03-13-2023, 08:17 PM
The carburetor on these saws are nearly identical to ones on the kart engine. The particular model carburetor on this saw was HL-65A. I did not like the wear it showed, so swapped it out to an HL-92A. If these carburetors did not have a model number stamped on the flange, you could not tell them apart.
The models HL-35, 45, 64, 65, 66, 73 and 92 are almost identical. These carburetors are common on David Bradley chainsaws. What few differences there are would not change how they work on any of the chainsaw models they were equipped on.
The HL-66 is actually the correct model for AH-58 and AH-82 kart engines. Except for the throttle shaft being slightly different, only one orifice size have been changed in the fuel circuit compared to the chainsaw carburetor model HL-65. Of these carburetors listed one model may have a larger air bleed to the low speed circuit, while another one may have slightly larger high speed needle seat.
The difference in these two carburetors for my saw were in both air bleeds. One was slightly smaller, while the other was larger. The newer HL-92 had low speed air bleed only 0.001" smaller, but the high speed air bleed was 0.009" larger.
The size difference in high speed air bleed size would be very significant on the low speed circuit, but not so much on the high circuit. But even that is not so much of a deal considering some of the carburetors are not even drilled with the low speed air bleed.
The HL-88 used on Westbends, and HL-93, used on Macs do not utilize the air bleed orifice. Either of these carburetors could benefit with having it added.
![[Image: attachment.php?aid=5301]](https://www.vintagekartforum.com/attachment.php?aid=5301)
Tillotson HL-65
Wow it looks just like any other Tillotson you have seen. This carburetor looks almost identical to the very first model in 1953, the HL-1A.
Tearing the carburetor apart, I needed to remove the welch plug. Super simple but super easy to screw the carburetor up too.
![[Image: attachment.php?aid=5302]](https://www.vintagekartforum.com/attachment.php?aid=5302)
Welch plug 1
You drill a little 1/8 inch hole in center of welch plug. But only drill far enough to barely penetrate the thickness of that plug. BTW it is very thin, so be careful. If drill bit catches as it penetrates, will pull the tip into the body and go right thru. Then you have junk.
![[Image: attachment.php?aid=5303]](https://www.vintagekartforum.com/attachment.php?aid=5303)
Welch plug 2
With a small pointy object, like this pick you see here, just pry it out. Almost impossible to screw things up doing this. But there was this one guy.....
![[Image: attachment.php?aid=5304]](https://www.vintagekartforum.com/attachment.php?aid=5304)
low speed circuit
You see those three very tiny holes down in the low speed cavity. The left on is low speed jet, middle is the transition jet, and that last one at the edge on right is the air bleed.
![[Image: attachment.php?aid=5305]](https://www.vintagekartforum.com/attachment.php?aid=5305)
Fuel circuit
You see how these are drilled from the angles. That extra one you see is the fuel pickoff. It feeds fuel to both low and high speed circuits. Fuel flows down into another drilled passage connecting both circuits together. Newer model carburetors have separate pickoffs that better control amount of fuel to each.
![[Image: attachment.php?aid=5306]](https://www.vintagekartforum.com/attachment.php?aid=5306)
high speed air bleed
Here you see how the high speed air bleed is drilled. The actual orifice size is abound 0.028". But it is recessed in a pocket drilled out larger to 0.060". This larger pocket helps direct air into the smaller hole instead of just passing over.
The fuel flowing thru high speed jet drilled straight down in venturi is controlled by size of high speed needle seat. The size of it is not so critical, but usually around 0.062" on many of the models I have checked
Not all of the carburetors utilize the air bleed for low or high speed ircuit. What the air bleed does is mix incoming air with the fuel being drawn in from engine. This actually leans down the mixture a lot. It also better atomizes the mixture too.
The high speed jet on these early carburetors is just a straight drilled hole. Later models utilized a brass check valve. In the late 70's John Hartman contracted Tillotson to build the HL-307 eliminating the check valve going back to a straight drilled hole.
He had already done the same thing earlier with the alcohol carburetor models HL-293A and B. The model "A" had a check valve, model "B" was straight drilled. So which one was faster on the track?
