Compression Test 101

[cpostis]

take the reeds out,retest the compression.
you will find it did not change.
however with the reeds out you wont move fuel.
the intake and ex ports will supply enough air at cranking speed to test ring sealing ability.

over the years playing with the two smokes I have found it really does not matter hot or cold or throttle open or closed.

only thing to watch for with the motor warm is a brass tester fitting inserted cold will expand in the threads.
makes it a bugger bear to get it back out.

Thanks Rodbolt, appreciate your passing on experience!

[Jason2tpa]

over the years playing with the two smokes I have found it really does not matter hot or cold or throttle open or closed.

only thing to watch for with the motor warm is a brass tester fitting inserted cold will expand in the threads. makes it a bugger bear to get it back out.

That's a pearl right there bolt. Good stuff, thx.

[99yam40]

Someone brought up the open/closed throttle on a 2 stroke before, so I tried it both ways with my Snap on gauge and I did not see any difference between open and closed throttle

[TownsendsFJR1300]

Someone brought up the open/closed throttle on a 2 stroke before, so I tried it both ways with my Snap on gauge and I did not see any difference between open and closed throttle

It was post #2, from the shop manual (mentions Yamaha's specifically and shows 2 stroke engines) Chuck posted

There may be no difference (as you pointed out) but, doing anything (holding the throttle @WOT-NO RESTRICTIONS) you can do for an accurate reading, why not?

It doesn't take a second to move your throttle lever forward for the test. Should your particular engine (carb) limit air into the combustion chamber, your readings could be incorrectly reading low...

IMO, the more accurate, the better..

[99yam40]

It was years ago that I did this because someone mentioned a 2 stroke will get enough air from the exhaust port to get accurate compression reading without opening the throttle, so I tried it.

I was just agreeing with what Rodbolt posted and saying I also tried it myself with no difference found

[cpostis]

It was years ago that I did this because someone mentioned a 2 stroke will get enough air from the exhaust port to get accurate compression reading without opening the throttle, so I tried it.

I was just agreeing with what Rodbolt posted and saying I also tried it myself with no difference found

Aha, now I understand. Being that the exhaust port is higher in the cylinder than the intake port, and when the "per-compressed air/gas" enters the chamber through the intake port and some fuel/air mixture is wasted out the exhaust port, no compression is begun until it passes the exhaust port. Theoretically, and actually there should be no difference in pressure when a reed valve is open or missing for that matter.

So, in actuality in doing a compression test (or a leakdown test) , there is no effect from activities below the piston.

Now, what are the first signs of 2 stroke worn cylinder components, I've heard poor idle due to the low compression at the low rpm (higher rpm seems to run good)? So, if you have poor idle and suspected poor air/fuel delivery; it may be poor compression. With these symptoms, you may just want to do a compression test before looking at air/fuel delivery?

Some on other threads have suspected clogged carburetor passages, cleaned them and still have a poor idle.

What ya think?

[boscoe99]

Just because one Yamaha does not indicate any change in compression pressure with the throttle opened or closed, do not make the mistake of assuming that they will all operate that way. It would be a good habit to always open the throttle wide open when doing a compression test on any brand/model of motor. If it does not matter then it does not matter on that particular model of motor. On some models it might matter. Not opening the throttle on those models can give an incorrectly low test value. Possibly leading to much angst.

Now as food for thought, when the piston of a two stroke is descending it is pressurizing air within the crank case. That pressurized air is then forced into the combustion chamber when the intake port opens. Does the pressurized air result in a higher compression value than non-pressurized air would (say the reed valves are not installed or the throttle is closed)?

Maybe. Maybe not.

[99yam40]

All I am saying is next time you are checking compression on a 2 stroke, check it both ways to see if it makes a difference.
It suprized me with the results, I have always checked with throttle open in the past and probably will still do that

[TownsendsFJR1300]

Now, what are the first signs of 2 stroke worn cylinder components, I've heard poor idle due to the low compression at the low rpm (higher rpm seems to run good)? So, if you have poor idle and suspected poor air/fuel delivery; it may be poor compression. With these symptoms, you may just want to do a compression test before looking at air/fuel delivery?

My neighbor gave me a Ryobi weed eater 31cc(2 stroke) that wouldn't run over 3,000 RPM/bad misfiring as well. Cleaned the carb, no change. He told me, he pulled the cylinder / head (one unit, and that there was no carbon build up on the rings..(which I believed him).

I still couldn't get it running properly and decided to do first a compression test, then a leak down. Compression was around 135 PSI. Sounds good! Checked the compression in another similar machine to compare, 195 PSI!! Hummm, somethings up.

I pulled the cylinder and the rings were carboned up big time... I don't know what he was looking at previously ... I had a parts cylinder / piston / rings which I just swapped out.

With those installed and the carb fine tuned, runs like a top, well over 7,000 RPM...

Re idling issues. Different machine, they use TWO bolts (not enough) to hold the cylinder head to the block. The idle started acting up and couldn't get it to idle steady, up and down, very erratic. I notice the block to cylinder surface wet with a little bit of oil... Crank it up, spray some soapy water, bubbles big time!. The bolts loosened up over time/vibs, was sucking in air leaning out the mix especially at idle.. Fixed the issue with a new gasket, lots of RED loctite(temps good to 400F I believe) but the engine block barely see's 200F. My favorite machine, runs like a top.

*Point being, ANY vacuum leak, especially on the lower end WILL affect the idle mix (and the rest of the range)..

[99yam40]

friend brought me a PoulenPro leaf blower that his son bought new and used less than a year that would not start.
had spark, and compression(do not remember the #s), so I cleaned the carb.
still would not fire off.
bought a new carb for it,
still would not fire up.
took piston out and cleaned the ring and grooves.(ring was stuck with carbon).
Fires up and runs great now.
compression did come up a lot.
I did not realize how much they were suppose to be running.

Had an old craftsman chain saw my dad had from back in the 70s that would only fire off if you shot some fuel into the open carb warm or cold and run only at high RPMs.
found the pulse line to the automatic oiler had fell apart leaking vacuum enough to keep the carb from sucking enough fuel to get it to fire up at cranking speed

I have never seen a 2 stroke motor that wore out, only ones someone killed by running the wrong fuel mix( no oil),overheated(water cooled that did not get enough water) or with a plugged up carb leaning it out until it melted or screwed up piston/cylinder

[TownsendsFJR1300]

Just to add, (slightly off track), these little machines are set super lean from the factory (thank you EPA) and after maybe a year won't run from lack of fuel.

There's usually two small adjustment screws in the carb body, "H" for high speed, "L" for low speed.

9 times out 10, adjust that H screw out(usually a half turn), richen up the mix and its running better than new... It'll now idle back down(vs a lean, running out of gas, higher RPM) also..

[rodbolt17]

as it was pointed out, when the intake port is open, so is the ex port.
means not much pressure in the cylinder from the incoming charge.
not much compression at all until the ports are sealed.

in about 30 years of dealing with marine two stroke I have only seen two wear out.
I have seen them corrode away.
overheat and burn a piston.
run lean and detonate one.
water intrusion.
lack of oil.
but the only two I ever saw that simply wore out beyond any reasonable attempt at an overhaul were both late 70's early eighties 70 johnsons.
both were in service 15+ yrs.
only outboards I ever saw that I had to use a ridge reamer on to remove the pistons.
I figure they had over 15,000 hours based on some of my other commercial guys with hour meters that did about the same routine.