Well I almost made it to the finish line. OK, only metaphorically speaking. Not on the track, but more like in the pits. But not really there either. Actually on the work bench this time.
Another one of my project chainsaws was begging me for a little bit of attention. It has been feeling neglected for way too long. After tripping over yet another box full of parts too many times over the last ten years, this one project finally made it to the work bench.
I like to mess around with these old dinosaurs and actually put them to use making firewood. I recently installed a pot belly stove in my cabin, so this saw has a purpose. My preferred choices are David Bradley and McCulloch gear reduction chainsaws. Mainly around the time karts made their debut.
Todays project is a David Bradley 5G21, using the Power Products AH-58 engine. This translates to 5 horsepower, gear reduction with 21 inch bar. It was picked up more than 10 years ago and been sitting around collecting way too much dust. The powerhead of this chainsaw was also used on karts. At least a slight variant of this engine was used.
I tore this saw down to find out why it died. Did not take long to figure out the problem. The engine still had some compression, and showed very slight scoring on the exhaust side piston. After removing the intake manifold and reed plate, there it was.
broken reed plate
reed parts
You can see the broken reed petal. Also how small the openings on plate are.
These engines are mostly solid. But the reed plates are a very weak link. So far, half of my David Bradley projects were dead from a single broken reed. Others were from weather damage or just sitting up too long. Rarely ever do I find one melted down or with broken rod.
Being gear driven makes this engine left hand drive. Most of these old David Bradley chainsaws are powered by the AH-47, 58 and 81 engines. Attempting to use one of these engines for a kart is not the best choice on earlier models. Not unless you want to make some seriously needed improvements first.
After the first generation of engines was adapted for karts, they showed their weaknesses immediately. Mostly made of magnesium and ba1ance issues plagued them severely. With the lack of ba1ance, the engines would shake themselves to pieces.
Vibration at higher rpms than originally designed, caused some of the bolts and studs to work loose and strip out. The cylinders were removable and the ears would break off sending them flying into oblivion. Think rocket launcher or punkin chunkin here.
It didnt take very long before most of the problems were resolved. The new models were cast from aluminum and heavily reinforced compared to the older ones. Originally a three bolt for engine mount, now has four mounting bosses.
Another major improvement was material removed from the sides of crankshaft rod journal for better ba1ance. Also larger diameter of shaft on flywheel side so it did not shear off. These improvements substantially increased the reliability of this model engine.
But on my little project, these improvements would not keep the reed assembly from failing. Once a single reed petal breaks off, the engine is out of service. If engine was running at full speed when it let go, that broken petal would make its way to the intake ports and lodge between piston and cylinder wall.
Fortunately that did not happen on this engine. But after tearing it down, I also noticed the piston rings were also worn out. Overall the rest of this saw was in good shape. The piston rings are only made from cast iron, and not chrome plated. So they dont last all that long anyway.
I decided to do an upgrade and see if this reed plate could be improved. One thing I always thought is the stock reeds are way too small for most of these saw engines. The rest of this engine is not too bad overall to make some decent power.
reed plate slotted
So I took the stock reed plate and cut out the center section leaving two ribs. Originally it had six very small openings that barely worked out to be much more usable area than venturi of carburetor. If reed opening is too restrictive, the carburetor cant draw enough to utilize the venturi to pull in adequate air and fuel mixed to produce much power.
carbon fiber reed
I used a single piece of carbon fiber and cut it to fit over the larger opening in reed plate. It now runs full length instead of split in half. I also have the reed petal opening so the rotation of crankshaft does not work against it.
side view reed plate
In the second pic above of reed plate disassembled, you can see the two long slots on each side of bolt holes to retain the reed stop. This presented a problem that was easily resolved. What needed to be done is machining down the thickness of reed plate so the one piece reed would seal. I removed about 0.045" of thickness from reed plate until the two pockets were eliminated. Then sanded on a slab of granite to make sure was flat.
clearance for reed screws
With the reed plate thinner now those screws stuck out. Instead of cutting them shorter, I drilled some pockets for them to clear in the manifold. You can see the holes that were punched in gasket that match the holes in manifold. But not so clear in this pic. maybe I should take a better pic of this.
rings
The piston rings were noticeably thin on the opposite side of gap. You can see the difference between the old rings and a new one here. After assembly the engine had decent compression. But other problems plagued this project.
