OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Victa Two Stroke Engines Victa Super 24/600 - PowerTorque decompressor issues

Hi Gadge,
Any thoughts on how to slow down the rate that the valve closes, I believe this is a major part of the problem I am having with these PT's on the 24 slashers for some reason. I have fitted probably 4 different motors to these over the time and I have this problem with every one of them kicking back. I am convinced it is because the valve shuts too quickly before the motor has enough momentum to over ride the compression. What I do not understand is if I fit that motor onto a standard 18 inch base it works fine. Make no sense but that is the way it is.

This is just thinking out loud Norm, but you know how some of the decomp nozzles have the little brass restrictor and others don't? I think it was to prevent a backfire causing the valve to smash into the 'ramp' in the cylinder and break the stem.

In theory there would be a slight difference in how the vacuum is delivered to the diaphragm - slower or faster

Don't know if the difference would be appreciable though

Hi Tyler,
Yes I had given that some thought, I had also removed the hose from the decomp thinking that without vacuum it may hold open a fraxction longer but that made no difference the second the piston starts coming up on compression it shuts the valve and instantly it locks up. I was thinking of fitting one of the lower comp heads to see if that helps but that sort of defeats the power to run these big slashers. It is a decomp issue but what I don't understand why it only causes these issues on this mower/slasher combination. Real mystery

G'day folks,
It would be appreciable, I'd say, given the very small area of the restrictor hole vs an unrestricted deco nozzle. Slower, for sure.

It's all about gas flowrates here, just at less than atmospheric pressure!

BTW, I'll move these posts on deco 'fine tuning' to a new topic shortly - just so that the info is readily 'findable'.

Hi Gadge,
Would be a good idea to move these post away under another title so as to keep your original topic clear. My belief is that the problem I am having has nothing to do with the amount of vacuum, it is the piston coming up on compression that is closing the valve too early, but why is it only on this configuration with the motor turning a belt. The weight of the blade carrier should compensate for this but it doesn't and why do I not get any of this sort of problem with the F/C ones. It is doing my head
Another thought I have had is trying one of the F/C decomp type that can be opened up and fitting a stronger spring
This is maybe another idea
https://www.ebay.com.au/itm/2pcs-De...XP-350-STIHL-029-066-MS260/323779330084?

G'day folks,

Posts split from the Victa Deco Valve Reco How-to thread , as they're off topic for that one.

G'day again,

After a bit of thought about this issue, that 'black nozzle' restrictor should slow down the valve stem movement in both directions [opening and closing], providing the diaphragm is sealing properly, on both the valve stem and the edge of the diaphragm chamber.

I see Gadge - because the vac from the intake manifold cant suck the valve up as quick, and the piston pushing upwards would be met with more resistance as the valve faces more resistance to closing (more resistance in the top end of the decomp as the diaphragm can't push air back through the smaller nozzle)

Norm, I have a Pace with the low comp head - still goes hard. But I don't cut tall weeds with it - wonder what difference it actually makes

At least on the 24 you have a big disk for more reciprocating mass

I'm wondering if by blocking of half the holes in the housing may slow down how quickly the diaphragm can be pushed up by the compression pushing on the head of the valve, in actual fact it might mean that only one hole could be left open, maybe they all would need sealing over. I have nothing to loose by trying. I will seal up a valve and depending on how it goes, drill open one at a time till it seems to work as it should

What about a piece of rubber or rubberised cork gasket cut in a ring?

Not sure what you mean Tyler? I have covered the holes with JB Weld so I will see how that goes in the morning. This is the original decomp valve from this motor and I know how bad that was originally. All I can see that the vacuum does is flatten the curve to even out the valve so it doesn't pulsate on every downstroke of the piston, not primarily to hold the valve shut, compression stroke does that easily

Sorry Norm, I meant put a donut piece of rubber under the foam filter in the decomp - it will act like a one way valve - let air in fast but seals and slows on the way out




This will probably be the Australian phrase of 2020 (closely followed by social distancing) haha

Thought I would just throw it in there to see if anybody noticed. I think the foam under the filter would help hold the valve shut which is exactly what I don't want it to do. And I think I have probably thrown out all the low comp heads I had, thought I would never be using those

Hi NormK,

Do you always experience that issue off the bat with every engine/replacement part? Or do you notice it over time? And to be clear, you're talking about when you're starting and when it first kicks you can't pull the starter anymore, it binds, as if the engine had no decompressor?

I recall a mower I picked up a while back, I can't remember which one or if/how I resolved what was wrong with it, but I recall that around the valve on the engine side there was a thin piece of wire wrapped around the stem, as to ensure the valve didn't fully close. It certainly wasn't a 24, but if I had to guess it might have been an old trick used to overcome the issue you're referring to?

