The question is often asked. Will a standard late model J engine be competitive with other engines? The simple answer is no. How can the engine be made competitive? It is usually suggested to have your engine blueprinted. Does this mean my engine will be as fast as everyone else?
The answer here is not so clear cut and depends on a lot of variables. The major influence on the power of the engine is the transfers in the cylinders and here you are at the mercy of who ever cast the cylinder. If the cylinder has all the right stuff and you carry out the right procedures then yes you can be equal to the best. I have written a book Go Kart Engine Building The Yamaha J that describes and illustrates these cylinders and tells you how to get the best out of these cylinders as well as how to dramatically improve a standard engine with an ordinary cylinder. Unfortunately these super cylinders are becoming quite rare and are mainly to be found in the hands of the top teams.
Whatever you do don’t give up because while they may not be super cylinders there are still some very good cylinders floating around. It is often people don’t recognize them. If you know what you are doing you will be surprised by how much you can improve the power of a not so good standard engine with relatively simple techniques. There is a great deal of personal satisfaction in making an engine that is competitive from a so called dud.
At present I am driving around at the local track with a J that I cannot get to go no matter what. I have done more to it than some of the engines I build with the super cylinders. It is easy to get an engine with a good cylinder to be fast. It is another matter with a dud. The best I can get from the engine I use is 9.8hp measured on the same dyno where one of my engines measured in excess of 11hp. It is very frustrating.
This particular engine I am using is a late model engine and it has one transfer cast slightly larger than the other. I have found this to be the case on all the late model J engines I have seen. There is published research around from reputable institutions to indicate that irregularities in the transfer ports, almost invisible to the naked eye will cause a considerable loss of power.
Pic 01. Poor flow pattern seen on this cylinder. It does not flow directly out the exhaust but is skewed to one side.
Out of desperation I got out my little right angle drive die grinder and made the smaller port the same as the bigger. The difference is about half a mm in width. There was no change in the roof or reshaping of the ports. It took about ten minutes and was very simple. Most of the time was taken up marking out, that is how I know they are different. I ran the engine on my little dyno and when I took the head off there was a change in the transfer pattern.
Pic 02. Change to the transfer port.
I don’t race this engine I just drive around to test and because I like driving a kart. It is hoped that in the future there will be rule changes that will allow the same grinding of the transfers as is allowed on the KT 100S engine.
Pic 03. Corrected Head.
It is quite amazing the change in transfer pattern. Theoretically the transfer pattern was now pretty good, flowing equally about a line centered on the centre of the exhaust port. I was all fired up to take the engine back to my dyno man for a comparison test only to find out he had sold his machine.
Unfortunately now I will never know if the engine makes more power but what I do know is relatively small changes in a small section of the transfer port can make a dramatic change in the transfer gas flow.
This cylinder is no longer AKA legal but I’m happy that I have proved to myself that my thinking about transfers is on the right track. The engine is now extremely fast when run on a practice day.
During the ten years I was building, developing and racing quarter midgets in NSW I learned a great deal about engines. There were no engine rules and absolute power was the name of the game. I made a 115cc reed valve KT 100S. Quarter midgets are speedway cars powered by kart engines. www.nswquartermidgets.com.au. They now use Rotax and PRD engines.
Pic 04. KT 100S reed valve motor I made for my quarter midget.
Besides building Kart engines for people I know or who are recommended to me I build little buggies based on midgets but without engines. I make them with some of the kids at the local school where I work as a casual teacher. I don’t charge enough to make a living out of kart engines. We use the buggies in the downhill billy kart derbies. At the moment we are exploring the possibilities of putting a motor bike engine in one and running in some the local light car club events.
