OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Victa Two Stroke Engines Diagnosis/troubleshooting guide

My current experience with an old Victa 2 stroke has got me wondering a bit about the diagnosis procedure with small engines. I wonder how the pros go about diagnosing problems. I realise that over time a certain amount of intuition comes into it but for those who are starting out, what is the best pathway. Here's what I do:
Check for spark, easy enough by grounding the plug onto the body of mower.
If okay, move to carburettor, a little bit of petrol into the spark-plug hole to see if it fires up. If it does, investigate carby and repair.
If no firing up occurs then suspect compression, will try and check with a compression gauge but notice that Victa, Briggs recommend checks other than with compression gauge. Just curious about how others do it and in what order?

Hey Tiger ,
My Method for getting a Victa Engine going -
I don't normally attempt to start them until i --

1. remove and clean the carby
2. clean and adjust points
then if it fails to run i normally dismantle and inspect rings and crank case seals.

Good info Mitch, something to bear in mind when I do the next one.

Hi All, I notice with most 'power-torque' engines the 'O' rings also fail, particularly the one under the starter, and of course the carby 'O' rings.
Another good tip is to remove the muffler and have a good eyeball through the exhaust port at the condition of the piston and rings.

Good tips DJ, when I posted this initially I was directing it toward all mowers/small engine stuff not just Victa but wasn't sure where to put it so I put it here. You're right about the o-rings with the powertorque, that engine works on vacuum so the slightest air leak causes problems on those.

First thing I check on a two stroke is compression, carb and all the rest can be repaired. If the piston is damaged or scored, it is often terminal. I've spent too many hours stuffing around with carbs trying to get units running only to find scratched/ scored bores and pistons! Pull off the exhaust and check piston condition and if the rings are free bin their grooves....

Keep in mind also, that on a piston-port two-stroke engine, it must have decent crankcase compression, as well as cylinder compression, to operate correctly.
Leaky crankshaft seals are a prime cause of loss of crankcase compression.

They are easily checked; by positioning the seal uppermost, giving it a squirt of fuel or light oil, and turning the engine over.
Bubbles, or the fluid getting sucked into the engine, = knackered seal.

Hi Gadge, can you please explain a little bit more the last one - checking crankshaft seals? Thanks

Righto, here we go.

I don't own a 2-stroke mower these days, so I can't do pics of one! Could use my chainsaw if we have to, but see how we go from a verbal description.

Neoprene elastomer lip seals are universally used on the crankshafts of 2-stroke engines, these days. And very effective they are too, in comparison to earlier types.

Their function is to provide an airtight seal, between the moving crankshaft and stationary crankcase. Single-lip seals, the most common type, use a coil backup 'garter' spring around the outer circumference of the 'lip', on the crankcase side, to ensure effective sealing when the crankcase pressure is less than atmospheric; i.e. a 'partial vacuum' exists inside the crankcase.

So when one of these seals fails in service, it will leak air, in one or both directions. The test I describe above uses a low-viscosity fluid, with the crankshaft in the vertical position, as a leak check.

There is enough of an annular void, in the tapered gap between the crankshaft and seal lip, to hold a small quantity of the test fluid. So it can be easily observed if air is bubbling outwards, or the fluid is getting sucked inwards.

This is something of a 'trick o' the trade', for this diagnosis.

I've seen manufacturers' 'recommended service tool' pressure testing setups; complete with blanking plates, squeeze bulbs and pressure gauges, demonstrated at service schools.

But the simple test is just as reliable, and a lot less mucking about. All you need to have on hand is a 'squirt bottle' of some sort, with a bit of 2-stroke fuel in it...

Hummm...as I understood, I need to remove the blade plate (and what ever is under it) to access the bottom crankshaft seal, position the lawnmower upside down, put some 2 stroke fuel on the seal and try to spin the engine (correct me if I understood wrong)
Will need wife's help to do this - she will have to hold the lawnmower while I pull the cord

Not sure if I'm gonna do this, since engine is starting easily (found that out accidentally after I've inspected state of the piston & cylinder through exhaust hole, cleaned exhaust hole of carbon deposits and tried to clean the scraped material by puling cord - and not previously removing the spark plug cap :D)

Anyway, more concerned now about the state of the piston, rings and cylinder - will post pictures soon in new thread...

Will inspect carb and upper seal first (under the starter)...

Cheers

If it starts easily, seal leakage is very unlikely, in my experience of 2-strokes.

So you've learned a lesson about removing the ignition lead from the plug - bet you won't forget that one!

The lower seal is much more failure-prone on mowers, as string, hay band etc. left lying around, winds around the shaft, and eventually eats the seal.

This ODK thread has some more detail on that problem; https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=62257#Post62257

Edit: The bottom seal check is more easily performed if the flywheel cowling has been removed, so that the flywheel can be turned by hand.

Good tip Gadge, appreciate it