Hi all,

With the advent of the new year, I've decided to treat my bike to an engine rebuild. I've done other bikes before, but never a 315...

Is there anything I should watch out for, or is it fairly straightforward? The engine sounds noisy, not rattly as such, but just like it's not running smoothly. Any idea on whether top/bottom end is most likely to have gone? Are the Montesas particularly susceptible to either? It's certainly noticeably more noisy than other Montesas I've listened to.

Also, whilst I've got it in bits, is there anything I can do to the internals to improve power??

Any help would be greatly appreciated.

Cheers,

Ben

Edited January 12, 200917 yr by Ben315r

What sort of noise are you hearing? A re-build sounds pretty drastic and costly action to take when you aren't sure what the problem is. Mont 315r's were generally pretty good at holding together, but obviously a couple of drownings may have lead to all manner of crud getting into the motor, and then problems can begin if not cleaned out properly.

If you're sure its bottom end noise then obviously a re-build is likely to be needed. If it's top-end, surely whipping the head & barrel off is the first thing to do. Check the state of the piston, rings, small end bearing and pin. If these are kn*ckered, change them and try it out again to see if the noise has abated. If still no better, then that would be the time to start splitting the thing and putting new mains, seals, con-rod (?), etc.

unless you use very high octane gas it will sound like hammered ****!

unless you use very high octane gas it will sound like hammered ****!

WHY?

Hi all,

With the advent of the new year, I've decided to treat my bike to an engine rebuild. I've done other bikes before, but never a 315...

Is there anything I should watch out for, or is it fairly straightforward? The engine sounds noisy, not rattly as such, but just like it's not running smoothly. Any idea on whether top/bottom end is most likely to have gone? Are the Montesas particularly susceptible to either? It's certainly noticeably more noisy than other Montesas I've listened to.

Also, whilst I've got it in bits, is there anything I can do to the internals to improve power??

Any help would be greatly appreciated.

Cheers,

Ben

Ear plugs, seriously if its just a bit noisy I would think twice.

unless you use very high octane gas it will sound like hammered ****!

What difference would high octane petrol make?

What difference would high octane petrol make?

Higher octane has less knock.

What difference would high octane petrol make?

OK, so what does that mean then?

trials bikes have a high compression ratio to help with low end power, higher compression engines need higher octane fuel to reduce or prevent pre-ignition or "knock"

I haven't done a bottom end on the 315r, so I can't advise you on that, but I have done a top end a few months ago. It is straight forward, so if you've done a top end on any modern 2-stroke, then it will be no surprises for you. Unbolt skid plate at the front, remove both frame downtubes, and radiator comes right out and gives you plenty of room to access the head and cylinder. The only tricky bit I remember while putting it back together was trying to torque one of the head bolts to proper spec. It was the rear-most bolt, and there just isn't much room to get at it. Getting the skid plate bolted back up was a little tricky too, so I had to get creative with a large C-clamp. But I have an aftermarket skid plate, so I don't know if the stock one is equally challenging.

BTW, you can remove magneto cover, grab a hold of the flywheel, and see if you can move it up/down or left/right. That will give you an idea of whether or not main bearings need attention, and only costs you a magneto cover gasket.

trials bikes have a high compression ratio to help with low end power, higher compression engines need higher octane fuel to reduce or prevent pre-ignition or "knock"

So are there any other reasons for this "knocking" or is it purely down to the fuel octane level?

Well, its easier to put some race gas in it to see if it solves the problem than it is to rebuild only to find it still rattles to beat ****!

Thanks for all the advice guys. I'll pull the head/cylinder off and have a look. I was perhaps being a bit over-enthusiastic when I said engine rebuild...!