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

GNT Biker · Member

Greetings from sunny but seized Spain

TTRfan · Super Guru

Welcome to the forum G! I have ridden down Malaga way with Lyndon at ToroTrail back when he used all TTR250s and also Baz Fleming at the Adventure Rider Centre. Fabulous trails around the mountains biggrin biggrin

Sorry to hear of your TTR's problem.

Let's look at doing the easy checks first.

1. Take off the start gear cover and check for debris, chipped, broken teeth or whetever. If nothing obviously damaged then pop out the gear and check if you can spin the little gear that connects to the sprag. It should spin freely one way and be solid the other. If it does then it is likely the sprag clutch is still OK.

Sprag_clutch_repair 001.jpg

2. If all OK there then next up is to remove the camshaft cover and make sure that the camshafts are still properly aligned, With the engine at TDC, the camshaft sprockets should be lined up as in the photo. Check the camshafts are both perfectly positioned and there is nothing obviously amiss with them in terms of scoring, broken caps, debris, etc

3. If everything looks OK there then my next suggestion is to remove the spark plug and check it looks OK and has not been damaged by anything loose in the combustion chamber that shouldn't be there. Try and have a look inside the head if you can - difficult I know.

4. If everything is checking out up to this point then put the TTR in 6th gear and very gently rock it back and forth - and I repeat, very gently! What happens - if anything?

Perhaps have a go with these suggestions and let us know what, if anything, you find.

Brian

GNT Biker · Member

Hi Brian
Thanks for the swift and detailed response. I will get the tools out at the weekend and report back.
G

GNT Biker · Member

Hi all again from Costa del Sod it!

so managed to follow instructions from Brian and images below to show the current state of play. I am no expert but it does not seem like good news.
first pictures show the starter clutch parts and housing. No signs of damage or broken metal or shards and the gears look ok, in fact they look pretty new to me.

The seconds set of images shows the cam position with the the single timing line lined up with the engine casing notch and the second st shows the cam positions when the engine locks up, the H shaped timing mark is then as shown.

If I put the bike in 6th gear it turns until it reaches the second position then jams like before.

I am guessing the cam chain has slipped and the piston is jamming on the valve but Not sure if this can be repaired without splitting the cases.

All advice welcome and particular thanks to Brian for such clear guidance.

G.

TTRfan · Super Guru

Hi G

Good news ref. the starter clutch assembly! biggrin

Hoping the rest may not be too bad - a bent valve could be all. Just got to hope the piston isn't damaged.

Unless anyone else thinks different, I reckon you are going to have to get the head off to check - a simple visual guide is here but the TTR250 manual is pretty good if you have it. Steve has a downloadable copy here.

At a minimum you are going to need a head/base gasket set but let's see what you find once the head is off.

If, as seems to be the case, the timing chain slipped was this caused by a valve "dropping" or did it jump the sprocket(s) causing the valve to make contact with the piston? The answer lies "within" I hope.

Brian

GNT Biker · Member

Hi Brian
Thanks again for your advice and support. I think the next move is to ship the engine back to the UK and dismantle it a bit further there as you have suggested. I know from experience here that parts are a nightmare to source and if I tried to get this done at my local workshop I will be waiting a long time. Also I can access engineering etc in UK if I need it a lot easier.
As soon as I get it back to the Uk I will strip it down and update you.
G

GNT Biker · Member

Hi All,

update from sunny Spain regarding the recent issues.

Thanks to Brian who provided really clear instructions I have now managed to delve deeper into the engine.

So bad news is both inlet valves bent but the valve guides look ok but will not know for sure until I get the valve springs etc off and can get at the guides themselves. On the upside I think the piston is ok as I can only see a small nick on the piston crown where the valves have touched but I am not sure what the piston surface and cut outs should look like. I have posted the best pics I can get and welcome any advice.

I am fairly sure the chain is worn and stretched as the tensioner was at full extension but the slippers are intact, they are quite firm but Don’t look worn so I think they are ok. No obvious wear on the crank gear and I think the cam Sprockets are ok as they do not seem worn and teeth look fine.

So in summary I am thinking that two new valves with seats cut and lapped as long as the guides are ok and various bits for the rebuild, gaskets, o rings new chain etc and I may be lucky. I have checked the tensioner is working properly so I think that should be ok but happy take any advice or thoughts.

So with much thanks to all the help received so far I would welcome any advice on other things to check/ replace while I am inside and any thoughts on the piston condition.

Really grateful that these forums exist and that there are still some great people willing to help others. I thank you all.

G

skinut · Member

Hi, from your photos, there are no clean areas on the piston that would indicate oil passing the rings, so it looks like the piston is ok. However, the photo does seem to show a couple of ' eyebrow' marks in the valve cut outs on the back (inlet side). This would indicate the inlet valves have been hitting the piston, caused by incorrect valve timing (late).
Regardless of whether you think the valves are ok, I would strongly recommend that that you replace the inlets and possibly if your budget can stretch to it, the exhaust ones too.

Have a good check on how much 'wiggle ' there is between valve guides to valves as the inlets are likely to have been damaged by the valve trying to bend in it. Again if in doubt have them replaced by a competent machine shop. The exhaust guides can have a little more clearance as they run hotter and oil does need to get down there or rapid wear will ensue. If you are not sure, the machine shop will advise you. If you do have any of the guides replaced, the valve seats will need to be re-cut to ensure they are correctly aligned.
The machine shop will normally do this as a matter of course.

On the rebuilding, side, with new valves, seats re-cut etc the valve clearances will need to be checked and shims changed to give the correct clearance. I would recommend that you do this before re-fitting the cylinder head and only do one cam with the other cam/ valves safely closed ! Once all this is done, align the cam marks but when re-fitting the head to barrel just have the piston a little way down the bore (on the upstroke) to make sure nothing can collide. Once the head is torqued down, rotate the crank so the TDC marks align, the refit the chain etc.
Hope this helps.