Compression

[J_Walker]

Simple. Question.
how do you know if an engine isn't getting enough compression? because Ive cleaned my carb, it wasn't even dirty. soaked it in carb cleaner. Ive checked my spark, its very bright yellow/white spark. so ima shoot for, it has to be the compression that's keeping the engine from running. What does it sound like/do when it doesn't have enough? or describe it. Thanks - John

 

[devino246]

Simple. Question.
how do you know if an engine isn't getting enough compression? because Ive cleaned my carb, it wasn't even dirty. soaked it in carb cleaner. Ive checked my spark, its very bright yellow/white spark. so ima shoot for, it has to be the compression that's keeping the engine from running. What does it sound like/do when it doesn't have enough? or describe it. Thanks - John

Lack of compression can be caused by several things: Valves not sealing, worn piston rings, bad head gasket.

To check compression, you need a compression tester. Autozone has ones that they'll loan out(pay full price, they give you your money back when your done). The one I have was like $30 or $40. You remove the spark plug and screw the tester into the hole.

On engines with compression release, you have to put a wrench on the PTO bolt and spin the engine backwards to get an accurate reading. It should read somewhere around 60-80. If its low, you can take the tester out, put in a bit of engine oil, and retest. If its higher, your rings are bad.

 

[J_Walker]

what if it was running fine before, when the clutch went nuts, and started making a horrible noise, I replaced it, than well yeah. It kinda just stopped working. than I checked everything by taking it apart. Couldn't get to the piston itself because the F*%& bolts were stripped to all he**! but everything looked fine, other wise. I have a Gx200.

PS: I put new oil in the engine, but it hasn't turned over sense I put it in, would it be fine if I drain it. and than reuse it? its leaking from the crankshaft case, the gasket is bad I think. And were should I store the Oil if I can reuse it. its been about a week, sense I put it in.

 

[devino246]

what if it was running fine before, when the clutch went nuts, and started making a horrible noise, I replaced it, than well yeah. It kinda just stopped working. than I checked everything by taking it apart. Couldn't get to the piston itself because the F*%& bolts were stripped to all he**! but everything looked fine, other wise. I have a Gx200.

Did you replace the head gasket?

 

[Doc Sprocket]

I second that- If you reused it, there's a chance it's damaged.

You can save and re-use the oil. Use any clean container you like- a good choice would be the container it came out of.

 

[J_Walker]

Did you replace the head gasket?

that's my point, I couldn't get to the head gasket because the bolts to the thingy are stuck on there, and stripped to all hell So I don't know, I suppose its stuck on there. It should be fine? other than the rings or something. FML idk what to do!

 

[fowler]

do u have a welder

weld a nut to the top of the bolts and hey presto

i have to do it a bit to get sump plugs outta mine trucks

 

[Doc Sprocket]

You're jumping to conclusions.

Here's what I want you to to. Remove the spark plug, pour a teaspoon of fresh fuel in the hole, quickly replace the plug, and give a few tugs on the starter. Report back.

 

[karl]

Im not shure if this is such a accrute test, but in a small engine book i have it says that if you spin the flywheel backwards and it bounces back on compression stroke it has enough compression to run.

 

[Don Qui Racing]

easy way to tell, pull the plug and put your thumb over the hole. turn the motor over, if it doesnt blow your thumb back out with good force than you have an issue. Start checking valve lash, maybe a bent rod.

 

[devino246]

easy way to tell, pull the plug and put your thumb over the hole. turn the motor over, if it doesnt blow your thumb back out with good force than you have an issue. Start checking valve lash, maybe a bent rod.

Im sorry, but thats BS. You wont be able to hold your thumb over the hole even with crappy compression.

 

[Rustydog2010]

Im sorry, but thats BS. You wont be able to hold your thumb over the hole even with crappy compression.

I beg to differ. I was working on a friends RM250, it had about 70psi compression and I could hold my finger on that. Once it had a new set of rings @170psi, I couldn't hold my finger on that.

At 70psi it wouldn't run, at 170 it fired right up.

Not sure if its the same as small engines, but it worked for me.

Jeremy.

 

[oscaryu1]

I beg to differ. I was working on a friends RM250, it had about 70psi compression and I could hold my finger on that. Once it had a new set of rings @170psi, I could hold my finger on that.

At 70psi it wouldn't run, at 170 it fired right up.

Not sure if its the same as small engines, but it worked for me.

Jeremy.

I can back that up as well. Granted, my experiences lies within small bore 2 strokes, but the end result remains the same.

 

[devino246]

So if the hole has an area of 1/2 sq.in.(probably about that much if not more) and the engine is developing 50psi, that means its applying 25 pounds of force on your thumb. I dont think my thumb could hold that much force, maybe its just me. I do however see a flaw though: You can hold your thumb on it with 70psi of compression, with which a small engine will run just fine.

 

[Rustydog2010]

I do however see a flaw though: You can hold your thumb on it with 70psi of compression, with which a small engine will run just fine.

Not sure if its the same as small engines, but it worked for me.

Never said I new it would work with small engines. Just that it has worked for me in the past.

But I see where your coming from, we did a compression check on a digger at work. We didn't have an adaptor to screw the compression tester into the glow plug, so we had to hold it down by hand. With 300psi developing it had enough force to make the compression tester and my hand jump up.

Jeremy.

 

[Don Qui Racing]

Well to get back to helping people and not bashing stuff you cant comprehend, cause u havnt tried it yourself, i say to the other gentlemen who have done it yes, it is a way to check. The point is not to try to prevent the pressure from escaping. If it blows you thumb back, then great, thats a simple indication of good compression. if not then thats a bad indication. I did this on a 125cc 4 stroke engine, and my thumb didnt buge and all pressure blew back out valves and carb. So i would say from that SIMPLE test with my so amazing thumb, that i could safely say that there was a problem other than fuel and spark.

DON'T KNOCK WHAT YOU HAVEN'T ROCKED!!!