XR650L: Mild Performance Boost?

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  • michaeln

Posted February 19, 2005 - 08:35 AM


Hi Folks

I have a 1998 XR650L. It has a number of modifications already:

Keihin FCR41 flatslide pumper carb
White Brothers header and E2 muffler
UNI Filter

It has about 37,000 miles on it, and the engine is stock internally. A couple weeks ago I opened it up and replaced the cam chain, both cam chain sprockets, and all of the cam chain tensioner stuff. It had been making a clicking sound at idle when cold, and inputs from here led me to believe it was a worn cam chain. That diagnosis turns out to have been correct, because after doing the work the noise is gone.

While I had it apart I noticed the cam had some scoring on the lobes, although the rocker arms looked OK. The engine uses a little oil, and it puffs some blue smoke when I first start it up after sitting all night, so I believe the valve guides or the valve seals may be shot (not surprising with 37K miles). There isn't any visible smoke when it's warmed up and running, but a friend who rode behind me last week said he could smell oil burning when I back off on the throttle on decelleration (again, probably valve guides or seals).

I checked the compression before pulling it apart with the engine fully warm. With the auto decompressor still active it was 82PSI (95 is Honda's spec), and about 140 with the decompressor deactivated (RH exhaust valve loosened way up). Honda says 199PSI in that mode. I didn't do the old "oil in the cylinder trick" to see if it was valves or rings. I loosened the RH exhaust valve quite a bit, likely quite a bit more than Honda's recommendation for this test, and don't know if that might have caused the PSI reading to be artificially low.

So, compression is low, although the motor runs great. However, I know it's time to refresh the top end. I would like to get a little more power at the same time, although I don't want to negatively impact reliability or lose the great bottom-end torque I have come to rely on in my riding style.

I intend to get a good 3-angle valve job done and replace whatever needs replacing in the head. I will also have a mild porting job done. Those two alone should give a bit of performance boost without impacting reliability. I will replace the piston & rings (and rebore to 675cc or whatever the max
is if I go with a stock piston), and also replace the cam.

But here is my quandry: If I go with a high compression piston (Thumper Racing's 10:1 Wiseco or a standard 10.25:1 Wiseco), I know it will run hotter (I would consider adding an oil cooler), and the electric starter may have problems cranking it. The normal wisdom seems to be to combine the high compression piston with a stage 1 cam like the Hot Cams one in order to decrease the dynamic compression ratio a bit due to the overlap period of the cam.

I have a preliminary report from William ("Mountainrider") who has just done this to his bike, and he reports that the motor now doesn't have much low end but screams at midrange and top end. Unfortunately, that isn't the kind of motor I want (my bike is my only form of transportation and is a daily rider used for commuting).

So I am thinking of other ways to go... such as just going with a Honda 8.3:1 piston in the largest overbore size and a new stock cam and rockers. There are also thinner base gaskets available that should result in a mild compression boost. Anyone have any thoughts on using those?

I think Thumper Racing has a 102.4mm 9:1 piston too (part of their Baja 600 kit for low octane gas), and if that piston would work in the XR650L that might be a way to go.

I don't want to stray too far from what makes this bike great though. I know it's not a racebike, and I don't want one anyway for what I need the bike for. Just looking for suggestions on a mild performance boost really.

I'd appreciate your thoughts.



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