Big Drag Carb

#1
This is a project thread showing how a carb can be altered for specific applications. The carburetor is for a well known builder and racer who needed a richer piece for a monster GX200 drag engine. This is a large bore Italian PCR carburetor and was too lean for the racer. The first thing you will see is the map of the wet side showing the size of the circuits and needle holes. One problem we saw is that the low side needle hole is not very big and does not allow sufficient fuel to flood the low side well. The first thing to do is to mill a trench and enlarge the hole.



In the next picture you can see the trench milled and the low side hole opened up. this will dramatically increase the volume of fuel to the needle holes. What you also see here is the three holes in the fuel well plugged. this was done by tapping 0-80 and 1-72 threads in the two larger holes and using brass screws to plug. The small .020 idle circuit was plugged with .020 carbon fiber rod coated with marine epoxy. The well was then cleaned up with an end mill. Keep in mind this is all being done in ahole the size of a penny.



The reason for plugging these holes (which are drilled through to the bore) is so we could fabricate a nozzle to allow a larger fuel dump on launch and to get a better signal off the bottom. The nozzle was made out of a brass machine screw. A .125 hole was drilled in the well as far aft as possible. Then it was tapped and the screw inserted with red loctite so that it protrudes .040 into the bore of the carburetor. Once the hole was drilled to .082, the nozzle was milled flat and a small chamfer was cut for better flow.

Nozzle installed:



Nozzle as seen in bore:



This custom blueprint job is the stuff I enjoy doing since it will be used for flat out racing. Using a nozzle on the low side is not something you see very often. I got the idea from another carburetor I did for the same customer. It was a swiss carb and works really well for drag racing. I will post more alterations made to this carb as I go along. The real test will be on the track, so we will have to wait and see how things go. thanks for reading.

Anyone who needs any help or is looking for special work on Tillotson or similar carburetors can feel free to give me a PM.
 

C9H13NO3

Active Member
#3
This is a project thread showing how a carb can be altered for specific applications. The carburetor is for a well known builder and racer who needed a richer piece for a monster GX200 drag engine. This is a large bore Italian PCR carburetor and was too lean for the racer. The first thing you will see is the map of the wet side showing the size of the circuits and needle holes. One problem we saw is that the low side needle hole is not very big and does not allow sufficient fuel to flood the low side well. The first thing to do is to mill a trench and enlarge the hole.



In the next picture you can see the trench milled and the low side hole opened up. this will dramatically increase the volume of fuel to the needle holes. What you also see here is the three holes in the fuel well plugged. this was done by tapping 0-80 and 1-72 threads in the two larger holes and using brass screws to plug. The small .020 idle circuit was plugged with .020 carbon fiber rod coated with marine epoxy. The well was then cleaned up with an end mill. Keep in mind this is all being done in ahole the size of a penny.



The reason for plugging these holes (which are drilled through to the bore) is so we could fabricate a nozzle to allow a larger fuel dump on launch and to get a better signal off the bottom. The nozzle was made out of a brass machine screw. A .125 hole was drilled in the well as far aft as possible. Then it was tapped and the screw inserted with red loctite so that it protrudes .040 into the bore of the carburetor. Once the hole was drilled to .082, the nozzle was milled flat and a small chamfer was cut for better flow.

Nozzle installed:



Nozzle as seen in bore:



This custom blueprint job is the stuff I enjoy doing since it will be used for flat out racing. Using a nozzle on the low side is not something you see very often. I got the idea from another carburetor I did for the same customer. It was a swiss carb and works really well for drag racing. I will post more alterations made to this carb as I go along. The real test will be on the track, so we will have to wait and see how things go. thanks for reading.

Anyone who needs any help or is looking for special work on Tillotson or similar carburetors can feel free to give me a PM.
Very nice! I need to send a carb to you that I have. It is a 166b and is going on a stock flatty. Think it'll work ok?
 
#6
A couple of questions Korny...

The marine epoxy.
Is that a polymide epoxy the kind used on hulls?
How resistant is that to Methanol ( and the red loctite too is it effected by methanol )

Now speaking hypothetcialy of course ( no oe here builds anything lol )
Lets say a fellow had some thing larger than he originaly intended that flows MORE fuel than he wants. How would a fellow go about tightening up a tilly fuel curve?

Somethings like the H needle seem clear. A blunted one is what see more often for meths so would going to pointed one help fine tune that?

Time permiting I have a pop off gauge I need to post a picture of. Shoudl provide some cheap laughs.....
 
#7
A couple of questions Korny...
The marine epoxy.
Is that a polymide epoxy the kind used on hulls?
How resistant is that to Methanol ( and the red loctite too is it effected by methanol )
The Marine Tex is excellent epoxy and has been used for years by racers in gasoline and methanol environments. I use red loctite on shutter screws and seat gaskets and have to use a torch to remove them. That said, the hole I plugged with the rod was .020 and the rod was .020 almost snug. The coating was less than half a thou or so. I don't use the stuff for larger holes as you can see by my threaded plugs, but it is the best epoxy I know of for this work. The red loctite on the nozzle was to keep it from spinning in while I finished drilling the fuel port. The threads on that nozzle were 80% interference fit, so the loctite is just a little overkill insurance.

Now speaking hypothetcialy of course ( no oe here builds anything lol )
Lets say a fellow had some thing larger than he originaly intended that flows MORE fuel than he wants. How would a fellow go about tightening up a tilly fuel curve?

Somethings like the H needle seem clear. A blunted one is what see more often for meths so would going to pointed one help fine tune that?
Sometimes a needle change will get you a little more adjustment since you will have more needle in the well. But, that's not going to get you from alky to gasoline by any means. What you see in this fuel well is what needs to be done to reduce hole size. BUT.....the tough part is closing up the needle holes. You have to make drill and tap guides, plug the holes and re-chamfer the hole to accommodate the taper of the needle. It's very time consuming only worth the time if you are set-up to do it and have a bunch of free alky carbs to convert to gas.

Time permiting I have a pop off gauge I need to post a picture of. Shoudl provide some cheap laughs.....
 
#17
good stuff k-dog. i like the pictures of the work you do. glad you are not afraid to show what you do. i cant wait to get my carb back from you. if you have time to post up pictures of it here, do it. it lets us all see your handy work, and what a tilly or ibea, etc looks like inside.
 
#19
good stuff k-dog. i like the pictures of the work you do. glad you are not afraid to show what you do. i cant wait to get my carb back from you. if you have time to post up pictures of it here, do it. it lets us all see your handy work, and what a tilly or ibea, etc looks like inside.
Thanks. will do.
 
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