XR600 big bore questions

TexKDX · Mar 26, 2003

Hey all - looking for XR600 experience. I am getting a '97 XR600 with quite a few hours on it and it will need freshening up. While I have it down, I am considering a 660 piston, 9:1 low compression so it will run fine on pump gas. Also maybe have the head roto-rooted some. Plan to run a fairly quiet exhaust and DS the thing. I just want to get it to run 100 MPH at altitude around 8k feet. The bike has been GPSed at 93 MPH WFO at 2.5k feet in current form. My friend that had the bike has owned several 600s and that this one was one of the slower ones he's had.

So, about the 660 kit. Does it add much vibration to the bike, seeing that I'll be DSing it? Do I really need to go to the lower compression piston, or can I use the normal big bore kit 10.2:1 piston and be OK with pump premium then dump some octane booster in it when I have to use low test or Pee-Mix gas in it in Mexico? Specific to fuel questions, understand this bike will be in the Rockies most of the time and I want to be able to burn pump premium up here which is like 91 octane.

Thanks for your comments! Leo.

beer_stud_76 · Mar 26, 2003

texkdx -

the XR600 piston it a huge hunk of metal anyway so i can't imagine a 660 kit would make her vibrate intolerably. my 600 is butter smooth. i've never noticed a vibration from it.

about the low compression big bore kit...(while i have no first hand experience) it just seems like a good idea. i really don't know if a 10.2:1 piston is enuf to make it a picky eater, but if you can get the same performance out of a low compression unit, then thats the way to go. seriously, who wants a picky XR600? lower compression will make your engine last longer too, especially if you are going to run WFO for long stretches.

about getting to 100mph... a smaller rear sprocket might just be enough to get you there. just FMI, the difference betweem 93mph and 100mph off road isn't really much. i understand you're a a high elevation, but is it really worth all the money and trouble to squeeze out that last 7mph?

jeremiah

junkjeeps · Mar 27, 2003

I have a XR600 that I was working on at one time and is now shoved in the corner. I do remember when I was on it that a mechanic told me while I was in the motor to change/rebuild the oil pump. He said the XR600 is famous for engine failure due to the oil pump. Hope this helps.

Mark

SFO · Mar 28, 2003

It seems like there are a few objectives here.
One is to rebuild a tired xr, another is to go 100mph, and also have it run on pemex. My experience with high mileage xrs that have been raced or spun hard has them with stretched rods or ovalized small ends. If you want to build a 100mph bike I might consider a carrillo rod as cheap insurance before going any further. I also might keep an eye on cycle news for some used parts or nos hrc parts for your bike. There are a lot of different ways to build these motors, but given your parameters, I would consider keeping the stock bore and building a good head add a good rod and put a spinner cam in it.
I know Rich has built a few of these too and there are a million recipes.
I think it could be a can of worms to just add a big bore kit to a tired xr and go thrash it in baja expecting a 100mph ride, not that it couldn't be done, but I would have some serious reservations.

Rich Rohrich · Mar 28, 2003

Mike Hobbs did a similar project.

http://www.off-road.com/dirtbike/xrproject.html

motometal · Mar 30, 2003

I don't think you want to go to a 9:1 piston...that would be a step backwards. I just put a flat top (so called 10.25:1 Wiseco) in my XR650L. The stocker was a dished 8.3:1. With the new piston and port work it runs great on 91 octane, and I am at a lower elevation. cam is stock. I think on your bike, going to 9:1 you would give away some power. I like the idea of Rich's link, retarding the cam a bit or using an aftermarket cam, which would also tame compression a bit, although cams for these bikes are a bit spendy.

SFO is right...look out for tired components and don't forget about the valve springs, particularly if you go with a cam with quicker ramps or more lift.

Regarding vibration with the larger piston, my Wiseco (same bore) is about 10% lighter than the stocker and I don't notice any vibration, is your engine counterbalanced? So maybe a 660 Wiseco would weigh about the same as the stocker..?..

I expect my bike will run a bit hotter with the mods, but haven't noticed it yet (no hot weather yet). Trimming the bottom rear of the front fender helps in this area, also make sure your jetting isn't too lean!

I've had my bike over 100 mph maybe once on the interstate before the mods (it was close to stock), and it was no big deal although I don't think it would do much more than that (and how do you know how high is ok to rev?). The guys that do over 100 off road on these are crazy! :aj:

motometal · Apr 1, 2003

along those lines, what about taking a stock type head gasket, which is laminated, and peeling off a layer or two? is this possible? Didn't save my old one to look at. My theory is that this would retard cam timing a bit and boost compression in the process. Hopefully the cam chain adjuster could take up the extra slack.

Has anyone tried this?

beefking · Apr 2, 2003

Don't mess with a head gasket; they are designed to hold in ALOT of pressure, and if you figit with it it will probably blow within a few minutes of riding. However, you CAN use a thinner base gasket. They even sell ultra-thin base gaskets so you can raise compression a bit without spending big bucks, and it is easy to change if ya don't like it. I have also heard of some people using poster paper for base gaskets! I'm not sure if this is a good idea, but in theory it could work. Not much different then what a normal base gasket is. ;)

junkjeeps · Apr 3, 2003

You think pulling the cylinder and changing a base gasket is easy? On top of the fact that you have to pull the motor from the frame to do any of what you mentioned? I've got one you can come and work on if it's that easy.

