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XR 400 Compression

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I cannot seem to get a straight answer on what the compression should be on this bike, or how to do a compression test because of the auto decompression. Does anyone know

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I have the same issue and no manual, any chance posting the compression test proceedures...I know how to check the compression, just not sure how the auto decomp fits into the proceedure.

Thanks

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But the problem as I see it, is the auto decompression is connected to the kick starter. So once you kick it, it is going to decompress and not give you an accurate reading. It should be substantionally less

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Before I was going to do a compression check, I would check/set valve clearances and the propper amount of free play at the decomp lever on the handlebars. Those 2 procedures go together. Only then would I trust anything a compression test said.

I believe I remember reading that the decomp mechanism on the cam doesn't activate below a certain rpm, hence the 450 rpm spec. You just have to make sure the mechanism isn't being activated manually because of lever freeplay or valves being out of spec.

If you do test compression first and the reading is low, first thing to check would be valve clearances and lever freeplay. Then if still low, pouring a little oil into the cylinder thru the spark plug hole, test again. If pressure goes up, you may have some bad piston rings.

If tests high, that suggests carbon build up in the combustion chamber or piston crown, changing the squish band.

To do the test the engine should be at normal operating temp. Connect the gauge, with choke off and holding wide open throttle, kick the engine over several times with gusto, watching the gauge.

But the problem as I see it, is the auto decompression is connected to the kick starter. So once you kick it, it is going to decompress and not give you an accurate reading. It should be substantionally less

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Before I was going to do a compression check, I would check/set valve clearances and the propper amount of free play at the decomp lever on the handlebars. Those 2 procedures go together. Only then would I trust anything a compression test said.

I believe I remember reading that the decomp mechanism on the cam doesn't activate below a certain rpm, hence the 450 rpm spec. You just have to make sure the mechanism isn't being activated manually because of lever freeplay or valves being out of spec.

If you do test compression first and the reading is low, first thing to check would be valve clearances and lever freeplay. Then if still low, pouring a little oil into the cylinder thru the spark plug hole, test again. If pressure goes up, you may have some bad piston rings.

If tests high, that suggests carbon build up in the combustion chamber or piston crown, changing the squish band.

To do the test the engine should be at normal operating temp. Connect the gauge, with choke off and holding wide open throttle, kick the engine over several times with gusto, watching the gauge.

What do you think of this...I have the valve cover off, the valves are not opening during the kick cycle and still it kicks easier then an 80cc.

Let me first confirm that the plunger and spring under the cam is some sort of auto decompression, if not then it's either the valves or the rings.

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What do you think of this...I have the valve cover off, the valves are not opening during the kick cycle and still it kicks easier then an 80cc.

Let me first confirm that the plunger and spring under the cam is some sort of auto decompression, if not then it's either the valves or the rings.

ur cam chain came off

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I don't think your XR400 has an auto decompressor, my stock '00 just has a little lever and cable on the handlebar that goes to the head. I looked in my shop manual and saw nothing that connects the kickstarter to the camshaft rockers. I did a compression test on mine awhile ago. With the choke off and the throttle wide open, give it a few good kicks. MM

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Maybe I'm misunderstanding, But the valves are not going to open with the valve cover off. The rotation of the cam transmits movement to the rocker arms in the head cover and those actuate the valves. I take it you mean that with the cover off and the valves not being actuated by the cam when the engine is kicked over, they should be seated, not allowing pressure to escape from the cylinder. Got cha.

The auto decomp mechanism is on the camshaft itself, cam sprocket side. It's a series of 2 small cams and a one way clutch.

I can't think of anything "below" the cam that you're talking about as a decomp mechanism. The only thing that comes to mind that you could be talking about is the auto cam chain tensioner. That has nothing to do with decomp, totally different system.

The manual decomp mechanism is that arm and spring in the right side of the valve cover.

I agree, with the cover off, if compression is too low, it's either rings, valve seating, valve seals/guides, cylinder head gasket. I'd try alternate putting a few drops of oil around the valves and retest compression, capfull down the spark plug hole, retest and so on until you find the culprits.

What do you think of this...I have the valve cover off, the valves are not opening during the kick cycle and still it kicks easier then an 80cc.

Let me first confirm that the plunger and spring under the cam is some sort of auto decompression, if not then it's either the valves or the rings.

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Maybe I'm misunderstanding, But the valves are not going to open with the valve cover off. The rotation of the cam transmits movement to the rocker arms in the head cover and those actuate the valves. I take it you mean that with the cover off and the valves not being actuated by the cam when the engine is kicked over, they should be seated, not allowing pressure to escape from the cylinder. Got cha.

The auto decomp mechanism is on the camshaft itself, cam sprocket side. It's a series of 2 small cams and a one way clutch.

I can't think of anything "below" the cam that you're talking about as a decomp mechanism. The only thing that comes to mind that you could be talking about is the auto cam chain tensioner. That has nothing to do with decomp, totally different system.

The manual decomp mechanism is that arm and spring in the right side of the valve cover.

I agree, with the cover off, if compression is too low, it's either rings, valve seating, valve seals/guides, cylinder head gasket. I'd try alternate putting a few drops of oil around the valves and retest compression, capfull down the spark plug hole, retest and so on until you find the culprits.

Thats what I thought, valves or rings, at least that's what I am trying to determine...I just want to make sure that the low compression was not due to any type of auto decompression.

I was under the impression that the plunger and spring

(a little pin and spring that sits below the cam right under the auto decomp thing on the cam) "it may not have anything to due with auto decompression...that's what I am trying to determine...it's function.

I went to Bike bandit and checked the schematic and found the plunger and spring under 03 xr 400 cylinder head...part number 4 and 5 which lists them as "plunger, cam setting" and "spring plunger" I know the lobe on the cam auto decompression pushes the plunger in and out as the cam rotates...but I have no clue what it's true function is and if it's connected to some type of auto decompression.

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I see what parts you're talking about. The hole that the spring and plunger go into fall outside the diameter of the cylinder. You can see that by looking at the underside of the head where the valve seats are. I can see no passage related to the spring and plunger.

The spring loaded plunger must act as a guide for the decomp cam. The service manual is vague on it's function.

I know the lobe on the cam auto decompression pushes the plunger in and out as the cam rotates...but I have no clue what it's true function is and if it's connected to some type of auto decompression.

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I see what parts you're talking about. The hole that the spring and plunger go into fall outside the diameter of the cylinder. You can see that by looking at the underside of the head where the valve seats are. I can see no passage related to the spring and plunger.

The spring loaded plunger must act as a guide for the decomp cam. The service manual is vague on it's function.

I agree, and I'll confirm that tomorrow, I was just thinking it might be linked to one of the valve ports.

Then for sure it's either the rings or valves.

Thanks for the help and info !!

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Just to conclude what I found.

The spring and plunger is not involved in bleeding off compression.

I was able to produce 100psi on the compression tester, low but enough to fire it up.

I narrowed the problem down to the carb not flowing gas, the floats were put in wrong...after that simple fix she fired right up.

The low compression issue I'll leave up to the owner if he wants to pull the top end apart.

Thanks for the help!

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