OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Victa Two Stroke Engines LM carby

Thought I would start a new page on these troublesome carbys and the previous discussions have become a little cluttered.
Upon studying this design for some hours I have established the cam in the stop position has the poppet valve in the full open position. On moving the throttle towards the run position the poppet valve closes and remains closed all the way till it reaches the run position where it goes to full throttle. This is why there is no throttle control so my idea is to grind a taper on the cam to allow the poppet to open as the throttle is opened so that the revs can be raised and lowered as it should have been designed in the first place. Anybody have any thoughts on this and want to run with me on this idea

If the poppet valve is in the full open position when the throttle is in the stop position you have a major problem,something in the carby would have to be broken,missing ,backwards or installed
incorrectly,near the stop position is the choke position so the poppet valve will be open more than
the idle position.One thing to look out for is where you locate the outer throttle cable in the plastic body,there are two positions the cable can be installed, if it is in the wrong one you may not get full throttle or it may not get to the stop position completely and there is a cable adjuster
at the other end of cable so I always check the cable is working correctly and adjust it if needed.





When the 3 leg plastic part is on the ramp that is when the cable will decrease RPM or Increase RPM ,depending which way you move the cable.

If your RPM is to high ,once I had a poppet valve that was longer than it should be so I replaced it and some of the 3 leg plasic parts
have a sholder that sticks out stoping the poppet valve opening to much.

The first drawing is a profile of the cam that raises and lowers the poppet valve, the second is of how I believe it should be to make it work giving full throttle control. Does anybody have any thoughts on this before I modify this cam?

Thanks Max, but as you can see from my drawings when you move the cable to the engine stop position the three leg piece drops into the deep vee allowing the poppet to go wide open, but you do not notice this because in that position the ignition has been cut,so the motor stops, if you have the ignition wires disconnected it will go full revs.My idea is to file a slope between the start position and the run position so that a gentle transition can be achieved with the poppet from closed to open and this does away with the governor which effectively does not work. Thoughts?

My idea is to file the blacked out section from the cam in the three places that raise and lower the cam. Any thoughts?

One of my pictures shows the throttle position in the stop position and poppet is not fully open
if it is something is wrong with your carby.

Max I know I have something wrong with my carby, and all 10 of them are exactly the same. The only reason your poppet will not go full open in the stop position is because the cable is preventing it from going all the way.If you have it all disassembled you can see what I am getting at. It is the design problem I am trying to solve

I have worked on hundreds of these carbys and never had this issue you are describing,
my first image of the carby shows full throttle
the second image shows half throttle
the third image of the carby shows stop position,the plastic part can not move any further in the clockwise direction.

Max over revving power torques drive people nuts around here, flat out or nothing, no throttle control
There has to be a fix for these dog of a carby

I never have those issues, if I stretch the outer long spring on the diaphragm so it has more pressure
to open the poppet,I can disconnect the vacuum line to the diaphragm and run it as a manual carby like the first Victa carbys with throttle control.If you decrease or increase the spring tension on either
side of the diaphragm you can change the way it runs.I just started the last mower that I put the
carby together and throttle control was good,has a good idle you can change throttle speeds and it
revs out but not over revving.I have noticed with old mowers with much lower compression readings
that they have less manifold vacuum so if you don't disconnect the vacuum hose,the vacuum hose seems to suck the diaphragm so the poppet is closed.

I believe most people leave the throttle position in the stop position when not using the mower,this compresses the outer diaphragm spring causing it to contract and loose its tension,thus making the
mower rev slow on full throttle.

Max, I like your thinking we might be closer to an answer, as you say they are not over revving but revving way to high if you are just running over a lawn and you see and hear it all the time with these power torques, revving their guts out and for no reason. What I am looking at on the cam is the long flat section that gives a complete dead spot in the throttle control. And yes we have tried new springs and they made zero difference. I agree the governor tube to the diaphragm should be able to be blocked off and the carby used as a complete manual one is what we are aiming for. I haven't come across them running slow on full throttle but I can see how it is possible. There has to be a simple fix so everyone can follow. I will discuss my cam alteration with Bumps and see what he thinks, can't be any worse

