Decompressor questions (400)

Guys, I was going to search, but it's down for maintenance...

I was wondering if you XR400 guys have the same automatic decompressor mechanism in the head/on the cam... like I have on my 400EX?

If so, tell me something...

Why would this thing get all mad and start clacking/tapping on me after every routine oil change?

I don't really know a thing about it, how it works, nothing...

But it's mechanical right? A mechanical thing pressed on the cam shaft, and some sort of spring and plunger? So it's not hydraulic or anything? What's the connection?

It sounds bad... scary bad... and clacks/taps at random when at idle. It starts up after an oil change and lingers a while at random... long after the oil has already recirculated everywhere. Then it will go away. But sometimes for a bit it seems to surface on cold starts for a little while after an oil change. Then I don't hear it at all for a while until the next oil change.

I've tried different brand oils, different viscosities, and I've even tried draining but not changing the filter. It's been doing it for a year.

I don't know... it has me dumbfounded...

That sounds like what XR4 has started doing. I thought it was my cam chain so I bought a new one and got started on changeing it. Only to find that still have another half inch or so of travel left in my tensioner. So I don't think it was the cam chain making the noise.(gonna change it anyways seeing as I'm halfway there)

The auto decomp makes the clacking/tapping noise when the idle speed is too low! (possibly fresh oil) Only had mine do it when idle speed is too low!

If you have a manual decomp lever, remove the auto decomp.

It can be pressed off the cam by any competent mechanic/person that knows what they're doing.

I'm having mine removed at the moment while the motor is being rebuilt.

Mine has done it too.

I have seen two Decomp,come apart on high miles Xr,s .If I do a rebuild,I remove and weld up the open oil holes,throw away the spring,and dowel.Also makes for easy starting.

I have seen two Decomp,come apart on high miles Xr,s .If I do a rebuild,I remove and weld up the open oil holes,throw away the spring,and dowel.Also makes for easy starting.

When you say you remove it, are you refering to the manual decomp. or the automatic decomp. or both?

I thought that they were there to help make starting easier, and by removing them it will make things less complicated and more reliable, but might make starting a littler harder, if anything.

So if I understand you right, you removed the decomp. (either manual or automatic) and it made starting easier for you?

I've been thinking about removing mine.... I like things simple.... more reliable.... but hesitate that it might make it a little harder to start.

I remove the auto decomp.Keep the manual,makes for easy start,even after the bike is on top of you.

That sounds like what XR4 has started doing. I thought it was my cam chain so I bought a new one and got started on changeing it. Only to find that still have another half inch or so of travel left in my tensioner. So I don't think it was the cam chain making the noise.(gonna change it anyways seeing as I'm halfway there)

Let me know... I know it's gotta be one or the other...

I don't think it's the chain anymore like I did at one time.

I'm in about the same place... chain stretched/worn... but still within the service limits...

The auto decomp makes the clacking/tapping noise when the idle speed is too low! (possibly fresh oil) Only had mine do it when idle speed is too low!

If you have a manual decomp lever, remove the auto decomp.

It can be pressed off the cam by any competent mechanic/person that knows what they're doing.

I'm having mine removed at the moment while the motor is being rebuilt.

One guy I had look at my EX raised my cold idle a hair and it silenced.

But it is still coming back each oil change... strange...

The EX does not have a manual decompressor...and neither do the aftermarket cams... you just take out the spring and plunger with them.

I have seen two Decomp,come apart on high miles Xr,s .If I do a rebuild,I remove and weld up the open oil holes,throw away the spring,and dowel.Also makes for easy starting.

Like stated above, the aftermarket cams don't have them...

Do you XR guys have to kick them over?

If not, you may want to try an aftermarket cam w/rebuild.

You said open oil holes... could you elaborate a little more there?

Would that be the connection between the oil drains and the noise?

Grasping for straws here...

When you remove the decomp(if I remember you need a press) you will see 1 or 2 oil holes,after you remove it.We weld up the holes ,since the decomp does not need oil anymore because you removed it.This info is for xr2&xr4 .Hope this helps you.When they ware out you hear a rattle sound

Ahh...

So it gets oil to it somehow through the cam?...

So maybe it clacks a while after an oil change because maybe it's not getting enough oil?

Or you just think it is worn out?

If it keeps clacking even after it is warm it is worn out.If your not getting oil the cam woud go first and rockers

Here is a video I shot of it last year...

You can hear it come and go...

And I appreciate you guys... a lot.

It really helps be get a better prospective by talking to you dirt bike guys.

The same has been true for both the Honda and the Kawasaki.

I'm glad both share the same basic engine with the dirt bikes.

If it keeps clacking even after it is warm it is worn out.If your not getting oil the cam woud go first and rockers

It shuts up after it warms up after the oil change.

Sometimes I hear it after that on a cold start, but it has been rare.

Definitely connected to the oil change somehow... or maybe the oil drained back off things?

Have you adjusted your valves? It sounds like one is not seating properly causing the rocker arm to slap. I had the same sound on my XR and it came and went, then finally stayed. The one valve where the decompression lever is was not seating all the way and was very obvious when trying to adjust it. Did a valve job and all is good.

That was the first thing I checked a year ago when I first heard the noise.

They were all dead in spec.

Might be worth re-visiting though...

Ticking is very common on XRs - most people initially suspect the rocker arm adjustments, auto decomp cam and then the cam chain (in that order). They supect these things in that order because the respective remedy for each becomes sucessively harder and more expensive to implement.

My guess however is that your cam chain tensioner is shot. For very low $$ (and very little effort) you can put in a new one and I bet the ticking will go away; I have done it on many XRs so far and on each occasion the ticking noise went away or was significantly reduced. When installing it be sure to first wind it out fully and then wind it in SLOWLY once the bolts are tightened. The manual gives the dimensions of a key that should be used for this purpose; I ground one into shape using a thick feeler guage blade.

Every one is correct - the auto decomp does tick when it becomes old (but not very loudly though) and it CAN fail. The bike is better without it.

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