11-22-2020, 10:04 PM
(This post was last modified: 11-22-2020, 10:23 PM by Terry Bentley.)
Chapter 68
Before I actually braved myself to butcher the HL-66 in to the new dream carburetor, the other lathe fodder had already taken a transition from stock to not salvageable. Tho neither of the first two donors were usable anyway.
Originally I made attempts to bore out one HL-15. It had a small piece broken out of the body, so was completely useless. It became my test subject just to find out what was way too big.
![[Image: attachment.php?aid=4793]](https://www.vintagekartforum.com/attachment.php?aid=4793)
HL-15
At first I tried opening up the throttle bore. The stock diameter is 13/16". There was no trouble taking this out to 7/8". So with this one modification, I could convert most earlier carburetors to the newer ones that were now available with the 7/8" throttle bore. There was little or no difference in size of orifices feeding the fuel circuits.
Other than minor differences between models, you could upgrade most of these chainsaw carburetors to the newer ones on the Westbend, Power Products and Mac engines available for kart racing. So that was not any big deal. Drilling out some of the orifices of the earlier models was simple enough. But there probably wont be a real big difference in performance on the track. The carburetor was still limited to the same small 11/16" venturi.
I wanted to find out how much bigger these earlier models could be bored. My next attempt was to open up the throttle bore larger like the new HL-115. Even tho the HL-115 is advertised as a one inch bore, that is not completely accurate. The outer face of the mounting flange is bored up to one inch. But the actual throttle bore is stepped down to only 15/16".
Now after pushing thru a larger 15/16" reamer, I found the limits to these earlier carburetor bodies. At this bore size the low speed circuit cavity was cut thru and now unusable. So I know the throttle bores can only go up to 7/8".
![[Image: attachment.php?aid=4792]](https://www.vintagekartforum.com/attachment.php?aid=4792)
HL-15 bored thru
It looks like most if not all of the earlier castings will be limited to the 7/8" bore. The HL-115 is a different casting that has more material to allow for the larger diameter throttle bore.
The next thing I wanted to find out is how much bigger the air horn could be opened up. This worked out with good results. The stock diameter of the air horn on all of the models I have checked is 7/8". The first attempt was to push a 15/16" reamer in the horn. It went well with no signs of cutting thru anywhere.
No reason to hold back. Next I went ahead and tried a one inch reamer. This worked well and there is enough wall thickness to be acceptable. One step closer to opening up these earlier models to possibly match the performance of new HL-115. It has a one inch bore air horn with a lot to spare.
Even tho the body is very thin around the air horn, I pushed it up slightly larger. Using a number 4 morse tapered reamer, I was able to open up the air horn another 0.015" and it tapers out larger to 1.030" at the flange. This should be good enough. There is a very thin wall now on the body around the air horn. This is as large as I am going. There is not much wall thickness, but no signs of being a problem.
Using that long tapered reamer from mounting flange side of carburetor body, the throttle bore was taken to as large as possible at 0.880". With the long shallow taper of this one particular reamer it transitioned smoothly down to the the bore size of venturi. It appears the venturi could be taken up to almost the throttle bore size, but that would likely become useless. Since the venturi would now be much less effective.
![[Image: attachment.php?aid=4794]](https://www.vintagekartforum.com/attachment.php?aid=4794)
Enlarged venturi/airhorn
Side by side one is stock and the other is bored out as much as I see is possible.
After some trial and error, I was content with the venturi opening up to about 0.830". This was achieved by using that long tapered reamer for throttle bore. It was run in deep enough to open up the venturi. With the gradual taper if ended up working perfect to take the venturi out to 0.830 and smooth transition back to the throttle bore.
I am now more or less satisfied with the results of air horn and venturi. There is still more to work on here. But for now I am ready for the real working finished version of this carburetor.
Most of the carburetors I have looked at, the throttle bore is just a straight cut in to the body, just far enough for the butterfly to clear. With this one tapered reamer, instead that step is now a smooth transition angled down to the diameter of venturi bore.
I felt like it was worth the trouble doing. This may not be a performance enhancement in actual function, but a psychological standpoint it was all there. The original stock float bowl carburetor that came on the Clinton A-490 had a very small 1/2" venturi. That carburetor was longer than a Tillotson, and had a very long shallow taper from throttle bore choking down to the venturi.
