Pistons - fried like an egg!

[armchairarborist]

ok checked mine and got 30psi first time, then noticed the decompression valve had been depressed:blushing: (not by me, i don't use the 'press here if you are gay' button), then got 90psi before finding the precision instrument was leaking, can't get readings over 100psi (even on a new saw) so pressure gauge must be duff:banghead:, better try a different gauge eh?

[spudulike]

Right -No 1 - Thanks Matt for the good rep - your 372 is coming along nicely - found some interesting stuff wrong with it.......

 

Pressure checking, firstly absolutely NOTHING to do with compression or the performance of the top end of the engine.

 

A two stroke engine pulls fuel oil mix from the carb, into the crankcase and then pushes it up through transfer ports in to the compustion chamber, this action also expells the exhaust and then once the piston clears the exhaust port, we are all ready for a good bang and lots of smoke.

 

The air is controlled by the carb and the integrity or the seal of the engine is critical to its correct running, if the crankcase seals, impulse line or carb boot are leaking, you will pull air in to the engine in varying and unwanted amounts and this then can cause a very weak mixture. If you are lucky, the dodgy idle will warn you about the problem, but if you just adjust it out and carry on then the next thing that happens is the exhaust side of the piston melts on to the cylinder wall - a full or partial engine seizure.

 

So - if an engine has seized, new parts are fitted and a pressure check ensures that it wont happen again as long as this is followed by the correct adjustment of the "H" carb screw - tach tune or tune by ear!

 

The early signs of air leaks are poor idle, an idle that has increased unexpectedly - like when your saw runs out of fuel - that increase in speed just before it dies - you may also get the saw hitting higher revs and sounding like a demented wasp you could also get that flat spot between 3,000 - 6,000 revs - like the 345, 350, 346, 357 Huskys when they are fully cold but it will not clear after 30 seconds running - thet wohh wohh sound in the midrange!

 

So - a pressure check = an important bit of the saws health and not engineering sphericals - this single check has saved the 066 Magnum and 048 on my bench further damage after repair.

[spudulike]

ok checked mine and got 30psi first time, then noticed the decompression valve had been depressed:blushing: (not by me, i don't use the 'press here if you are gay' button), then got 90psi before finding the precision instrument was leaking, can't get readings over 100psi (even on a new saw) so pressure gauge must be duff:banghead:, better try a different gauge eh?

 

Think you had better read my last post my friend - compression testing is NOTHING to do with pressure checking.

 

FYI - compression in a decent running saw with a flat crown should be 150PSI - 180PSI, a saw will run with as low as 130PSI but usually badly. Maximum PSI I have recorded was 240PSI with a vintage Jonsered with a domed crown.

Exceptions to this rule are vintage Poulan Countervibes where they are renowned for 135psi being normal and good.

Generally, good compression = good power

[spudulike]

Oh - I gather you salvaged that comp gauge off the Ark Royal - think you may need a new one - the Gunson one usually does the job - much read about Schrader valves and the like but mine works just fine.

[armchairarborist]

i get it now, you could have told me sooner haha:biggrin:

[miker]

i have just fitted a new pot and piston on a husq 51 and im only getting 120 psi ???

[Mesterh]

Spudulike, since you seem to have quite a bit of knowledge about the old two stroke can you clear something up for me.

 

I have came across a couple of ms200t cylinders that have lost some of the nikasil coating just and only below the intake port. I assume this is only due to contaminated air ie poor filter cleaning/damaged filter etc. Or could it possibly be anything else?

[spudulike]

i have just fitted a new pot and piston on a husq 51 and im only getting 120 psi ???

 

Check your gauge with a known good saw that is run in and working well - if it pulls the same then all is well and your gauge is reading low.

 

Standard test is to test a cold saw (a hot saw lowers compression), it should have been run if possible so normal levels of fuel/oil cover the piston/cylinder, the saw should be put on the fast idle setting and the choke taken off, the first pull should give around 100psi and then around 4-5 more pulls should maximise the compression reading which should be as originally specified in my earlier posting.

 

120 psi is low and you may have damaged the ring on assembly, does the saw run - 120 psi and it would be touch and go as to whether it runs. Does the 51 have a number of piston options - perhaps you have fitted the wrong one? I think they came in 44 and 45mm versions!

[spudulike]

Spudulike, since you seem to have quite a bit of knowledge about the old two stroke can you clear something up for me.

 

I have came across a couple of ms200t cylinders that have lost some of the nikasil coating just and only below the intake port. I assume this is only due to contaminated air ie poor filter cleaning/damaged filter etc. Or could it possibly be anything else?

 

If your wear is around the inlet port then it may be years of fine wood dust etc coming through the filter.

 

The cylinders have a finite life, the plating will wear out in time, you may get away with a little wear below the line of the exhaust port but the compression area above the exhaust port must be free of scores and have no damage.

 

If the cylinder has the plating worn below the inlet port then I think it is time to fit a new P&C or buy a new saw - you have probably had your money out of it:thumbup:

[Mesterh]

Cheers for the info.

 

Have you found anyone selling non oem cylinders?