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Compression testing.

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Hi all.

Starting issues again. I thought I'd got the hang of starting my fairly new to me 2001 XR400. But it seems not:bonk:

I took it for a run the other day, it started well, after half a dozen prime kicks and then 2 or three full kicks. Warmed her up and took it for a nice run round some tight twisty tarmac. Everything was going great until I stopped for a short break, killed the engine using the kill switch. And that was that. :thumbsup:

I kicked and kicked and kicked. I tried bump starting and bump starting and bump starting. I tried kicking and kicking and kicking. I left it for 30 minutes and kicked and kicked and kicked. I took the plug out, thinking maybe I had flooded it. Not an ounce of moisture on the plug. Plug colour looked ok though. I tried bumping it again and it did actually start, and ran for about 30 seconds then conked and never started again. :thumbsup:

It had a spark.

It had fuel.

I rescued the bike with my van after thumbing a lift home.

It is currently jetted 60/162 with no snorkel, standard pipes with VW tip, and K&N. But, when I got the bike it was jetted 55/162 with a power bomb header pipe and FMF tail pipe.

I'm thinking maybe 55/162 was too lean and maybe damaged valves or something. And hence my engine doesn't have enough suck to draw fuel in.

I'm seriously thinking of junking the auto decompressor and just using the manual lever. (My DR350 never had auto decomp and it started just beautifully. Wish I'd still got it I think.)

I would like to do a compression test on my engine. The manual says to use an adapter with the compression tester.

Does anybody know what this adapter is or what it does?

Can I use an ordinary compression tester?


How can I test the compression when the engine has an auto decompressor?

I thankyou in advance for any thoughts or advice, and would appreciate your help.

I have not been able to pluck up the courage to enter the shed and go try starting it again. Or even take tools to it yet.


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The adapter they speak of is just a fitting that screws directly into your spark plug hole and it also has a hose that connects up to the gauge. Since the RFVC head is so deep for the plug, you have to have this.

The XR400 has an auto decompression mechanism built into the cam. I don't think I have heard of these going bad. It only kicks in when the crank/cam reverse directions.

The manual decompression level may be suspect. A lot of folk have found that the lever is out of adjustment, causing the valve to remain open. Make sure it is loose, and disconnect if in doubt.

If you are certain you are getting spark, then there is not a short in the kill switch (happens often).

I would also try turning up the idle screw a few turns (turn to the right = more fuel/air).

Make sure choke and fuel are on. If you have a hill, try rolling down it and opening the throttle till it starts.

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My friend bought a really nice CR250R for next to nothing.

The kill button was stuck. $10 later, the bike doubled in value.

Hint. Start there.

On XR's the manual compression lever will move around and engage the manual decompression. Make sure the lever on the engine is not being pulled forward. Make sure there is a little slack in the cable.

I bought a cheap compression tester. It came with an adapter for smaller spark plug holes.

Compression testing.

Screw it in.

Hold the throttle wide open. (or it will read low)

Kick until the guage stops going up. (6 to 8 kicks)

My TL125 was testing at 60 psi. (dead)

I put in a cap of motor oil.

Re-tested the compression.

It went up to 125 (I think 130 is pretty borderline)

This showed the rings were shot.

125 psi? Why not try! It acutally ran until the oil burned off.

It's in pieces now awaiting a full rebuild.

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Thanks guys,

I'm pretty sure my manual lever is okay, but I will double check.

My kill switch is perfect, so I know it's not that.

I thought the auto decompression worked forwards aswell as reverse upto about 800 rpm. I didn't realise it was just reverse.

That changes my thought process some what.

Thanks again.

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