This float bowl carburetor was something manufactured about ten years earlier. Likely before someone figured out it didnt really make enough difference in performance to go to the effort with that smooth transition. But now, after almost doubling the horsepower and pushing the rpm way above the intended usage, I dont see how it could not be of some improvement. Even if it was only a slight gain.
Before I actually braved myself to butcher the HL-66 in to the new dream carburetor, the other lathe fodder had already taken a transition from stock to not salvageable. Tho neither of the first two donors were usable anyway.
Originally I made attempts to bore out one HL-15. It had a small piece broken out of the body, so was completely useless. It became my test subject just to find out what was way too big.
HL-15
At first I tried opening up the throttle bore. The stock diameter is 13/16". There was no trouble taking this out to 7/8". So with this one modification, I could convert most earlier carburetors to the newer ones that were now available with the 7/8" throttle bore. There was little or no difference in size of orifices feeding the fuel circuits.
Other than minor differences between models, you could upgrade most of these chainsaw carburetors to the newer ones on the Westbend, Power Products and Mac engines available for kart racing. So that was not any big deal. Drilling out some of the orifices of the earlier models was simple enough. But there probably wont be a real big difference in performance on the track. The carburetor was still limited to the same small 11/16" venturi.
I wanted to find out how much bigger these earlier models could be bored. My next attempt was to open up the throttle bore larger like the new HL-115. Even tho the HL-115 is advertised as a one inch bore, that is not completely accurate. The outer face of the mounting flange is bored up to one inch. But the actual throttle bore is stepped down to only 15/16".
Now after pushing thru a larger 15/16" reamer, I found the limits to these earlier carburetor bodies. At this bore size the low speed circuit cavity was cut thru and now unusable. So I know the throttle bores can only go up to 7/8".
HL-15 bored thru
It looks like most if not all of the earlier castings will be limited to the 7/8" bore. The HL-115 is a different casting that has more material to allow for the larger diameter throttle bore.
The next thing I wanted to find out is how much bigger the air horn could be opened up. This worked out with good results. The stock diameter of the air horn on all of the models I have checked is 7/8". The first attempt was to push a 15/16" reamer in the horn. It went well with no signs of cutting thru anywhere.
No reason to hold back. Next I went ahead and tried a one inch reamer. This worked well and there is enough wall thickness to be acceptable. One step closer to opening up these earlier models to possibly match the performance of new HL-115. It has a one inch bore air horn with a lot to spare.
Even tho the body is very thin around the air horn, I pushed it up slightly larger. Using a number 4 morse tapered reamer, I was able to open up the air horn another 0.015" and it tapers out larger to 1.030" at the flange. This should be good enough. There is a very thin wall now on the body around the air horn. This is as large as I am going. There is not much wall thickness, but no signs of being a problem.
Using that long tapered reamer from mounting flange side of carburetor body, the throttle bore was taken to as large as possible at 0.880". With the long shallow taper of this one particular reamer it transitioned smoothly down to the the bore size of venturi. It appears the venturi could be taken up to almost the throttle bore size, but that would likely become useless. Since the venturi would now be much less effective.
Enlarged venturi/airhorn
Side by side one is stock and the other is bored out as much as I see is possible.
After some trial and error, I was content with the venturi opening up to about 0.830". This was achieved by using that long tapered reamer for throttle bore. It was run in deep enough to open up the venturi. With the gradual taper if ended up working perfect to take the venturi out to 0.830 and smooth transition back to the throttle bore.
I am now more or less satisfied with the results of air horn and venturi. There is still more to work on here. But for now I am ready for the real working finished version of this carburetor.
Most of the carburetors I have looked at, the throttle bore is just a straight cut in to the body, just far enough for the butterfly to clear. With this one tapered reamer, instead that step is now a smooth transition angled down to the diameter of venturi bore.
I felt like it was worth the trouble doing. This may not be a performance enhancement in actual function, but a psychological standpoint it was all there. The original stock float bowl carburetor that came on the Clinton A-490 had a very small 1/2" venturi. That carburetor was longer than a Tillotson, and had a very long shallow taper from throttle bore choking down to the venturi.
This float bowl carburetor was something manufactured about ten years earlier. Likely before someone figured out it didnt really make enough difference in performance to go to the effort with that smooth transition. But now, after almost doubling the horsepower and pushing the rpm way above the intended usage, I dont see how it could not be of some improvement. Even if it was only a slight gain.