low power need help

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matiks

New Member
Apr 25, 2012
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gilbert,az
I just put a new 66cc motor kit on my bike and noticed that it had two base seals/lower gaskets. On my first ride it put 89 octane mixed at 24-1 and the bike would only go 10 miles an hour at wot and shut off if i let off the gas. I also filed down the piston skirt on the intake side so it would open fully. Could the extra gasket cuase low proformance? or me filing the skirt?



after though i should of ran it stock for awhile then filed but it was apart and clean...opps
 
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JonnyR

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May 13, 2012
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ronkonkoma, new york
welcome to the forum

there is a few things that could be wrong lets start with does it idle right

also did you put the rings back in correctly they have pins on the piston they line up with

when you took it all apart did you damage any gaskets the base gasket can be replaced with RVT black silicone to form a gasket and it is thinner than stock single gasket

how much did you take off the piston when you ported it you could have taken to much off

there are other things but we can start here
 
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matiks

New Member
Apr 25, 2012
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gilbert,az
i left about a 1mm showing when at tdc as to not screw things up. rings i lined up with the pins befor i put it in. no damage to the gaskets and no leaks.

It will never rev up all the way when riding but if stopped and clutch in if held at wot it will slowly rise to a full rev.

it seems that its easier to pedal start then my old motor so thinking it has lower compreshion i just do not know if two seals would make a noticeble differances. I guess i might as well remove it and see if it fixes my problem. any info is allways appreciated.
 
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M

matiks

New Member
Apr 25, 2012
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gilbert,az
ok i will remove the tank one thank you for the sugestion. I am 2 stroking and from looking at the plug it runs between lean and good
 
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matiks

New Member
Apr 25, 2012
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gilbert,az
I took the extra bottom gasket out and it runs better and has more power but it has a metal slapping noise at certain rmps, mid range(i think). At full rev it sounds good but i seem to be missing the torque to get up there. I now can pull in the clutch and it will rev up quickly but dose not like to idle.

What I am thinking is that i must have cut the piston skirt to short becuase now it goes a hair past the intake port at tdc becuase i removed the gasket.

if any one has cut it to short or knows what i am talking about i would like to hear any advice
 
crassius

crassius

Well-Known Member
Sep 30, 2012
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STOP

that slapping noise may be nothing to worry about, but you should really check for piston hitting head, or a ring slightly off the locater pin before you run it too much
 
M

matiks

New Member
Apr 25, 2012
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gilbert,az
I took it apart this monaring and there seemed to be no contact between the piston and the head no marks of any kind. When looking at the cylinder i noticed that the line cuased from the carbon build up around the very top the the cylinder were the rings do not go up to was a little higer on one side(front right of the piston if your sitting on the bike.

SO it seems that my piston was comming up slightly crooked the deifferances is less then a mm.

I checked for what may be the cuase and the only thing i can come up with is either slightly lopesided bearing or where the skirt was shaved(on the intake side to match the intake port) its slightly higher on the right side less than a mm my thought is that the right side of the cylinder is easier to push down becuase its has ??? slightly less ressistance????

I orderd a new piston so I will see if that corrects the problem. then trim the skirt a little at a time to see if it betters its proformance.
 
crassius

crassius

Well-Known Member
Sep 30, 2012
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USA
The acceleration on the piston when it has to reverse direction at the top is incredibly high. A bit of weight difference left/right could cause what is known as 'piston slap'.

With these engines being so primitive & sloppy in their manufacture, I doubt I'd ever bother to try to get that intake opened more than usual. A lot can go wrong & much better engines can be had than the standard ones that come with the kits.
 
fatdaddy

fatdaddy

New Member
May 4, 2011
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San Jose, Ca.
If ya only took some off one side of the piston then it's off balance. You'll need to take an equal amount from the other side to get it closer to balanced. The heavy side is pushing it further up on that side when at TDC, and further down when at BDC. I know it's chinese made but the piston still has SOME balance from the factory. The metalic slap sound may be coming from that.
fatdaddy.
 
2door

2door

Moderator
Staff member
Sep 15, 2008
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Littleton, Colorado
Even though fatdaddy and I usually agree I'll have to step in here and say that I trim the intake side of all my pistons to match the port and I've never experienced anything like uneven cylinder wear, piston slap or loss of compression. My engines all run strong and most have accured hundreds and in a couple of cases thousands of miles with no ill effects from trimming the skirt.

My guess here is that the OP might have an ignition timing problem that accounts for his severe lack of performance. Possibly a magneto rotor or crankshaft keyway that was cut wrong retarding the ignition. It wouldn't be the first time. I had one engine that I found was 4 degrees retarded from others and it displayed some of the same performance issues described by the OP.

Checking magneto rotor index against top dead center is a tedious job but I've used it to get the spark to happen where I want it to and to help diagnose low performance issues on engines.

Tom
 
fatdaddy

fatdaddy

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May 4, 2011
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San Jose, Ca.
And you may be right 2door. I've shaved a little off a few pistons myself, But I guess I'm just too anal to NOT even it up on balance.
A weak spark (coil) or timing issue is just as likely. A quick check for timing, The key should be at 2 oclock with the piston at TDC. There are more acurate ways, but this is just a quick check.
And like I said, it could also be a weak coil. I KNOW, everytime a bike don't run right I cry COIL. I've had a LOT of bad experiences with those cheap chinese coils and it's an inexpensive thing check. I think everyone that owns a MB should have a spare one anyway, cause 90% of the time IT'S THE COIL. It's the other 10% that gets interesting.
Another thing. If you took the magneto(the spinning magnet thing) off at any time it may be on backwards. I think Al. Fisherman has the best thread on setting this up, check it out. I learned some from it myself, great thread.
fatdaddy.
 
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matiks

New Member
Apr 25, 2012
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gilbert,az
ok i will check the timing and i have hear of lots of people having good results with triming the pistion. But i think it was trimed just right with both base gaskets and then when i removed one to get better ompression then piston skirt trim is to high. I am just woundering if that could make that big of a differance??

also if the skirt is un trimed and it closes sooner and that would raise the compreshion a little and noramally compresion=torque? and thats what i am after...does that sound right or do i have no clue.

new piston is here the 13 so i will know if it was the skirt or something else.

UPDATE the new piston came in and i put it on the bike and it would not start
I had : spark, gas, air, good compression, good plug and wire,...... and it would only sputer for a secount for about an hour pedaling/ trouble shooting....turns out i had a vary weak spark and i had to replace the magnet and now its working with the new piston (untrimmed skirt).
 
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