spudulike

It will be even less. What brand is this tester? 10 years abuse is good for any saw, invest in a new one and keep the old one as a backup. With 90psi compression, the saw wouldn't run.

You need the top end off (including the muffler and carb), inspect the plating on the cylinder for damage and wear. It may have worn through to the aluminium if the air filter is a wire mesh one. Place the piston rings in the bore checking the end gap. Measure up the piston especially the lower skirt. Replace parts as necessary. If the cylinder is OK (hope it is), try to source a Meteor or Hyway replacement as they are pretty good replacements to OEM. Go careful on the size as there have been a number of variations of this saw. Other than that, if the cylinder is shot, try for a secondhand OEM part or a kit from the same manufacturers as mentioned earlier.

As above, if the saw has relatively low compression, it will drop off further with running and this will cause hot start issues. Other than that, the fuel tank breather can be checked out as if it is semi blocked, the vacuum caused through running may cause an air lock. Also check the carb settings/colour of the spark plug. If the saw is running lean on the L screw, it may be difficult to start when you aren't using the choke. On an old machine, the compression is the most likely as it will have many hours use so it will be worn and may have been nipped up at some stage. You really need 150psi+ when cold, more would be nice. From your description, it sounds like the engine doesn't cut out in mid cut but only when you switch it off or it just stalls when idling. If it was cutting out mid cut flat out, it could be a coil/HT issue as heat can stop the coil functioning if faulty but it could do it mid cut or as yours does - had a 460 once like that and it was the coil to flywheel gap being too large.

Google reckons - Griselinia littoralis.....I reckon it is an evergreen shrubby bush thingy!

Ah, OK, very difficult to tell what the issue is then. All you can do is to do the leak down test, fit the new piston and cleaned up cylinder, check out the fuel system and go from there. When you try to start it, don't use the decomp valve. The extra compression in not using it will help the machine fire.

Has the saw run with the aftermarket kit on it - you can see if the piston crown has carbon on it. If there is plenty of crud around the coil and the saw has run in its current state then it is probably the fuel system. Flywheel keys do shear but it isn't that common. You often get a few backfires or severe kickback when a key goes. Personally I would have whipped the carb apart, cleaned and inspected it and more often or not, that would get it running. You can often tell from adjusting the carb if the saw is leaking air but it does take some experience to spot. It takes a BIG air leak to stop a saw firing completely - I once had one with no seal on the clutch side and it still ran....badly. Make sure the carb screws are out 1 1/2 turns each, they should end up at around 1 turn each once a final tune is done.

It may have already been flooded, the spark plug may be oiled up, the OP may have bunged too much/too little fuel down the hole. I usually stick a squirt of GT85 or WD40 down the throat of the carb, that usually fires most times. Most people don't work in our sort of environments and posses our diagnostic skills so things we do and get the effect we need/want, sometimes just don't happen when tried....look at the compression test....the times people buy an eBay special for cars and big engines and get a poor reading.

You need a device that can produce vacuum as well as pressure - most use Mityvac but you can use home cobbled gauges and brake bleeding kits/bicycle pumps if smart enough. You basically plug all the engine orifices and then push air or draw out depending which test you are doing. Most will use a modified spark plug with a tube down the middle of it or you can feed air in through the impulse connector on some machines. Probably easier to give it to someone that can do the tests. Some plug the orifices, put pressure in to the engine and then dump it in a bath of water - better clear that one with the other half before trying it, especially if they are in it at the time!!

Difficult to tell but in my experience, the most common issues on saws and other hand held two stroke kit is the fuel system probably followed by seizure but this is less common now with limit caps, rev limited coils and auto-tune. Your machine is pretty old and with old machines, it is common to have a load of old issues, typically wear and abuse from not being regularly cleaned and serviced. A lemon would consist of major cylinder damage and a REAL lemon would be a bent crank or failed big end. I have only ever seen one 372 with a blown crank and that was from a US ported one where the impulse line came off mid cut and it detonated. I did manage to get it back together again for a reasonable cost - secondhand OEM crank etc. The top end was perfect - shows a bit of porting does protect the engine.

