mystery engine problem. (low power)

[mechanickid]

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]

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

Jim

 

[mechanickid]

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

 

[mechanickid]

btw its a 66cc china.

 

[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

 

[mechanickid]

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]

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]

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

 

[toytime]

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]

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.

 

[toytime]

I was thinking more along the lines of them not charging you anything but I guess it helps to get to know them. "My guy" just screwed me , I guess. I never asked him what he would charge me to make a gas tank and when I went to pick it up he said " I don't know, how about a hundred bucks. I didn't let on but I almost dropped dead when he said that. The tank turned out perfect though and it was way better than I expected and it is the last thing I needed to finish the bike so I'll get over it.
I wish you luck with your bike.

 

[emdude]

how did you manage to get get the engine to work with out the head gasket? I've tried this several times because I'd like all the compression I can get, lots of hills where I live here, but the piston hits the head on my engine. I have an 80 cc that is I assume just like everyone else's but I just can't do without the head gasket. How'd you do it?

 

[mechanickid]

perhaps you have an older engine, the one i got seemed to be a little different than most of the ones people talk about on here, like my piston skirt doesn't cover up a little of the air intake. i still have a gasket on the bottom, maybe you could try putting two gaskets on the bottom and see if that improves anything timing wise.

 

[emdude]

Well, maybe they di change the engines. On mine I DO have the lower part of the piston skirt still in the intake with the piston at TDC. The other problem is that the piston at BDC I can see the top piston ring in the intake. Sadly this does no leave much room for a taller base gasket or even a spacer under the cylinder.

 

[mechanickid]

oh well thats too bad, but if you piston is already almost touching the head than you dont need to get more compression.

 

[mechanickid]

okay so i found something a little new, when im turning the engine with the spark plug off it will stop a little at the bottom dead center if its moving slow enough. moving a little fast and it doesn't seem to have much resistance. it does have a bit of a rattling sounds though, kinda like a clankity like somethings hitting something, but its not the piston hitting the head.

 

[toytime]

I wonder if your engine has the needle bearings instead of just the bushings and one of the needles is messed up? Sounds like your engine will go past that point at speed but when trying to start it, it just doesn't have the momentum to get past that point.
It's really amazing that it runs .

 

[mechanickid]

its very subtle, i dont think it would be that because it wouldn't only occur at bottom dead center i dont think. boy this thing really finds a lot of problems to have!

i started a new thread Clankity rattling sounds because people dont seem to be looking at this one much anymore and i really want to find the problem.

thank you though Toytime

 

[toytime]

I saw your other thread, I think it's more a matter of not knowing what to tell you.
OK, maybe I'm wrong but what else can it be? Is the piston or the rings bumping up to a ridge or whatever as it reaches it's bottom stroke? Is there some sort of problem with the teeth on your clutch and crank gear? The only thing we are sure of is that you dropped some metal into the cylinder. Normally that would call for splitting your casing apart but like I would most likely do , you took a chance and hoped for some luck but in the end, you may have paid for it. It's my life story! ..lol
I'm just thinking aloud now but would removing your small gear and turning the clutch a few teeth and then re-installing the gear help to figure things out help?

 

[mechanickid]

well guess what? im stupid. it was the head, it was out of alignment and the piston was just barely touching it. still low power but no rattle, now im going to fiddle with the needle, i could have this licked soon.