Big Bore Carburetor Mod

Fielies created the topic: Big Bore Carburetor Mod

This thread discusses the Content article: Big Bore Carburetor Mod

1. Scott, when cutting the taper on the carb bigger to move the apex forward, do you do it on a milling machine, or free hand?.

2.Do i understand correct. If i cut towards the high speed hole i should stop before i get to the high speed hole. The idea is to get the high speed hole close (before) to the smallest part of the apex, looking at the carb from the bell shaped side.
#5694

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Ed Sidders replied the topic: Re:Big Bore Carburetor Mod

Scott
As always excellent article , these guys that have been cutting a radius on a stock 257 bell and selling it as a modded carb just learned something :ohmy:
#5697

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Scott replied the topic: Re:Big Bore Carburetor Mod

Fielies wrote:

This thread discusses the Content article: Big Bore Carburetor Mod

1. Scott, when cutting the taper on the carb bigger to move the apex forward, do you do it on a milling machine, or free hand?.

2.Do i understand correct. If i cut towards the high speed hole i should stop before i get to the high speed hole. The idea is to get the high speed hole close (before) to the smallest part of the apex, looking at the carb from the bell shaped side.


I do them FREE HAND, The deciding factor on how far forward you more the apex depends on the venturi diameter that continues to grow bigger as you move the apex forward. Have found venturi bores any larger than @.540" wont flow any more because of throttle shaft restriction within throttle bore.

** So if your back boring gets at the delivery hole and you still smaller than .540" ... GREAT, but if you are getting near the .540" diameter and still cannot get the apex where you want it, call it close enough :cry:

If you just cut slowly it really is quite an easy task :dance2:

2 stroke Engine Tech of 30+ years, Prop tweeker, Chronic tinkerholic, Home of Motor Heads RC Racing Engines ...
8 time NAMBA National 6 lap heat racing record holder.
#5699

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INEEDSPOTS replied the topic: Re:Big Bore Carburetor Mod

true mod carb has the shaft and butterfly massaged. This is one i messed with, have not messed with the screw. Stock bore 257.


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#5700

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Scott replied the topic: Re:Big Bore Carburetor Mod

Then again, you can gain flow like this ...

2 stroke Engine Tech of 30+ years, Prop tweeker, Chronic tinkerholic, Home of Motor Heads RC Racing Engines ...
8 time NAMBA National 6 lap heat racing record holder.
#5704
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Scott replied the topic: Re:Big Bore Carburetor Mod

another view

2 stroke Engine Tech of 30+ years, Prop tweeker, Chronic tinkerholic, Home of Motor Heads RC Racing Engines ...
8 time NAMBA National 6 lap heat racing record holder.
#5705
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Scott replied the topic: Re:Big Bore Carburetor Mod

With now sharing the next step in flow improvement by thinning throttle shafts ... Be afraid ... VERY afraid in the very distinct possibility that you can now get shaft breakage !!

** With MANY of the linkages used in the hobby, Most rotate the shaft from the opposite end of the throttle stop.
So the shaft is being rotated threw the throttle bore and then bottoming out the stop. ** NOTE: If your servo end point is not set too JUST get the rotation of throttle at WOT just at the stop, it will try and rotate the shaft further causing it to break mid bore !!
In most cases it will break AT the screw hole allowing the STEEL screw and butterfly to be ingested into the engine :bugface:

Not good !!

2 stroke Engine Tech of 30+ years, Prop tweeker, Chronic tinkerholic, Home of Motor Heads RC Racing Engines ...
8 time NAMBA National 6 lap heat racing record holder.
#5707

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TedRoslund replied the topic: Re:Big Bore Carburetor Mod

A few weeks ago when I had a carburetor problem, I was able to borrow a carburetor from Scott that had been modified in the just prescribed fashion. Using that carburetor I was able to place third at the grand-prix with a boat I have had for 14 years that had rarely even finished. I saw the ability to idle slower and better out of the hole acceleration. I am now in the process of modifying all of my carburetors to the same specifications. :)
#5741

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marc replied the topic: Re:Big Bore Carburetor Mod

That's great feedback Ted! :drinks:

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#5878

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Steve Seebold replied the topic: Re:Big Bore Carburetor Mod

A simple solution for not bending or breaking the butterfly bar when you hit the throttle stop is to simply remove the throttle stop or replace the brass butterfly bar with titanium or stainless steel.
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