Boosting a 40 year old 2-stroker ?

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Boosting a 40 year old 2-stroker ?

Post by copterpack on Sun Sep 20, 2015 5:55 am

Hello,

newbie here...and in 2-stroke technology. I have an old ( 1960's ?) 148 cc Rotax single cylinder engine which I like to use for a rather weirdo project.
According the very scarce info I found, these engines where used in early snowmobiles and as industrial units (water pump-generator etc...).
In theory, the 148 cc delivered around 6 hp at 6000 RPM which probably made these engines last forever.
I would like to boost that "meager" 6 hp to a more realistic "modern" figure. Having said this, I realize that without complicated (expencive ? ) modifications the 1hp per 10cc, giving a theoretical possible 15 hp, will be very hard to accomplish and probably not needed for me.
Let me clarify that I'm not an engineer and being retired doesn't really help with the financial back-up so the modifications need to be as "simple" as possible and "affordable", two very relative meanings......I wouldn't mind to try to fab my own tuned pipe and/or buy a suitable ignition module and /or suitable carb.
What I'm after is more power/torque in the high end RPM range without going the insane high ( 12000-15000 ? ) RPM route.
I have gone through a few publications on 2-stroke tuning but this often made it more confusing for me than clarifying things. Since the engine is just part of the project I don't have the devotion to dive into the art of tuning.
Problem is that I have no clue where to start but I assume that gather info/figures is a good beginning.......
What I have so far is a theoretical RPM of 6000 and 148 cc, not much but "something".

Any suggestions as how to procede from here on ?

Kindest regards,
Gilbert

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Re: Boosting a 40 year old 2-stroker ?

Post by Admin on Fri Oct 16, 2015 11:19 pm

the most bang for your buck is just lathing down the mating surface of the head to increase compression. Make sure that you don't reduce the piston to head clearance to less than .8mm
You can actually do it yourself by moving the head around on sandpaper that is on glass. Takes a lot of work to grind off 1mm and it is very tiring on the arm but I have done it before (and wish I'd of just taken it to the machine shop).
All other modifications are more involved but offer a great opportunity to gain experience such as porting and making your own pipe.

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Re: Boosting a 40 year old 2-stroker ?

Post by copterpack on Sat Oct 17, 2015 3:18 am

Hello,

thanks for your reply. When I removed the cylinderhead to check the condition, I was surprised of the thickness of the old gasket. I assume that using a copper gasket might boost the compression without going through the hassle of "machining" the head by the method you suggested, or to machine a bit off the head in my old small metal lathe.
When I test ran the engine (have no clue how many years it hadn't run) the noise it produced was beyond what I expected ! The engine came without exhaust pipe and carb but I was lucky enough to have an old Bing carb that I could use. I quickly "fumbled" a muffler together from some old stainless steel cones (weighs like a boat anchor !!) I had laying around and again to my surprise, the noise was supressed by at least 50%. I also noted that it took more time to rev up the engine so it is kinda slow on the throttle now.
The Rotax 148 cc engine is rather rare and a quick search on the net revealed that spare parts are likewise to find.
Having no experience at all in porting an engine it might be tricky to experiment on the cylinder, having no chance to try again on another cylinder.
I think I'm going to take the "easy" route and try to gain as much as possible from adapting the cylinder head and making an exhaust pipe. All I have to loose on the exhaust is some metal and time, the former is cheap, the latter is free LOL !!







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