Slowly trying to sort out my montesa in tatty condition. The fool who had it before me looks to have been riding it with no bash plate and caught the bottom causing a small hole in the engine casing ( rear brake side ). I only noticed this when I put fresh oil in and when I cleaned it up the oil had petrol mixed with it. I can only assume this is the reason the bike bogs out really bad between half and 3/4 revs. Is this causing an air leak if so whats the best way to identify the leak and ways to fix it? Any help would be massively helpful as im itching to get out on it. Cheers folks!!

is the hole in the centre case or the clutch cover?If the oil has petrol mixed with it I would say you have a clutch side crank seal leak allowing some of the crang charge to be pushed throught to the gearbox and mixing with the petrol

If petrol is getting in the gear oil,it must have a crank seal gone.The air is not coming in through the hole,its bogging cos it's sucking gear oil in and burning it,does it smoke badly?If it's really tatty be prepared to spend a fair bit on it to get it useable.Sorry for the bad news

Cheers fella's.

Yes its smoking I presumed it was just too much 2 stroke oil in the mix. The hole is under the casing. I took the cover off to see if there was any damage and the seal around it looked stretched. Its a 99 and I only bought it to practise on pprobablybuy a newer model. Been looking at casings on ebay for

Remember there is a vent in the top of the gearbox that is a designed in "air leak."

I agree with others, sounds like seal failure.

You should get that hole fixed to prevent loss of oil and gearbox failure. 3options:

Best = New Case

Next best = weld up the hole

Cheapo solution = Clean surfaces really well and seal with metal filled epoxy if the hole is not huge. I know people who have run bikes for years that were fixed this way.

Cheers for that. Two people have told me about that epoxy. Ill give that ago first see how it goes. Will it just be one seal on the right casing or could it be both? And if possible does anybody know where I can get these seals from?

http://www.jimsandiford.co.uk/ will have seals or http://www.ebay.co.u...=item35d0b576fc if you want to try the epoxy, jb weld is good I did a Honda tlm crankcase with same problem as yours worked a treat

Edited January 16, 201412 yr by mont315

There are seals on both ends of the crank.

Left side is behind flywheel which needs a special puller. No oil outside the seal.

Right side is behind clutch basket. May need a clutch holder to get the nut off.

Your location "Northwest" doesn't give us much help to direct you to parts. Northwest UK, US, Australia?

Northwest uk chaps. Im in work this weekend ill crack on with it through next week. Cheers all for your advice