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Post by theswede on Feb 11, 2011 5:50:11 GMT -5
Last fall I had a engine breakdown, described here: scooterrebels.proboards.com/index.cgi?board=50ccscooters&action=display&thread=1034. With the riding season (hopefully) arriving in a month or two, I went ahead and tried to fix the broken engine. As it seemed to have overheated, I suspected the valves so I replaced the head. Unfortunately that was not the issue, as I found out last night. The engine starts easily and runs with good power, but with a fairly loud clatter. So it's the old engine out of the frame again for a tear-down  I did check the cylinder when I had the head off, but could not see any scratches or anything that could indicate I had a broken piston ring or a partial seizure. Camshaft bearings also looked and felt ok. So it would seem the crank bearings are damaged... I do have my spare engine still, and just ordered a 72cc kit incl head with larger valves from Autotech355 (eBay) for it. But I had really hoped to get the old engine fixed... Anyone have any hints on how to go about identifying what is wrong with the old engine? Do I need to install new oil seals in it if I split it? I assume I need new gaskets.... |
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Post by Rebel on Feb 12, 2011 0:42:20 GMT -5
Loud clatter does seem like valves to me. A bottom end sound would be a lower pitched knocking than a top end problem. If you have a mechanics stethoscope I'd listen around the motor with it running and see where the loudest noise is and figure that is where the problem is. You might pull the head and cylinder off and then grab the piston and see if there seems to be some side to side play in it. If you can feel any I'd think it was certainly a crank bearing.
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Post by theswede on Feb 12, 2011 11:02:44 GMT -5
Yes, I too was pretty certain it was the valves but I did change the head and the clatter is still there. I suppose I could have set the valves wrong when I installed it but I checked several times. Oh well, the engine will come out of the fram next week and my living room will return to the workshop state it had the last few months.  A mechanics stethoscope is a good idea, I'll pick up one asap. |
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Post by gmoney on Feb 12, 2011 11:15:51 GMT -5
No need to buy one, just use a long screw driver the longer the better, and place the metal end on where you want to listen to, the other end pressed against your ear. You'll hear with it just as well and save the time, trouble and expense of buying one.
Gary
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Post by kz1000st on Feb 12, 2011 15:29:07 GMT -5
The easiest way to tell is to get the head and barrel off and pull the piston up and push it down. It almost sounds like the big end bearing is shot. Also see if you can hold the connecting rod and try moving the piston and see if the small end bearing is gone. Remember Doug these motors spin pretty quick so a connecting rod bearing in a small, fast spinning engine would really raise a ruckus (sorry for the pun).
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Post by theswede on Feb 20, 2011 13:06:46 GMT -5
I believe I have found the issue. Today I removed the head and cylinder, and could not find any evidence of any bearing damage. No play or grinding noise or feeling anywhere in the crank or crankshaft. However, while checking the bearings i noticed the seemed to be a slight play in the flywheel. And when I removed the cooling fan I saw that one of the rivets had snapped of! Further checking showed that there is a slight play, maybe .5mm between the inner and outer part of the flywheel. I believe this was the source of the intermittent rattling noise I have been hearing. So now I need to get a flywheel puller and a new flywheel... Picture:  I do have a question for the experts though: The piston looks slightly scratched, both on the intake and exhaust side. There are _very minor_ scratches in the cylinder, on the intake side only. The piston also seems discolored on the intake side. Is the piston and cylinder shot, or is this just common wear? Intake side:  Exhaust side:  I have been using Castrol GPS 10W-40 synthetic motorcycle oil. The engine got pretty hot due to a very restricted exhaust system last summer (the performance system I had installed came to pieces, and I had to use the original system for a while). |
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Post by Rebel on Feb 20, 2011 13:23:36 GMT -5
Looks like a little wear to me. Since the exhaust side is the affected side, I'd wonder about that temp being the culprit. Have you checked the wear on it? Measured with a micrometer? We had a member a year or so back post some fotos of a mod he did to the piston to help control this. He drilled a few holes in the skirt of the piston so that oil could splash through and lubricate the piston better. He said he was a scooter mechanic as I recall and that all the scooters he did that mod on did not wear like this. I think it was three holes on both sides. I'll look for the post for you to check out. here is the thread scooterrebels.proboards.com/index.cgi?action=display&board=125plus&thread=1009&page=1 |
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Post by theswede on Feb 20, 2011 13:48:16 GMT -5
Thanks Rebel!
The most wear and discoloration is actually on the intake side of the piston, not the exhaust side. And it looks like there is minor traces of binding there too, now that i have looked closely at it. There is an area on about 5x5mm where the scratches are continuous, deeper and vertical. And it matches the scratches in the cylinder... I don't have access to a micrometer so I can't measure it.
I suppose it might make sense the issues are on the intake side as the exhaust side gets more oil than the intake side, it being on the bottom side of the cylinder.
That piston mod looks interesting, thanks for the link!
I have a completely new cylinder+head kit from Carlos (autotech355 on ebay) with larger valves I am going to install, I think I will make this mod on it.
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Post by Rebel on Feb 20, 2011 14:02:38 GMT -5
Good luck to you with that mod. It makes some sense to me.
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Post by theswede on Feb 21, 2011 12:45:33 GMT -5
All set:  I hope this works...  Just waiting for some parts and then I'll put the engine back together. |
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jmkjr72
contributing staff
Commander 132nd Northern Cav. Division
Posts: 2,779
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Post by jmkjr72 on Feb 21, 2011 13:15:14 GMT -5
looks like the top of that new piston could use a bit of work to polish it and smooth it up good
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Post by Rebel on Feb 21, 2011 15:18:57 GMT -5
I don't want to be overstating the obvious, but you did do both sides of the piston didn't you?
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Post by theswede on Feb 21, 2011 16:25:34 GMT -5
I don't feel confident that I can manage any real improvement to the surface on top of the piston with just my Dremel, so I have decided to leave it as it is. And yes, I did do both sides of the piston . 3x1.6mm holes on each side. |
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jmkjr72
contributing staff
Commander 132nd Northern Cav. Division
Posts: 2,779
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Post by jmkjr72 on Feb 21, 2011 16:55:09 GMT -5
if you have pollishing pads on your dremmel thats proably all you need
i have some fancy air tools but more often then not i just use my dremmel its quiter and more convient
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Post by Rebel on Feb 21, 2011 20:38:13 GMT -5
My dremel warms my hands up nicely too. I've got one I bought in about 1975, it is still going. I wired up a sewing machine foot pedal to a electrical outlet, stuck it in a box and plug my dremel into that so a cheap one speed became a variable speed dremel, works pretty well.
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CAN YOU HELP ME OUT MAN,AS i LOST ME JOB DUE TO NOT BEING ABLE TO GET THERE?ANY INF