mystery engine problem. (low power)

mechanickid · #1 03-11-2009, 09:56 PM

well im back, i've posted so much with my issues im practically a spammer.

but anyway, this is a very strange problem.
the engine runs pretty well, revs high, but doesn't idle very low, has great compression, (removed head gasket) but has very little power. i recently rebuild the engine with seals and a new ring, but got some metal filings in the bore, and they scratched it up a bit, but i've washed them out by poring gas in and moving the engine a lot.

it gets up to speed, but has very little power, im running 32/1 with new 89 oct gas and the plug is quite new and a dull gray, has spark, usually at that mix its chestnut.

the piston moves freely and so does the clutch and bike.

everything seems well with the new carb.

exhaust cap is off and clear.

i cant imagine what the problem could be!

once again, any help is quite welcome

Creative Engineering · #2 03-11-2009, 11:05 PM

Does it seem like it is getting harder to start? Not in terms of time, but effort.

Jim

mechanickid · #3 03-11-2009, 11:18 PM

more time than effort until i took the head gasket off, then more time AND effort.

mechanickid · #4 03-12-2009, 12:18 PM

btw its a 66cc china.

**Norman** · #5 03-12-2009, 12:31 PM

scratches in the bore means its not getting max compression/power the scratches will allow blow by and a power loss if you replaced the seals you might have a seal failing and that will add to the problem if you have any black oily goo around the head or cylinder gaskets or the crank seals that is a sign of a leak and that is not good need to fix them
you might be able to keep riding it and may get the thing to improve but if you have scratches in the cylinder you might want to replace the cylinder and piston/rings.
you can also check on buying just an engine and part out the one you now have or save it for parts.
with metal shaving in the crankcase it will damage the bearings one small flat spot on a bearing and soon it will let go.
did you get a good seal on the crankcase sealing gasket? pressure up the crankcase and spray the case with soapy water and see if it bubbles the hard part is the area behind the engine case that the clutch and mainshaft for the rear drive is. not much of meat for a good seal and if your gasket isn't sealing there there is going tobe a leak that will allow the fuel mix to excape should see signs of an oily goo coming out behind the clutch and rear drive sprocket.
good luck keep us informed

mechanickid · #6 03-12-2009, 01:55 PM

Quote:

Originally Posted by Norman

scratches in the bore means its not getting max compression/power the scratches will allow blow by and a power loss if you replaced the seals you might have a seal failing and that will add to the problem if you have any black oily goo around the head or cylinder gaskets or the crank seals that is a sign of a leak and that is not good need to fix them
you might be able to keep riding it and may get the thing to improve but if you have scratches in the cylinder you might want to replace the cylinder and piston/rings.
you can also check on buying just an engine and part out the one you now have or save it for parts.
with metal shaving in the crankcase it will damage the bearings one small flat spot on a bearing and soon it will let go.
did you get a good seal on the crankcase sealing gasket? pressure up the crankcase and spray the case with soapy water and see if it bubbles the hard part is the area behind the engine case that the clutch and mainshaft for the rear drive is. not much of meat for a good seal and if your gasket isn't sealing there there is going tobe a leak that will allow the fuel mix to excape should see signs of an oily goo coming out behind the clutch and rear drive sprocket.
good luck keep us informed

well it has had great compression after i replaced the seals, i didn't take the crank case apart, it was clean and didn't show any signs of leaking, i took it off the check everything. no oily goo around the head or the behind the clutch area. im sure i dont have a leak because when putting it back together i broke a ring and ran it with one, and it ran well, just a tiny bit less low end power. then i replace the ring i broke and got some shavings in at the same time (trying to tighten the head bolts into the case using pliers) and it ran crappy, so i took it apart again and found the scratches and ran it with the one old ring, and it ran just as bad. then i cleaned out by taking the spark plug off and poring gas in and moving the engine. but it still doesn't run much better. even though the piston is moving
easily and has good compression.

:[

the scratches are pretty small so they might wear away.

thanks

**Norman** · #7 03-12-2009, 04:54 PM

get a compression guage and see just what is the max pressure I'll go check a couple of mine and I'll tell you what you should have in the way of pressure. off to the shop.
didn't do a cold test on the 80cc
here is the reading of my cough cough 80cc hot throttle closed 90 psig this is a well used engine
reading on my dax 70cc cold throttle closed 115pisg pretty new engine
reading on my dax 70cc hot throttle closed 100 psig
so if you have a reading somewhere in that range you should be good.

mechanickid · #8 03-12-2009, 09:55 PM

im afraid i dont have a compression guage available to me. nor have i the expenditure to purchase one.

toytime · #9 03-13-2009, 11:36 AM

Why don't you take your bike to a small engine repair shop? or a local rental shop that rents motorized tools may help. That's what I do, I've made friends with a guy who repairs the tools and I drop by for those times of need. Maybe even try to find the local guy who repairs lawnmowers in his garage by looking on Kijiji or craigs. Every person who works on engines will have one and it only takes one minute to do the test.
I don't know if it's worthwhile on these kits but does anyone ever hone these cylinders?
It may help him, I don't know.
You said you had the "exhaust cap is off". Others may not agree, but I think that is not a good idea, you need back pressure.

mechanickid · #10 03-13-2009, 01:51 PM

i could do that when i have some money... its a good idea. i've heard that honing these engines doesn't work.
as for back pressure, that might help but not in my situation. i've run it with it off before, just gets more power. i do agree that back pressure is good though.