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Discussion Starter · #1 · (Edited)
Compression testing is a good way to test the internal condition of your engine. It's easy and it only takes a few minutes. Here is a quick guide on how to do a compression test. You will need a spark plug socket and ratchet or a spark plug tool to remove your spark plug and you will need a compression tester gauge. You can get a compression tester at your local auto parts store or you can order them off the internet. Ebay would be a good choice. Wherever you buy one, just make sure it's not only for automobiles, but ATVs as well. Make sure it has the proper threaded adapter to fit your spark plug hole. Also, I would stay away from the ones that you just hold against the spark plug hole. Get one that will thread into the spark plug hole. You can find them for $30-$40, probably a little cheaper, but don't go too cheap because you usually get what you pay for. Here is a picture of a compression tester gauge.




BEFORE BEGINNING

Before you begin, make sure you have a good strong battery and make sure it's charged up. If you have a bad or weak battery, you may want to replace it because if it doesn't crank good and strong, you are likely to get a low, inaccurate reading. Make sure your engine is warmed up to operating temperature(if possible). About 10 minutes of riding should do.

First, take out the spark plug and thread in the adapter for the compression tester. Make sure you have the correct size adapter for your particular ATV. Slide your kill switch to the "off" position. Some ATVs won't crank over with the kill switch in the "off" position, so if yours is like this, then you will need to either unhook your ignition coil or ground the end of the spark plug wire to a good ground. You can use a jumper wire with alligator clips on each end to ground it. Next, make sure the throttle is in the wide open position. You can either hold the throttle lever with your thumb or you may be able to tape it or use a zip tie to fasten it to your handlebars to hold it in the wide open position. If you don't have the throttle in the wide open position, you will probably get too low of a reading. Also, if you are testing a newly rebuilt engine, the engine needs to have been run for, at least, 30 or 40 minutes or you will probably get too low of a reading.

NOTE: Before you begin with the actual test, make sure the threaded adapter is screwed in good and isn't leaking any air out around it.

ACTUAL TESTING

With the throttle in the wide open position, push the start button and crank the engine over until the hand on the gauge stops moving. Each time the engine turns over the hand should raise a little more until it reaches the maximum compression of the engine. When it stops, that is your compression reading. This usually takes no more than 10 seconds. Try to avoid cranking an engine for more than 10 seconds at a time as this is hard on the starter and the battery. Now, push the relief valve on your compression gauge and that will reset the hand back to zero. It's a good ideal to repeat the test a couple or three times to make sure you get an accurate reading. On kick start models, it will be the same procedure, but obviously you will be kicking it over instead of using a start button. Worn piston rings and cylinder walls will increase the number of strokes it takes to reach the maximum reading. If you're kicking, it could possibly take as many as 10-20 kicks to get the highest reading.

THE READING

You will need to check your repair manual for your particular model for the correct compression specifications. See note below. Usually, an engine will run OK if it has at least 100 PSI of compression. Most engines will have somewhere between 100-250 and some as high as 300 PSI, depending on the engine. Sometimes they will run with under 100 PSI, but usually not very well. If you get a low reading, you can do a "wet test" to try to help determine the problem.

If your reading is too high, then you probably have carbon built up on your piston and combustion chamber.

NOTE: You may get a low reading on some engines because some engines have a decompressor assembly built into the camshaft. Check the service manual for your quad to see whether or not your quad has a decompressor assembly built into the cam.

WET TEST

If you got a low reading, pour about 1-2 teaspoons of clean motor oil down into the cylinder through the spark plug hole and do the compression test again. If your reading increases, then your rings or cylinder walls are probably worn. If your reading doesn't increase, then it's probably your valves. You could have a bent valve, you may have leaky valve seats, or your valve clearance may not be adjusted properly. Also, low compression can be caused by a blown head gasket.

CAUSES OF LOW COMPRESSION

*Worn piston rings or worn or damaged cylinder walls
*Leaking valves
*Valve clearance not properly set
*Blown head gasket

CAUSE OF HIGH COMPRESSION (stock engines)

*Carbon buildup in combustion chamber and on piston

NOTE: Compression testing is a good way to keep track or "gauge" the wear in your engine. When you first get your ATV or when you rebuild the engine in your ATV, you can do a compression test and then later on, you can do them periodically. This will help you determine the wear in your engine each time you do a compression test and will guide you in knowing when your engine needs rebuilding.

