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Post by jdb4whlr28 on Mar 11, 2010 0:09:32 GMT -5
My 50cc Chinese moped randomly stopped running one day. I was going full throttle, then went to coast to a stop and the engine idled down then died. I assumed at first it was the plug, so I bought a new one and had no luck. So I tried running it on starting fluid assuming it wasn't getting fuel. Again no luck. I then did a test to verify spark and it was getting plenty. So if it was getting spark, and fuel, my next guess was compression. I took the plug out and put my thumb over the hole and it seemed rather weak, but then again it was a small engine. So i ended up taking the whole thing apart and inspected the piston and rings and they both seemed fine (its usually pretty easy to tell if the rings are going to cause the engine will run like crap for awhile before they let go?). I ended up putting some liquid gasket between the cylinder and the head (where the head gasket goes) and it seemed to boost the compression a little bit but still the moped refuses to start. Any ideas?
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Post by shiftless on Mar 11, 2010 1:18:57 GMT -5
Check the valve lash .004 intake and exhaust
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Post by jdb4whlr28 on Mar 11, 2010 1:25:22 GMT -5
This would stop the engine from running even on starting fluid?
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Post by Fox on Mar 11, 2010 10:03:47 GMT -5
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Post by jdb4whlr28 on Mar 11, 2010 21:17:52 GMT -5
I just got finished adjusting the lash on the valves.. Still won't start. Any other ideas?
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Post by 90GTVert on Mar 11, 2010 21:47:43 GMT -5
Since you have fuel and spark the main things left are compression and timing. You sound unsure about the compression it has, so I'd explore that route before looking for timing issues.
Did you check the piston ring end gap or just look at the piston and rings? The top compression ring's end gap would ideally be about 0.006" and the second compression ring would be about 0.008". If the gaps are too wide they won't seal. You should have seen evidence of blow-by on the piston if that is the case.
You can try squirting a little oil into the cylinder to see if compression increases. If it does, your rings need to be replaced or you have some seriously worn cylinder walls.
Are the spark plug threads in good shape? If they are damaged or worn it could leak compression there.
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Post by jdb4whlr28 on Mar 11, 2010 22:19:41 GMT -5
The piston didnt really show signs of bad rings... The thing that confuses me is that it just stopped suddenly. It ran perfectly fine until it died. Wouldnt it burn oil or something if the rings were going bad? That makeshift gasket i replaced the head gasket with seemed to boost the compression. And i actually got it to backfire recently. As far as the timing, I redid that using the timing marks provided on the flywheel and camshaft.
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Post by drawkward on Mar 11, 2010 22:27:00 GMT -5
And you're completely sure you are getting spark? I had a bad stator that gave me spark one time, then none the next... Just a thought.
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Post by jdb4whlr28 on Mar 11, 2010 22:38:16 GMT -5
Yea, I grounded it to the cylinder and it was sparking. It seemed to be a strong spark, but I'm not an expert when it comes to engines by any means, so I'm just guessing.
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Post by Fox on Mar 12, 2010 2:37:02 GMT -5
CDI or a sheared Woodruff key. Maybe...  |
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Post by 90GTVert on Mar 12, 2010 21:17:45 GMT -5
It could burn oil if it were the rings, but it also wouldn't be the first time one died from sealing or compression issue with no warning. Not saying that's definitely what you're looking at here, just that I don't believe that option can be eliminated. I don't suppose you have access to an actual compression tester to clear up any doubts? Maybe use the loan-a-tool program at an auto parts store?
You said it backfired. Was it through the exhaust or is it spitting fuel and air back out of the carb? If it's coming out of the carb, you probably have a timing issue.
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Post by jdb4whlr28 on Mar 12, 2010 21:36:55 GMT -5
I believe it backfired out of the carb. I ordered a new CDI last night. Just in case that was the issue. Luckily they are relatively cheap. After that comes in, if it still won't start, I'll get a compression tester. I was thinking of just buying an 80cc big bore kit for it anyway and that would take care of any compression/ring issues...
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Post by 90GTVert on Mar 12, 2010 22:08:39 GMT -5
Backfiring through the carb could point to a few things. Your ignition timing could be causing the spark plug to fire while the intake valve is still open. Valve timing could be off causing the intake valve to be open when it should be closed. The intake valve could be stuck open, not sealing well, or burnt. Usually you would burn an exhaust valve before an intake valve. Any major sealing issues here would leave you with little to no compression. The ignition timing issue mentioned above should still allow you to have decent compression because the piston and valves still operate in the normal manner, the spark is just wrong. Could be an exhaust valve not opening for some reason, but I doubt that's the case here. you should be able to feel nothing or next to nothing coming from the pipe when you crank it over if that were the case. A spare CDI is a great thing to have around since they can't be tested without some specialized tester that no one usually has. Far be it from me to discourage anyone from a big bore kit, but before you buy a big 80cc kit have a look around this thread... 49ccscoot.proboards.com/index.cgi?board=4perfrom&action=display&thread=55&page=1 |
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Post by jdb4whlr28 on Mar 13, 2010 2:17:19 GMT -5
So what is your overall opinion on the big bore kit? Does it actually make the crank bearings go after 2,000 miles? When I bought my scooter it had around 1100 miles and i think i only put 300 on before it died..
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Post by 90GTVert on Mar 13, 2010 3:54:14 GMT -5
I think I'm going to stay away from them. There is enough evidence out there of bearing failure with 80+cc kits that I don't believe the power it will make over a 72cc kit is worth the risk. The 72cc kits usually give you a nice boost in power and torque and will achieve what most people are looking for.
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