Jump to content
  • Sign up now

    Registration is quick and easy 

Leak down testing


Mr Fro
 Share

Recommended Posts

Right then. I noticed the other day that when switched off and parked in gear on a decent slope that there was a slight hissing noise coming from the engine somewhere. I did a compression test and all was well. I'm not using oil or spitting it out from anywhere so I'm thinking that I may have some iffy valve seating.


The next step is leak testing but I don't have the kit or any experience of them. Naturally, I don't want to spend a fortune on my 500 quid track toy - nor do I have a compressor any more...


Has anyone got any brilliant ideas as to how I might go about this without chucking a load of cash at it?

My thought so far go like this... Get cylinder in "closed" position, stick bike in gear and lock the back wheel, chuck a nugget of dry ice down the plug hole and screw in a compression gauge then note the max pressure and any loss.


Cheers everyone,

Fro

Link to comment
Share on other sites

Get a bike place to do a leakdown test. It just pressurises your combustion chamber with compressed air, then you listen at the valves and engine breathers to work out where the worst leakage is from. Only other way is to make a connection via the spark plug hole to a spare tyre?

Link to comment
Share on other sites

Just might be the motor coolant and oil cooling - settling down. When you switch off the motor the water and oil pump stop straight away, some of the fluids pool up in the hot parts of the engine resulting in these fluids getting very hot hence the hissing. The expansion bottle for the coolant "If water cooled" is install on the bike this purpose. If there's no external signs of leakage, no signs of water contamination in the oil and running temperature is normal l would say all's ok.

Link to comment
Share on other sites

Cheers guys. I considered the spare tyre idea but it needs to have quite a high initial pressure ~100psi I think.


The bike was cold at the time and the noise disappeared when I dipped the clutch so I'm certain there is some leakage somewhere. I can't complain about the performance though as it runs like a champ - could well be I'm being a bit picky but it wouldn't hurt to give the other half a good reason for stripping it down. ;-)

Link to comment
Share on other sites

Could it be a blocked breather on the tank?

Nope, it was related to the forward thrust of the bike. I reckon it's probably a valve that needs lapping or - with any luck - just a valve clearence issue.


I've dug out a couple of regulators so I'll try cobbling something together. Don't know what I'm going to do about the pressure source yet though...

Link to comment
Share on other sites

Cheers guys. I considered the spare tyre idea but it needs to have quite a high initial pressure ~100psi I think.


The bike was cold at the time and the noise disappeared when I dipped the clutch so I'm certain there is some leakage somewhere. I can't complain about the performance though as it runs like a champ - could well be I'm being a bit picky but it wouldn't hurt to give the other half a good reason for stripping it down. ;-)

I can't complain about the performance though as it runs like a champ - could well be I'm being a bit picky but it wouldn't hurt to give the other half a good reason for stripping it down. ;-)

Chris wrote: Well if it's not broke don't fix it!

Link to comment
Share on other sites

because you go higher then the normal pressure of the cylinder


100 psi is usually higher then most motors if it holds pressure ok at 100 psi then all is will


eg 10 psi wont be enough to show a leak on a weak piston ring :wink:

Link to comment
Share on other sites

if you have done loads of leak down tests then you will know that leaks can occur at different pressures!


hence why they test above what the cylinder pushes if it holds 100 psi then it will have no problems with the 90 psi they are rated at


its like testing breaking strength of rope if it breaks at 100 tonne then the safe working load could be 90 tonne


remember leak down testing is to make sure its at its optimum and within a certain spec if you have a 10% drop at 100 psi that could mean a full engine rebuild to some manufacturers


but if your testing at say 50 psi you may only get a 5% drop as the pressure isn't there so you could end up with false readings


IMO do it by the book if your going to do it and if it within spec forget about it

Link to comment
Share on other sites

Doesn't necessarilly need a leakdown tester, just a way of applying compressed gas into the combustion chamber. If this is done with a cold engine, without a layer of oil on the rings, he only then has to listen for air flow from the valves or crankcase breathers to get an idea where the worst of the leaks is coming from. If it leaks at 50psi, it's gonna leak worse at 100psi. If it doesn't leak at 50psi and your still worried, then find a higher pressure gas source. I have made adaptors from old sparkplugs and used a minipump to achieve higher pressures. You are not looking for a precise reading at first, just the source of the leak. :mrgreen:

Link to comment
Share on other sites

Thanks for the input guys!


The doc Fred put up is pretty much what I had in mind and, of course, what Stu said about overload testing is - for me - on the money too. I'd rather go over than under seeing as this is for the track bike.


So far I've acquired a few quality gauges and some connecty-pipey bits so I just need to trawl ebay for a compressor on the cheap and I'm well on the road! :-)

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...

Important Information

Terms of Use Privacy Policy Guidelines We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.