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

Moose · Member

Compression

domski1 · Senior Member

As far as I am aware most ttr's don't have a decompression device, some have the actuator arm in the head but its a manual cable operated device. I recently carried out a compression test on my ttr due to starting problems and achieved 120 psi, still under the specified figure.

barra8 · Guru

Doomski is correct...the only device that it '''may''' have is the decompression lever.

You will need a tester that ranges to around 200psi to test this properly.

If you have bad compression check that the actuator lever is not getting caught (if fitted)

Do a cold & hot test & see how ya go.

....................

Jarrah.

Moose · Member

The tester i have goes to 300psi...
Think i am over it and will just re ring it for the fun of it..
It does not have a decomp lever

barra8 · Guru

Alot of that compression loss could be from the valves not sealing & simply needing re-shimming. 10 thou is bugger all so i would not think it would be rings.

If you are doing them to be sure it would pay to do your valves at the same time as these are the most important part of the engine.

You can test if it's rings by doing a cold & hot test.

If it is low on compression when cold but goes higher when warm then it is highly likely that your rings need replacing.

If however you get the same reading cold & hot or it only has the slightlest change between a hot & cold test than valves will need work.

.................................

Jarrah.

Moose · Member

barra8 wrote:
Alot of that compression loss could be from the valves not sealing & simply needing re-shimming. 10 thou is bugger all so i would not think it would be rings.
If you are doing them to be sure it would pay to do your valves at the same time as these are the most important part of the engine.
You can test if it's rings by doing a cold & hot test.
If it is low on compression when cold but goes higher when warm then it is highly likely that your rings need replacing.
If however you get the same reading cold & hot or it only has the slightlest change between a hot & cold test than valves will need work.
.................................
Jarrah.

Will also do the head if going to the trouble of doing rings but might try and do a decent test once i have the motor on the bench.. Will start a project post in that section once i get a minute

barra8 · Guru

Moose wrote:
barra8 wrote:
Alot of that compression loss could be from the valves not sealing & simply needing re-shimming. 10 thou is bugger all so i would not think it would be rings.
If you are doing them to be sure it would pay to do your valves at the same time as these are the most important part of the engine.
You can test if it's rings by doing a cold & hot test.
If it is low on compression when cold but goes higher when warm then it is highly likely that your rings need replacing.
If however you get the same reading cold & hot or it only has the slightlest change between a hot & cold test than valves will need work.
.................................
Jarrah.
Will also do the head if going to the trouble of doing rings but might try and do a decent test once i have the motor on the bench.. Will start a project post in that section once i get a minute

Yeah sounds like a plan....It also sounds like you are going to be using the solvent in the ports trick to see if the valves are sealing? Or are you going to run the bike on the bench?

Sounds like a great idea to start a thread.....Maybe i have inspired someone??? wink

Good luck with it all buddy.

..............................

Jarrah.

Padowan · Senior Member

Bumping this topic.

I just got a new compression tester and did a wet and dry test on my TTR. The results were incredibly low, 85 and 95psi dry then wet. This seems waaaay too low, but the biks is running OK, perhaps lacking a bit of punch, but it starts and runs OK. I know the low range is 145psi, so I'm really surprised the bike runs at all - does anyone know what the lowest compression is that the engine will run just OK?

I'm going to take the head off and lap the valves (whilst doing the stem oil seals, as it blows some smoke on cold startup), but with a compression readong so low I'm thinking either my new tester is rubbish or something else is wrong.

As the 2 readings (wet and dry) were reasonably close (albeit low) I'm thinking that the bore is probably OK and that I must be getting leakage from the valves (the clearances are OK btw), but I'll check out the cylinder and piston whilst the head is off.

Any other thoughts?

TTRfan · Super Guru

Hi Simon

That's very interesting.

I have a newly rebored (+.05mm) engine that you are welcome to check your compression tester on? I'd be interested to see what the 325 checks out at as well wink

Brian

Padowan · Senior Member

OK so I'm in the process of taking the top-end off, I'm trying to get the cam-caps off and the bolts won't budge. Service manual says these should have been torqued up to 10Nm, they must be tighter than that or are they some sort of anti-vibration bolt that you wouldn't expect to come out easily?

I'm using a 6-side 1/4" socket and small ratchet on them, and I'm worried that I'll either shear the bolt or round the head off with the force it feels like they're going to need to shift them.

Any advice please guys, or just man-up and get an extension on my little ratchet?

Thanks

TTRfan · Super Guru

That's an oddity on the TTR engine Simon.

Despite torquing them up to only 10Nm they seem to need a breaker bar to undo them so. yes, man up and give them some wellie - diagonally of course

Brian

Padowan · Senior Member

OK, thanks Brian. Await the next post where I shear one off.......

Padowan · Senior Member

Success. Thanks Brian - only needed a 6 inch copper pipe extension - I didn't have the reducers to fir a 1/4 drive socket to my 24" breaker bar!

Flying Dodo · Veteran Member

Interestingly I also had the same problem - actually broke an allen key tip off - thankfully it didn't tumble down into the abyss! Had to use an allen key for the socket set and a lot of wellie - this was after I had refitted the head and torqued them (incorrectly) to 20 rather than 10.

Padowan · Senior Member

So the engine is reassembled.
* New piston rings
* Lapped valves
* New head and base gaskets

Everything oily (so a wet-test equivalent) and turning it over on the starter I got 135psi on my tester.

Considerably better than the figure I got previously of 95, but still really low, making me think that my tester (at £20) is just cheap and badly calibrated - which is dissapointing. Perhaps I'll put it on my compressor and see if it reads anything like the pressure on that....

So if I assume that the calibration is poor that's still an improvement of almost 50% more compression, that's a huge difference. I#m looking forward to breaking it in and seeing what sort of improvements I see.

Will finish putting it all together this evening and fire it up.....

Padowan · Senior Member

So it ran straight off the button, much less turning over and choke than before.

Ran for a few mins and checked everything was OK. Reassembled fully and took out for a gentle ride. Certainly feels tighter and punchier, felt like it had much more torque - was short shifting to keep the engine at lower revs during break in and it pulled better than before in a higher gear.

Once back home there was an oil weep from the pipes, so I've replaced the copper washers. Need a few more gentle rides before I open it up fully but I'm expecting good things!

TTRfan · Super Guru

Excellent news Simon - good job biggrin

Brian