10-21-2020, 02:45 PM
(This post was last modified: 10-26-2020, 09:37 PM by Terry Bentley.)
Chapter 66
I finally rigged up a quick way to mount up these Tillotson carburetors to the small Craftsman lathe. It is just a flat plate with the right bolt pattern drilled and tapped with two studs. It is also centered to the lathe spindle. So makes it real simple to get the carb running true.
The throttle bore is easy enough to machine out larger. These castings are very thin where the butterfly fits. Opening the bore out to 7/8 inch, the wall thickness is less than 1/32". But there is plenty of meat around the flange and bosses so not to become an issue.
It is really straight forward to bore out the air horn. Hopefully it can be taken out even bigger than 15/16". One thing sticking in my head is wanting to add an extension to the end of the carb face. Then machine it to blend to the bore size of air horn. Also radius the outer face to help with the air flow. There are also several different shapes of the venturi on these. They all obviously work, but dont really know enough to choose one shape over the others.
From the transition point of venturi, or the large diameter before choking down, it will taper out bigger to the outer face of air horn. The small extension will continue this taper even farther before the large clean radius to outer face of extension.
This idea is sort of like bolting an Azusa velocity stack to the carb. But instead of being that long, will only lengthen the carburetor about one half inch. I wonder if there are any specific formulas that could get me an approximate length of carburetor or anything about length of air horn. It would make sense that there would be a certain length for optimum air flow at a particular rpm range.
The Azusa velocity stack looks really cool. But sure seems long. You gotta wonder if someone did all the leg work on it to come up with the length? Maybe this is something to run past Frank for some input, next time over to his place.
So my first major modification for the carburetor will be to machine a 1/2 inch thick aluminum spacer that is 1 3/4" outside diameter. Then drill the two mounting holes that measure 1 3/8" bolt circle. On the lathe it will be machined to blend the extension matching the air horn bore.
Wallah, I have something cool looking if nothing else. If there is a formula to follow, that would be great. But for now this is good enough to start with. The added length should improve incoming air speed at a given rpm and maybe increase the amount of flow, before compressing thru the venturi. Sounds plausible, and if I am on the right track, so to speak, may improve the performance of the engine enough for the effort.
Checking dimensions of some carbs in catalogs, I went with these more common ones. The HL-63 HL-87, HL-88, HL-92, HL-93 and HL-101 for comparing between each of them. They are all the same in basic appearance. Most of the difference is in how the fuel is being fed thru the passages. Below are the dimensions of some of these:
throttle bore venturi air horn
HL-87 13/16" 11/16" 7/8"
HL-88 7/8" 11/16" 7/8"
HL-92 13/16" 11/16" 7/8"
HL-93 7/8" 11/16" 7/8"
HL-101 7/8" 11/16" 7/8"
After a lot of trial and error and a good bit of luck, I managed to bore the first carburetor out with success. Like a lot of success.
So basically all of the above carburetor bodies are of a similar casting. Some minor differences, but nothing affecting how much one can get bored.
I ended up with these finish dimensions using both HL-15 and HL-63 bodies practicing just for lathe fodder.
New Old
Throttle bore .890" .811"
Venturi .780" .687"
Air horn 1.015 .875"
I am so stoked about this first bored out carburetor. It turned out great. Unfortunately the completely finished carb body was no good before I started, since there is a big chunk broken out. It was only the first one to try and find out just how big it could go. The other one is not far behind to being finished out.
After figuring out how much I could get away with boring these, then added the air horn extension and went even farther. So one carburetor body is now bored with the extension and one without.
The first usable carb also has the air horn extension. I was able to open it up to 1.050" and taper down to the finished ID of carb body at 1.015". I dont have a clue yet if all of this modifying will actually help or not. But I can only hope. The next practice day cant come soon enough.
All of the holes drilled feeding the fuel circuit on these different carb models were mostly the same size. It looks like the best idea is to go with the hole sizes from the HL-93. Since it is the first carb built specific for the McCulloch Mc6 and Mc10 kart engines.
The fuel circuit is drilled very straight forward. Nothing like the earlier models used on a lot of chainsaws. Especially on what I discovered on the Tillotson HL-19, 63 AND 87. There are like 4 different variations of the one HL-19 carburetor. It was used on many of the earlier Super series chainsaws.
Just looking at how the fuel passages were routed thru the carburetor body to feed the low and high speed circuit is nothing that makes much sense. Especially in how simple the newer model HL-93 and some others I have torn apart are drilled. I dont understand why some of the these models seem to be so complicated.
Maybe this is something I can run by Frank. You know him being an engineer of some kind. Engineers are people who supposedly solve problems. I would be curious what his take is on why some of these earlier carbs seem more complicated of how the design is in fuel passages.
I ended up matching the fuel circuit to the HL-93. The throttle bore is about the same size. But the venturi and air horn are bigger. So if the engine wont stay running at idle from being lean, should be easy enough to open up the idle hole to the next drill size. If the engine starves thru the midrange when trying to accelerate, then the transition hole can be opened up a little bit more.
The same fix if motor falls over or acts like is starving for fuel at top speed. The high speed circuit can be opened up slightly more. This is also taking account for both high and low speed needles stay with in a certain range of adjustment. I hope it works out that easy. If I am right about this or can figure out if much different, can dial this carburetor in after some track time.
