Leak down testing

[Mr Fro]

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

[megawatt]

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?

[Chrissb6]

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.

[Mr Fro]

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.

[Stu]

Could it be a blocked breather on the tank?

[Mr Fro]

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...

[Chrissb6]

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!

[Tango]

Could you not use talcum powder to show where the leak is.......as demonstrated in this youtube clip?

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[Mr Fro]

Could you not use talcum powder to show where the leak is.......as demonstrated in this youtube clip?

 

I'd worry that it would mix with the oil...

[megawatt]

dunno why you need 100 psi. Any air pressure in the CC will do, with piston at TDC and all valves closed. Just listen????

[Stu]

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

[megawatt]

If it's leaking, it's leaking. I've done loads of leakdown tests on my dragrace engines at less than 100psi. The rings will leak when dry and cold, just depends how bad the leak is.

[Stu]

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

[megawatt]

If he hasn't got 100psi, he will need to do it at a lower pressure?

[fredc]

If you want to make a leakdown tester have a look at this word file

Leak Down Tester.doc

[megawatt]

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.

[Mr Fro]

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!