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Collection, Preservation and Display of Old Lawn Mowers

Re-starting a warm 2 stroke engine

On a few occasions my Qualcast mower with a prewar Villiers 2 stroke engine has stopped during the cutting of the lawn.  For example I didn't have enough fuel in the tank or I had it idling at too slow a speed when talking to the wife etc.  When coming to restart the mower on these occasions I couldn't until the engine was cold.  Is this a feature of 2 stroke engines or is there something amiss with it?  Is there anything that can be done so this won't be a problem in future?


wristpin Mon, 18/04/2016

Usually a sign of a worn engine. May be worn rings or drawing air around worn crankshaft bearings and seals.

If the mains aren't too bad, seals may help as may new rings in a glaze busted bore but for a lasting result a cylinder re- bore and reground crank  may be the only options.


pyneapplechunks Mon, 18/04/2016

The engine was stripped right down,cleaned and rebuilt with new piston rings and any wear there was, was minimal.  There are no seals on the crankshaft. 

wristpin Tue, 19/04/2016

May be the new rings have not bedded in, particularly if the bore was not glaze busted when they were fitted. Did you measure the bore for wear and "out of round" and what were the end gaps of the rings when installed in the worn bore?

With no seals, and the crank running in plain bushes it could be another wear situation allowing excessive ingress of air. Another possibility is primitive cooling leading to  heat soak in the stopped engine causing premature fuel vaporisation.

pyneapplechunks Tue, 19/04/2016

I glaze busted the bore as best I could and fitted new rings.  I did not test for "out of round".  The end gaps in the rings were 8 thou (the piston dia. being 2").  Could it be that a slight amount of air could be coming in through the crankcase and cylinder gaskets?  Maybe they are bedding in and need re-tightening. 

hortimech Tue, 19/04/2016

This sounds like worn main bearings, if you try and lift the flywheel, can you feel any movement between the crankshaft and the main bearing, there shouldn't be any appreciable amount. When I started work as an apprentice, there were still a lot of these two stroke Atco's about, most got scrapped for the problem you are facing. They started when cold, but wouldn't start from hot, the cost of replacing the crankcases (the bush was part of the case) out weighed the value of the machine.

Now I am not saying you should scrap yours, but it may be that you need to find someway of replacing the main bearings.


hillsider Tue, 19/04/2016

Correct me if I am wrong but I think I remember the subject of air leakage around crankshaft bearings being raised previously and the question was raised about the use of modern two stroke oils being a contributing factor. The answer being to revert to old style SAE30 oil without any additives, the theory being that the modern oils are less able to sustain the sealing effect on the bearings.

wristpin Tue, 19/04/2016

Definitely subscribe to old spec oils for old engines, eg  a non detergent SAE 30 or 40 mono grade. Millers do a two stroke mixing oil specifically for vintage engines.

pyneapplechunks Tue, 19/04/2016

There is little or no up and down movement on the main bearings and just a little side play.  I am using SAE 30 oil mixed with the petrol in the proportions originally specified.

hillsider Tue, 19/04/2016

Just a thought, how about raising the carb needle up a notch as an experiment to see if it improves things.

Failing that try using the dedicated mixing oil that wristpin mentions.

pyneapplechunks Tue, 19/04/2016

I found that the cylinder head nuts and crankcase nuts weren't fully tight so I tightened them up again.  I will move the carb needle up a little and see if this has any effect next time I cut the lawn.  If there was any unwanted play in the bearings perhaps this could be taken up by using a thicker oil eg. a 40 grade as suggested.

wristpin Wed, 20/04/2016

Should have asked from the outset - is a full tank enough for your usual mowing session? If so start with a full one and don't worry about it!