There was a ton of carbon buildup behind rings, so took some effort scraping it out using what was left of an old ring.
As I was about to bolt on the intake manifold, noticed the threads were stripped out in the crankcase for one of the studs where it fits. This is a common problem if engine has been torn apart a few times. Magnesium doesnt like being tightened up too many times. Most people usually over tighten these small 1/4-20 screws anyway.
If using a torque wrench to spec these, you would swear they were not tight enough. Thinking back now, I remember this intake manifold stud was missing when engine was originally torn down. I should have checked it over more closely. But the rest of the saw showed little signs of being worked on before.
Since the saw was now mostly assembled back together, I attempted to heli-coil the stripped out hole. On these magnesium crankcases, there is very little material around the threaded hole. Even less when drilled out and tapped for a heli-coil.
The hole is also not so deep. Even using a bottom tap, the threads will taper down slightly. No problem here until attempting to screw in the stud. It started in and immediately got tight. Another partial turn of the stud and I heard the dreaded noise of the crankcase cracking.
Even tho the heli-coil is held in well enough to risk, I didnt want to take the chance. That small chunk usually breaks off and stud will stay in place. Right up until it fails sometime later on. I didnt want to spend all this time and knowing it would eventually break out. Welding this spot does not hold up so well either, since magnesium usually ends up porous and weak.
Since the damage was done and no way of repairing this, I walked away. The next day I decided to swap out the crankcases. This was a good time to replace the crankshaft bearings while it was torn down this far. Something I noticed on most of these engines, they have no end play on crankshaft. The last two of these I have torn apart, the case halves had a very slight air gap when bolts were removed. But not really measurable even with the thinnest feeler gauge.
It took almost no pressure to mate the surfaces together, but still not good in my mind. I changed the crankcase and the new one was the same way. I guess when up to operating temp it is not a big problem. For a racing engine, I would have to machine the bearing pockets slightly deeper for at least a minimum bit of movement.
I spent some time to check out other David Bradley projects. Then noticed two of the AH-47 crankcases needed the bearings replaced. There was no end play at all and bearings were very noisy.
closeup
So here is the stud boss that is very weak. Some other magnesium cases I have laying around have been repaired and threaded to 5/16. The Heli-coil was a failure, so options are limited. On newer aluminum cases, not so much of a problem.
cases
The crankcase I swapped, is a newer one with the four bolt mounting. But it is still magnesium. All of the kart engines with this newer style crankcase are aluminum.
Going to all of this trouble I had to make it look pretty. Fortunately the old paint is easily removed. Adding a few ounces of lacquer thinner to kerosene or naptha, while washing the parts, old paint comes off fairly easily. For tough spots, straight lacquer thinner did its job.
So after doing a lot more work than planned, but that is usually the case, I finally made it to the end. Everything checks out and carburetor was disassembled, welch plug removed and all orifices were cleaned out and sized accordingly.
Now the saw is close to as new without being on display in the lawn and garden department at your local Sears store back in 1959.
The next step is filling gas tank with some real gas mixed with 4 ounces per gallon of blue oil. If there are no unforeseen problems, this saw may become a productive part of society once again.
I'm in need of 1 Rupp Uni Mount to complete a kart for this coming season. Any help would be greatly appreciated in obtaining one. I have parts to trade also. Kindly call or text at 585-748-5470.
Nevermind.
I found the answer to my question "back end frame height of a GK800/1200".
Thanks for looking!
Hi everyone,
Greetings from St. Louis.
My name is Steven Lord and my question:
Could anyone provide me the distance between the ground and front half of the frame on a GoKart 800?
I would also like to know the same for the back half.
I'm building a GoKart 800 replica. I have most measurements except frame height.
I'm thinking I saw somewhere the front end rests about 2" above the ground but again, I might be mistaken.
I need the back height of frame to ground as well.
Curious if anyone remembers, has information/pictures of a sprint track at the North Platte, Ne airport. It was called the Central Nebraska kart track and I have looked high and low for any information without any luck. My family used to race there in the 1960's. Thanks for any help, Dean.
