rs 125 engine rebuild???

arronb · Derestricted Danger Joined: 16 Jan 2013 Karma :

I have rebuilt my rs 125 engine as it over heated, I fitted new crank with new bearings, but while I was stripping it I cracked the left side engine casing so I bought a new one, fitted it and put it all back together and now its in my bike but when trying to start it its rotating real slow as if they aint enough power in the battery so I power the battery up with a very powerful booster and its still struggling to rotate round, but before I built my topend onto my bike I could spin it freely with my hand... does anyone know the problem?

thanks

P.addy · Red Rocket Joined: 14 Feb 2008 Karma :

Remove the starter and plug it directly to a battery, does it spin freely?

arronb · Derestricted Danger Joined: 16 Jan 2013 Karma :

I took the plug out and its spinning a abit faster as it dosnt sound like its struggling as much but still not fast

P.addy · Red Rocket Joined: 14 Feb 2008 Karma :

Drop some petrol down the spark plug hole and pop a new spark plug on there.

arronb · Derestricted Danger Joined: 16 Jan 2013 Karma :

but if its only turning over slowly so that wouldn't make it spark enough to fire it up

P.addy · Red Rocket Joined: 14 Feb 2008 Karma :

chris-red · Have you considered a TDM? Joined: 21 Sep 2005 Karma :

Charge the battery
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arronb · Derestricted Danger Joined: 16 Jan 2013 Karma :

ive charged the battery, could it be my starter motor? it could of weakened by pushing it with pressure quite a lot? plus with a new battery?

P.addy · Red Rocket Joined: 14 Feb 2008 Karma :

Take it off and see if it spins faster Thumbs Up

chris-red · Have you considered a TDM? Joined: 21 Sep 2005 Karma :

arronb · Derestricted Danger Joined: 16 Jan 2013 Karma :

the battery is fully charged mate and ive tried turning the flywheel and its quite tight to turn, and I cant bump it as its thick snow :/

P.addy · Red Rocket Joined: 14 Feb 2008 Karma :

You sure you built it correctly? Set the correct squish etc? What base gasket did you use?

spaz · Nova Slayer Joined: 29 Apr 2012 Karma :

Did you fit new crank seals?

If so, it sounds like you've fitted them so they are butting up against the main bearings. There needs to be a clearance between the seals and the main bearing otherwise it will inhibit the free rotation of the crank shaft.
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arronb · Derestricted Danger Joined: 16 Jan 2013 Karma :

Yeah im sure i built itpropperly and i bought a full gasket kit of ebay for 28 quid& yeah i fitted new seals

P.addy · Red Rocket Joined: 14 Feb 2008 Karma :

spaz · Nova Slayer Joined: 29 Apr 2012 Karma :

Well in that case it sounds like a strip and rebuild is in order.

Thumbs Up

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Robby · Dirty Old Man Joined: 16 May 2002 Karma :

I assume the bearings are pressed into the cases, the crank is fitted, then the whole lot gets bolted up.

The usual trick I've heard of when assembling 2-strokes like this is to give the end of the crank a knock with a hide mallet before bolting it all up, to seat the bearings. Otherwise you get tightness from poorly seated bearings.

arronb · Derestricted Danger Joined: 16 Jan 2013 Karma :

It roatates nice and freely without my barrel on, only when ive got my topend on it gets tight, does that mean the conrod could be bent?

P.addy · Red Rocket Joined: 14 Feb 2008 Karma :

You done a full strip/rebuilt and not bought the conrod?

What was the reason for the strip?

arronb · Derestricted Danger Joined: 16 Jan 2013 Karma :

I fitted a crank but wasnt brand new, the guy said was in good condition but looks like it cant be

arronb · Derestricted Danger Joined: 16 Jan 2013 Karma :

I fitted a crank but wasnt brand new, the guy said was in good condition but looks like it cant be

spaz · Nova Slayer Joined: 29 Apr 2012 Karma :

Is the piston installed the correct way round?

If not, the piston rings could be fouling the exhaust port. Or, the piston skirt could be fouling the conrod.

What about the rings? Are they installed the correct way up?
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bikersupermot... · Could Be A Chat Bot Joined: 26 Jul 2008 Karma :

When you fitted the crank to the conrod did it move easily on its big end bearing?

If the conrod spins ok then your problem lies either with the conrod big or small end bearing or the piston not sliding freely up and down in the cylinder. I'd take the head off and check where it's binding in one or more of these places.

Let us know how u get on.
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andy_uk · World Chat Champion Joined: 13 Aug 2011 Karma :

Pull the top end off again.
Did you get the right piston size? How easy did it slide out?
Make sure the crank still rotates freely.
Assuming everything's okay so far, gently push and pull the conrod to make sure there isn't any play in it.
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arronb · Derestricted Danger Joined: 16 Jan 2013 Karma :

the piston is the same one as its not got any damage from when I overheated it and I got told by fitted an new engine casing wont work cause it wont line up properly? apparently it has to be a matching pair of casings otherwise the crank will tighten up?, the casing I bought is for thr rotax 122 engine etc but the guy said it don't matter? any ideas,

cheers for the help guys