The Bench Racers Journal

[Terry Bentley]

Chapter 67

After all the effort I put into the first two practice carburetors, it was time to get some outside input about what has been done on them. So the only person convenient that I know with higher learning and kart racing experience is Frank. At least someone to have a discussion with about any possibilities of them working, or not working, from the mods that have been done. I dont know what to expect from this, and really no one else to turn to about it. Just about anything is better than a blank stare when I ask anybody else. But what Frank has to offer could be just as useless.

Today Frank was out in his garage again. He was intently focused on his pride and joy Dart super prototype kart. This is sort of a first to see him actually working on something at all. Except for his engine sitting torn apart on the desk during my last visit, I have not witnessed him doing real constructive work on anything. Maybe Frank has some hidden skills I have yet to discover. 

So I walk up behind him in the garage and spit out my usual, "Hey Frank". He just about jumps out of his skin. He then turns around and raises up his hands into some kind of judo or kungfu stance I have only seen in Japanese movies. Never in real life before now.

My first thought was maybe I am going to be made into chop suey if he had followed thru with that specialized lethal stance he was about to engage with just then. I did raise my hand up to block his karate chop. I didnt really block it more than just stopping him from slicing me in half. Not really, he didnt seem to have enough body strength to follow thru with his hand ax to cause any physical damage. Or maybe just being so disciplined that he was able to stop immediately when realizing that maybe I might not be a dangerous threat to his livelihood.

OH CRAP! Sorry bout that kid. Its my naturally ultra fast reflexes. Sometimes they are so automatic, even surprise myself of how quick I am. My instinct usually keeps the helpless and innocent from getting really hurt. Guess you are in that safe crowd or things might have ended up really bad for you right now. So whats up? I assume you need something, or is this an unlikely social visit?

Gee Frank, social is something too. So I guess you can say it is both. I just wanted to show you some things I have been working on. But I see you are busy working on the kart. I dont want to interrupt anything important. No problem to drop by some other time.

No trouble. Just decided to tear down the kart and do a real thorough super precision detail on reassembly. I realized after you finding some things that were questionable, maybe I should go thru the whole system. Since the motor is now ready to prove its worth, no reason to put it on a kart that isnt completely set up top notch for racing. The better this kart is set up now, the less effort it will take me to drive out there on the track. Ya know what I mean? So whatcha got goin on today?

Frank, promise you wont laugh. I have been beating around some ideas in my head for most of the summer. Finally focused some time on them and got a couple sorted out. These are two dead carburetor bodies. I have a lot of different models from various chainsaws. For the most part they all very similar. These here I went ahead and bored out just about big as I could go without cutting thru the casting anywhere. What do you think?

Well I gotta say they look really neat. I am guessing you have not tried them out yet. What formulas did you use to remap the fuel circuit? I assume you did recalculate everything, so the fuel delivery is correct . I already said you cant just run a bigger drill bit thru a hole without changing how the rest of a carb works.

Of course Frank, I realize that. It was surprising to me that there is very little difference in size of most of the fuel passages on any of the carbs I checked over before modifying these two here. It didnt matter what size throttle bore or venturi, most of the low and high speed orifices were basically the same size. My plan is to stick with how the HL-93 carburetor is drilled to start with. Then make adjustments from there if something does not work out so good.

That sounds like a solid plan and all, if you were a simpleton. But you do have to realize there are hundreds if not thousands of hours in research and development to successfully build just one of these carbs. Likely a whole fleet of specialized design engineers working for months or longer just to develop any of them. And that would be for a very specific application.

Things really go haywire when you throw a monkey wrench in there with these modern racing engines. The demand for properly metered fuel is so much more complex now for real on track race conditions. The engineers would have to rewrite the book on what they have developed up to date for industrial and commercial use of those carbs there.

I dont want to burst your bubble, but its like a one in a million chance either of these may actually work. I can almost guarantee they wont even work as well than before you got happy with a big drill bit. Things are just not that simple. If they were I would be long out of a job. So good try at doing some pretty machine work on what you got here, but dont expect too much when they get tested on the track.

