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Old 5 Jul 2003
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Join Date: May 2001
Location: Manchester, UK
Posts: 381
XR400 top end rattle

It seems my purchase of XR400 for the Trans Am Trail could have been a better one!!

I have now fixed my clutch and crossed the rockies, but am hanging around Moab , Utah because of a really unhealthy sounding top end rattle. Before I dismantle the bike I would like to know about any common problems or ideas.

The rattle happens predominantly at lower revs and seems to be a result of the front right valve, suggesting to me either a broken valve spring or problems with the decompression system. I have pretty much ruled out the valve spring issue and am fairly sure the cam chain isnt to blame.

The problem comes because i dont know muck about the decompression system fitted to kick start bikes.

The noise is a loud rattle from the top end of the engine. It makes the noise individually when I turn the engine over on the kickstart without the engine running as well as when its running normally.

When I remove the front right valve cover and turn over the engine I see a very odd cycle of events. The valve pushes and releases twice as you would expect (smoothly) but then a minor compression occurs hardly enough to open a valve and then as this releases the valve seems to hold down for a few fractions of a second before snapping back to the 'off cam' position. It seems to be this snapping back that is making my rattle. It is generally there, but every now and again on a slow tickover it sort of fades in and out in an intermittant manner.

I know diagnosing engine sounds over the net is a nightmare but do any of you have any ideas?

I really need to get this fixed and do some riding in some of the most spectacular desert/canyon scenery I have ever seen!!!!!

Many Thanks ( A tortured)

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Old 5 Jul 2003
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Join Date: Jun 2003
Location: Eureka, CA USA
Posts: 326

Sorry to hear about you luck. First of all I am not an expert, but it does not sound good to me. Since it happens when the engine turnes over I would do two things. First get someone that really knows his business to look at it. Second since you are on a long trip through the desert I would pull the engine apart and have a good look. Getting stuck in that desert in July can kill you real fast. I know this isn't good news, but it would be prudent. Also carry extra water with you when you head out. I can and have put things together in shall we say ingenious ways, but a rattling engine in the desert is not something I would play with. I have been in the outdoors all my life and most people don't realize how quickly you can get into trouble. Sorry I wasn't much help.


"The older I get the better I was"
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Old 6 Jul 2003
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Join Date: Jun 2003
Location: Motor Racing Capitol of the World
Posts: 51
It sounds very well like it could be the decompression device. It works by propping the exhuast valve open at very low speeds(kicking speeds) this helps bleed off compression. On that bike it uses a centrifugal device. It needs to be checked for proper action.

BUt, thats a guess. I would do as already suggested. Find an expert. MAny problems found early will cost MUCH less. The top end on that bike is easy to inpsect with out major dissassembly.

You need to do this regulary.
1.) CHange oil and filter a bunch.
2.) Listen and check valves clearances a lot.
3.) clean air filter A lot.
4.) lube chain a lot.

This bike is very reliable. As long as its not over revved/over heated with wrong gearing or somehting. OR the oil isn't changed frequently there shouldn't be many problems.

NOw you need to check.
1.) valve clearnace.
2.) Auto decompression operation.
3.) cam lobes for wear.
4.) cam followers for wear.
5.) condition of cam chain/tensioner.

This bike is very expensive to repair if you have cam journal failure(but shouldn't have to with maintenance). The cam doesn't use a sleeve style bearing. The head itself is machined to carry the cam. Mess up the cam journals mess up the 500 dollar head and 100 cam cover.

IF you need parts over night or some thing to a shop. Let me know. I might have some in stock. Tuesday at the soonest now.

Todd Bellew
Thank You,
Todd Bellew
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Old 11 Jul 2003
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Join Date: Jul 2003
Location: Springer, Oklahoma, USA
Posts: 2
Hey David,
Sorry to hear about your bike. I happen to know a good mechanic in Moab. If you need some help shoot me an e'mail and I'll put you in touch. Good luck
Eric grampes@yahoo.com
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