So I'm leak testing my new engine. My leak testing rig is working great and is air tight. Almost 100% my engine is air tight ***BUT*** I can't quite get a good seal on the exhaust side. Not willing to settle for mostly air tight maybe air tight or even probably air tight. I need to see her hold pressure before I proceed.

I currently have a rubber bath plug that's flat on the bottom being pressed down by a flat piece of metal that goes on the exhaust studs and gets tightened down.

I tried grease, it helped but I could hear little grease bubbles. Tried an O Ring under the plug but that didn't quite work either.

Here's the question. Do most people fit some sort of bung or just use a piece of rubber? If bung, how do you get one that's oval shaped for the port? If rubber, how do you hold it down and what type of rubber i.e. thickness etc. ?

I found plenty of threads of people describing their rigs, but no one really went into how they block the exhaust port.

Thanks

Clancy

hey bro, i use 10mm thick off cuts of rubber held down by some rough-as metal plates left over from when i made my flywheel holding tool.

The rubber was originally an anti vibration thing for the engine cover of a ride-on mower. I guess hardware stores might have something like that but i've never seen any if they do.
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After years of moaning about immigrants now i am one...

Be aware of the air pressure that you are using and always keep an eye on your gauge.
It will eventually leak with lots of air pressure and may damage your crank seals in the process...

I have blocked it with success using only the stock exhaust flange and an air rubber from inside a tire cutted to fit between cylinder and flange and then screwed a plate to put pressure on the flange.

Thanks guys,

Looks like I need a nice flat piece of rubber.

I have a nice low pressure fuel gauge for my system. It actually does vacuum too, but I haven't figured out how to actually make a vacuum - maybe I'll just try and suck on it.

It maxes out at 15psi and I've never gone above about 7 so far.

Also it leaks out within a minute or so at the moment with the leaky exhaust so I'm pretty sure it will be ok.

Any other tips would be appreciated.

Yeah i test no higher than like 7 or 8 on the gauge. Have you blanked one side and left the other open and put a test on to see if you're losing anything through the centre seal? You must do this test in 'both directions' too so: blank the left side and test whilst the right side is open and vice versa. If you test whilst both sides are blanked you will eventually pressurise the whole engine through the centre seal (if its leaking) and it will appear to be perfectly airtight. Though the tell tale sign of something being wrong is when you take the carb blank out of the side you aren't pumping up and find it gets forcibly blown out by the pressure behind it.
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After years of moaning about immigrants now i am one...

The other side is definitely open. It doesn't even have a barrel on!

I wanted to leak test it at the earliest point possible so until I can get the left side of the engine to hold air, I'm not continuing.

Very confident the cases and seals aren't leaking. I had a lot of leaking around the insulators. Now they're threebonded, the next weakest point appears to be my exhaust plug.

Hopefully once that's sorted I will get the confirmation I've been waiting for.

Well...

My old bath plug to blank the exhaust was actually working fine!

Turns out air comes out past the power valve shaft if you haven't bothered to put the seal in yet.

I'm an idiot. That's what I get for trying to leak test with the bare minimum components on the engine.

Surprised it actually held pressure (mostly) without a powervalve seal.

5 PSI and holding!

Now.... how to test vacuum....

edit:

I got a tiny screw driver and held the valve while I sucked on it. Got it down to almost 5hg of vacuum - whatever that means!

Holding vacuum too. We're looking good.

Skyntara wrote: I had a lot of leaking around the insulators. Now they're threebonded.

Lol, yeah i had a go at getting the insulator to seal with only the gasket and failed - a wipe of 1207B soon righted it!
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After years of moaning about immigrants now i am one...

These bloody insulators!

Found out the "better" one of the two I had was actually split on the right hand boot.

Probably been like this all along, just adding to the problems.

Got the other boot on, waiting for the threebond to set before I can leaktest again.

If I've learnt anything from this rebuild it is LEAK TEST!!!!

Best $20 you can spend on your bike is a dodgy, home made leak tester. Will save a lot of time and $$$$ if you do.