Wow Frank, you sure know how to stroke someones ego. With all that confidence buildup, I might as well go jump in front of speeding locomotive. Just go get it over with already. I wasnt expecting these carburetors to be a bolt on and go faster super trick. But I dont have any problem working on figuring out what it will take to make them perform a lot better than the way they were when stock.

There is one model I have several of. I am hoping these will be my focus when everything has been worked out. You know all those highly technical engineer calculations you are talking about. The HL-87 does not have provisions for the choke assembly like all the others. That would be two less holes needing to get plugged for a clean pathway thru the air horn.

Tell ya what kid, I have a super duper deal for you. If you like playing around with these carbs so much, there are a couple of them around here you could have cheap. I mean like really cheap. So dont try your pathetic sales pitch on me this time. My Frank Price is firm. Well worth it considering what they cost new. All I want is a dollar for each of them. Nothing more and nothing less. Take it or leave it.

Frank goes over and starts shuffling thru things under his work bench. Then comes back with a very small cardboard box. He pulls out three Tillotson carburetors and sets them down in front of me. They look complete, and nothing in particular stands out about any of them. I check each one over carefully. These all have the long standard style throttle shaft. Easy to use on a kart engine by bolting on a small arm with swivel. I dont see any reason to pass these up.

What do you think kid? Interested or not?

I look at the model number stamped on each flange. One is an HL-35, an HL-65, and HL-66. So long as none of these has been butchered inside, I dont have a problem with snagging them up. OK, Frank, its a deal. Do you take personal checks?

Huh? You going to write me a check? Even if you do have a checking account, which is highly unlikely, H, E, double hockey pucks, NO. I dont want a personal check. Washingtons in silver or silver certificate. That is what I would prefer for payment. No haggling this time either, just real money. Got it?

Dont you mean hockey sticks Frank? Alright, here you go. Payment in full. Dont say I ever leave a debt hanging loose in a noose. Are these the only carburetors you got? You know so long as I am throwing George down the drain, what are a few more?

Before I take your money kid, just to be clear....I have tested quite a few carbys in real world race track conditions. Spent plenty of hours back to back with these very same carbs. None of them was nearly fast as that one over there on my personal Westbend 580. So if you think anything different about these, since they came from me, I am clearing the air about it right now. Dont make any assumptions since I am fast on the track. Just because they are in my spare parts bin, doesnt make them anything special. To me they are past tense.

Gee Frank, dont be so modest. I know anything coming from you is much better than what I own. All my parts have already been thrown away at least once before. So heck yea, these should be in near perfect operating condition. You know good used carburetors in top notch shape. So we have a deal. I like what I see. There are two here I dont think are in my stash. If nothing else I can surely use some parts from them. The throttle shafts are straight and dont have any play. Saves me a little bit of money not having to buy new ones when putting some of my other carburetors back together.

Oh well, I didnt think about keeping them for spare parts. Guess some of the pieces might be interchangeable. But I see the throttle shafts are different. What makes you so sure they would even work on the ones for yours you got there? I mean they dont just make a bunch of different parts that can all do the same thing. Too many pieces to keep in stock or have to be available in inventory for the manufacturer and repair shops. Just doesnt make a lot of sense if either shaft would work in place of the other one.

Dont know what to tell you Frank? But looking at the throttle shafts on your carburetors, they should bolt up right in place of the missing ones on mine. But it dont really matter. It they work then fine. If not, I will find something else. Hey Frank since we are talking carburetors look at what I got here. What do you think about how the fuel circuit is run in one of these HL-87 carburetors?

I am completely lost on why they designed the fuel circuit this way. Just does not make sense to me. But then again I am not a college educated engineer. I have not figured out why that brass tube is pressed into where the high speed needle goes. This is one thing that is a hold up on boring any of these particular ones out just yet. I want to understand how it works first. Then if no special reason, go ahead with my plans.

I would like to use this model carburetor since there are several in my pile of parts. Also the holes for choke shaft are not drilled like I already mentioned. One less thing to plug and machine back out. But if something is weird about the high speed circuit, then will just pass on them and use some of the others.