This is about all I can think of. I hope I didn't leave anything out and I hope this helps everyone with their compression tests.
 

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Thank You bro for the awesome write-up I know this will be a good starting platform for people to go by. I will add though if your quad is a kicker the compression gauges that have the rubber end you just stick in the hole are going to be a pain in the rear to hold in and kick..lol Thank you again Helmut. I stickied this thread for people to come back to as a refrence.
 

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1 thing I read in there that won't work on every machine is having the throttle wide open. I can't remember exactly what happens but on the TRX450ER you CANNOT push the throttle and the start button at the same time. It will cause damage. It states it on the throttle and in the manual.
 

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Discussion Starter · #4 ·
Thank You bro for the awesome write-up I know this will be a good starting platform for people to go by. I will add though if your quad is a kicker the compression gauges that have the rubber end you just stick in the hole are going to be a pain in the rear to hold in and kick..lol Thank you again Helmut. I stickied this thread for people to come back to as a refrence.
You're welcome. Yeah, I said in the first paragraph, to stay away from the testers that you hold against the spark plug hole. They're more trouble than they're worth in my opinion.
 

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DUH!!! LMAO I must be really tired.... worked all day came home dug up me septic tank yesterday had it pumped out today and covered it back up, and went fishing...lol
 

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Discussion Starter · #6 · (Edited)
1 thing I read in there that won't work on every machine is having the throttle wide open. I can't remember exactly what happens but on the TRX450ER you CANNOT push the throttle and the start button at the same time. It will cause damage. It states it on the throttle and in the manual.
All I can find in my 450R/ER manual is that after 2005, they have a "closed throttle position detection system". This is what it says:

"The closed throttle position detection system is an
ignition cut-off system that will prevent the engine
from running at any speed beyond idle while the
throttle lever is fully closed.
Both the throttle lever and carburetor throttle drum
have a position switch to detect their fully closed
position.
If the throttle drum "sticks" in an open position (carburetor
switch OFF) and the throttle lever is fully
closed (throttle lever switch OFF) the ICM will cut off
the ignition to prevent engine damage or an accident."

It doesn't say anything about the throttle being in the wide open position.

According to my manual, you're still supposed to hold the throttle to the wide open position. This is what the manual says:

"Shift the transmission in neutral.
Open the throttle all the way and operate the
kickstarter forcefully several times until the gauge
reading stops rising."

It doesn't say anything about the start button, but this is a 450R/450ER manual, but maybe the ER is different and they forgot to mention it in the manual.

It also says that the compression will be comparatively lower on these models, because the camshaft has a decompression device installed.
 

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1 thing I read in there that won't work on every machine is having the throttle wide open. I can't remember exactly what happens but on the TRX450ER you CANNOT push the throttle and the start button at the same time. It will cause damage. It states it on the throttle and in the manual.
All I can find in my 450R/ER manual is that after 2005, they have a "closed throttle position detection system". This is what it says:

"The closed throttle position detection system is an
ignition cut-off system that will prevent the engine
from running at any speed beyond idle while the
throttle lever is fully closed.
Both the throttle lever and carburetor throttle drum
have a position switch to detect their fully closed
position.
If the throttle drum "sticks" in an open position (carburetor
switch OFF) and the throttle lever is fully
closed (throttle lever switch OFF) the ICM will cut off
the ignition to prevent engine damage or an accident."

It doesn't say anything about the throttle being in the wide open position.

According to my manual, you're still supposed to hold the throttle to the wide open position. This is what the manual says:

"Shift the transmission in neutral.
Open the throttle all the way and operate the
kickstarter forcefully several times until the gauge
reading stops rising."

It doesn't say anything about the start button, but this is a 450R/450ER manual.

It also says that the compression will be comparatively lower on these models, because the camshaft has a decompression device installed.
This is an ER(electric start) only issue. 69quadracer knows what it will do, I can't remember. It causes something to kick back and cause some damage. Like I said, I don't remember. Hopefully 69 will chime in.

I know on mine if I hit the throttle while hitting the start button it will make a clucking noise.
 

