Proper Steps for Compression Testing?

[DeePa]

I bought a compression guage at the local pep boys, and I let the engine warm up on my 85 200s for 20 mins or so. I then proceeded to pull the plug and screw the hose/guage into the plug hole. This took about 10 mins or so seeing as it was my first time trying to get it hooked up.

I put the bike in neutral, gave it full throttle, then pull the pull stater 5 times.

It came up with 90 lbs of compression, and in the manual it says it should be around 140-150ish.

Did i do this right?

Do i have to unhook the decompression cable? Where should the choke be? Do I have to hold it full throttle?

I ran the test again 5 mins later and it was down to 60 lbs of compression.

Could it be because it was about 5 degrees F out that the engine had cooled before i had a good chance to test it, and then further cooled to get the 60 lbs reading?

The engine runs fantastic, although she likes to backfire once in a while if i rev her up in neutral.

Thanks,
Mike DiPasquale

[bigredhead]

process sounds good.

make sure the choke is not on.. ( it would restrict air ) hold the throttle on full while you crank i over.

Might want to remove the bolt that holds the gas tank so that once you shut it down it's quicker and easier to move the tank over a bit to twist the tester into place.

About the compressions release: if you bike has it ( not too many do ) it should be unhooked.. that would defeat the purpose.

Ever adjust the valves ?

[hondaATCman]

The compression should be read when the engine is COLD. I never had an automatic compression release machine, but I would think you have to unhook it to run a test. Hold the throttle wide open and crank it until the needle on the compression gauge stops moving.

[bigredhead]

The instructions on my tester recommend getting the engine to operating temp.

you sure about that one ?

[slippy4]

you would want to test it when it is warm, because that is where you are running the engine most the time and that is when you need the compression. your engine should have more compression when it is cold then warm. always test with the choke off and wot, warm. should be between 90-130 or so. 90 on the low end.

[DeePa]

ok, so should i unhook the decompression cable or not? I think it would probably help.

when i am home either tomorrow or friday ill run it again and see what happens.

No i have never adjusted the valves and have no clue how to do so.

Thanks,
Mike DiPasquale

[HaggLE]

I think the name of it suggests that you do. Its a DECOMPRESSION cable. It removes compression to make it easy to start. So yes disconnect it.

http://www.3wheeler.org/dirtwheels/6-87~tuneupindex.html
This will tell you how to adjust your valve clearances.

[thBriGuy]

you would want to test it when it is warm, because that is where you are running the engine most the time and that is when you need the compression. your engine should have more compression when it is cold then warm. always test with the choke off and wot, warm. should be between 90-130 or so. 90 on the low end.

So... I take it I need to get the motor running before I can check the compression??? Or can I get a decent enough test just cranking it over cold, since it doesn't run...yet.

[MTS]

So... I take it I need to get the motor running before I can check the compression??? Or can I get a decent enough test just cranking it over cold, since it doesn't run...yet.
you can get a decent reading cold, if it is above 100 psi its good to go, and lower and its recomended you rebuild it ;)

[hrc85250r]

i checked my 250R after riding it for a couple hours(letting it sit for a while to cool down of course) and i got 190 psi, and i forgot to open the throttle all the way...will i get more psi with the throttle open? is 190 good?.....thanks

[md1985250r]

Compression tests should be done with the engine at operating temperature.

hrc...a stock 85 250r should read between 170 and 199 psi.


GREG

[hrc85250r]

i cant remember where, but i read somwhere 150psi stock for a 250r....???

[mbg75]

readings will vary depending on altitude. the higher up, the lower the psi reading.

you can also get an inaccurate reading if your using a long hose between the gauge and head on a small cc motor. it adds to the combustion camber size, reducing comp. ratio.

one trick you can use if you are getting low #'s,
is to perform a compression test and note the reading. then perform the test again, but this time pour a capful of motor oil into the cylinder before testing. if the reading goes up significantly then the rings are worn.

[md1985250r]

i cant remember where, but i read somwhere 150psi stock for a 250r....???

Not to sound rude but I know where I just read it ,in my Honda manual. 170.7-199.1 psi chapter2 section 9



nbg said,
"you can also get an inaccurate reading if your using a long hose between the gauge and head on a small cc motor. it adds to the combustion camber size, reducing comp. ratio."

Sorry but I have to disagree. The hose should have a check valve on the end, therefore the length of the hose is NOT a factor.

