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

G'day folks,
As there doesn't seem to be a detailed procedure for this on ODK, I've decided to write one. So here goes.

One type of Victa decompressor valve, the 'long' type P/No EN72135, as fitted to the full-crank engines, is now NLA from any trade supplier. So rebuilds will become increasingly necessary, to keep these older mowers running.

As part of a minor overhaul of a 160cc full crank engine that I currently have as a 'work in progress', the deco valve will be rebuilt, using a rebuild kit supplied by the ODK shop; it's HERE

Here's the deco valve, as removed from the engine. NB: Only deco valves [like this one] which have 3 spring clips around the periphery of the diaphragm chamber can be rebuilt. BTW, removal of this valve requires a 13/16" AF open end spanner. The later short types have a 3/4" hex.



The white nylon 'decompressor nozzle' often doesn't survive having its hose removed, and this one hasn't. However, black nylon replacements are available, cheap, and now listed by the ODK Shop HERE.

Next step is just cleaning up the valve in the parts wash, so I won't put up a pic of that...

Part 1 Dismantling/Cleaning

First step is to pry off the 3 clips. This has to be done with care, as these ones are the original 'jesus clips'.
Those are the ones that cause one to exclaim 'Jesus, where did that go', as they fly off to who knows where.

A sharp pointed scriber is my tool of choice.



It is used as shown, under the outer edge.

NB: It is only shown facing the camera for clarity; this job is best done with the clip facing away from you, pointing towards a catch container, or backstop, to trap any flyaways.



First view of the valve innards.



The valve components, arranged in order of removal, top to bottom, then left to right and up. Note the carbon deposits visible on the stem of the poppet valve. The foam filter is generally cactus; a wipe with a petrol soaked rag will help in removing any sticky remnant bits of it.



The retaining clip at the top of the poppet valve stem is trickier than it at first seems; it has an inner flange to lock it into the small spring. To remove it, the small spring has to be compressed a bit first. The points of a pair of dividers work well here.



The poppet valve; cleaned up by wire brushing. I used a vise with soft aluminium jaw covers to hold it during this process, and a hand wire brush.



It's also worth de-coking the valve bore above the poppet valve seat; a 9mm or 23/64" drill bit is a very good tool for this job. This valve wasn't at all badly coked up; we used to see much worse, back in the days of BP Zoom 25 pre-mixed fuel.

Part 2 - Checking parts and Reassembly

Here's an exploded drawing, from Gregory's Victa Workshop Manual No. 103.

This is for the long type deco valve; the earliest models used a screw to retain the vacuum hose to the deco nozzle, as shown here.

The latest type of deco nozzle uses a barb to retain the hose, and has a brass restrictor.




Checking of parts, to assess suitability for further service, is chiefly concerned with the deco nozzle and poppet valve.

The nozzle must be a tight fit in the diaphragm cover, so as to avoid vacuum leaks.
Vacuum leaks can have the effects of the poppet valve not closing fully, and even leaning off of the fuel/air mixture, for really bad leaks.

The poppet valve must slide freely in its guide in the decompressor body.

It's also necessary to check that it seats properly, i.e. that the valve head is not bent.
This is easily done by smoking the valve face with a lighter or match, so that it has an even coat of soot. Or you could pinch some of the wife's lipstick, if you're game...

Insert the valve in the deco body, and use its stem to pull it against its seat, while spinning it around. When it's removed, it should show a bright ring where the soot has rubbed off, around the circumference of the face. If there's any break in that ring, the valve head is bent, and the valve must be replaced. It's NLA as a separate part, so it will be necessary to find a used one, or a complete valve assembly.

Bent valves are the reason for the brass restrictor fitted to the late black decompressor nozzles.

With the earlier unrestricted type, a 'carby side backfire' can slam the poppet valve head against a ledge in the cylinder head [which itself prevents the valve falling into the combustion chamber, if the valve stem fractures], and bend it.

Reassembly

What's in the repair kit:


The larger diaphragm fits this 'long' deco valve; the smaller fits the later 'short' type.

Reassembly is straightforward, with attention to the following points. Use steel wool to clean the mating flange surfaces, then commence assembly as per the workshop manual drawing above.

Ready to install the poppet valve retainer, with dividers for a spring compressor.


Ready to install the diaphragm cover.


Job done. Note that the diaphragm chamber clips are installed 'barbs up'; they are easier to tilt into place, if it's done this way. Just place the nozzle in position, but do not seat it fully, at this point. This is so that it can be 'timed', after the deco valve is screwed fully home in the cylinder head.

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.

G'day folks,

Just thought I'd 'sticky' this one, to make it easier for novices to find.

Any suggestions for other Victa 'how-to's' worthy of 'stickying' in this section, while I'm at it?

Have to recommend Norm's Carby mod - best thing to do with a cantankerous g4/lm

https://www.outdoorking-forum.com.au/forum/u...re-g4-lm-carby-repair-modifications.html

Done! I think I'll put up a sticky here, with links to the 'DIY Victa Special Tools' posts over in Tech Talk, too.

Righto folks, there has been ample time for comments I feel, so I'll now lock this thread.

As always, re-opening can be requested by Notifying the Mods.