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Discussion Starter · #8 ·
1 thing I read in there that won't work on every machine is having the throttle wide open. I can't remember exactly what happens but on the TRX450ER you CANNOT push the throttle and the start button at the same time. It will cause damage. It states it on the throttle and in the manual.
All I can find in my 450R/ER manual is that after 2005, they have a "closed throttle position detection system". This is what it says:

"The closed throttle position detection system is an
ignition cut-off system that will prevent the engine
from running at any speed beyond idle while the
throttle lever is fully closed.
Both the throttle lever and carburetor throttle drum
have a position switch to detect their fully closed
position.
If the throttle drum "sticks" in an open position (carburetor
switch OFF) and the throttle lever is fully
closed (throttle lever switch OFF) the ICM will cut off
the ignition to prevent engine damage or an accident."

It doesn't say anything about the throttle being in the wide open position.

According to my manual, you're still supposed to hold the throttle to the wide open position. This is what the manual says:

"Shift the transmission in neutral.
Open the throttle all the way and operate the
kickstarter forcefully several times until the gauge
reading stops rising."

It doesn't say anything about the start button, but this is a 450R/450ER manual.

It also says that the compression will be comparatively lower on these models, because the camshaft has a decompression device installed.
This is an ER(electric start) only issue. 69quadracer knows what it will do, I can't remember. It causes something to kick back and cause some damage. Like I said, I don't remember. Hopefully 69 will chime in.
Yeah, I was just going to say that it must just be the ER models. It looks like they would mention that in the manual with the compression testing procedure, but it's not the first time I've seen repair manuals leave out important stuff.
 

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Discussion Starter · #9 · (Edited)
I wonder if it would do it with the ignition coil disabled, though, because if it doesn't try to start, it shouldn't try to kick back, should it?

Those 450R and ERs are a little more complicated than most ATVs.
 

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Yeah, it is funny how many people say that the ER's don't start for crap. I bet you most of those people are giving it gas when try to start. It even says on the throttle "leave closed during start up"
 

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EPIC guide. I will be printing this one out and sticking it in with my 400ex manual and Lectron guides for my carb. A monthly or bi-weekly Compression test is a great idea to knowing what the hell is going on inside the motor.
 

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on the 450er model if you open the throttle while starting it it can kick back and destroy the one-way starter clutch, i've had it happen on me a couple of times:icon_ nono:
 

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Discussion Starter · #13 ·
on the 450er model if you open the throttle while starting it it can kick back and destroy the one-way starter clutch, i've had it happen on me a couple of times:icon_ nono:
When doing a compression test, you won't be starting the ATV, just turning the engine over.
 

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my 250 doesnt start wide open either. maybe its like that for all of them. also if you rev your engine right after starting theres no oil pressure yet and can score things ie crankshaft. thats a good post for compression testing!!
 

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hey helmut, i was wondering what you said about the decompression assembly on the camshaft. my 01 400 ex has that decompression assembly on the stock cam along with that little bity decompression plunger and spring and if you lose that little bity pin, the quad wont even start. apparently that little plunger reacts with the decompression mechanism on the stock cam and slightly opens the intake valve for easier starting. I guess thats one of the reasons why people install hotcams to avoid having to use that little plunger. i was wondering since i do have the stock cam with decompressor, what is a good compression psi?? thanks again!
 

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Discussion Starter · #16 ·

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thanks and me being new to the site, what do i click on to check and see if my messages have been replied to. im jumping all around this forum reading everyones topics and trying to give my 2 cents as well. i absolutely love this forum. best forum ever! plus you guys are really insightful and knowledgeable and really really helpful. thanks alot!
 

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Discussion Starter · #18 ·
thanks and me being new to the site, what do i click on to check and see if my messages have been replied to. im jumping all around this forum reading everyones topics and trying to give my 2 cents as well. i absolutely love this forum. best forum ever! plus you guys are really insightful and knowledgeable and really really helpful. thanks alot!
Just click on "My Account" and then "Edit Options" and you can subscribe to the threads under "Messaging and Notification".
 

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hello there guys i have a 02 honda rancher 350 whihch i have done a full rebuild on the motor i had it runnin and it run fine on the choke but wont idle good took plug out and its grey and also i have done a compression test and all i am gettin is 60 psi is this do to the decompression plus when i give the bike gas its like it only runnin on the idle jet is this possible also
 

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hello there guys i have a 02 honda rancher 350 whihch i have done a full rebuild on the motor i had it runnin and it run fine on the choke but wont idle good took plug out and its grey and also i have done a compression test and all i am gettin is 60 psi is this do to the decompression plus when i give the bike gas its like it only runnin on the idle jet is this possible also
60 p.s.i. is low in my book. did you check your valves when you had it apart..and see if they were bent ??.
 
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