Honda XR 400 Automatic Decompressor Removal

So after last summers engine failure on the Honda XR 400 I have been working most of the winter and cause of trouble getting parts and some setbacks it has taken a lot longer then I would have like getting it back together.

So when it came to the cam axle the auto decompressor feels quite loose:

From what I can read most people who remove it find the bike easier to start and it has been a bit cranky getting it started so decided to remove it, there is a manual decompression option also, on mine that cable and lever has been removed but should it be hard to kick the bike I will look at adding that option again.

Since I don’t have a press at home that would have made removing sprocket holder a lot easier a bench wise and a small sledge hammer was used, changed washer on top for a piece of aluminum flat bar since washer kept jumping away, had some help from my dad (long time carpenter that really knows how to wield a hammering tool) needed hard swings to get moving, press would have been a lot better:

Hammering the cam gear holder out

Once it was a little out the spring that is activating the auto decamp is exposed, put aside and kept hammering:

Auto decomp

To get it the last bit of the axle a puller was rigged up, might have been possible to use all the way but was hard to get it rigged in place and aligned:

Pulled the last bit

With all the bits and pieces off the axle:

Decomp removed

There are two holes lubing the auto decomp and backfire safety that needs to be covered so the rest of the cam won’t be starved for oil, some just weld them shut or use epoxy but since that is harder to reverse I went with manufacturing a sleeve that will cover the holes, found a pipe that was near in size and visited a friend with more tools, opening up the inner diameter some on the lathe:

Decomp removed

Was also cut on the lathe and a hole was drilled:

Drilled hole

Angle grinder to open up the slot to the hole and an opposing slot for the auto decamp cam, have to confess that it was a tiny bit to big so had to rig it up in the lathe and shave off a little, by measurement it should be 24,28 mm and ended up very close to that once done:

Decompressor cover sleeve size

Punched out a few plugs out of some rubber and was thinking about adding that but choose not to, the sleeve should hold back the oil enough to ensure proper lubrication:

Ready to assemble

Since there was a press there to I went ahead and pressed the sprocket holder in place:

Pressing sprocket carrier in place

Pressing sprocket carrier in place

All done:

Decompressor removal cover

Decompressor removal cover

With backfire safety removed the stop for that sitting it the cylinder head needs to be removed also, make sure to get the spring out also:

Spring and pin

2 comments to Honda XR 400 Automatic Decompressor Removal

  • frank

    hello nice bit of work on with problem very like, thinking of getting hot cam they seem to of took the decompressor off did you just want to keep the old cam, was it in goood condition did you need to cut a bit out for the plunger or what was the bit you cut out for

    • Johan

      The cam was in good condition and Im not interested in searching for more power out of the little XR, have the KTM 990 Adventure when I need more power.

      Had to cut openings in the sleeve I fabricated cause of the pins pressed into the cam to stop the auto decompressor from spinning.

      Must admit that the poor XR is not very easy to start, need to try and adjust the manual decompressor better something is not right, the option is to reinstall the automatic one.

      And I will probably try to sell it since it gets ridden so little.


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