Category: Tech

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Saving Those Heads
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Saving Those Heads

How to Fix Destroyed Vintage Cylinder Heads

By Len Emanuelson

One of the real issues with running vintage kart motors is
that the parts are hard to find and are getting very expensive. There are a few
engines like TKM and a few IAME based engines that you can still buy new
replacement parts, but a typical cylinder head can be $250 or more. So it pays
to try to salvage what you have if you can. In most cases all that’s required
is a manual lathe and a $60 arbor from LAD.

I’m no machinist, but I have a lathe, and at the rate I blow
stuff up, I fix my own engines. In this case a friend brought me a TKM 150cc
cylinder head that had been pretty well beaten up by a piston failure. As you
can see in the “before” photo, we mapped out our strategy by numbering the
sequence of the surfaces to be machined. The objective is to end up with the
squish band the same relative distance from the sealing surface as it was
before machining. Another objective is to remove the least amount of material
possible, because it is the head’s total mass that acts as a heat sink, and
keeps your air-cooled motor cooling as efficiently as possible.

Follow along as we bring this cylinder head back
to life in less than one hour of time in the shop.

Step 1

First, inspect the damage to see if the head is even repairable without welding. We decided to first machine the cylinder sealing surface (#1) .020”, the head-to-muff surface (#2) .020” second and the squish band (#3) .020” third. If the squish band did not clean up at .020” we would have started over on all three surfaces with an additional .010” cut. The object is to remove the least possible amount of material and still restore the surfaces.
First, inspect the damage to see if the head is even repairable without welding. We decided to first machine the cylinder sealing surface (#1) .020”, the head-to-muff surface (#2) .020” second and the squish band (#3) .020” third. If the squish band did not clean up at .020” we would have started over on all three surfaces with an additional .010” cut. The object is to remove the least possible amount of material and still restore the surfaces.

Step 2

Here is the LAD cylinder head mandrel. It fits into the lathe chuck or collet and then the head screws on to the mandrel via the sparkplug threads.
Here is the LAD cylinder head mandrel. It fits into the lathe chuck or collet and then the head screws on to the mandrel via the sparkplug threads.

Step 3 & 4

Step3
The mandrel should stick out of the lathe chuck or collet just enough to screw on the head and clear the fins. This provides the least amount of runout for accurate machining.
The mandrel should stick out of the lathe chuck or collet just enough to screw on the head and clear the fins. This provides the least amount of runout for accurate machining.

Step 5

We first machined the head-to-barrel sealing surface. To do this we ran the lathe in reverse (clockwise) so that we could easily see the cutting tool. Important because the tool must cut all the way into the corner of the “step” for the liner. The cylinder must be very tight on the mandrel or it will unscrew from the force of the cutting tool.
We first machined the head-to-barrel sealing surface. To do this we ran the lathe in reverse (clockwise) so that we could easily see the cutting tool. Important because the tool must cut all the way into the corner of the “step” for the liner. The cylinder must be very tight on the mandrel or it will unscrew from the force of the cutting tool.

Step 6

Next we cut the surface above the muff. We ran the lathe in the normal rotation (CCW) for this operation.
Next we cut the surface above the muff. We ran the lathe in the normal rotation (CCW) for this operation.

Step 7

The angled squish band is machined with the “compound” part of the lathe. Foreign motors like the TKM, most Komets, etc. use approximately an 11.5-degree angle on the squish band. A good way to check the angle your compound (without the lathe running) is to touch-off the cutting tool on the inside edge of the squish band, back the tool carriage off .010” and then move the cutting tool to the outside edge of the squish band with the compound. It should be .010” away from the squish band if the compound angle is set correctly. If not adjust.
The angled squish band is machined with the “compound” part of the lathe. Foreign motors like the TKM, most Komets, etc. use approximately an 11.5-degree angle on the squish band. A good way to check the angle your compound (without the lathe running) is to touch-off the cutting tool on the inside edge of the squish band, back the tool carriage off .010” and then move the cutting tool to the outside edge of the squish band with the compound. It should be .010” away from the squish band if the compound angle is set correctly. If not adjust.

Step 8 & 9

Step8
After cutting the squish band the bowl-to-band parting line was still in pretty rough shape. We readjusted the cutting tool and took a light cut on the outside edge of the bowl area until the damaged area was gone.
After cutting the squish band the bowl-to-band parting line was still in pretty rough shape. We readjusted the cutting tool and took a light cut on the outside edge of the bowl area until the damaged area was gone.

Step 10

A piece of 400 grit sandpaper was used to blend the angle cuts and damage in the bowl area. Probably not how your shop teacher showed you how to do it, but as you can see in the finished photo, it works.
A piece of 400 grit sandpaper was used to blend the angle cuts and damage in the bowl area. Probably not how your shop teacher showed you how to do it, but as you can see in the finished photo, it works.

