The Bench Racers Journal

[Terry Bentley]

Chapter 121

After school today, I spent some time going over how the fixture needed to be built so the carburetors can hopefully be bored out fairly soon. I sort of worked out something in my head, but still not sure about the details just yet. There needs to be some type of plate to bolt the carburetor down regardless of how I build the fixture. Which as of now I am completely in the dark. So at least this is something that can be made at home tonight for starters.

One piece of that scrap aluminum I picked up last week from Mr Sadeskey is 1/2 inch thick plate. I am fairly certain this one part of the fixture wont change so much before figuring the rest of it all out. There will be two sets of small holes drilled so the carburetor can bolt to from flange side and from the air horn.

There will be a hole drilled in the center large enough for that one long tapered reamer to pass thru. The size and shape of this plate is not important yet, just so long as it is larger than the carburetor bolted to it. For now it can be square or rectangle and should work. Then cut down any excess after I figure the rest of this fixture out.

While the flat plate was clamped in my small drill press vice, something hit me like a ton of rocks. Lets say that figuratively so it doesnt cause a major headache or concussion. I mean just like that I saw something without it being there in plain sight. This prompted me to hurry up and finish drilling out the two sets of bolt holes and a large one in the center.

My first thought was hoping this drill press might use number 3 Morse tapered mandrels just like most of the larger drill bits and reamers I am using to bore out the carburetors. With this flat plate now finished, I see how easy it might be to use the drill press on very low speed to modify the carburetor bodies.

Before destroying a good carburetor, I should do a test run with a junker first, just to make certain it does work. If successful, I can finish these tonight. Then just chuck up on the lathe and polish the air horn blending it in to the venturi.

After some in depth thoughts in my head, I realized all that needed to be done is set the travel of whatever reamer or bit being used. Then swap out carburetors one at a time. The flat plate gets clamped in my small vice and carburetor body is centered automatically when reamer is fed down. So long as depth is correct, then all three carburetors will end up exactly the same.

First by feeding the reamer down, it centers to existing bore, then I clamp the vice in place on drill press table using a big C-clamp. So much for semi mass production techniques on the shops lathe in school. Of course this looks like it will work, but I am still thinking about the rough sketch Mr Sadeskey had drawn out for me. Something is making more sense now that I have part of the fixture worked out like he was trying to explain to me at school.

Now that I am seeing things more clearly, this fixture really isnt that complicated. What I kept missing on is how the carburetor body would be centered when chucked up in the lathe. The carburetor still needs a way to be moved around on the fixture itself to get centered to the throttle bore or air horn.

I decided to scrap the idea of using the drill press tonight. Instead go ahead and work out any bugs on the main fixture. It sure isnt complicated as I first made it out to be. So long as there is enough adjustment, nothing even has to be running perfectly true when mounted in the lathe chuck. Well it does need to be close. I will try to make it run true as possible with what there is to work with here at home.

It amazes me sometimes how much work can be done without constant interference and distractions. Tonight I managed to complete the fixture and so far should work great set up on one of the lathes at school tomorrow. Using my small lathe here in the garage, there is too much guess work and compromises trying to hold those big reamers centered, since they dont fit the tail stock. Otherwise I did manage to work thru all the steps and figured out how to do everything on the scrap carburetor.

OK, I know what your next question is. You want to know why I am going to all this trouble if I have already bored one out on the small lathe here. Well that first carburetor was all guess work. I was doing the work in small steps. So nothing was set up for a repeat performance, so to speak.  With the fixture and now sure of which tools I need have eliminated any problems attempting to repeat what I already tried on that first carburetor.  

This morning I am waiting for Mr Sadeskey to unlock metal shop. He accidentally locked the keys in his car. He did say a spare car key was hidden, but did not tell where. With last nights confidence boost, I am really hoping to finish these carburetors today before school starts. The teacher finally returns and now it is time to show him the fixture.

