Low compression numbers

[holeshot]

Although I've recently put a piston and rings in my 125 (3 engine hours ago), I thought it still didn't have as much "bark" as new, so I bought an inexpensive tester from Harbor Freight.

The numbers I kept coming up with (throttle wide open, engine warm) were only about 114 PSI (the manual calls for 129 to 199 PSI). Being the parnoid type, I immediately tore tore my bike apart and found - nothing. No damage, no excessive blowby, original hatch marks still showing in the cylinder (I didn't " ball hone" it at replacement time).

Is this a bogus way of determining your engine condition? :think: I wonder how accurate these testers are?

I should have measured it when new and then made a comparison. :bang: Next time.

I'll replace the ring and base gasket and run it.

I wonder when the '05 four strokes are coming out? :eek:

 

[Layton]

I don't think I would put to much faith in an inexpensive testor from Harbor Freight. You get what you pay for.

 

[marcusgunby]

The 03 kx 125 has a very loose ring from new-they are like a worn one already, i use a vertex or wiseco setup, the 04 rm 125 is now the same with a loose ring, so we have CRs,RMs and KXs all come with a worn ring from new-not sure of the logic myself.

 

[holeshot]

I re-assembled the motor, took another compression test and got 115 PSI (probably 1 PSI extra from the well oiled cylinder).

This isn't the first wild goose chase I've been on, and it probably won't be the last. I will remember this one for awhile, though......

 

[steve125]

So how many times are you kicking the bike??? I use a Snap-On gauge, with the throttle WFO, i kick it quickly until the gauge will go up no more. My 03kx125 I was getting around 190 with a fresh top end and PV closed. Open, it usually drops around 10 psi. On my built motors I almost always go for 200 psi, with race fuel of course.

 

[marcusgunby]

You need to check another bike to verify its not the guage.

 

[holeshot]

I'm kicking it rapidly with the throttle WFO. Kicking it slowly brings about 110 PSI - kicking like a madman brings only an additional 4-5 PSI. The compression gauge may be a POS - I'll see if I can verify it's accuracy.

 

[KX02]

I have heard that cheaper gauges read lower. I bought one from a parts store, I think it was Actron or something? It looks like an OK unit. I only got about 160-170 out of my KX125 with a fresh top end. I did expect more but the bike runs fine.

 

[reelrazor]

Good idea to shop for a guage with a very short hose for two strokes. Also verify the powervalve is in the low speed/closed position.

Also, forget the 'hold in place on the spark plug hole' compression guages, they ALL suck.

 

[JasonWho]

Why the short hose? I have a Craftsman, but I have never got around to using it.

 

[Ol'89r]

The compression gauge may be a POS - I'll see if I can verify it's accuracy.

Ron.

If you want to compare your gauge with another, I have one you could use. Stop by and pick it up. :cool:

Terry.

 

[holeshot]

Ron.

If you want to compare your gauge with another, I have one you could use. Stop by and pick it up. :cool:

Terry.

Terry:

I think I'll do that. :cool:

 

[reelrazor]

Why the short hose? I have a Craftsman, but I have never got around to using it.

A short hose is a good idea on any compression guage, but it truly becomes a factor when testing two strokes.

A two stroke's compression stroke is really only started when the exhaust port closes. This can often be half of the measured stroke length, and on radically ported top end engines(or with powervalves 'open') can be closer to a third of the measured stroke. This means a 2st engine will only displace and compress the volume of air above the exhaust port into the compression guage.

Long hoses can quite often get close to equalling or exceeding that volume, especially on smaller engines.

Since a 4st engine displaces/compresses air for most of the compression stroke from bdc on up, the volume issue becomes less of a factor.

A long hose is effectively an extension of the combustion chamber and therefore will read/measure/show as if the compression ratio is lower than it actually is.

Ever had a cylinder head milled, even slightly, on a two stroke? Pretty noticeable difference even skimming .020" or less.

Imagine stacking 8 or 9 or 10 head gaskets between your cylinder and head. That is what a long(high-ish volume) hose mimics.

 

[holeshot]

Some very good points reelrazor. Especially true with a small bore two stroke.

 

[JasonWho]

How long is too long? My compression tester hose is around 10 inches.