Step 11

The final check is to make sure the squish band is the correct diameter for your engine’s bore. We are using a 58.8mm piston, so the 58.85mm size is just about right. If your diameter is too small, cut the squish band deeper. If it is too big, cut the cylinder sealing surface some more. After you do a couple of these you’ll get the hang of juggling the dimensions for the desired results.
The final check is to make sure the squish band is the correct diameter for your engine’s bore. We are using a 58.8mm piston, so the 58.85mm size is just about right. If your diameter is too small, cut the squish band deeper. If it is too big, cut the cylinder sealing surface some more. After you do a couple of these you’ll get the hang of juggling the dimensions for the desired results.
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How To Set Pop-Off Pressure
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How To Set Pop-Off Pressure

By Len Emanuelson

First, let me start by stating that I’m no 2-stroke carburetor expert. However, I’ve been forced to learn certain things about how to setup and service my carbs as part of my race prep. One of those procedures is checking and setting pop-off pressure — the pressure required to unseat the inlet needle and seat, or in some cases, the carburetor’s gross jet. What this affects is the carburetor’s calibration (rich or lean) especially in the lower rpm ranges. There are as many target pop-off pressure settings as there are opinions and engine combinations, but the generally accepted settings are 8.5-10.5psi for gas and 5-7.5psi for methanol. You can’t go wrong picking a mid-point in those ranges as a starting point.

After you remove the pumper stack, what you see is the “brain” of the carburetor, the fulcrum arm/inlet seat assembly. It’s hard to believe, but this crude little lever spring arrangement operates at engine rpm, or in my case 15,000.
After you remove the pumper stack, what you see is the “brain” of the carburetor, the fulcrum arm/inlet seat assembly. It’s hard to believe, but this crude little lever spring arrangement operates at engine rpm, or in my case 15,000.

There are a couple of other factors that affect low-rpm mixture control. The most important is the fulcrum lever height. A good starting point is with the top of the arm adjusted (bent) so that it is flush with the adjacent “flats” on the carburetor. If you bend the arm up, you richen the mixture. Typical settings for raised fulcrum arms (at least on my carbs), is .030” for methanol carbs and .060” for my gas carbs. Another consideration is the type of inlet valve, needle and seat or gross jet. My Buller methanol carbs run dual-ball gross jets that flow more fuel (even at smaller openings) than needles and seats. My gas carbs use needles and seats that run cleaner on the bottom end and seem to give snappier response.

The way you check pop-off pressure on a typical Mikuni or Tillotson pumper carburetor is to remove the pump “stack” breaking the carb down to the carb body with it’s inlet valve, fulcrum arm and spring. Next, you pour some type of liquid in the inlet valve well. Kermit Buller recommends and sets his carbs up with Marvel Mystery oil, but gas, solvent, WD40 or most lubricants will work. Then you take your pop-off gauge and insert it in the passage that leads to the chamber below the inlet valve (see photo), and slowly pump pressure into this chamber. Keep watching the fluid around the inlet valve for bubbles if the gauge doesn’t seem to be holding pressure as you pump it up. Dirt or a bad gasket or surface under the inlet valve often leads to leaking here (you will see bubbles around the inlet valve).

Most inlet valves, needle and seats or gross jets require a liquid to provide a seal. I use Marvel Mystery Oil because it’s what Buller recommends for checking gross jets, but lighter oils like WD40 or your actual premix fuel work well too.
Most inlet valves, needle and seats or gross jets require a liquid to provide a seal. I use Marvel Mystery Oil because it’s what Buller recommends for checking gross jets, but lighter oils like WD40 or your actual premix fuel work well too.

When you get to the set pop-off pressure the inlet valve will open releasing pressure (usually spraying oil all over). If you are using a needle and seat, it should be an audible “pop”, initial pressure release and then the gauge stabilizing at a “holding” pressure. If you are using a gross jet, the pressure will simply release at the pop-off number and then stabilize at a lower number. As Buller’s info sheets describe it, the gross jet acts more like a fuel regulator.

To change pop-off pressure you need to adjust that very tiny coil spring under the fulcrum arm. It’s good to have a large supply of pop-off springs as they have a nasty habit of launching, never to be found. For a given spring wire diameter etc., the length determines the pop-off pressure. I purchase my springs from Comet Kart Sales, but they are available from others such as E.C. Birt. As they come from Comet they are approximately .750”-long and produce a needle and seat pressure of 9.5psi in a Mikuni carb. That is nearly ideal for many gas foreign motors. By compressing the spring (squeezing it several times to coil bind between my fingers) to .570”, the result was 6.5psi in my gross jet methanol carb. It’s strictly a time-consuming trial and error operation. However, after you establish the correct length spring for your application, you can usually hit the pop-off number in just a couple of tries.

