OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Victa Lawn Mower Frames Power torque engine speed problems

I am trying to understand this machine! Several members have all sorts of ideas. The issues that still bother me are:
*What sort of fuel level pins are available, I have noticed several colours.
*What do the brass shims look like, that Bruce is talking about, that act on the main spring on the diaphragm to adjust the applied force on the poppet valve.
*Is there a modern universal type tool that can test all sorts of problems on small engines for electronic problems?

Thanks dudes for all your help so far! What a fantastic site!

Thanks chris, I am on a work for the dole project, rebuilding mowers and line cutters for charity. We have plenty of second hand parts but no cash for basic essentials, screw drivers, O rings etc. Thanks for the advice, I must remember that some models have an adjustable cable! I only just discovered that the poppet valve had different settings. I would really like to know about a cheap electronic RPM counter. Also the electronic manual support on this site does not have specific instructions to trouble shoot ignition problems. However I have figured most of that by reading here and else where. I forgot to mention wanting to know more about two stroke engines running at four strokes and eight strokes, perhaps Grumpy can explain. I must upload a video of one of my own power torque utilities, with sound to get an opinion. With ear protection on of course. I already have tinnitus. That idea of feeding nylon lead into the combustion chamber; and the business card magneto clearance measurement, thanks again dudes. Cheers.

A 2-stroke engine fires every revolution, which means every two 'strokes' or transits of the piston from one end of the cylinder to the other. A 4-stroke fires every second revolution. Most crankcase induction 2-strokes have very poor scavenge capability, which means they do not fully expel the burned gas after the power stroke. A puff of new gas from the transfer port pushes some of the burned gas out, and at full throttle this is fairly effective, but at very light throttle there just isn't enough puff from the transfer port to do the job. This means that ordinary 2-strokes have a barely combustible mixture at light throttle - like having the exhaust gas recirculation valve on a car engine stick open, for instance. So we are all accustomed to hearing 2-stroke mower engines run in an extremely crippled and erratic manner at light throttle, then as the load comes on and the governor opens the throttle, they clean up their act and start to purr. There is one exception to this lousy-scavenging-at-light-throttle problem - the old Puch 250 'split single' motorcycle engine, which had two cylinders with one common combustion chamber across them, and two spark plugs. The intake and exhaust ports were on one cylinder, and the transfer port on the other. The result was a pretty decent scavenge efficiency. They would almost 2-stroke at idle, and purred from there on up. But I digress.

When a crankcase induction 2-stroke misses, it leaves an almost combustible mixture in the cylinder. On the next rotation, when it gets a new puff of mixture from the transfer port, the scavenge efficiency is low again but it only needed a bit more fuel to be able to fire, because the mixture already in the cylinder contained a fair amount of oxygen and fuel. So, it fires every second revolution, fairly regularly, just like a 4-stroke. This is called 4-stroking, and makes a 2-stroke at light throttle sound like a 4-stroke - but it isn't completely regular, and it occasionally misses twice in a row (6-stroking) or fires twice in a row (2-stroking), making it sound pretty horrible.

By pure luck, at some load conditions you can get the threshold of combustibility for the gas in the cylinder surpassed after the same number of scavenge cycles pretty regularly. We've all heard lawnmowers do this when they have a pretty steady light load - they 4-stroke, maybe even 6-stroke or 8-stroke, pretty steadily and sound sort of OK. Of course it takes a miracle to 8-stroke or 10-stroke steadily, because the amount of burnable gas in the cylinder changes by a smaller proportion on each successive stroke. 4-stroking is very common, 6-stroking happens as a one-off event but is seldom steady, and 8-stroking smoothly is almost unimaginable.

I'm not really certain that the vintage Victa we saw on this board recently was 8-stroking rather than 6-stroking at idle, but my guess was that at nearly 3,000 rpm, it was firing too slowly for 6-stroking. To have an engine 8-stroke steadily at no load seems remarkable to me, unless it was specifically designed to do that. That is why I commented on it.