Big Bore Carburetor Mod

Tech Scott Schneider


Learn how to do a modification on a "Bored Out" big bore carburetor for consistent performance and reliability...





In the 20+ years working as a commercial small engine mechanic, a fellow shop owner and I built competition racing chainsaws as a side business.  In doing this, the designs and specs on the parts changed a bunch.  Carburetor issues were a huge hurdle to overcome when we tried to achieve maximum CFM while maintaining correct mixture control by opening up venturi bores.
Part of this learning process was trial and error with a quite a few trashed carburetors as a result.
The operational fundamentals of a "Pumper Carburetor" had to be completely understood. Those teachings and understanding came from years of collaboration between service/technical personnel from the Walbro Corporation, Husqvarna, Stihl and many others.



The explanation...

So we enter the realm of performance "Big Bore" carburetors that are further modified by some who have the best of intentions, but fall short of understanding what they get from the modifications done, other than a higher CFM flow rating.  In other words, what they can disrupt in their quest for getting the higher flow rate by doing the Bore out & Intake bell modifications the way they do.

For quite few years now I have applied versions of this modification to help any carburetor that has its HIGH speed fuel delivery port in a less than optimal position for high rpm use.

"Pumper" carburetors by design are basically metering devices that sense the pressure drop within their throttle bore/venturi. As fuel gets drawn out by the pressure drop of air being drawn through the bore/venturi, fuel levels drops within the metering well and is then replaced by the fuel pump as the inlet needle is lifted by the descending diaphragm.
* The High speed circuit being the MOST important in terms of a stable flow of fuel is also the most sensitive to an inconsistent vacuum signal.

Bored carburetors are sometimes being modified in such a manor that the intake bell shape and the reduction in diameter moving down into the carburetor's bore is moving the apex of the venturi further inward, going deeper than the high speed fuel delivery discharge port. (The APEX being the point at which the intake bell reaches MINIMUM diameter, thus creating the actual venturi).
In such a shape the low pressure created by the venturi falls downstream of the delivery port.  While the pressure in general is lower than atmospheric pressure near the venturi, the High delivery port still delivers fuel as it should, just without much constancy relative to being placed at or slightly behind the apex.


*This information is being shared due to so many variables in the way some have gone about creating a "Big Bore" modified Carburetors.

While this MGB article has some meat and potatoes "How-to", whether or not it applies to a Big Bore carburetor you may have needs to be determined by a few simple measurements and visual inspections.

Carburetors that I have seen and reworked have been Walbro 257's, 488's, 771's and some with numbers ground off.  Tuners who have done them are not going to be specified due to finding inconsistencies from enough to prompt this article.  No mud slinging or bad mouthing intended...just straight up information on how to fix one if done in such a way that this article addresses.   

Does YOUR "Modified" big bore need help ?

You determine this quite simply by:

#1 Measuring the venturi bore's minimum diameter and noting where within the bore it is in relation to the high speed delivery hole.
#2 Visually looking at the intake bell of the carburetor straight on, noting if you can or can not see the high speed delivery hole obviously being within the modified bell shape of the intake area.

** If you note either of these two following items, your modified Big Bore can gain performance by this fix !

#1- When measured the smallest diameter is DEEPER in the throat than the High delivery hole

#2 - You can obviously see the delivery hole is within the bell shape entering the throat.

PS. This modification can also be applied to get better fuel metering from the stock Zenoah WT-644 Carburetor. ( the following two pictures show this done to a Walbro WT644 ) ( Only the slot behind delivery hole is required )

Scotts camera dump 01-06-07 139 Scotts camera dump 01-06-07 138



How to fix it...

Reworking the throttle bore/venturi only takes one cutter. A straight shank 1/8" diameter BALL end carbide in a SINGLE CUT configuration.

Remove the butterfly from throttle shaft and remove the shaft. I personally take apart the entire carburetor so that I can clean it up completely after the work is done. (For more information on re-assembling the carburetor butterfly and shaft see THIS ARTICLE).

