What's on your bench today?

[Craigb]

11 minutes ago, spudulike said:

Good work getting one of these sealed. The inlet manifold and crank seals on the clutch side are usually difficult on these machines.

no bubbles whatsoever on the pressure and no drop on the vac test.

[Craigb]

This test got me wondering. What sort of pressure and vac are developed during use in the crank case? Must be less that the usual 10psi test surely.

[spudulike]

Good question but can't answer it. I have often wondered the same but on the top end. The compression must be phenomenal compared to the start up pull over at 14,000 rpm.  

I guess if you tried to measure it, all you would get is an average pressure or vacuum as it all happens so quickly.

[Craigb]

Under the spark plug must be ridiculous at wot. Completely useless information but still interesting. Someone must know.  I’ll ask Jeeves.

[pleasant]

3 hours ago, spudulike said:

I suspect the fix may have been something much more simple so blocked exhaust/spark arrestor is possible as is carboned up exhaust port.

I think ADW was pretty close with a clogged muffler/spark arrestor, but as Spud specifically said 'blocked exhaust port' then he takes the gold house point.

 

Well done.

 

Don't see this often so much with modern fuel/oil mix as I used to, and haven't seen one as bad as this ...ever. See if you can spot the piston skirt in the first pic.....you can just make it out if you zoom in. No tricks of the light or shadows anything- that first pic is genuinely how carboned up the exhaust port was. The escape route for the gasses was less than the diameter of a biro tip. Second pic is taken at exactly the same angle, and shows how it should look once we did a clean up.

 

My recommendation was to check the oil/fuel ratio and to ditch the Stihl Red they were using, and upgrade to the low carbon HP Super.

 

 

Edited June 8, 2023 by pleasant

[bmp01]

9 hours ago, spudulike said:

Good question but can't answer it. I have often wondered the same but on the top end. The compression must be phenomenal compared to the start up pull over at 14,000 rpm.  

I guess if you tried to measure it, all you would get is an average pressure or vacuum as it all happens so quickly.

Old tech by now, cylinder pressure measurement in high speed 4 stroke engines was done with piezo electric pressure transducers couple of decades ago. Getting a real pressure trace was possible, relating pressure trace to crank angle less convincing.

Ball park peak pressures 100 -110 bar at max torque, engines had static compresion ratio 11, 12, 13, that sort of thing.

I'd punt at 50-60% of that for decent brand 2 stroke engine, maybe 75% for a tuned 2 stroke engine.

[Craigb]

41 minutes ago, bmp01 said:

Old tech by now, cylinder pressure measurement in high speed 4 stroke engines was done with piezo electric pressure transducers couple of decades ago. Getting a real pressure trace was possible, relating pressure trace to crank angle less convincing.

Ball park peak pressures 100 -110 bar at max torque, engines had static compresion ratio 11, 12, 13, that sort of thing.

I'd punt at 50-60% of that for decent brand 2 stroke engine, maybe 75% for a tuned 2 stroke engine.

 

eould that make it around 1000psi?

[bmp01]

14.5 psi to 1 bar

1000 psi = 69 bar in the ball park, maybe a little high.

 

[Aled Hughes]

On 19/09/2011 at 23:09, miker said:

if you filled crankcase up with fuel,could this be a method to test crank seals, ie if it leaks out over say 12 hours? or is it more of a pressure issue??

You could immerse the saw in water, pressure the crankcase whilst piston is at bdc and look out for bubbles from your air leak.

[Will C]

20 hours ago, pleasant said:

I think ADW was pretty close with a clogged muffler/spark arrestor, but as Spud specifically said 'blocked exhaust port' then he takes the gold house point.

 

Well done.

 

Don't see this often so much with modern fuel/oil mix as I used to, and haven't seen one as bad as this ...ever. See if you can spot the piston skirt in the first pic.....you can just make it out if you zoom in. No tricks of the light or shadows anything- that first pic is genuinely how carboned up the exhaust port was. The escape route for the gasses was less than the diameter of a biro tip. Second pic is taken at exactly the same angle, and shows how it should look once we did a clean up.

 

My recommendation was to check the oil/fuel ratio and to ditch the Stihl Red they were using, and upgrade to the low carbon HP Super.

 

 

You sure I wasn’t right on the cause of using engine oil instead of 2 stroke 🧐  I’ve ever seen one like that! Amazed it still ran at all.