Cheers,

pau13z

Hi pau13z,
This problem I have only ever experienced wit PT's on the 24's and every 24 I have had has had it from the get go when I picked them up. Hundreds of PT's on standard bases and never a problem. The other thing is they don't start anywhere like a standard mower that should start first pull every time. These kick and buck and just because you had it running 2 seconds ago does not mean it will start again and it will go through its performance of kicking back, but then it might just start straight up or just decide to try breaking your fingers. If you are talking about a piece of wire to hold the valve open that is just going to burn the valve out fairly quickly I would think but it seems like somebody had a similar problem

Hmmmmm that does sound really odd. Knowing that the mower needs the blades to be balanced correctly as referenced previously, could that be the reason? It's the only one with a pulley from the crank to the blade disc, isn't it? Maybe flex in the belt is causing that issue to be exacerbated? Essentially throwing off the balance?

Do you experience the same issue on firm ground, level ground, or gradients? Does the same issue occur if you pull the starter very slowly vs quickly?

I'm thinking... if it's not flat level ground and a firm surface, if there's anything that could be touching or interfering with the entirely free rotation of the disc or blades, that could play a part with that balance issue. Similarly, pulling the starter hard and fast, flexing the belt more and again causing the balance to be off further, that might do it? Just throwing out ideas, if I think of any more I'll flick them through.

Ok, this is the cause, don't ask me why. I JB welded up the 4 holes in the back of the decomp valve. No kickback but it immediately blew the plastic cap out of the valve, kept running, I pushed it back in and held it, this then blew the JB Weld off 2 of the holes I had covered over. With only 2 holes covered the motor will now start fine without any sign of it kicking back. I now have to find a way of covering 2 holes without it blowing off. Maybe cut a curved strip to go around and cover 2 holes and hopefully the JB Weld can hold it on

Amazing. I would have thought that would make it worse
How does no decompressor go. Should be a lot worse.

Jeff

Pity you can't pull the PT deco valves apart - fitting a cover strip over the top side of those holes would be the shizz here.

Jeff, not a chance in hell of pulling it over with the decomp plugged off
Gadge, I could put a full crank one on it but I want to hang onto as many of those as I can, never know when I will need those.
I wiped the decomp down with metho before I put the JB Weld on but it just blew it off easily, no real attempt to hold.

Norm,

Does the FC decompressor solve the problem? I have a few of those even in Eden where the 24 is.

I would be prepared to go to that much trouble to improve the 24 a little more.

Jeff

Norm, I'd clean the metal with Acetone, and then let the JB Weld set up for a few days - it takes that long to set up fully hard and reach maximum bond strength, IME.

Jeff,
just tried a F/C decomp and the kickback is the same. What I was thinking with the decopms that can be pulled apart it would be easier to plug the holes from the inside. I tried to rivet a plate on the back of one with JB Weld under it but now the decomp won't close. I was very careful to just go through with the drill. I have done another one and just put a patch over the 2 holes and glued them on with Araldite. See if that holds tomorrow

Thanks Norm,

I guess a FC decompressor might be easier to modify. Perhaps a stronger spring or a washer under the spring.

Jeff

Gadge,
the problem with blanking the 2 holes off is that while the valve is still open you get compression pressure up under the diaphragm. With a bit of luck the Araldite might hold

Hi Norm,

I was just thinking maybe a manual operating decomp valve would possibly solve the problem.




Cheers
Max.

Hi Max,
I gave them a thought a few days but I think they are probably 10mm and I would have had to make up a bush, but if it would solve it it would be worth it.

Hi all,

How does that work? Must have to push the knob after pulling the cord. Is there time and the valve would have to be very close to hand.

Another idea is they only kick back because they fire backwards.

. They will not fire if the ignition is off. Try a pull with ignition off then switch on when turning properly.

Probably not a workable solution
There would be a way of getting DC out of the primary coil windings. Have to be the older style magneto with the separate module.and would need some clever electronics to operate an electrical valve.

Another idea that will not work.

Persevere with the decompressor valve I think Norm.


Jeff

thre common on chainsaws, you press the decomp button and it releases the compresssion until the motor fires which pops the valve shut again.

Jeff,
the decomp valve with 2 holes covered solves the problem, I just have to work out what will hold on to the block the holes. This is not a problem caused by ignition firing early, it is a compression issue that must be compounded by the belt driven blade carrier. Got me stumped why but this is what it is. I can't believe these machines have been around for years and nobody has come here asking about it. The bloke who dropped it off got a full crank one from me some time ago admitted this one has been a pig of a machine, but it belongs to his father