Set the length of cutter in collet so that the ball end is approximately 1/8" or more beyond the delivery port when inserted from the manifold surface of the carburetor.
* You will want the front edge of the collet that's holding the cutter to bottom out on the rim of the throttle bore. And MAKE SURE the bare shank is riding on the edge of throttle bore and not cutter teeth!


The FIRST step is to create a back taper from the .625" throttle bore in behind the venturi apex and in reality moving the apex forward towards the High delivery hole.
By holding the cutter against the outside edge of the throttle bore at the manifold end, the cutter will be on a slightly tilted angle, this is what you want!
Slowly work your way around the bore making a light cut going round and round several times. You will see the apex point of the intake bell to bore start moving forward towards the High delivery port. The OVERALL bore diameter is increasing at the same time!
*Below are a few pictures to show where the bare shank of the cutter needs to rest while it gets rotated around the bore making the back taper.

Scott camera dump 2-27-09 182 Scott camera dump 2-27-09 183 Scott camera dump 2-27-09 154



Ideally you want the apex to get within .030 - .040" of the delivery hole or closer. NOTE: you don't want the venturi bore diameter to exceed approximately .530" -.540", so go slow and measure it as you work it forward.



Once you're done with this step, you have the MOST CRITICAL step to perform...a steady hand and accuracy for this step is paramount!

* Sight down on the carburetor and get the cutter placed perfectly straight in line with the throttle bore.
Again with the shank of cutter resting on edge of throttle bore slowly cut a slot right behind the delivery hole having the tip of the ball shaped cutter JUST touch the delivery hole. Make the slot just deep enough that it ALMOST is equal to the throttle bore.
**** DO NOT go so deep that you contact the smooth .625" portion of the throttle bore ****

Notice in pictures below: The machining  marks left from cutting the back taper and intersect point with intake bell.

Scott camera dump 2-27-09 145 Scott camera dump 2-27-09 146

Scott camera dump 2-27-09 149 Scott camera dump 2-27-09 148


Polish up the front bell and the back taper with a scotch bright pad. Wash out the carburetor and you're done.

If you accidentally damaged the gasket surface a tad from the collet running on the edge of throttle bore, just use some 180 grit wet/dry sandpaper on a piece of glass or surface plate and smooth it back out.



Scott Schneider
MGB Technical Advisor



Log in to comment

Mholst replied the topic: #6216 12 years 7 months ago
I have modified my Zenoah 260 the rpm with this engine in my sprintcat 42`` is 18500 with a PS 7016/2 ss prop.Speed with a PS 7018/2 was 103 km/t.
I have tried several types of Zeonah carbs and the 813A works best i got almost 800 rpm more with this carb than with a modified 257.
I also found out that there are no gain by soldering the butterfly plate i just grind down the head off the screw as much as possible and cut the bytterfly holding screw (its to long)
The most inportant is to line up the carb with the isolator plate and inlet port.I have also instaled zero drag zeals.And lightened the piston.I also driled four holes just below the piston ring to get better flow.And raised the exhaust port and modified the transfer ports.

My homepage.
02cummins replied the topic: #6021 12 years 8 months ago
I am going to try this carb mod, has anyone done this or run the 813 carb with any results
INEEDSPOTS replied the topic: #5966 12 years 8 months ago
theres no screw and blade coming out of mine... i epoxy the blade to shaft, then epoxy the screw to blade and shaft. Ive yet to have one loosen up or come apart. Far too many bone stock carbs are having the screw loosen up and not let carbs close to just take them out of the box and bolt it on and run.
Steve Seebold's Avatar
Steve Seebold replied the topic: #5965 12 years 8 months ago
Scott wrote:
If it looked good and in reality was just a shiny low penetration joint, swallowing the butterfly at 17K+ is going to hurt ... A LOT !!

:clapping: Scott is absolutely right. And if you mess with the screw in the butterfly, you will most likely end up with the same results. :diablo:
Scott replied the topic: #5946 12 years 8 months ago
dkf wrote:With the carb body acting as a heat sink, it would be VERY difficult to get a good bond between the butterfly and shaft.

If it looked good and in reality was just a shiny low penetration joint, swallowing the butterfly at 17K+ is going to hurt ... A LOT !!

User Login


You are not logged in.

© 2017 All Rights Reserved.