Help with a 50cc liquid cooled yamaha

Maritime

Maritime

Over 10,000 Posts
OK, my buddy's son came over to work on his DT50 LC we added a capacitor in place of the roached battery and it smoothed out the electrical. We also did a compression test and only got 80 PSI. I know if the bike was a 4 stroke it wouldn't even run but the little thing fires right up and carries my fat ass to 45 mph so I am wondering what a proper PSI is on these little motors. I am also wondering if it needs a top end redo. 2 stoke folks chime in please. Google fu failed me on the specs and he forgot his manual at home.
 
80 is worn out bad, it should be in the 150 range easy
but did you do it warm at full throttle ?
do a case pressure test then tear it down, new piston, rings, pin, bearing,clips and gaskets MINIMUM
but check the bore
inspect the reeds etc
clean the entire muffler/chamber/sparkle arrester system out as well
clean the carbon out the exhaust port
 
thanks, yep did it after he road it to the house full throttle and I kicked it over 4-5 times, then I put it in gear and just pushed it and couldn't get it any higher so will need to look into a big bore kit.
 
80 is really low, my MB5 calls for 185. I surprised you got it up to 45 mph with the compression so low, that's really good.
 
HerrDeacon said:
80 is really low, my MB5 calls for 185. I surprised you got it up to 45 mph with the compression so low, that's really good.
Click to expand...
Yep, me too. I think the actual compression must be higher but there may have been something I wasn't doing 100% right. First time testing a 2 stroke. I have done plenty of 4 strokes and it should be the same. Throttle wide open kick-push until needle stops wouldnt go higher the 80. I should check the guage on one of my other bikes I know has good compression.
Sent from my MB525 using Tapatalk 2
 
Actually I wonder if these little guys have a decompression switch?

Sent from my MB525 using Tapatalk 2
 
I wonder because it seemed to build pressure fast then just kind of stop and it was hard to tell if there was a leak? It was also a bitch to get the tester into the head, had to take the bike half apart LOL. I will look at the parts fiche and see what I can find.
 
Hmm I don't see anything. I am baffled how the bike can show 80 PSI and still run as well as it does. With the owner on it who weighsa maybe 140 lbs it will do 80 KPH or 50 mph, rated top speed I believe is 55 or 60 so I would guess comp is low ish but more likely in the 100 range, engine has no valves but can there be a leak spot like valves I am missing? Anyway, he is riding it until it gets colder then we'll haul it in for a winter overhaul, he wants to put a 70-80cc kit in for next year, then the year after we are going to build him a 250-500cc road bike and his little brother will inherit the DT, then My son 2 years after that. We just need to keep them from blowing it completely up LOL.
 
xb33bsa said:
80 is worn out bad, it should be in the 150 range easy
but did you do it warm at full throttle ?
do a case pressure test then tear it down, new piston, rings, pin, bearing,clips and gaskets MINIMUM
but check the bore
inspect the reeds etc
clean the entire muffler/chamber/sparkle arrester system out as well
clean the carbon out the exhaust port
Click to expand...

Case pressure test, I missed that part when I read this the first time. I forgot 2 stroke cases build pressure, Would this be in the service manual? The very first thing we are going to do is read the section on engine refresh, pull it apart and look over all the bits and measure etc. then go from there.

Edit. found the how to on Dan's MC pages and it is a moot point as if there is a case pressure issue it is 99% of the time crank seals that are old and warm up and work better when hot, so crank seals are on the to do list for winter anyway. But I learned some more stuff about 2 strokes.
 
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