OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Victa Two Stroke Engines Full crank decomp valve

I am wondering if anybody has ever looked into making an M&F adapter to extend the PT decomps so they will fit the F/C's. I'm thinking of getting some brass bar and turning some up. I do have a few of the F/C ones here but they are slowly vanishing and I have countless numbers of P/T ones here. Anybody know if Delrin would stand the heat because that is super easy to turn and thread?

Hi NormK,
my knowledge is limited but my common sense says anything other than metal won't withstand heat from an engine head.

I'm sure you are right MF, I think Delrin can handle about 250c so I have decided to get some aluminium rod and make some out of that. Problem is it is going to cost me $40 for a button die to cut the male thread. I already have a 14mm tap here. People are going to need these decomps to keep these F/C motors running

If it can handle that much, give the delrod a go before moving to the aluminium but the $40 should pay for itself quickly if it effectively gives you free FC decomp valves, plus it will look better.

Hi Norm and Mf,

Not sure why you would need to make an adapter Norm as you can buy a repair kit on ODK.

https://www.outdoorking-forum.com.au/forum/u...a-decompressor-valve-rebuild-how-to.html

https://www.outdoorking-forum.com.au/engines...a-decompressor-repair-kit-en72684-dec605

I wouldn't use Delrin or Aluminum in something that is like a spark plug thread as if you have to undo it later on it can snap.

The Vacuum diaphragms are different sizes for the PT and FC .

If you are missing the complete decompressor for a full crank you could just cut and shut a PT decomp and weld in an extension.

Another problem with an adapter is the loss of compression as you are increasing the combustion chamber size.The extra
squish volume wouldn't be that much but probably like putting in an extra head gasket .

If I had a motor with a non working decompressor I would block the hole with a N21 ( long reach plug Thread reach: 19mm.)
and grind off the end tip off the plug ,also snap the porcelain off the other end to shorten the plug.

Some of the Power Torques didn't come out with decomps as the hole in the head wasn't drilled so I don't
really see the need for one if you don't have a weak arm. Some motors it may help to start like a Victa 24.

As these mowers aren't worth much ,the spark plug blocking the decomp hole is all I would do to get one running
unless someone is too weak to start the mower.

Cheers
Max.

Hi Max,
I don't have a problem as I have a few decomps here but I often have people coming here wanting them for the F/C's and I know they are only going to get harder to get so I was thinking of a way to solve a problem in the future. I know if the comp is low you can get awau with a spark plug in the hole. I use my mower for many years like that. I haven't tried a F/C with new rings and no decomp but I'm pretty sure it would hurt

On a fresh rebuild without a decompression valve they definitely bite. I ended up with a pretty sore hand, arm and shoulder.

I wouldn’t use delrin, you’re either going to end up with an awful mess, a fire or an explosion when it melts and gums something up.

Last time I checked brass was more expensive than steel so I’d be turning parts from steel. Though I’m not turning anything at the moment, I haven’t made any real progress on my access driveway to get the machinery in to start work.

The Victa, Series 70 MKI 160cc Engine didn't have a decompressor ,starting the mower without a decomp sounds like
a good work out ,I never had a problem starting new Power Torque motors without a Decompressor.

If making an adapter for the PT decomp to fit a FC you would want to charge $30. for the adapter otherwise you
may as well machine something else up that you can easily sell, in my opinion.

Cheers
Max.

Hi Max,
This bloke from Ballarat says that with a plug in the decomp his 24 nearly breaks 3 fingers when it kicks back, in the same way I found the PT's on a 24 bite. I fitted a F/C on a 24 recently that was obviously before they started fitting the decomp valves and there is no sign of kickback, but thinking about it it might only be a 125 but it certainly handles the tough stuff the same as the 160's When I get it back from my mate I will check it out.

Hi Norm

Yes with all the problems that you get with kickback on the 24s it certainly pays to use a decomp on these mowers with less rotating mass on the crank Norm. I put up a Youtube video of a Powertorque with no blade plate and started the mower many times without the slightest kick back.The engine was off a 460 Utility and had a slightly heavier crank hub than normal.

One problem could be the standard 160 on a 24 uses a high comp head and when using another engine from
another model it will have a lower comp ratio.(low comp head)


Cheers
Max.

Hi Max,
Another one I had severe kickback problems with was a F/C edger. I ended up putting from memory an 800 gram weight on the bottom of the crank and it still kicked back. I then fitted a decomp with the brass restrictor in it and problem solved.

I’ve not rebuilt a powertorque so I can’t speak about them but I have rebuilt a few full crank 160cc motors. I tried swapping in a Victa 18 style manual wind pull start on a late 60’s corvette motor I rebuilt some time ago which had no decompressor fitted. It was a sufficiently unpleasant experience that I won’t try it again.

Well-worn motors haven’t ever posed a problem but fresh rebuilds are a different proposition.

Are you only concerned by the boss that screws into the head or something else?

Hi Ironbark,
All I want to make is a 20mm long M&F extension to screw into the head to allow the PT decomp valve to clear the head fins. I have some ally rod and a 14mm tap, I just need to get a 14mm dienut and I can make one up. Once the extension is screwed into the head there should be no reason for it ever to be removed. A mate is going to check and see if he happens to have a dienut in his boxes of stuff otherwise I will just buy one