[mbg75]

the check valve would have nothing to do with that, because as the pressure pushes into the hose the valve opens. once it is open the volume would be the same as if there was no check valve at all.

[bigredhead]

but after the 1st compression storke, the hose's volume is part of the 2nd compression stroke. and you have to go thru a 1/2 dozen to get a total reading. Thus the hose lengh would have no effect, as long as it's ridgid and not inflating.

But then again, it's late, and i'm very tired.. so i might be totally off on that one ! LOL..

[mbg75]

but after the 1st compression storke, the hose's volume is part of the 2nd compression stroke. and you have to go thru a 1/2 dozen to get a total reading. Thus the hose lengh would have no effect, as long as it's ridgid and not inflating.

But then again, it's late, and i'm very tired.. so i might be totally off on that one ! LOL..
exactly, thats why the reading increases as you keep turning the motor over, until it finally peaks.

[kilabeez0]

what is the point of holding the throttle open?

[bigredhead]

what is the point of holding the throttle open?

Allowing as much air into the cylinder as possible, the more air you compress into the same space, the higher amount of pressure you create.

[kilabeez0]

Allowing as much air into the cylinder as possible, the more air you compress into the same space, the higher amount of pressure you create.


Throttle pulls the needle in the carb up to allow fuel. If the test is for air, why dump fuel into the engine ? If the fuel isn't going to get burnt or combusted what is the point?

[hondaATCman]

Why would you want to check the compression once everything has heated up and expanded? Are you telling me that I should just leave my Suzuki LT 250r Quadracer alone because when its cold it won't start unless it is pulled by something and has a compression reading so low that the numbers are only in the double digits, but when it gets warmed up and expands it will kick start fine and has an almost normal compression reading? The engine has to be cold if you want accurate results.

Also - Kilabeez - you said that holding the throttle open doesn't affect the amount of air going in the cylinder. How did you come up with that?? When you open the throttle the slide moves up in the carb and lets more air go through into the combustion chamber. Think of the slide as some kind of restrictor plate. When in the closed position (idle) almost all air is restricted from entering. When the throttle is wide open (WOT) there is pretty much no air restriction since the slide is all the way up.

If you want further information about compression testing here is a professional article from Duncan Racing:

Compression testing your ATV engine is an important and easy way to acquire and keep track of engine data needed to make jetting adjustments, gauge engine wear and adjust engine for altitude changes.

Following are some helpful tips to get consistent and accurate measurements.

Name Brand: Always note the tester brand and condition. DRI recommends using a Snap-On Compression tester. Try to always use the same tester.

Tester Probe: Many compression testers have different probe designs. It is very important in achieving a correct reading that the threaded end of the probe is the same length as the spark plug. A probe too long will give a reading to high and a short probe will give a low reading.
It is also important to note the location of the one-way valve in the probe. It is best if the one-way valve is at the end of the probe.

Consistency: The most important issue is consistency. To be able to obtain good useable data it is important to achieve constancy in your test method.

Engine Temperature: Engine must be cold to get proper results. Room temperature is best. Engine needs not to have been fired for at least 45 minutes.

Throttle Position: Throttle must be held wide open to get correct results. If test is performed with throttle valve closed a potential low reading will show on gauge.

Number of Kicks: If the machine being tested is of a kick starter design it generally takes 15-20 kicks to get the engines maximum compression reading.
If the machine uses an electric starter, machine should be turned over for at least 10 seconds or longer until needle on compression gauge stops moving.

Number of Tests: DR recommends doing at least 3 tests, 5-10 minutes apart on each cylinder. Multiple tests will help insure that correct test results are achieved.

Helpful Hints: For best results, it is best to test a fresh engine after it has been run at least 30 minutes.
Testing a fresh motor before it has been run can and will usually lead to a low reading.

After engine break-in is complete it is advisable to do a compression test on your engine and record it for future comparison. Try to always use the same tester, and if possible have the same person perform the test.

[a-camp]

My gauge dose not work with the hose on it . I have to use the solid nozzle. Take off the tank. My gauge is screwed with the hose im not saying they all are but mine is to the lb with the press on and hold on shaft ( against other gauges ) and 60lbs or so off on the hose so dont trust yours till you prove it . I always tested cold for numbers . Is it good to lose that much comp. when it gets warm ?? I would not think so ,a few pounds .