98 XR600R lacking top end power

I have recently purchased a 98 model XR600R and it appears to be lacking top end power. I have had a friend who has had a number of these bikes who says that it is running like his de-restricted XR400. In addition I have taken this bike to the local Honda shop here on the Gold Coast, to be told that there could be a myriad of problems causing this lack of power.

This bike has an original 5000km on the clock, which is confirmed by the lack of wear on all of the external components. I bought this bike off the original owner who rode it mainly on the road, and changed the oil/filter at 500km intervals. He said he used Fully synthetic oil since new.

The checks I have performed have confirmed the following.

Exhaust and Header have been removed and inspected to confirm that any factory restrictions have been removed, including rear spark arrestor.

Carburettor has been removed to confirm that inlet manifold rubber restriction has been removed.

Rocker cover has been removed to confirm valve timing is stock and hasn't jumped.

Spark plug has been removed and gap set.

Valve clearances have been checked 0.004 inlet and 0.005 exhaust.

Aftermarket air cleaner has been cleaned and oiled and airbox has been checked for restriictors (none found)

Main jet in carburettor has been checked to find it is a 165 main jet.

All of my riding will be vitually at sea level.

This problem is s__tting me to tears. I suspect a jetting problem.

Can anyone help??

Check you intake and exhaust valve springs. Maybe a spring is out. Check for a flat lobe on your cam. You could also have sticky valve, or a plugged-up main jet. Make sure the choke plate moves out of the way then the choke lever is down. Make sure that the carburator doesn't have any leaky gaskets.

If your valve clearances are in inches, then they are correct.

Intake: .10mm +/- .02mm

Exhaust: .12mm +/- .02mm

Make sure some stupid rodent didn't crawl up, past where the spark arrestor used to be, and make a nest in one of the headers. If one is present, shoot it!! :banghead:

Does it miss fire at high RPM or is just weak?

Have you checked the spark plug coloring since youve ridden it? That will shed much light on whats going on...

My '93 has a WB aluminum exhaust with a stainless header, drilled airbox and I have a 155 main jet. Has the bike been sitting? Many guys don't clean out the carb properly when needed. I'd start there. If that doesn't do it, check all of your electrical connections.

I recently owned a stock engined but derestricted '95 XR600. There is a few things to check as follows-

*Make sure the entire snorkel off the top of the airbox is removed

*Re-gear the bike with 14/48 for dirt riding or 14/46 for more open riding

*rejet carb with the following- 155 main, 62 pilot jet (stock), needle 3rd clip position (stock), airscrew 2.5 turns out, float level 14.5 mm (stock)

*ensure inlet manifold matches the inlet port in head.

If all of these things check out, theres not really anything else you can do. This may come as a surprise to you, but an XR4 and the XR6 are close in top-end acceleration, and if you find it hard to pull away from the XR4 that's normal! Believe me i've owned both of these bikes and have tested this theory!

I recently dragged my buds 97 xr400 against my 94xr600r, on a medium pack dirt road, and was surprised to see them tie ! He weighs about 160 lbs and I 300lbs ish. I'm sure if we swapped bikes it would be real different with my big butt ! But I wander , when the sand/mud gets thick, is when the cc's on the 600 really shine . Because xr400 bud says my bike is so fast. Or he 's not twisting enough when he should ?

The power drops off sharply in the high midrange. It doesnt misfire at all.

I havnt yet checked the plug colour, however at my next opportunity I have it planned to plug chop and alter jetting accordingly. I am a novice at this, although am very mechanically minded.

Thanks for all your help guys. I'm off to take the carb off and to replace and check and clean carb jets float internals etc.

Just one other question also

Do these bikes have a small amount of puff through the crankcase breather hose as mine does. Not a lot, however a small amount.

Also if I get the kick starter to top dead centre (compression stroke), if I push real real hard, I can push through the compression stroke without the decompression lever. I heard they have a automatic decompressor also???

Can you enlighten me??

This "puff" of air out of the crankcase breather is completely normal. All XR600's from 1988 on have an auto decompressor lobe on the cam that flicks open one exhaust valve at cranking speed, it is normal for these models to be easy to kick right through the stroke 3 to 4 times before they hit any sort of compression.

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