OK, sounds like you are using a compression gauge for a car which will have a much heavier rated schrader valve which isn't a problem for a 1.0 litre engine with electric start but put it on an engine 20 times smaller with a rope and pulley........!! I would still hazard a guess that the main problem won't be an air leak. It may have one but I have only found them ever leaking slightly around the clutch seal, never a major leak. The 372 is a solid machine. If the thing has a spark and sounds like it has compression, the thing to check is the fuel system including the impulse line that has been known to come off. It is always worth doing the pressure and vac test on a wreck as you don't want to get it going only to take out all the work you have put in but don't reckon it will have a big leak unless someone has done something silly. I would be giving the carb a look at, diaphragms are OK, the diaphragm and gasket fitting order is OK, the gauze strainer for blockage, the needle valve for sealing and pressure check this once back together. Check the fuel line (pressure test it), fuel filter and impulse line is OK and connected.

Yup, if you get duff info then you get duff diagnostics. I was working on something going severely wrong with the top end causing the massive drop in compression - a ringless piston would make 75psi. As you said, that 5 mins in your hands usually tells a big story and usually gives you 75% of the story in most cases. The pressure and vac test is the foundation to any good running machine and starts any decent diagnosis on an old machine like this.

I think the Soviet steel thing was never real and it was just down to the Alfa Naples factory being demotivated and pretty much like British Leyland in the mid to late 70s. Not sure about the Fiats - the 2.0L twincam was a beast though! Lancia....Beta...OMG, engines falling out!!!

So the compression reading of 75psi may be 170psi? What is the gauge...make, type etc If you lift the saw up by the starter handle, does it tumble to the ground of hang there on each compression stroke?

With 75psi??....it would need around 130psi bare minimum to fire and run. The 372 bottom ends are pretty solid. The seals tend to be pretty good from past experience. The only thing I ever found was very slight air leaks down that sleeve that goes round the crank on the clutch side or a lightly leaking clutch side seal but never had a major leak. The compression is non existent, unless the gauge is a car one and the compression figure is much higher but hasn't the gauge been tested on another saw??

Show us the damage to the piston. The front looks OK so it is possible the circlip has come out, the end has broken off or the big end is kicking out white metal. Get this diagnosed before putting it back together again.

I thought rot on cars like this was a thing of the past. Having purchased old bangers through the 80s, I was quite used to also purchasing thumping great tins or Upol filler, primer and spray paint. Not seen rust like that since I was looking at purchasing an Alfasud..nice green cloverleaf but it was rusting in weird places...around the filler cap, windscreen wiper holes etc. Didn't bother in the end, great cars but the rust!!!

When you are young, you don't look at the danger or the consequences of doing something a bit stupid and dangerous. When you get a fair bit older, you cling on to what you have in life and know that life is difficult enough with just normal aches and pains let alone ruptured spleen, fractured pelvis, shattered vertebrae, muscle and nerve damege, broken bones etc.....if you are unlucky. Also with age, your balance, strength and general fitness won't be as good as a 20 year old. I will still get up a ladder and don't think too much about it but take less risks than I used to.

The gasket should be between the muffler and heat shield plate and none between the plate and cylinder. I wonder if a circlip has disintegrated between the piston and cylinder fitted or the big end is breaking up. 75psi is almost non existence as 150-175 is more normal on one of these saws. The best option is to clean the OEM cylinder and to fit a new Meteor/Hyway piston. Re-using a seized piston is never a good long term option. The piston skirt does improve compression and does cut down the piston rock at BDC.

Great, lets check out the method of measurement......we have seen so many use a car gauge and register low compression when even a saw with 200psi will register 75psi. If the test shows the saw is at fault, 75psi is VERY low so the top end will need to be removed and the piston/cylinder inspected for damage etc. On heating the plug.....I did that today and bingo, a flooded saw becomes a running saw again!!

I had 5L of brick cleaner left for me by the previous owner of my old house, with a decent amount of HCL in it, it seemed rude not to use it!

The black looks like the residue left when you use acid on the aluminium transfer. The black usually comes off with a bit of rubbing with emery but wouldn't do any more of this unless the black part is a high point.

It looks like piss yellow or I guess you could call it flat cider as well. I wouldn't paint my living room or wear a shirt that colour but guess this isn't what you meant.

Warming the plug is an old school method of starting a reluctant or semi flooded engine. It has the effect of drying a wet plug and also helps vapourise any wet fuel that lands on it when pulling over the engine. Oh....and it works rather well!

This every day of the week. I stick to Volkel taps and use the tapered type as in the pic if the receiving thread is still marginally there. Make sure it goes in dead square and DO NOT buy a cheap kit off Amazon or eBay.....the cheap taps are exactly that, cheap and will destroy your cylinder.