HL-87

Well this is a strange duck. I dont know why it is done this way. Let me study it for a moment. How about snagging me a beer out of the Frigidaire. A cold one sure can help the ole brain do its job sometimes. Just make sure the bottle doesnt get all shaken up there. Dont even think about pulling that one on me. I wasnt born yesterday. There is a bottle opener hanging on the wall next to the fridge. Go ahead and pop the cap, just so there is no funny business.

Frank I am hurt. How can you think I would do something childish like that? Its not like I am some depraved 10 year old child that is desperate for attention. I might actually think about doing something like that. But at 13, it would only be a passing thought. I could never act on an impulse like that no matter how strong the urge is. Well so far as you know I wouldnt.

During the few minutes Frank studied this Tillotson HL-87 carburetor, he downed that first beer in not much more than one big gulp. Then two more right after that, before going into depth about the workings on the fuel circuit. Well I will tell you something right now. This carb looks like a complete custom job. A real professional build too. Probably built engine specific for a particular purpose. Do you know what type of racing machine is it built for?

I am fairly certain it came off a McCulloch chainsaw Frank. The chainsaw actually fell off a work truck. Owner didnt want to spend what the broken parts would cost to replace. So just went ahead and bought a newer saw. Its one of their lower end saws too. I believe it is a model 1-41.

Well one thing for sure I will tell you kid, this carby cant be stock. No way. Just from how it is designed. It would cost a ton of money to mass produce them like the way this one here is made. It also looks like specifically developed for high rpm use. There doesnt seem to be any way to feed fuel to the low speed circuit. Just look at how the passage is run right here. Take my word, this baby has to be a one off special.

Are you  for sure it came off that particular saw? Maybe someone was building a special competition saw or something like that. I have worked over plenty of carbs in the last few years. I dont normally miss something like obvious as this. Converted a couple over to alky too. Nothing like this one will function on a stock engine. Another thing you mentioned about the choke. Mark my words, I know what I am talking about. They dont build production carbs without chokes.

Whatever you say Frank. I will keep that in mind. So whats the story on these old carburetors of yours anyway? Do you remember much about testing any of them? Like this one HL-35? I see the low speed needle is screwed in all the way. The head of needle is almost touching the body. I didnt think they would screw in that far. Looks a lot more than the other needles will go.

Oh yea, that one is a complete dud. Must be a factory glitch. You know they sometimes slip thru quality control. It sure didnt want to idle at all. Just loaded up and died every time I tried to get it going. When I finally got it to somewhat idle in the pits, that sucker would just lay flat over and die when giving it gas. If I could ever make it out on the track, it just would not run worth a damn.

Then if I get it working out on the straights, any slow turn at all, would usually be running too rich to clear out and not accelerate again. Just didnt bother past that. I had others that were ready to try out. So there was no reason spending any time with that one. Just too many issues from the start.

You see the red one right there came off my first kart. It was one of those from Sears. Talking about one heavy tank! I learned how to really drive well on that kart. This old red carb was the only thing left undamaged after the motor finally cratered. It was a Power Products AH-82. One of those earlier styles made of magnesium. That engine was horribly out of ba1ance when you put the revs to it. `Bout shook my teeth loose every time I tried to drive it. After the motor went caboom, I bought the Westbend 580. Good engine that one is.

Last summer I was running it and just remember hearing a loud pop! Then that very carb in your hand shot forward and landed right in my lap. I was really hauling ace down the straight that one time. Nearly leading the race too. Fortunately there was enough speed for me to coast it back all the way to the pits. I looked behind and the jug had broken off and was gone. I mean literally gone. As in missing. What was left of the piston and rod had kept rotating and slinging around and nearly cut the crankcase in half. Sure made a lot of racket. There were bits and pieces of that engine all over the track.

I tried that carb on my Westbend once, but for some reason, could never get it to even fire up. Like it would not pump fuel. I never tore it apart past the one time to see what was up about it. Lost interest after that. The Westbend has a newer style bigger carb. An HL-88. Saw no reason to test anything else. You get a really fast package, then no reason to screw things up by changing parts out all the time. Eventually something goes wrong and another dead motor. Racing can get real expensive that way.

Alright Frank, thanks for all the information, but I gotta cut out. So you think it is just a complete waste of my time modifying these carburetors? Is that what I understand? You dont expect me to have any luck making something from them. OK, makes a little bit of sense. But I am still having fun working on them. Maybe just for kicks, I will get one together and try it out next time there is a practice day. If it sucks really bad, you can be an "I told you so". Wont hurt my feelings at all.