 

[holeshot]

My hose is about 10 inches as well. :laugh:

 

[JasonWho]

CONCRETE THING THAT HOLDS BACK WATER

I knew someone would say something. :-)

 

[reelrazor]

How long is too long? My compression tester hose is around 10 inches.

That's not bad. Some are 25" long and that is just TOO long unless you are using it on an openclasser.

I have a couple I use. One is made as an Echo(weed whips/backpack blowers/chainsaws) special tool and only has a 2 inch hose(just enough to thread in and clip on the guage laying on the edge of a bike's cylinder head). The other has about an 8" hose.

I have played around with the Echo unit as it's quick coupler fits on all the hoses I have tried it on. The numbers vary quite a bit, sliding down the scale the longer the hose goes.

This is why using someone else's compression reading isn't that great a way to evaluate your engine.

I use them mainly to see where an engine is at(baseline), and what it does in the future(forecasting). For instance a newly rebuilt/refreshed engine will show an increase as the engine breaks in, peaking when break-in is complete, and (hopefully) maintaining that(or close to it) level for a while before starting to slide down the scale. That downward slide is the signal to order parts for the next refresh. A rapid drop may mean blown gasket, ring sticking etc. and can save you money by catching catastrophic failures before they occur.

 

[JasonWho]

I need to look at mine. Maybe a shorter hose coud be installed. I know the hose is not connected to the guage part.

 

[holeshot]

Correction: just measured the hose at 15". :eek:

 

[MikeT]

I tend to think it is the gage. Obviously the hose plays a part. I would say your gage is just reading low probably in the neighborhood of 40 to 50 psi.

I am starting to remember some Physics here. I think I can figure it out with some time...... You are really concerned with the pressure in the cylinder when the spark plug is installed so that would eliminate the extra volume of the hose.

The formula I remember is P1*V1 = P2*V2.... I think that is right So that would mean that if you took the pressure reading from your gage times the volume of the hose and divide that by the volume of the cylinder, you could get the actual pressure in the cylinder.....

You would think that the gage manufacturer would think of that, but the gage usually has a short "no hose" attachment and a long hose attachment, so some sort of correction factor should be used when the different hoses are used.

I may be off base but I think it's Boyles Law seeing that the tempreture remains the same.

 

[marcusgunby]

I think they are designed for car use really, the vol of the cylinder is so huge compared to the vol of the hose it makes no difference, me and steve came across this problem before, he got readings of about 200psi i think and im only getting about 160psi, i verified the guage accuracy and its ok, his is a good quality setup and mines a cheapo, i can only think the hose is the difference, maybe a cheapo one expands more as well?

 

[reelrazor]

Yeah marcus, the hose quality is a factor as well. Most good guages have pretty stiff hoses with small i.d., the cheapo ones don't.

It really doesn't matter as comparing numbers with unkown other guages is iffy at best.

The pressure in the cylinder is affected by the return wave of the pipe to a large extent. This can be seen even at foot kicking speeds and with no combustion. Test an engine with no pipe, install the pipe and test again. The numbers change.

 

[darringer]

I would think with all things being equal, except hose length, a compression gauge will read the same with either a long hose or a short one. With a longer hose, there is more volume to fill and pressurize. Therefore, it would take more kicking to bring the gauge to maximum. Eventually the same reading will be reached, but it will take longer to get there. Is my logic flawed?

 

[holeshot]

Darringer:

The problem is that the gauge (at least my cheap 10$ one) has in the check valve at the top of the hose near gauge itself, not in the part that screws into the head, so the volume of the hose must be considered as part of the head volume. The air in the hose acts as a cushion or spring on the compression stroke (like you've dramatically increased volume at TDC) and then springs back during the downward stroke .

Just picking numbers out of the air, if the volume you are displacing with the piston is 70 cc on 125 two stroke (because it doest compress until it covers the exhaust port) and the volume of the hose and chamber at TDC is 70cc, then you've only have a compression ratio of 1:1

If the check valve were at the bottom of the hose (where it screws into the head), you would get a much more realistic reading, because pressure would be allowed to build in the hose. Maybe some of the more expensive testers are made this way (I'll check).

All of this is pretty moot anyway - soon all of us will be riding auto decompress four stokes. :joke: