Hi guys Just wanted to share my experiences going the conversion route. Inspired by the engine conversion section on RCU I picked up 'troy built' 25cc trimmer/weedwhacker engine on ebay for as little as 35 usd. Cut off all the excess metal - lightned it as much as I could, got a prop hub and EI for it. Gutted the stock muffler - its crimped together so I opened it and took out all the internal stuffing -wire mesh spark arrestor etc. Changed to a larger walbro carb. All up weight was around 1300 grams or so. Top rpm on an '18x6 apc w' was abt 6400 rpm
I have been using this engine on a 1/4th scale giles for about a year and a half. Performance may be described as 'sports aerobatic' - no unlimited vertical and definitely no 3d.
But the great thing abt a conversion is that it is always a work in progress. So after a rough landing/stall in a head wind, which broke the landing gear I decided to pull the engine and make some 'improvements'
I noticed that more weight could come off the prop hub So I took it to a local machine/car repair shop and asked him to turn it down on a lathe. Cut some of the crank off that was extending behind the case. Now it weighs in at jsut over 1kg or 2.2 lbs
I also 'blueprinted' the engine to find out my exhaust and intake timings. These were rather conservative so I took a small file and increased the port size - top of the exhaust port and bottom of the intake port until my timing was 155 degrees on the exhaust and 135 on the intake. I also got rid of the head gasket to increase compression. Resealed using a 'liquid gasket maker'
well yesterday I ran it with some apprehension - but the thing seems to have a lot more power and pulls hard on an apc 17x6 I need to try some different, lighter wooden props on it. Without a tach (which is high on my shopping list now) its not possible to know the increase in rpms
guess when I go to the field Ill see if anyone can tack it for me - but the proof of the porting is in the flying!
Hope this might help someone get into gassers really cheaply. Besides I get a lot of satisfaction in seeing my grass cutter fly!