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Post by tygertung on Jun 29, 2012 16:49:56 GMT -5
How much compression should I have on a 50cc 2 stroke? I had my Suzuki apart, and when I had assembled the motor again, there was only 100 psi. I had read on the Mineralli builds for the 100cc engine that our mate GTVert was getting 175.
Do I need to reduce the combustion chamber volume to get the PSI up? Would 150 psi be safe?
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Post by skuttadawg on Jun 29, 2012 17:06:59 GMT -5
Some have diff compression ratios for example I looked up a Kymco it was only 7.2 to 1 while mine is 10.5 to 1 unsure of its PSI
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Post by 90GTVert on Jun 29, 2012 17:20:52 GMT -5
Not really sure on your model, but 100psi is pretty much the minimum to expect it to run. I'd like to see 150psi or above for a strong running minarelli. Did you do a rebuild with a new top end or why was it apart?
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Post by skuttadawg on Jun 29, 2012 17:23:23 GMT -5
How many miles on it ? Try new rings perhaps
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Post by felliott on Jun 29, 2012 18:52:49 GMT -5
If you put in new rings did you hone the cylinder?
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Post by tygertung on Jun 30, 2012 0:17:08 GMT -5
It was very recently rebored, I had done about 50-100 kilometres on it (30-60 miles), so it was just run in.
I had it apart because I fully rebuilt the engine, I had shortened the barrel by about 2.5mm or so, made up a 1.6mm base gasket spacer, and had spaced the barrel up enough to get the squish clearance to 0.5mm using varying size base gaskets.
I did this to get the piston about level with the bottom of the ports at bottom dead centre, as factory it was shrouding them by about 2mm. However this increased the port timing too much and the engine became very peaky and was very unstable in running at part throttle. With the heavier weights in the variator though it would hit about 75 kph (47 miles per hour)
It soft seized going down a 60kmh road at full throttle riding home, so I pulled it apart.
There was some very light scuffing at 4 corners so I gave the cylinder a very light hone, and polished the scuffing marks off the piston.
I also put a greater chamfer on the edges of the piston to prevent oil being scraped off the bore. I made up a 1.6mm head gasket and used thinner base gaskets, however the squish clearance was a wide 0.7mm.
I was planning this time of machining the squish band recessed into the head so that I can bring the barrel down a bit more, piston up a little and then the compression will increase. The motor is still a bit peaky you see, and I would like to use some heaver roller weights to get the top speed back up to 75 kph, so if i lower the barrel maybe 1mm the low down torque should increase.
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Post by skuttadawg on Jun 30, 2012 0:25:21 GMT -5
Do a plug chop as running lean will soft seize a 2T
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Post by tygertung on Jun 30, 2012 2:35:42 GMT -5
Since I pulled the motor down and made the changes it has not had any tendancy to soft seize. The plug is a medium brown colour if you inspect it, which would indicate that it is fine on the jetting. I suspect it might be a touch rich as it tends to run better and make more power when cold.
I forgot to mention that I had a NGK number 6 plug in it which is probably a bit hot, but I didn't hear any detonation or anything like that. There was no evidence of detonation when I pulled down the engine. I was running an 8, but I have changed back to a 7 which is what I usually use in this engine, as the 8 had a tendancy to misfire at high speed.
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Post by felliott on Jun 30, 2012 17:53:29 GMT -5
I f you machined the top of the cylinder and put a shim on the bottom, you actually moved the exhaust port closer to top cylinder. If that happened you loose compression because you have less space from the top of exhaust port to the head.
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Post by tygertung on Jun 30, 2012 17:55:31 GMT -5
Correct. I was wanting to get the ports flowing properly, as the piston was still shrouding the ports at bottom dead centre.
I will perhaps have to reduce the combustion chamber to increase the lost compression.
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