OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Victa Two Stroke Engines From working to zero compression

This lawnmower was working in that it started fine but I guess I never really put it to the test by mowing a complete lawn. Anyway yesterday I tried to fire it up and I found it hard-starting but it did fire up briefly and then it felt/sounded like backfire and I haven't been able to get it to start again. Spark is there, carby fine I measured the compression via a compression gauge and it reads zero. I have another Powertorque and it reads 90 psi. I know that compression gauges can't be relied on with these machines but have 2 questions.
1. What could have happened here, head gasket blown?
2. Does a compression gauge take into account crankcase compression? Could a bottom seal have gone and that stops it from firing up?

Decomp valve?

Thanks Norm, how do I test for this? In the past when the decomp valve has had it, it's really difficult to pull the starting cord. On this one the cord pulls like normal.

Pull the decomp out and put your finger over the hole and turn the motor over, that way you will feel if the compression has returned

Tried it again tonight, this time it started first time, can't work it out. Starting to suspect the coil.

Looks Like a turbo disc? Lacking in power even on a good day so I found out running one myself and is just my 2c.. They must have been a city mower back in there day say 20 millimetres max high grass?...But yeah on a high note the turbo disc flies really well down the paddock over your left shoulder haha.. Sorry I cant be of more help tiger in working out what's up with yours

Coil won't cause you to loose compression

Normally compression gauges will not register crankcase compression as your measuring the combustion chamber pressure. With a reading of zero compression it can be a few things. Rings that are stuck can cause low to no compression. Blown head gaskets should still produce some compression if your compression gauge is sensitive enough (one for testing small engines and not for autos). A decompressor stuck open could very low compression.

What is making no sense to me is the zero reading as there should something; unless the rod was broken or a hole in the piston which with it starting up again rules this out.

still seems to me as though the decompression valve is sometimes sticking open or the diaphragm is perforated.,

Did some further investigation, took the muffler off and piston is in pretty good condition, cleaned the carby as well just to be sure. The muffler outlet tube had a little carbon buildup so scraped that off. Put it all together, hooked up the compression gauge and again zero. Tried to fire it up and starts first time. Turned it off, tried again and started first time. Yes it's a turbo, this is the first time that my compression gauge has let me down.

Are you disabling the decompressor valve, when doing the compression test?

Either by applying suction to the vacuum tube, or removing the valve and plugging the mounting hole with an old spark plug.

Hi Gadge, I'm not disabling the decompressor, but I don't do that with other powertorques and they read as high as 90 psi. Is it possible that the decompressor valve functions a little different on this mower?

Very likely - if it's sticking open [as they do, when they get gunge buildup on the poppet valve stem, or a hole in the diaphragm], you will get reduced/no compression.

Have a look at the deco valve diaphragm chamber - if it has 3 spring clips around the periphery, it's rebuildable; kit is available from the ODK Shop. If not, it will need replacement.

Took the decompressor off and tested it, it seems to work fine. Stuck a spark plug into the decomp port, very difficult to pull cord, anyway the compression gauge read 75 psi.

How did you test it?

Gadge, i tested it with some tubing connected to the decompressor and sucked on it to see that if the valve moved in/out and it did.

Good so far; the final test is to suck, then seal the end of the tube with your tongue - using new tubing is a very good idea, unless you like the taste of petrol!

If it won't hold vacuum, and the valve opens again, either the diaphragm or tube connector is leaking, and needs to be replaced.

I kinda always liked the taste of Premium fuel here. Actually quite nasty tasting in my opinion.

But if you have access to vacuum/pressure tester it would make testing a lot easier as you can test using the specs and see the leakage rate. Here I use one to test crankcases and carburetors for leaks.

I like the idea of the vacuum/pressure tester, which brand to you use?

Here I use a MityVac Silverline 8500 model but I don't know if that manufacture's products are even available in your area.