What's on your bench?

[pbillyi69]

if they dont seal all the way say from carbon build up cant air be sucked back through it?

 

[drf256]

How?

Piston going up is compression. Piston going down is combustion. The cylinder pressure in a running engine is never negative.

If there is never any vacuum in the area the decomp valve lives, how can air be drawn in?

Just another folk lore I can’t understand and disagree with.

 

[drf256]

Sure, a decomp valve can make a saw fail a pressure/vac test. That’s why I plug for testing, or grease the stem.

They just don’t allow fresh air into a running motor.

 

[redline4]

How?

Piston going up is compression. Piston going down is combustion. The cylinder pressure in a running engine is never negative.

If there is never any vacuum in the area the decomp valve lives, how can air be drawn in?

Just another folk lore I can’t understand and disagree with.

I agree.
I think it's a difference in 2 stroke operation vs. 4 stroke operation that gets confused.

 

[drf256]

Exactly Ed.

 

[Johnmn]

Sure, a decomp valve can make a saw fail a pressure/vac test. That’s why I plug for testing, or grease the stem.

They just don’t allow fresh air into a running motor.

I get what your saying doc, I have seen them fail pressure/vac testing quite often. I have an ice auger that suddenly wouldn't start, after messing with the carb for way to long I pressure/vac tested it and the decomp leaked terribly. I plugged it and it fired right up and has ran good for two years now. I have been scratching my head with that one, yes it was leaking but it still had okay compression.

 

[drf256]

They can leak enough to cause hard starting.

 

[Johnmn]

Fully rebuilt Husqvarna 55 with Jonsered 590 closed port cylinder

 

[huskihl]

The leaky decomp, scored cylinder syndrome is comical. Should be pretty easy to test. Go get your saw warmed up, cut wood with it and then push the decomp in.

 

[pbillyi69]

my post was a question. i dont know

 

[Czed]

Has trouble tuning this 288 I rebuilt
Chain wouldn't stop spinning
figured a clutch spring was broke it was

But damn that was the toughest clutch I've ever had to break loose
I was certain the crank would snap
Ran the battery dead and it finally
Came loose.
I swapped out the 8 pin for a 7
It's a strong saw for a stock saw
He runs those damn puny 20" bar's on everything
They suck.

 

[Billy Currie]

A Partner 7000 I picked up for $100. Cleaned the carburetor, air filter, replaced fuel filter/fuel lines, and sharpened the chain(a little rusty). Bar oil tank was empty, so it's either a leaker, or the previous owner ran it dry.....

Oiler works and loads of compression!

 

[Czed]

Got a nice 288 lowtop in today
Just a few mismatched fasteners
He had it worked on before and they couldn't fix it.
He brought me this 272 with no compression
Just needs a piston kit,carb kit fuel line
He gave me this nice 257 great compression I'll fix it after the other 2 are finished.

 

[Maintenance Chief]

I don’t believe leaking decomp valves would ever ruin any engine. Lack of fuel from a leaking supply circuit will.

If someone wants to explain to me how a decomp valve can allow any air into a running engine I am all ears.

Agreed

 

[NateSaw]

if they dont seal all the way say from carbon build up cant air be sucked back through it?

I think you're thinking 4 cycle brother.

 

[pbillyi69]

i havent ever really thought too much about..docs post made me think a little. that's just what popped in my pea brain.

 

[NateSaw]

i havent ever really thought too much about..docs post made me think a little. that's just what popped in my pea brain.

This is how I think of it... 4 cycles, above the piston, are suck, squish, bang, and blow... On the intake stroke exh valves are closed, cylinder is empty, intake valves open and piston travels down sucking in air and fuel. That's how 4 cycles make so much vacuum, provided the throttle plate is closed.

 

[pbillyi69]

it makes perfect sense to me just never thought about it. i dont use them but as i age i think they may become necessary....just not yet

 

[Kiwioilboiler]

Is the exhaust in a 2T ever under wacuum? Not tryna start anything, genuinely interested.

 

[NateSaw]

Is the exhaust in a 2T ever under wacuum? Not tryna start anything, genuinely interested.

I don't see how. With an expansion chamber, sound pressure waves push air fuel mixer back into the cylinder when the stars align, (rpm, volume, frequency, port timing etc.) but that's a pressure wave too.. Like an echo.