If things work out, maybe there wont be any reason to buy a bigger carburetor just to keep up with the competition.
I finally rigged up a quick way to mount up these Tillotson carburetors to the small Craftsman lathe. It is just a flat plate with the right bolt pattern drilled and tapped with two studs. It is also centered to the lathe spindle. So makes it real simple to get the carb running true.
The throttle bore is easy enough to machine out larger. These castings are very thin where the butterfly fits. Opening the bore out to 7/8 inch, the wall thickness is less than 1/32". But there is plenty of meat around the flange and bosses so not to become an issue.
It is really straight forward to bore out the air horn. Hopefully it can be taken out even bigger than 15/16". One thing sticking in my head is wanting to add an extension to the end of the carb face. Then machine it to blend to the bore size of air horn. Also radius the outer face to help with the air flow. There are also several different shapes of the venturi on these. They all obviously work, but dont really know enough to choose one shape over the others.
From the transition point of venturi, or the large diameter before choking down, it will taper out bigger to the outer face of air horn. The small extension will continue this taper even farther before the large clean radius to outer face of extension.
This idea is sort of like bolting an Azusa velocity stack to the carb. But instead of being that long, will only lengthen the carburetor about one half inch. I wonder if there are any specific formulas that could get me an approximate length of carburetor or anything about length of air horn. It would make sense that there would be a certain length for optimum air flow at a particular rpm range.
The Azusa velocity stack looks really cool. But sure seems long. You gotta wonder if someone did all the leg work on it to come up with the length? Maybe this is something to run past Frank for some input, next time over to his place.
So my first major modification for the carburetor will be to machine a 1/2 inch thick aluminum spacer that is 1 3/4" outside diameter. Then drill the two mounting holes that measure 1 3/8" bolt circle. On the lathe it will be machined to blend the extension matching the air horn bore.
Wallah, I have something cool looking if nothing else. If there is a formula to follow, that would be great. But for now this is good enough to start with. The added length should improve incoming air speed at a given rpm and maybe increase the amount of flow, before compressing thru the venturi. Sounds plausible, and if I am on the right track, so to speak, may improve the performance of the engine enough for the effort.
Checking dimensions of some carbs in catalogs, I went with these more common ones. The HL-63 HL-87, HL-88, HL-92, HL-93 and HL-101 for comparing between each of them. They are all the same in basic appearance. Most of the difference is in how the fuel is being fed thru the passages. Below are the dimensions of some of these:
throttle bore venturi air horn
HL-87 13/16" 11/16" 7/8"
HL-88 7/8" 11/16" 7/8"
HL-92 13/16" 11/16" 7/8"
HL-93 7/8" 11/16" 7/8"
HL-101 7/8" 11/16" 7/8"
After a lot of trial and error and a good bit of luck, I managed to bore the first carburetor out with success. Like a lot of success.
So basically all of the above carburetor bodies are of a similar casting. Some minor differences, but nothing affecting how much one can get bored.
I ended up with these finish dimensions using both HL-15 and HL-63 bodies practicing just for lathe fodder.
New Old
Throttle bore .890" .811"
Venturi .780" .687"
Air horn 1.015 .875"
I am so stoked about this first bored out carburetor. It turned out great. Unfortunately the completely finished carb body was no good before I started, since there is a big chunk broken out. It was only the first one to try and find out just how big it could go. The other one is not far behind to being finished out.
After figuring out how much I could get away with boring these, then added the air horn extension and went even farther. So one carburetor body is now bored with the extension and one without.
The first usable carb also has the air horn extension. I was able to open it up to 1.050" and taper down to the finished ID of carb body at 1.015". I dont have a clue yet if all of this modifying will actually help or not. But I can only hope. The next practice day cant come soon enough.
All of the holes drilled feeding the fuel circuit on these different carb models were mostly the same size. It looks like the best idea is to go with the hole sizes from the HL-93. Since it is the first carb built specific for the McCulloch Mc6 and Mc10 kart engines.
The fuel circuit is drilled very straight forward. Nothing like the earlier models used on a lot of chainsaws. Especially on what I discovered on the Tillotson HL-19, 63 AND 87. There are like 4 different variations of the one HL-19 carburetor. It was used on many of the earlier Super series chainsaws.
Just looking at how the fuel passages were routed thru the carburetor body to feed the low and high speed circuit is nothing that makes much sense. Especially in how simple the newer model HL-93 and some others I have torn apart are drilled. I dont understand why some of the these models seem to be so complicated.
Maybe this is something I can run by Frank. You know him being an engineer of some kind. Engineers are people who supposedly solve problems. I would be curious what his take is on why some of these earlier carbs seem more complicated of how the design is in fuel passages.
I ended up matching the fuel circuit to the HL-93. The throttle bore is about the same size. But the venturi and air horn are bigger. So if the engine wont stay running at idle from being lean, should be easy enough to open up the idle hole to the next drill size. If the engine starves thru the midrange when trying to accelerate, then the transition hole can be opened up a little bit more.
The same fix if motor falls over or acts like is starving for fuel at top speed. The high speed circuit can be opened up slightly more. This is also taking account for both high and low speed needles stay with in a certain range of adjustment. I hope it works out that easy. If I am right about this or can figure out if much different, can dial this carburetor in after some track time.
If things work out, maybe there wont be any reason to buy a bigger carburetor just to keep up with the competition.