A forum for owners of Yamaha TTR250 trail and enduro bikes!

There have been a number of on-forum and offline queries about smoking engines in the last few days so I thought it might be worth putting my thoughts down here for you guys either to shoot down in flames or improve upon 

If the smoke is the only symptom and the engine isn’t rattling then there is a very good chance that it’s a relatively easy fix with just new rings and valve stem oil seals needed to stop the smoking.

You need to get the head and barrel off to see what’s happening. There is an “how to” guide on the forum here  to help with the strip down.

In terms of stripping the engine to examine the piston and bore and perhaps fit new rings and valve stem oil seals (worth doing when the head is off) requires reasonable spanner and socket sets, feeler gauges to check valve clearances and a decent scraper to get any old gasket off. Can’t think I use any special tools – except maybe a modified (shortened) screwdriver for the cam chain tensioner coz it’s a bit awkward! 

Assuming the piston and bore look OK with no finger-catching gouges in the barrel, or a break in the continuity of the carbon ring at the top of the cylinder, then new rings might solve your problem.

To explain the carbon ring comment above a little further, a worn piston can "rock" in the bore and the signs of this will be obvious as shown in the following pics:

If your piston and barrel are worn like this then your engine is due a new piston set and rebore. This engine definitely had a rattle, particularly as the revs built up

(PS I am pretty sure that the wear on this particular engine was caused prematurely by the float level being very wrong and giving an over-rich mixture. A very rich mixture will "wash" the oil off the barrel and cause the wear.)

To explain the rattle, when the piston "rocks" from side to side the skirt of the piston hits the side of the cylinder bore resulting in a tell tale noise called piston slap or, as I hear it, a rattle!

You may wish to double check the wear against the tolerances listed in the workshop manual but I have always found this difficult and instead rely on examining the pattern of wear on the parts. 

As a minimum, this is what I reckon you need if the piston and barrel look OK: 

• Top and bottom gaskets are normally all you need e.g. http://www.totallyttrs.com/#gasket_set
• Valve stem oil seals x 4 - about £25-30
• Piston ring set - about £30. Don't order until you have checked the piston to see if it is a Yamaha one and whether it is oversize. Buy the ring set that is correct for the piston.
• If your engine is still original it might be worth replacing the cylinder base O ring whilst the barrel is off as they get flattened over time. About £5-8

If the barrel needs a rebore then you will need an oversize piston kit e.g. http://www.ttr250.com/TTR_extras/#Wossner_Pistons_-_full_kit

A smoky engine will only get worse and allows the engine oil to get contaminate quickly thus exacerbating wear so needs to be dealt with as soon as sensibly possible.

Hope that helps somebody 

Brian