Norm, the problem may not be with your carby, especially if you've tried a few and all are giving you the same symptoms.
Over-revving is generally from a vacuum leak causing it to run super lean and rev it's guts out normally caused by carby a diaphragm that is not sealing correctly but i assume you've checked and double checked that on all of them, or by a dodgy o-ring somewhere. So i propose a couple of other things. 1)Is the pickup off the flywheel clear. Hook a fuel line up to it and see if it's clear when you blow through it. 2)Are both o-rings on the manifold in good order? 3)Starter o-rings? The big one and the small one on the shaft. Finally, the seal on the bottom bearing may be shot which will require a complete tear down of the motor and probably a bit more troule that it's worth.
Cheers, Ted

Hi Ted, Bumps has a motor that he has done a complete rebuild on (not sure why he did that but he did, he has been doing his head over this motor for 12 months now) and it has exactly the same symptom, with several carbs tried on it.The ones I have do exactly the same. Put it in start, starts instantly, start to move the throttle down and the poppet closes and the motor will die unless you go straight to the run position and then they go full revs, numerous motors and numerous carbs all with the same symptons. This is why we want to get to the bottom of it, it is a design fault in the cam and has probably sent more powertorque motors to the tip than any other problem with them

Hi Ted, just to clarify, the motor that Bumps is playing with is not a powertorque, it is an earlier motor, I think a full crank that he welded a flange onto the inlet spigot and then bolted on the plastic adaptor to take the LM carb.He regrets this but now the challenge is on to work out why these carbs won't work properly, it no longer is a case of just wanting to get a motor running, we intend to solve this problem before we die, whatever it takes. This motor he rebored, new bearings/seals and it runs well, but it performs in exactly the same way regardless of the carby we put on it. Mine all do the same. Disconnect the kill switch wire and in the stop position, full revs, run position, full revs and the motors will die anywhere in between

These things can be made to have throttle control and the only use for the stupid diaphragm and springs is to hold the poppet in position, they would have been a great carby but Victa got it all wrong, dug themselves into a hole and came up with all sort of mods to try and overcome the mess they had made of it

Norm no one here will disagree they can be a troublesome carb at times. You have started 3 or 4 threads on the same issue where we have all offered good advice, when are you going to take some of it? You continually bang on about how many carbs you tried and got the same result, think maybe it's not the carb then? They've been fitted to Victa's for over 35years now, the design is pretty solid i'd say.

Yes the poppet is open slightly when the throttle is set to off, it is supposed to be this way, you are cutting the engine off, not starving it. Why would you even been trying to make it run in the off position?

The diaphragm and spring control the poppet yes, this is the governor, why is this stupid? By all means modify your carb to your hearts content, mine work great just as they are.

Well Norm,
You have to keep in mind, that these carbies do work just fine when new. And generally for a considerable time thereafter.

So to my mind, the major source of their problems is going to be the elastomer components, which are the most prone to deteriorate over time. Chief among these are o-rings [on the engine side, including the starter o-rings on PowerTorques] and diaphragms. And often difficult to diagnose, too.

A similar type of problem is with diaphragm replacements in motorcycle CV carbies. The best known one is when using Stromberg diaphragms in Bing carbies on older airhead BMW flat twins. Ya gotta do both at once, or you won't be able to balance them; the Stromberg ones are a bit 'stiffer'...

The critical point about the 'stop position' of the LM carby cam, is that the poppet valve is not the only component controlling the air flow at this throttle position. The 'cam arms' close off the peripheral slots in the 'air chamber' under the cam.

This is an issue that can't be explained properly without diagrams; it really needs a hard copy of one of the workshop manuals in front of you. The online downloads just don't have sufficient image resolution to make out what's going on!

Dan it is of no interest to me about what you think, and they can be made to work with full throttle control and it has nothing to do with these "O" rings and crankcase leaks, diaphrams and all the other rubbish people spruke on about

I started this topic to see if I could draw out somebody who had intimate knowledge of these carbs and so far nothing, zip,zilch. Also at the start of the topic I deliberately drew the cam profile backwards expecting somebody to jump all over me about how I had it all wrong, once again not a peep about it. Bumps and I now have the answer to getting full throttle control on these carbs so unless somebody really wants to know I will make no further comment on LM carbs