Kid you dont just make something expecting it to work well without a lot of calculations to confirm the outcome is what you are after. You are just playing Russian roulette. Otherwise, if you like playing with them, well, more power to ya. The finish looks first quality on these two here. It is a shame things are not that simple. Otherwise everybody with a hand drill would be doing that work for themselves. The real ambitious ones would be trying to pass them off as something special and sell for a good penny or two.

Take it easy Frank. Check you later.

Back at home I went ahead and tore apart these three carburetors from Frank. My curiosity was up and really wanted to find out why the two of them were not working properly. It sure wasnt too difficult to see why the HL-35 was having troubles. After removing the welch plug, I found a tiny piece of the body was actually broken off. It is very small boss where the low speed needle seats. That explains why the needle looked like it was screwed in too far too. Because it was.


HL-35

Someone either bottomed out the low speed needle too hard or maybe tried to force a high speed needle in there by mistake. OK, one down. If the needles were left where he had last tried to run the carburetor, then explains a lot. The high speed needle is almost screwed in all the way. It was set way too lean. Might have been the only way to get it  running in the pits. Out on the track it would not do anything but starve for fuel. Now which of the other two should I look over next?

Frank said the red carburetor would not pump any fuel. Wonder exactly what he meant? I also decided to drill out the welch plug on this one. Then looked over everything under a really bright light and good magnifying glass. This carburetor is extremely clean looking inside. If I could remove whats left of the ugly paint, it should look like new underneath.


HL-66

After soaking the carburetor in lacquer thinner, I scrubbed it really well using a small paint brush with the bristles cut down very short. Works great to remove buildup this way. The paint dissolved quickly and was all washed off. Sure enough the carb does look just like new again. 


HL-66 clean

Alright why would this one not pump any fuel? It is the stock HL-66 carburetor for a Power products AH-82. Fuel passes thru this way and pumps down here and finally fills is the chamber. It was working fine when we had it on the engine before that thing blew up. Everything looks correct. Did Frank maybe put the carburetor on wrong? Naw, he surely would not overlook something that obvious. He is after all, an engineer.

I compared this carburetor to the other one which is an HL-65 These are very similar. They both have the same casting number. Also both are used on Power Products engines. It should be close enough for comparison. I looked over both of them carefully, but nothing jumped out. That is until I saw something different between the two. There is a small brass restrictor in the hole that feeds the pulse pressure from crankcase to pumper diaphragm on the flange.


HL-66-1a

The HL-65 carburetor has a standard 1/8 " hole drilled. It is just like most of the others. This red HL-66 carburetor has a brass plug pressed in that hole. There is a very small orifice drilled thru in center of the brass plug. It is only 0.040" in diameter. I checked it with the shank of a number 60 drill bit.

If this is the problem, then why is the plug even there? I looked thru some of the kart mags for possible answers. Sure enough there is an ad for the Sears kart with choice of engines. The PP-82 has a top mounted gas tank. Hey maybe that is the problem. If there is a top mounted tank, then it will gravity feed to carburetor. There is no need for a fuel pump. Or maybe it pumps too much when using it set up this way. So a restrictor was added to keep that from happening.

Franks new Dart super prototype special model has a seat mounted gas tank. The top of tank would barely sit much higher than the carburetor. But it would need to be filled up mostly full to feed the carburetor without a working fuel pump. Could it be something really this simple? Maybe I should confirm everything I am basing this on before making any conclusion.

After much thinking, I decided to use this HL-66 for the first carburetor to officially bore out for testing on an engine. After cutting up those two bad carburetor bodies for practice, this one with be the real deal. By drilling out that brass plug, to match all of the others, now the pumper diaphragm should work correctly. Then drill out some of the orifices in the fuel passages to better match the HL-93.

If this carburetor can fire up and run decent on an engine in the garage, then will be ready for the first track test. Well after some more time to get a baseline to work from. Get the kart running good consistent lap times, then swap out to this new to me bored carburetor and see what happens.