So I cleaned out all of these small orifices being very careful not to enlarge them. Some of these orifices are only 0.001" different from a similar model. I have logged hundreds of these carburetor sizes and from the first model to the latest with similar dimensions, all fall within a very narrow range.
The big differences are in how the fuel circuit is routed. A few of them look like an engineers pipe dream of complexity or just a nightmare on acid. Others are so simple, you wonder what those other guys were even thinking. Or maybe that was the problem, they were not thinking.
The models HL-35, 45, 64, 65, 66, 73 and 92 are almost identical. These carburetors are common on David Bradley chainsaws. What few differences there are would not change how they work on any of the chainsaw models they were equipped on.
The HL-66 is actually the correct model for AH-58 and AH-82 kart engines. Except for the throttle shaft being slightly different, only one orifice size have been changed in the fuel circuit compared to the chainsaw carburetor model HL-65. Of these carburetors listed one model may have a larger air bleed to the low speed circuit, while another one may have slightly larger high speed needle seat.
The difference in these two carburetors for my saw were in both air bleeds. One was slightly smaller, while the other was larger. The newer HL-92 had low speed air bleed only 0.001" smaller, but the high speed air bleed was 0.009" larger.
The size difference in high speed air bleed size would be very significant on the low speed circuit, but not so much on the high circuit. But even that is not so much of a deal considering some of the carburetors are not even drilled with the low speed air bleed.
The HL-88 used on Westbends, and HL-93, used on Macs do not utilize the air bleed orifice. Either of these carburetors could benefit with having it added.
Tillotson HL-65
Wow it looks just like any other Tillotson you have seen. This carburetor looks almost identical to the very first model in 1953, the HL-1A.
Tearing the carburetor apart, I needed to remove the welch plug. Super simple but super easy to screw the carburetor up too.
Welch plug 1
You drill a little 1/8 inch hole in center of welch plug. But only drill far enough to barely penetrate the thickness of that plug. BTW it is very thin, so be careful. If drill bit catches as it penetrates, will pull the tip into the body and go right thru. Then you have junk.
Welch plug 2
With a small pointy object, like this pick you see here, just pry it out. Almost impossible to screw things up doing this. But there was this one guy.....
low speed circuit
You see those three very tiny holes down in the low speed cavity. The left on is low speed jet, middle is the transition jet, and that last one at the edge on right is the air bleed.
Fuel circuit
You see how these are drilled from the angles. That extra one you see is the fuel pickoff. It feeds fuel to both low and high speed circuits. Fuel flows down into another drilled passage connecting both circuits together. Newer model carburetors have separate pickoffs that better control amount of fuel to each.
high speed air bleed
Here you see how the high speed air bleed is drilled. The actual orifice size is abound 0.028". But it is recessed in a pocket drilled out larger to 0.060". This larger pocket helps direct air into the smaller hole instead of just passing over.
The fuel flowing thru high speed jet drilled straight down in venturi is controlled by size of high speed needle seat. The size of it is not so critical, but usually around 0.062" on many of the models I have checked
Not all of the carburetors utilize the air bleed for low or high speed ircuit. What the air bleed does is mix incoming air with the fuel being drawn in from engine. This actually leans down the mixture a lot. It also better atomizes the mixture too.
The high speed jet on these early carburetors is just a straight drilled hole. Later models utilized a brass check valve. In the late 70's John Hartman contracted Tillotson to build the HL-307 eliminating the check valve going back to a straight drilled hole.
He had already done the same thing earlier with the alcohol carburetor models HL-293A and B. The model "A" had a check valve, model "B" was straight drilled. So which one was faster on the track?
So I cleaned out all of these small orifices being very careful not to enlarge them. Some of these orifices are only 0.001" different from a similar model. I have logged hundreds of these carburetor sizes and from the first model to the latest with similar dimensions, all fall within a very narrow range.
The big differences are in how the fuel circuit is routed. A few of them look like an engineers pipe dream of complexity or just a nightmare on acid. Others are so simple, you wonder what those other guys were even thinking. Or maybe that was the problem, they were not thinking.