or a check valve in the pressure line.

CAJUN [8D]
Lone Star Combat State
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NACOGDOCHES, TEXAS
MY MIND WORKS LIKE LIGHTNING,,ONE BRILLIANT FLASH AND IT'S GONE!!
Don Pruitt
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Post by Don Pruitt »

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There was a time when I though that I knew what was going on with carburetors and the like. However, as I get older I am beginning to have my doubts. I know that when I flew C/L combat years ago I could take the insert out of the carburetor on a Fox 36X and tap the back plate for a pressure feed to the tank and realize a dramatic increase in power/speed. Then recently at Tuscaloosa AL I checked out Lou’s Cyclone .15 and the venturi looked to be about 75% of the diameter of an OS LA .15. The Cyclone would turn 22K on 5%.

One of my recent experiments with a TT .15 was to remove the throttle barrel and JB Weld a plate over the opening where the throttle control arm was connected. This increased the venturi diameter by about 20%. I was running a bladder tank with the before and after and the result was that at peak I lost about 500 rpm.

I have always heard that an internal combustion engine can be viewed as an air pump. You must be able to get air in and out efficiently for best performance. Opening up the intake to get in more air should go along with opening up the exhaust path to let the exhaust air out. Note that the hole in the stinger on the muffler is relatively small and contributes to the pressure build up inside the muffler. Another thought is that with a given engine displacement and speed you will move a finite amount of air. As example, a 0.15 cid. engine turning at 10K will theorically move about 10.4 cu. ft. of air per minute. (0.15 X 10,000 /144) Moving the air through the engine is not 100% efficient. Some volume in lost by the restrictions in the airflow path. One of these restrictions is the carburetor intake. By opening the intake, you reduce the restriction and improve the airflow. However, the speed of the air moving across the needle valve opening is slightly decreased and the atomization of the fuel may suffer slightly.

I agree with Cajun that you should be able to improve performance at the higher rpm but at the expense of performance at the lower rpm. Obviously there is still a lot going on that I don’t understand. I am anxiously awaiting some input from other knowledgeable people.
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Don Pruitt
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sgilkey
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Post by sgilkey »

I really like the 32SX and 40FSR carbs on the 25FX. They both have the same choke diameter (.295"), 13mm necks, and will give about 300 rpm increase over the stock 25FX carb (.236"). Both needle superbly and give excellent fuel draw. A MCP gives very good backpressure and nice fuel draw, if you use a stock muffler with a reamed or removed tailpipe you may have fuel draw problems, even with the stock carb.

When considering carb restriction, don't forget the spraybar. The carb barrel opening may be huge, but if you have a big ol' spraybar across the opening, it reduces the effective area. The norvel big mig has a tiny choke (.206, about the same as a 40FP), but also no spraybar. the 32SX and 40FSR carbs have big chokes but also big spraybars.

A Nelson .40 pylon engine has I think a 9mm (about .350") choke and no spraybar, not real big when you consider the power and rpm it turns. The muffler stinger is only 1/4" ID, if I recall, not much bigger than a stock 25FX muffler. So huge chokes and big tailpipes are not necessary for big power. But just like on a street rod, if your engine can take more carb and freer-flowing exhaust, it is likely to help somewhat, as long as you can take the hit on fuel draw.

BTW the new Webra has a pretty big carb, and practically no spraybar. And makes bodacious power. If you put an ST.34 carb at .283" on a MAgnum 28XL, which comes stock with only .239, you will get at least 500 rpm- that engine really likes being uncorked. But at higher rpm, it really starts to vibrate, which leads to fuel foaming, which leads to sagging! So not always the best setup. in my experience, most combat engines respond well to bigger carbs.

Scott Gilkey
NAES
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Post by NAES »

I'll just have to find out sunday at the next contest

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