OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Victa Two Stroke Engines Victa Decompressor Valve - Rebuild How-To

Part 3 Function/diagnostics testing

Here's an instrumental test rig that can be used to test Victa deco valves. Any gauge that can read into the vacuum range a bit will work; this gauge is part of a mounted set of 4 that I have, for synchronising carbies/throttle bodies on twin to four cylinder motorcycles. The vacuum pump is a base model 'Mityvac' hand pump. The gauge needle deflects to the left here for vacuum readings; the outer scale is vacuum/pressure in inches of mercury.



One stroke of the pump will close the poppet valve, and reach a peak vacuum of ~5". However, there isn't a perfect seal between the valve stem and the diaphragm, so it leaks down to atmosphere in a few seconds.

That's the diagnostic check - the valve should open a bit slowly, not just snap open. If it snaps open, there is a leak. The usual suspects are the diaphragm itself, and the deco nozzle fitting.

The same test can be performed by mouth; sucking on the end of the tube, and then blocking it with your tongue while watching how fast the valve opens.

I strongly recommend fitting a new piece of tubing before doing it this way, unless you like the taste of two-stroke oil.

There is another diagnostic test, which only applies to the PowerTorque engine [with its exposed deco valve]; check for visible puffs of smoke from the underside of the decompressor, when starting or running the engine. These mean that the valve isn't closing fully.

The last task is to fit a replacement sealing washer to the body. 14mm spark plug crush washers are the OEM part, but I didn't have any new ones on hand. So I just used a solid aluminium washer, from the pack shown. Copper would be fine, too - that's what is used on aircraft spark plugs.


Well, that's this how-to done, I'd say. Comments welcome.