From: couyant@convex.com (George Couyant)
Subject: Project XR600
Organization: CONVEX Computer Corporation, Richardson, Tx., USA
Date: Thu, 27 Aug 1992 07:06:32 GMT
X-Disclaimer: This message was written by a user at CONVEX Computer
              Corp. The opinions expressed are those of the user and
              not necessarily those of CONVEX.

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*** Caution, some filth may follow ;-) The following article contains ***
*** certain references which may be interpreted as being filthy and   ***
*** amazingly perverse.  It may offend some readers, husbands, wifes, ***
*** uncles, aunties, pets and wall paper and may also encourage a     ***
*** healthy interest in people of the oposite gender. There are       ***
*** references to exhausts, heads, reaming, stroking, barrels,        ***
*** pistons, relief, breathing, inlets, water, porting, abuse,        ***
*** restricting, experimenting, choking, flowing, bending, power,     ***
*** creaming, grunting, sliding, watering eyes and heaven forbid,     ***
*** Mikuni carburetors.....Then again, maybe you won't be offended    ***
*** so it's probably worth while reading on.                          ***
***    This warning and the following article aren't meant to offend  ***
*** nor is it meant to alienate any readers (it's all toungue in      ***
*** cheek, ok? ;-^  ;-^  ;-^  Hope you had a chuckle. I did.          ***
*************************************************************************

A good riding buddy recently bought a new XR600 and over the last month or
so we've been doing a bit of work on it to make it a competitive race
mount. We concentrated mainly on making go faster without sacrificing
reliability and we think we've found a cure for the gearbox gremlins.

If you're interested, read on.....

Firstly, the stock gearing made its way to the dust bin and was replaced
with a 50 tooth rear sprocket.

Front and rear suspension went straight to XR's Only for progressive
springs and a revalve (the rotton sod wouldn't let me modify them..maybe
just as well...). The idea here was to keep its lovely slow speed damping 
and complience but firm up the mid stroke onwards so that it'll handle 
big hits properly. The result was amazing. It feels much firmer and more 
stable at speed. You can hit obsticles at speed and it takes it in its 
stride.

Off came the barrel and it was sent away for a 630cc kit. They wanted to 
stick a 12:1 high compression piston into it but we went for a "more sane" 
11:1 piston (the 11:1 will run better on 97 octane pump gas). When we got 
it back, we assembled the head onto the barrel to check valve - piston 
clearance with the cam we planned to run (mild one which has a big lift 
boost over stock, but timing and overlap are close to stock).  We were 
surprised that there was so little clearance and it meant we had to do 
something. The choices were to either recess the valves deeper into the 
valve seats or fly cut valve reliefs into the piston. I left it up to the 
owner and he tossed a coin...we fly cut the piston.

The cam was a little tight in its bearings and whilst it would free up
somewhat after a bit of running, I decided it would be best to give it a
bit of room. The owner is a fitter and turner and his shop has a neat NC
machining centre, so we spent a day farting around aligning everything up
and gently reamed the cam bearings. The cam now spins true and free. A
real improvement and well worth it.

Next we needed to improve the breathing. Another mate has a Muzzy motor in 
his XR and we got a chance to measure the porting on that. What they've
done is remove a lot of material in the inlet tracts to straighten the
flow path and made them a hell of a lot wider. If this had been a water
cooled head, I'd never have attempted this sort of porting for fear of
hitting water jackets....But port we did. The alloy is really soft (softer
than my TT600's) and I had trouble keeping the porting tools from clogging
and marking the finish (maybe it's time I overhauled my porting tool to
get a bit more speed out of it..). If you're attempting it, be careful.
The finished product looks good though and you can clearly see the inlet
valves through the ports. The exhaust ports were ok and just got a bit of
a cleanup. This was a really good casting with stuff all slag. Along with 
top quality silicon valve springs, the top end should be bullet proof and 
be able to hang together at high rpm.

The gearbox is a problem on hard working XR600's. The hard facing on
3rd and 4th gears (sometimes 2nd as well) wears and they break with
potentially disasterous results. Since it's damn near impossible to get
better quality cogs, the only thing left is to lessen the stress on them.
We had heard that a clutch from an NX650 can be used to increase the
primary drive gearing. This will lessen the load on the gearbox since it
will spin the gearbox drive shaft faster. You can make adjustments to the
gearing later by increasing the size of the rear sprocket to compensate.
He took the plunge and spent a heap of money on a new NX650 clutch
assembly and primary drive. Hope it's worth it....only time and much
throttle abuse will tell....

With everything bolted together, the bike ran well but it was clear that
the carby and exhaust was too restrictive. The thing would go like a
rocket and then run flat up top.

We first exeperimented with the carby by removing that shit choke
butterfly. This thing really restricts air flow and I still wonder why
they persist with it. The TT600's choke system is great in comparison.
This made a big difference to the top end. If you want instant noticeable
power increase, simply remove the choke butterfly! Only problem is that
it's a bear to start when cold. This would clearly piss me off, so I
borrowed a square slide 41mm Mikuni (the stock carb is a 39mm Keihin) and 
grafted that on. Talk about a difference! This thing hauled! I think he'll
buy the carby.

For the exhaust, we wanted to bolt on the exhaust off an early model XR
(pre '91) because they breathed better, but couldn't get one for a
"reasonable" price. My mate ended up making a great looking set of headers
out of larger diameter tubing. There are smoother bends and it flows much
better than stock.

So what's it like??  I was really impressed with the final product. It's
like a really strong XR with power everywhere. On it's first ride, the
mate with the Muzzy motor came along and whilst the Muzzy motor would pull
away at the very top end, this XR would cream it in the low and mid range.
Much easier to ride and dare I say easier than a stock motor. Because it's
got heaps of grunt and is very controllable, you can leave it in 3rd or
4th all day...and oh those power slides! I had about an hour of riding on
it on the day and it was hard giving it back. Lifting the front end for
logs and obsticles was a breeze. Just twist the throttle from zero rpm in
2nd or 3rd. No messy wheelspin, just gobs of tourque. Hit the open stuff and
your eyes water! Big 5th gear power slides without once stepping out of
line (mind you, I didn't want to drop his new pride and joy so I never
went *really* crazy)..

It was interesting for me jumping straight back on the TT600. It felt big
and whilst I have the TT set up stiffer, this XR seemed more stable when
cranked right over in a bend. The TT felt fat and heavier. The XR steered
better in the tight stuff too, though when flat out at speed, the TT was a
class above. Power wise, down low they were about the same (TT's motor mods
are similar) with the TT having an edge in the mid range. Up high though,
the XR would hang on for what seemed like another 500 rpm. 

One thing we did notice though was that a lot of oil was being blown down
the crank breather pipe. We're told it's common when revving hard and
there's a simple mod to the oil pump to fix it. I'm a bit concerned by it
because none of the other XR600's I know of have this problem. 

If you have some cash sitting around, spend it on these mods. You'll never
believe the difference. I want one.

George C.