I have a 1960s Kirby Lauson engined Flymo contractor that I use for the local churchyard, it starts second pull when cold and with great difficulty when hot . When hot, even after several applications of the primer the plug remains dry which suggests premature evaporation or vapour lock. I just accept that after a tank full it's done enough and in most cases so have I.

pyneapplechunks Sun, 01/05/2016

Whilst in conversation with Millers Oils, as suggested by Wristpin, I mentioned that I had been mixing SAE 30 gearbox /engine oil for vintage cars with the petrol to make my own 2 stroke mix.  This SAE 30 oil is different to the SAE 30 2 stroke oil and will not mix properly with the petrol.  I also checked the spark on the engine, and even though I had put a new or what I thought to be a new plug in the reconditioned engine it wasn't sparking well.  I then changed this for an old plug which I had and there was a noticeable improvement. Hopefully the problem has now been sorted.

gtc Sun, 01/05/2016

This SAE 30 oil is different to the SAE 30 2 stroke oil and will not mix properly with the petrol

Yes, you definitely need two-stroke specific oil.

wristpin Sun, 01/05/2016

Short memories!

When that machine was current the recommendation for oil would have been the motor oil of the day, specific mixing oils for lawn mowers  were not around until the 1960s . The recommended oils for machines of the day would have included Castrol XL or XXL both mono grade motor oils. The downside was that they needed a thorough mixing and then had a tendency to separate out if stored - hence the need for a good shaking of the can of mixture before use. To say that a non detergent SAE30 mono grade "will not mix properly" is patently untrue as that is what was being done from the early 1900s (or earlier) until the arrival of specific lawnmower  mixing oils in the 60s. It just needs a bit more effort !

The Millers offering is basically a similar product but with additives to overcome the mixing and separation during storing issues.

Spark plug issue. That doesn't surprise me as my Dennis "eats" modern NGK plugs but runs quite happily on old Loge and KLG plugs that are possibly as old as it is. I read somewhere that the porcelain internals of old plugs were glazed where as modern plugs are not and become contaminated . I will try to find the article.

hillsider Sun, 01/05/2016

I don't know if it is still around in the UK but many years ago as boys we had a variety of old motorbikes on the farm we had a gallon of Bardahl two stroke oil to mix up for the two strokes in the fleet.

Just as a compete aside one of the bigger machines in our fleet was a 600cc Panther sloper with side car attached, that thing had so much pull that one day one of the farm lads tied it to the draw bar of a four wheel trailer load of Cauliflower just to see if it would pull it - and it did! I wonder how many modern bikes would put up with that sort of abuse.

wristpin Sun, 01/05/2016

Bardahl - that's a name from the past, although apparently still selling additive products in the UK and the name is plastered all over US NASCAR racers etc. Have memories of those roller shuttered pump cabinets that used to stand on garage forecourts - sort of orange colour I think.

gtc Wed, 04/05/2016

Wristpin's points about ordinary oil are well taken but the point about requiring a good shaking is moot, especially if the tank isn't run dry between uses.

On the flipside: I have recently repaired a four stroke mower whose owner's wife filled it with some bizarre two stroke mix. I don't know what oil was used, nor in what ratio, but the resulting fluid had the consistency of castor oil.


Superscavenge Wed, 04/05/2016

A good point that Wristpin makes about old plugs. I have noticed that some old plugs give a white tinge to the spark colour. I have used Champion D16 which are supposed to be the replacement for the old Com 8 in stationary engines and they fail very quickly whereas a cleaned up old Com 8 seem to last forever.

pyneapplechunks Sun, 05/06/2016

Fingers crossed I think I have solved my restarting a hot 2 stroke problem.  My garage mechanic suggested changing the condenser and as this was a fairly easy thing to do by changing the whole brass 'pill box' containing the points and condenser,which were known to be sound and working, I was able to restart it hot.  This was confirmed today after cutting the grass.  After completing 1 hour of continuous mowing I stopped the mower and it restarted straight away with one turn of the handle, and I repeated this 4 times.

gtc Mon, 06/06/2016

My garage mechanic suggested changing the condenser.

Good tip and makes sense. Heat is the enemy of condensers (capacitors) at the best of times. If they are already dodgy, then heat will put them into failure mode.