DT125R ARCHIVE

zooankski · DTR Senior Member

New build top end compression

Victor · Guru

Could it be losing compression through the pv?

You did have the exhaust on right?

Did you start it up to get some fuel through?

-- Edited by Victor on Sunday 11th of May 2014 08:25:24 PM

zooankski · DTR Senior Member

No, bare engine sitting on the bench. Pv is fitted but nothing else.

i just hooked up the tester and spun it over using a battery

-- Edited by zooankski on Sunday 11th of May 2014 08:31:55 PM

Victor · Guru

Well that's your problem. It need's the exhaust on, and fuel probably adds a bit too

p.s I hope you put lot's of 2t oil in there before compression testing

-- Edited by Victor on Sunday 11th of May 2014 08:39:07 PM

dread123 · Moderator

rings on correct? with the expander ring too? make sure pv all tight and sealed also. Genuine or pattern headgasket? and torqued down correctly?

dread123 · Moderator

doesnt need exhaust to do compression test -although make sure you have throttle wide open when testing-could also point to dodgy crank seals :p

dread123 · Moderator

and +1 about the oil..:p

zooankski · DTR Senior Member

Makes sense to have the exhaust on doh, will try that. Rings are all correct. Lower ring has expander ring fitted.

-- Edited by zooankski on Sunday 11th of May 2014 08:55:48 PM

dread123 · Moderator

ignore me you do need exhaust haha

Victor · Guru

Lol you high dread?

Sorted it yet Zoon? You know we have a top-end section by the way

-- Edited by Victor on Sunday 11th of May 2014 08:58:44 PM

dread123 · Moderator

hahaha little tired... :p

zooankski · DTR Senior Member

Was in a rush , hence wrong section;)
Fitted the exhaust and no different. Compression still low.
Not sure on best course of action now to narrow it down. How could I check head gasket? All torque settings correct so maybe a slightly warped head?
Crank seals could be an issue maybe?

Victor · Guru

zooankski wrote:
Was in a rush , hence wrong section;)
Fitted the exhaust and no different. Compression still low.
Not sure on best course of action now to narrow it down. How could I check head gasket? All torque settings correct so maybe a slightly warped head?
Crank seals could be an issue maybe?

Crank seals wouldn't lose that much psi i shouldn't think, or a bad head gasket. You sure the tester is ok?

Any reason you decided to check compression anyway? did the rebore look bad or something?

Pretty sure you should be around 100 psi so 65 is really low. Maybe just try get it running

I think a cylinder full of fuel/oil will compress higher than just air aswell

Could possibly be a lose plug lol

-- Edited by Victor on Monday 12th of May 2014 12:01:32 AM

Calum122 · Guru

What is it like to kick over with your hand. Is there lots of resistance

I've never used the expander ring. The Wossner pistons don't come with them and I never fitted it to a Mitaka piston kit.

If you're sure you've built it correctly. Rings on right way up. Locked in correctly, Piston correct way round. The Head Gasket on the correct way up.

£10 seems cheap for a genuine head gasket. Make sure it's either genuine or Athena.

zooankski · DTR Senior Member

All spot on,piston arrow facing forward, genuine gasket too. One thing I have noticed today during rebuilding a second engine.
The mitaka piston has an expander ring and 2 rings the same width.
The engine with the possible problem has the expander ring in behind the lower ring which is narrower than the top ring??
Seems odd to me why this second ring kit is different.
A narrower width ring must surely have the expander behind it?
How would you put rings on the wrong way up? They are chamfered to align with the pin so unless you have a big ring end gap then I can't see how that's possible.

zooankski · DTR Senior Member

Rings on the left supplied by PJME as part of Mitaka kit. Note smaller width ring.

Rings on the right again with Mitaka kit but both the same width???

fitted expander ring below lower ring on both engines. Engine with rings the same size is ok so no idea why I have the smaller ring.Will ask PJME tomorrow unless anyone has any ideas?