Mr Sadeskey looks over everything and studies the fixture closely. I had already bolted up a carburetor body so he could see exactly how it fits together. While he kept his focus on the rectangular plate, I explained about the over sized holes and bolts that hold it together.


carb fixture 2


flat plate


carb fixture 1


carb plate screws

You may wonder why I went to so much trouble cutting these bolts down to work. Why not just drill the hole oversize instead? Well that is what I had planned to do. But while digging in my big bucket of bolts and beyond, I found these and thought how cool. They should work perfectly.

When the fixture is chucked up, I use either a center in tail stock or even a reamer that fits in one end of carburetor body to make sure it runs true. Then tighten the two special 1/4-20 screws that holds the plate. The shank of screws are undersized and holes in plate are drilled out larger. So there is some movement to get carburetor centered. You tighten the screws and start boring it out.

This is very well thought out Rick, urr, uh his brother. Sorry uh....Terry. You did a good job figuring out how to set it up and work around potential issues. I think you are on the right track to reaching your goal.

reamers

So these are tools you are using to machine out the carbs?

Yes sir. On the left is one just for throttle bore. The next one is for venturi. These next three will open up the air horn in stages. That big tapered one is only used to flare out the end of air horn.

With what I brought here today, the carburetor bodies should be all done with nothing else but finish work. That can be done at home if I run out of time here in school. I almost short cut the fixture and planned to use just that flat plate on the drill press last night at home. But ran into some snags and afraid they might not all turn out exactly the same. Or worse even messing them up.

It is good you thought all of this thru so there will be less chance of ruining them. You show good mechanical aptitude. I think your little project is going to work out well from what I can see looking over this fixture here.

If you want to go ahead and set this up and start, then get busy. I have some school work to attend to. But I will be back to see how things are going. That is unless you need my help directly.

No sir, I dont want to hold you up. I have worked out most of the problems in my head and dont expect any major issues than cant be addressed. If I do need your help, will you be around somewhere?

Oh yes, I will either be in the attendance office or next door in the teachers lounge. If you do need me, I can help you out, not a problem at all. Dont hesitate if something is amiss. That paperwork is not nearly important as teaching you kids. Well in my eyes it isnt, but the school secretary might not always agree with that.

Once again, I was left alone in the shop to work this morning. The last time here in the shop Mr Sadeskey actually spent most of the morning showing me how to operate this lathe and giving me ideas on how to build the fixture. It would not take too many mornings with his help to find out just how useful this shop could be for most of my other projects.

I looked over my shoulder and reminded myself of the near disaster where the chuck key was embedded in the wall. So chucking up the fixture in lathe, I used one of the reamers to center the carburetor. Then tighten up those two screws. Checking over everything again, making sure the fixture is secure in the chuck, AND setting the chuck key down on the work table first, it is time to power up the lathe and make some chips.

Working thru the usual trial and error, again heavy on the error part, I figured out it was better to bolt the carburetor on flange side first. The reason for this is to machine face of air horn to make sure it is flat and square to the bore. It seems that surface is not exactly square to anything.


Machine air horn

Next step is run the 13/16 reamer in and open up the venturi. As of now the biggest carburetor venturi is still at 3/4 inch. So this one is bigger. Follow that up with the 15/16, then right behind using the one inch reamer to open up just the air horn. This diameter stops then transitions down sharply right at the base of venturi.

I am using a slightly larger reamer next that dad thinks originally was for kingpin bushings on something. Not sure if a big truck or could even be for a forklift. He did not have a clue. This reamer is only 0.015 inch larger and tapering smaller right before blending into where the venturi chokes down.

bored out air horn

This is followed up with a #4 Morse tapered reamer. It is only used to taper the outer part of air horn. Just a slight angle like a funnel shape. This makes it a lot easier to blend everything together later using course Emory cloth. Then following that up with some finer grit for a smooth finish.


throttle bore

With the face of air horn now machined square to the bore and flat, I can flip the carburetor around and run a long shallow tapered reamer thru to open up the throttle bore measuring just over 7/8 of an inch. This is big as these chainsaw carburetor bodies can be opened up before breaking thru the low speed fuel chamber. The long shallow angle of reamer will taper down and blend smoothly to diameter of venturi.

