Engine Trouble Rebuild time?

Joined
Jun 6, 2018
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#1
I have a couple of 66cc china girls. The first one I built has around 3000 miles and has been running RICH its entire life. When the season ends and snow falls, I will tear down the engine. I already have new rings to put in, but should I put a new piston in as well? It also has a really loud knock at idle so I think I will replace the wrist pin and needle bearing. I ordered bits to solder and rejet to a leaner setting. I'm also replacing all the hardware since all the screws are chewed up and I had a stud snap on the new bike. Any other matinence that anyone recommends? I won't change the cylinder or head as they are fine and I don't want to spend the money (I'm in high school). I will also retard the timing by filing the woodruff key to prevent 4 stroking=more stress. I want to shave the head a bit and port/ramp, but with this old of an engine I think I'll just baby it. It still can hit 34-35 despite the age and running way too rich. I'm hoping the rings will bring back some compression as my new bike hits 40-41 with the spark retard, porting, and ramping. All stock parts. Over the 3000 miles, I have only replaced the clutch cable as far as break downs go. Originally built to ride to work when I was only 14. Now my friend and I race around the neighborhoods and trails and we ride them HARD. I still use them for trips that are ~3 miles. Too much fun for how cheap they are. I find them very durable if you maintain them and build them right. However, if I build another, I am replacing all hardware before I build and re-tapping the mounting studs for 8mm.
 


JerboaJohn

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Jul 29, 2018
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#2
3000 miles? :eek:
new jug + rings, make sure big end rod bearing is not played out. new clutch assembly, surprised yours lasted that long. I do not beat my 48cc and got 750 miles on the pads and only 800 on the assembly
 

LewieBike

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May 21, 2014
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#3
I'd like to have the luck of having a HT engine that runs great out of the box without issues. Having it go 3k miles, you're doing something very right.

I'd say keep it running slightly rich, a little trace of 4 stroking isn't a bad thing, I've owned a few 2 cycle motorbikes that would do some 4 stroking at cruise. Doesn't hurt anything except maybe a little extra gas consumption.
 
Joined
Jun 6, 2018
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#4
I'd like to have the luck of having a HT engine that runs great out of the box without issues. Having it go 3k miles, you're doing something very right.

I'd say keep it running slightly rich, a little trace of 4 stroking isn't a bad thing, I've owned a few 2 cycle motorbikes that would do some 4 stroking at cruise. Doesn't hurt anything except maybe a little extra gas consumption.
3000 miles? :eek:
new jug + rings, make sure big end rod bearing is not played out. new clutch assembly, surprised yours lasted that long. I do not beat my 48cc and got 750 miles on the pads and only 800 on the assembly
Well I finally tore it down to assess the damage. I can include pictures of a 3000 mile engine if anyone wants, but I am happy to say everything looks good. The jug still has its lining and there is some slight scratches on the intake and exhaust side of the piston and jug, but its really minor and could have something to do with the fact I ran the engine without first sanding the edges of the ports. Slight indication of blow-by on the piston exhaust side, but it was very minor for 3000 miles. I cleaned the piston and head with a wire wheel and put new rings in. I also shaved the head and will make a new head gasket from a pop can to boost compression slightly. The crank bearing has no slop, but the wrist pin is all but gone. There is slop between the wrist pin and needle bearing along with several gouges on the top side. I ordered a new one along with a needle bearing from china (too cold to ride so why pay more for fast shipping). I think I will slightly ramp the piston and trim the skirt on the intake side. I already closed the jet to 2 sizes smaller. I will also retard the timing by grinding the woodruff key. The only reason for doing this is I have 2 bikes and my buddy and I like to race, so I need to try to make this one faster to match my new bike. Plus it is way to rich and leaks gas and oil out of the muffler when I park it. My clutch pads still have plenty of life. I run mystic synthetic oil at 32:1 with 89 octane. Every couple tanks I out moly grease on the gears and the clutch arm (the thing by the drive sprocket). Chain and cable lube with PTFE that is kinda waxy for the chain. Also, my paint has held up and there is only one tiny ding on the magneto cover that I think I did. Custom spring tensioner that I built. On these bikes, there is a lot of things I do that simply aren't worth my time, but I like to build and do things myself plus it saves some cash.
 

darwin

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May 26, 2008
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#5
Well I finally tore it down to assess the damage. I can include pictures of a 3000 mile engine if anyone wants, but I am happy to say everything looks good. The jug still has its lining and there is some slight scratches on the intake and exhaust side of the piston and jug, but its really minor and could have something to do with the fact I ran the engine without first sanding the edges of the ports. Slight indication of blow-by on the piston exhaust side, but it was very minor for 3000 miles. I cleaned the piston and head with a wire wheel and put new rings in. I also shaved the head and will make a new head gasket from a pop can to boost compression slightly. The crank bearing has no slop, but the wrist pin is all but gone. There is slop between the wrist pin and needle bearing along with several gouges on the top side. I ordered a new one along with a needle bearing from china (too cold to ride so why pay more for fast shipping). I think I will slightly ramp the piston and trim the skirt on the intake side. I already closed the jet to 2 sizes smaller. I will also retard the timing by grinding the woodruff key. The only reason for doing this is I have 2 bikes and my buddy and I like to race, so I need to try to make this one faster to match my new bike. Plus it is way to rich and leaks gas and oil out of the muffler when I park it. My clutch pads still have plenty of life. I run mystic synthetic oil at 32:1 with 89 octane. Every couple tanks I out moly grease on the gears and the clutch arm (the thing by the drive sprocket). Chain and cable lube with PTFE that is kinda waxy for the chain. Also, my paint has held up and there is only one tiny ding on the magneto cover that I think I did. Custom spring tensioner that I built. On these bikes, there is a lot of things I do that simply aren't worth my time, but I like to build and do things myself plus it saves some cash.
Dude needs to be giving advice not taking it!
 

Lukesky36

Active Member
Joined
Oct 3, 2018
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#6
For how cheap these motors are its almost cheaper buying a new one then a rebuild but i got 1200 Miles on my engine checked the inside still pulls strong great compression but i got some blow-by near the exhaust. My crank keyway broke and i didn't have one laying around so i just swapped engines im still going to fix the older one still though when i replaced the jug i filled the case with oil and covered the needle bearing with oil and let it sit like that for days until i got a new jug great engine took forever too start up though with a cup of oil in it lol surprised i didnt hydrolock it
 
Joined
Apr 9, 2018
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#7
3000 miles is the amount for a car oil change. Kinda sad but if Japan made these they would surely last longer. 20k is possible for a Honda gx35 4 stroke or so someone said might try that setup not a fan of friction drive though. There are belt drive setups for $700 (ripoff).
 

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