So after tearing the HL-66 completely apart, there was something odd. It actually looks like someone ran a big drill bit thru the venturi. It measures just about 3/4 of an inch, but in a very rough looking hole. From the diameter of the venturi, it just goes straight back until the step for throttle bore. Thats not right. Someone has actually attempted to modify this carburetor the same way Frank was saying would not work. But he didnt mention any problems with this carburetor when it was used on his Power Products engine.

I measured the carburetor all over. Then made sure to record the sizes of each orifice in the fuel circuit. The air horn was 7/8", throttle bore was 13/16", and the venturi originally should have been 11/16". The size is cast into the body under the stacks in the fuel chamber. The red HL-66 and this other HL-65 share the same casting number of 011774 and both should have the same 11/16 venturi.

I tore the HL-65 carburetor apart and checked all of the hole sizes. Everything was the same as the HL-66 except for one hole. It is what I have seen referred to as an air premix orifice. It is drilled thru the outer ring face of venturi that feeds air to the main fuel channel in the venturi. On the HL-66 carburetor for a kart engine, the air premix orifice is drilled out larger at 0.037". This HL-65 carburetor, which is correct for a Power Products chainsaw using the AH-81 engine, is drilled to only 0.026".

So for the first official modified carburetor, this red HL-66 gets the full treatment. I was disappointed to find out the horrible drill bit job to make the venturi larger is offset a lot. I wasnt sure if it would even clean up being so far off center. I have not bored one out much larger than 3/4" yet. Since that is as big as the new HL-115 comes, I didnt know what to expect with going any larger. But this is uncharted territory right now. There really is nothing to loose going forward with any of these. I am going to just do this one really big.

For the tooling, I got lucky. Grandpa had a 5 gallon bucket full of old reamers. He had picked up from an auction thru work. Straight ones, adjustable, and tapered ones. All sizes from 1/8" up to over 1 1/4". After the first couple of attempts on those initial test subjects, I came up with only needing two reamers to do everything at first. That is to get just slightly larger than a 3/4 venturi. But since this one carburetor needs some cleanup on the venturi, I will have to bore it out on the lathe. Just big enough until it is running true again.

The first thing I tried was to use a 7/8" reamer in the air horn to help center the carburetor on the fixture. So no fine adjustments needed to getting it running true. Next was to run the 15/16" reamer in until it reached the base of venturi. Then follow up with a 1" reamer. I found an odd reamer that had a pilot. It looks like something that would align bore kingpins of some kind of big truck. It is another 0.015" larger and slightly tapered down smaller at one end.

Now the air horn is mostly finished. Before flipping the carburetor over, the venturi was opened up large enough running true with the bore. Now the carburetor was turned around and using a 13/16" reamer to help center it on the lathe. There is one very strange looking reamer in the bucket that has a long shallow taper from 1/2" to a 7/8" straight diameter. This one worked out perfectly to open up the throttle bore and then a gradual taper down to the venturi diameter.

Another thing done was to blend the base of air horn to taper sharply down to the venturi. A one inch diameter 90 degree countersink did the trick. I tried one more thing since going this far already. There was a few different Morse taper reamers. One of these was just larger than one inch at the small end. It is the number 4 Morse taper. This opened up the air horn slightly larger tapering out to the flange face. Now the only thing left to do was just blend and polish everything with emory cloth until it looked just right.  

With this first carburetor finished, below are the dimensions of the newly modified HL-66 carburetor.

HL-66     Old      New                       HL-115
Throttle  0.810   0.880"                     0.937"
Venturi   0.687"  0.830"                     0.750"
Air horn  0.875"  1.015 up to 1.030"   1.00"

I drilled out all of the fuel passages to match the HL-93. Comparing the dimensions between every one of the carburetors that I checked out, the HL-93 seemed to be an good average of all the others. A few orifices were slightly smaller and some larger. But none were very far off from the majority of the slightly smaller chainsaw carburetors. This leaves me with some room to drill out the passages larger if something still doesnt work right.

So that ugly looking red carburetor is now a beautiful work of art. Frank might be right. This carburetor could completely suck. But I am willing to stay with it until it does work. Hopefully a lot better than the original. And maybe even good as the new HL-115. A kid can still dream cant he?