2 stroke compression

[blownengine]

My buddy just dropped off his 3 horse, 2 cycle, Toro snowblower saying it wont start. I have very little experiece with 2 strokes so I used my knowage from 4 strokes.

I checked for spark and compression. It has spark and the compression read around 53 psi.

My question is how much psi should the compression be on this 3 horse 2 stroke?

Thanks, Dan

 

[cncsparky]

That sounds a little low. The jetskis I once owned were 110-155psi.

 

[mnibike]

two strokes should have more compression than four strokes

 

[Oldsalt]

Two strokes, with very little exception, can be expected to have far less compression, as measured with a compression gauge, than a like sized four stroke. The main reason for this is that the piston as it nears the bottom of its stroke opens a port to start allowing the exhaust gases out and a port to allow intake of new mixture. Therefore the piston cannot start compressing anything until it travels back up the cylinder quite aways. The most common won't start problem with a 2 stroke? The spark plug.

 

[Strigoi]

all i know is that big bore 2 strokes have a bunch. i bruised my foot too many times when i had mine.

 

[blownengine]

Well its only a 3 horse snowblower. I wasnt expecting much.
I will drain the gas today and put some fresh stuff in.

 

[blownengine]

I cleaned the carb last night. It had alot of oily sludge. I think my buddy made the oil/gas mix too rich.
Put it all back together today and it runs fine. So i guess the compression is ok. Weird.

 

[Olskoolmastertec]

A good rule of thumb

On a 4 stroke a minumum of 40lbs. on a 2 stroke a minimum of 80 lbs. A two stroke with less than that has a scored piston or stuck rings. Also check the oil seal on the pto side of the crankcase,I have seen them come out and just dangle on the shaft,this allows the charge that is supposed to be blown up into the cylinder get shot out the side where the missing seal is causing a no start.....NEVER EVER shoot starting fluid into a 2 stroke,that is a good way to wash whatever residual lubrication there is right off the cylinder walls allowing them to score and tear up the rings.

 

[blownengine]

On a 4 stroke a minumum of 40lbs. on a 2 stroke a minimum of 80 lbs. QUOTE]

Did you put those backwards accidently? Shouldnt it be 80 pounds for a 4 stroke and 40 for a 2 stroke.

 

[shoe]

Check the exhaust port to see if it is Clogged ,, Take the Muffler off , the screen might even be plugged up .. you can see in the Cyl and piston also to see if there are any scratches in the CYL wall or on the piston . If it is scatched it may not start

 

[Motra]

The Tecumseh service manual doesn't list a compression value for comparison but as shoe said, a visual inspection through the exhaust port should be sufficient. That engine has an automatic compression release for easier starting so I wouldn't expect to see a very high reading, 53 psi is probably fine.

 

[blownengine]

I already got the engine running, just a dirty carb

 

[Olskoolmastertec]

How I learned the hard way

My boss at the mower shop gets a snowmobile dropped off for repair. I am 15 and quite unknowledgable."I am now 51" anyways this twin cylinder 2 stroke snowmobile engine sat for about 2 years. I yank the rope and it "seems" to have good compression and great spark"cdi ignition" so I rebulid the single tillotson carb as my boss instructed. Put her all together and procede to pull the rope a bit...well it fires right up and then quits,After several rope yanks all it does is putter,mind you to me,the idiot on the end of the rope,the compression seems great,however...after sitting for so long the cylinders and rings were a bit rusty,after the initial fire up the rust fried the rings and compression dropped a bit. After a compression check,it had 60 lbs...both cylinders,we figured thats enough..not so...after a quick yank of the jugs,a mild deglaze,new rings and gaskets...that puppy had 155 lbs compression in each cylinder and ran great. Now over the years I have had a slew of 2 strokes come into my shop. And after initial checking of compression I develpoed these"rules of thumb" about compression. Yes it also has alot to do with displacement but in general,ya follow what I have learned and ya wont go wrong. As far as 4 strokes on small equipment,most of em got compression release mechanisims built into the cams,so ya gotta rotate em back wards to really get a good check. This is my 2 cents worth. It has worked for me and made many a happy customer. After all,re ringing a 2 or 4 stroke is not that hard as long as the cylinder is decent.

 

[65ShelbyClone]

Two strokes, with very little exception, can be expected to have far less compression, as measured with a compression gauge, than a like sized four stroke.

I agree. 2-Cycle engines often have very low compression ratios and likewise low(ish) cranking compression. However, I would expect even a healthy utility 20cycle to have something like 80psi or more.

On a side note, I just did a compression check tonight on a CBR600 F4i engine and got an average of 180psi dry/200psi wet. .....12:1 compression might have had something to do with it. :biggrin:

 

[jeffdubb8]

test. sorry just wanted to see if i could post...