ET: Engine (Twin) Leak down test

[Bill Cannon]

A while ago I noticed that the rear carb was not pulling as strongly as the front on my TPV 1200 egli. Tested by putting my hand over the bellmouth.

I borrowed the leakdown tester from work. I have never used one before and struggled to get any meaningful results at first. I realised that I was not doing the set up procedure properly. This sorted I got an excellent result on the front cylinder and a very poor one, about 70% on the rear. I didn't have a suitable piece of hose to detect where the leak was from.

So today proceeded to remove the rear head. This was made very difficult because one of the TP split cylinder studs did not want to come apart. I eventually managed this by raising barrel and head enough to get mole grips on the very bottom of the stud and then freed the top half.

Does anyone know the thread of the joint in the studs? I need locking nuts to tighten them again.

Anyway, head now off, see photos below. There's a lot of carbon in there for a new top end that's only done about a 1000 miles. Has that got to be oil or can an over rich mixture cause that, any thoughts?

Too hot to do anymore today so valves out tomorrow and see what they look like.

Cheers Bill

 

[Robert Watson]

The whole point of a leak down test is to listen.....

At TDC with both valves closed

noise from the exhaust means an exhaust valve is leaking

noise from the carburetor means the intake valve is leaking and noise from the crank case means the rings are not seated well.

Then you know what needs looking at.

As for that carbon, it is not uncommon to see on a street bike, but a bit of carbon might have come adrift and got stuck under a valve, so doing a leak down will tell you that! Or it could be that the rings never seated properly if this has only 1000 miles on it, and oil is passing up from the crank case, so doing a leak down will tell you that!

Cheers

Robert

 

[Robert Watson]

I should add that now that the head is off, try pouring a little petrol down the intake, and then the exhaust port and look to see if it passes the valve seat. That will tell you if one or both are leaking. The rings will always leak a little so it won't work there. If the valves are good then either put the head back on and test as above, or lift the barrel and look at the rings. What does the bore look like??

 

[Bill Cannon]

Thanks Robert.

I tried a short piece of rubber hose and wherever I stuck it (no I didn't try there!) I could hear a sound of air moving, so I tried just putting in my ear and the noise was there all the time!

Bore looks good and head seal looks OK as well.

I'll try the petrol before removing the valves.

Cheers Bill

 

[Robert Watson]

It can be a bit tricky to hold the cylinder being tested at TDC, so a little trial and error and patience (which I am always short of) is required.

 

[Monkeypants]

If you have hearing loss, as most of us do to some degree, you might make a trip to Costco or the UK equivalent. Purchase Seimens hearing aids. Download hearing aid App and set hearing aids for "stunned"
For the first time in years you get to hear the tinkle tinkle when your are taking a leak and a great many other things too.
Im only half joking as I'm more than half deaf and could not get much useful info from the leakdown tester without the use of hearing aids. As Robert says, it's mostly about hearing where the leakage is happening. The other part is that it does first tell you if there is a problem by showing leakdown percentages.


The hearing aids really should come with the leakdown tester as a kit!

 

[davidd]

In Bill's defense, 70% does mean a complete stripdown and examination followed by rebuilding and checking. So, no real lost time. I simply think back to this photo:

TDC is the correct place to check, but if you are out of patience you can remove the pushrods and check. The piston will go to BDC as it is pushed down by the air, but the sound detection should be very good as the valves cannot open without pushrods.

I had 60% with this:

Bill, best of luck,

David

 

[Black Flash]

Ouchh,
It really hurts to see a picture like this. But if you are lucky enough the bore survived it.

 

[Monkeypants]

My OZ Rapide was down around that percentage, however it all seemed to be leaking through the valves, according to the leakdown test. There was a loud hiss from both the carb and the exhaust on the rear cylinder. The front cylinder was marginally better but also showed leakge thru the valves.
So I knew beforehand that the problem should be with the valves and the cylinders most likely could stay on.
It turned out that the problem was due to carbon specs on all of the valve seats, worse on the intakes and the very worst on the rear intake.
Cleaning that off and lapping the valves to gas tight brought compression right back to the "excellent" area on the gauge, both cylinders.
When the compression dropped off I assumed that the rings and bores were worn and was about to order everything for a complete top end redo. After the leakdown test indication that idea was put on hold.
The carbon buildup was due to worn-out needle jets causing a very rich mixture.

Looking at the carbon deposits on Bill's 1200, this could easily be causing the compression loss there.


Glen

 

[greg brillus]

Middle threads are 10 mm x 1 mm pitch.