Get a good pressure check gauge. If you run 2-strokes, you’ll need it often. I found this one on the internet for about $35.
Get a good pressure check gauge. If you run 2-strokes, you’ll need it often. I found this one on the internet for about $35.
To check the pop-off pressure, slowly pump up the gauge until the inlet valve releases the pressure. This carb popped-off at around 6psi and held at 3.5psi, which was perfect for methanol fuel.
To check the pop-off pressure, slowly pump up the gauge until the inlet valve releases the pressure. This carb popped-off at around 6psi and held at 3.5psi, which was perfect for methanol fuel.

The photos here (iPhone7 camera) show me checking the pop-off pressure on an eBay carburetor I purchased (atomized methanol Mikuni) and also setting the pop-off pressure on an existing Buller atomized Mikuni I have been racing with that was too lean causing engine damage. It’s not a bad idea to check your carburetor’s pop-off pressure whenever you are installing a fresh diaphragm kit.

If the pop-off wasn’t correct, you remove the fulcrum lever and axle by simply loosing the Philips-head screw seen here to the right of the gross jet ball. Just loosen so that the fulcrum axle tilts up and slips out. If you remove the screw every time you need to grow a third hand to put it all back together.
If the pop-off wasn’t correct, you remove the fulcrum lever and axle by simply loosing the Philips-head screw seen here to the right of the gross jet ball. Just loosen so that the fulcrum axle tilts up and slips out. If you remove the screw every time you need to grow a third hand to put it all back together.
The stamped steel fulcrum lever is pretty fragile, so if you need to bend it to adjust fulcrum lever height (described further on), be gentle. The bottom side has a dome that helps retain the spring. Make sure the fulcrum lever rotates freely on the axle.
The stamped steel fulcrum lever is pretty fragile, so if you need to bend it to adjust fulcrum lever height (described further on), be gentle. The bottom side has a dome that helps retain the spring. Make sure the fulcrum lever rotates freely on the axle.
Spring length determines pop-off pressure. The longer the spring, the higher the pop-off. Once you determine your correct pop-off spring length through trial and error, use that as your baseline.
Spring length determines pop-off pressure. The longer the spring, the higher the pop-off. Once you determine your correct pop-off spring length through trial and error, use that as your baseline.
If you noticed leaking around your inlet valve during pop-off testing, or the inlet valve just wouldn’t build to a pop-off pressure, there’s a good chance it is leaking under its seat. As you can see the debris in my second carb here that was allowing the gross jet assemble to leak by it completely. I cleaned it with a small screwdriver and added a fiber washer.
If you noticed leaking around your inlet valve during pop-off testing, or the inlet valve just wouldn’t build to a pop-off pressure, there’s a good chance it is leaking under its seat. As you can see the debris in my second carb here that was allowing the gross jet assemble to leak by it completely. I cleaned it with a small screwdriver and added a fiber washer.
There is a love-hate relationship with both gross jets and conventional needles and seats. I’m an equal opportunity hater – I use gross jets for methanol and needles and seats on my gas carbs. The gross jet on the left is from Buller, the needle and seat in the center is from E.C. Birt and the need and seat on the right is from Speed Parts.
There is a love-hate relationship with both gross jets and conventional needles and seats. I’m an equal opportunity hater – I use gross jets for methanol and needles and seats on my gas carbs. The gross jet on the left is from Buller, the needle and seat in the center is from E.C. Birt and the need and seat on the right is from Speed Parts.
Fulcrum lever height plays a role in determining a carburetor’s inherent richness or leaness. Level with the surrounding “flats” is kind of the neutral position. This lever is actually raised .040” above the carb body for a richer mixture.
Fulcrum lever height plays a role in determining a carburetor’s inherent richness or leaness. Level with the surrounding “flats” is kind of the neutral position. This lever is actually raised .040” above the carb body for a richer mixture.
The best way to check fulcrum lever height is with the depth end of a caliper. My methanol atomizers are .030”-.040” higher, and my gas Mikunis are setup with the levers .060” higher.
The best way to check fulcrum lever height is with the depth end of a caliper. My methanol atomizers are .030”-.040” higher, and my gas Mikunis are setup with the levers .060” higher.
Here are some brutalized fulcrum arms in my collection. The one on the bottom with the sharp kink came off a carb with a gross jet. It is a good example of how to “dog-leg” a lever so that when you raise it, it is still parallel with the top of the carb. Likewise for the top lever.
Here are some brutalized fulcrum arms in my collection. The one on the bottom with the sharp kink came off a carb with a gross jet. It is a good example of how to “dog-leg” a lever so that when you raise it, it is still parallel with the top of the carb. Likewise for the top lever.