Normally most carburetor throttle bores are cut in straight with just a square edge. Since the air flow is from the other direction may not affect anything if done this way. Well that is what I am assuming since all of the Tillotsons are machined the same.

So I took a stock Tillotson like used on most chainsaws over the last 5 or 6 years opening the original throttle bore from 13/16 to just over 7/8 inch. The venturi could be 1/2, 9/16, 5/8 or 11/16 diameter on any of these carburetors. Now this one measures just over 13/16 inch when finished. The tapered reamer used for throttle bore blends to the 13/16 diameter that was first done and opens it slightly larger.

The most obvious difference in appearance is how much the air horn is enlarged. On all of these stock carburetors the air horn is 7/8 inch. After getting bored out, the air horn starts at 1.015 and tapers out larger to 1.030 inch. So nothing restricting air drawn in to venturi then pull fuel thru metering circuits.

I had just over an hour working in the metal shop here at school this morning. Mr Sadeskey left me to my desires. I have been successful in modifying three more carburetors today. There was enough time to use the Emory cloth and blend the different angles and transitions from venturi out to the face of air horn.

Mr Sadeskey came strolling back in shortly before the first bell even rang this time. I was already cleaning up the lathe and boxing all the reamers and parts back up. As he approached I handed one of the now finished carburetor bodies to him for final inspection.

I am very impressed Rick. Uh, anyway this looks like excellent work. It is a shame you are not enrolled in my shop class this semester. Your project would definitely get very high passing marks. I hope you put this much effort in other endeavors. If so, you should have no problem overcoming obstacles than may fall your way in life.

I will be very happy to help you in the mornings when I am available. Which is usually most days. I really enjoy working with students that come to school to actually learn things. Just give me a heads up when you want to use the shop. I am already looking forward to having you in class next semester too.

After the carburetors were bored out I made sure everything was cleaned and put away. Then wiped down the lathe with oil. I heard the first bell ring and decided it was time to head out. After washing my hands, I thanked Mr Sadeskey,  loaded up my toys then walked across the hall to draw more stick pictures in Drafting.

When we jumped off the bus after school, Steve followed me home again and in to the garage. Not so common on school days, but I can tell he is itching to get back out to the track and race again. He had time to finish most of his homework on the bus ride home. Any more progress? Steve asked me this since we normally sit together, but the bus was already at capacity and we were at opposite ends today.

Heck yea more progress. We are on the home stretch Steve. I hope by this weekend it might be possible for at least a practice at the track. I wish there was more time to do a couple more sessions, but I want to go race. The last few weeks with all this work is getting old.

Even tho I am enjoying working on the engines, it is time to put some laps down. I think everything is ready. Just need to bolt the engines together. I worked thru most of the holdups and will find out if everything actually works or not. I am leaning towards it working tho. Like in a big way.

OK, the carburetors are finally bored out so they can be put back together. Do you want to work on those today?

Heck no, I dont want to get near those things. There are too many little parts inside to screw up or loose. No, you keep up the fight, I will be here for moral support. And if you got a Coke or something, even a Mountain Dew, then I can grab a couple for us to drink.

Tell you what Terry, I will bust my butt when engines are ready to bolt up to the karts. I will even put your engine on for you. That is how nice of a guy I am. Hey man, got any more M&M's left? I sort of got a sweet tooth today. You know maybe trade for some work?

Wow Steve you are so helpful some times. Just hardly ever when you are over here for some reason. I reached in my pocket and pulled out a hand full of change from coke bottle deposits. Here, go double up on everything while I get busy. Steve snatched it out of my hand really quick. Dont open up my M&M's and eat any of them this time. Make sure not to put them in you pocket either, I dont like mine all melted.

Gee, you sure are bossy today. Maybe I should go home and come back over later in case this is a Twilight Zone moment taking place right now. Steve ducked outside and disappeared as I tried to take a swing at him.