Mark

cujet · Apr 3, 2003

I say go for the higer compression. Especially at altitude. If you are looking for 100MPH in the dirt, you will need to do some serious mods to the engine. Your power loss at that altitude is significant. Enough to bring a stock XR down into the 83 MPH range in dirt. Of course, the street is a faster surface and it is just a matter of gearing and ducking down to get 100MPH.

Chris

beefking · Apr 3, 2003

Originally posted by junkjeeps
You think pulling the cylinder and changing a base gasket is easy? On top of the fact that you have to pull the motor from the frame to do any of what you mentioned? I've got one you can come and work on if it's that easy.

Mark

Its not that hard of a job, especially with an XR wich is a pretty simple motor (don't need to pull the whole motor out of the frame for base gasket..???) I don't have experience with an XR600 in particular, but I have delt with a few other bikes (xr100, 200, drz400, dr350, other small bikes).

Back on topic, I forgot to add that since you will be riding in high elevations anyhow, the high compression will somewhat make up for the lower air pressure in the atmosphere. My cousin lived somewhere in Co. that was a few miles up, and he was running 17:1 compression on his old Husky!!! So I'd go for the high compression and just run higher octane gas.

PS Mark-just pay for the flights and I'll be there! ;)

junkjeeps · Apr 3, 2003

I have had the motor out of the frame twice on my bike. There is no way to perform any type of valve, piston or any top end removal work other than setting the vavles without removing the motor from the frame.

Mark

TexKDX · Apr 4, 2003

junkjeeps, I'll bet I can pull the top end with it in the frame. It sure was a no brainer on the 250. Once the valve cover comes off there is pretty good clearance to lift the head over the cam chain guide, since there are no bolts sticking up to interfere with sliding the head forward.

Thanks for the tips, guys. I'm not sure about the hours on the bike as I just made a feeble attempt to follow it around about 2.5 years ago at an event, the Terlingua dual sport, on my WR400. I'll look it over and figure out how many hours/type of riding is on it. I remember the owner's comments that it was not the fastest XR6 he had owned, kinda of middle of the road, so that just may be that bike and not because it has been flogged.

I'll definately go thru the head while it is down. That is, the guy who works it over will. As far as the compression, I think with Rich's advice I can probably lug around enough fuel addititive of some type to get me thru a week's ride on Pee-Mix fuel. I may also tie a small bottle on the back of the headlight for emergencies up here, say when the little operation on Taylor Park is out of 91.

Talked to the current owner yesterday - the bike should be crated and shipped next week, a few weeks ahead of it warming up around here, so I'll rip into it soon. As far as the oil pump, if the motor is like the 250 you have to pull it loose to get to the cam chain anyway, so it may be a good idea to go thru it as well.

Take care all, Leo.

TexKDX · Apr 4, 2003

Rich et al - a few questions:

- where do I get the slotted cam gear?
- How about the 628 vs. 660? Thumper just shows a 660 kit.
- If I go with the 10.2:1 660 piston, can I dial the cam timing back more for a trip where I know fuel will be an issue?

Many tanks and missles, Leo.

PS - did I mention I'm gonna lower the thing 2-3 inches also?

junkjeeps · Apr 4, 2003

Let me know how getting that top end off still mounted in the frame goes. Maybe you guys know something I don't. I've never messed with the top end of a 250 or 400, but I have on the 600. Best of luck to ya!

Mark

motometal · Apr 5, 2003

Not sure if and how the "L" model is different; on the "L" model the whole thing comes apart with the engine in the frame. Taking the head cover off is tricky (but not as bad as putting back in), everything else is cake.

I'm a bit skeptical that using a thinner head gasket would cause an immediate blowout. Why? Do you speak from experience?

I suppose given the pitch diameter of the cam gear we could calculate the amount of cam timing retardation with a .040" difference in head position.

SFO · Apr 5, 2003

Hey motoM, send me your cam gear and I'll bust some slots in it if you want, pm me for the address.
I have tried delaminating the head gasket by one layer before with no problems although now I just set my squish by clipping the base of the cylinder and leave the gaskets alone.

Vinnny Bob · Apr 6, 2003

I have an xr 600, and have rebuilt the thing from the trans to the head. there is no way to get the head cover off with the moter in the frame.
The 660 kit will just make your bike run hot, and that will cause you to loose power. I have a xr's only hrc cam and a puls charge intake on my bike. I ride at about 4500-5000 ft. My gps has read 94.6 mph with stock gearing. With taller gears and a little room Im sure I could push 100. It does not take much to make these bikes fast.
Call the guys at Baja Designs or XR's only they helped me a ton.
Good luck

TexKDX · Apr 7, 2003

Thanks Vinny for the feedback on pulling the motor. Yuk! Oh well, just a few more bolts I guess.

I tried BD's web site but the *******s re-did it and it looks like took all the 600 stuff off of it.

Heard the same on the 660 kit about heat from the guy I'm having do the work. We settled on a 628 11:1 piston and porting the head. He said at altitude you lose so much compression that the 9:1 piston would be a bad idea and gas would not be a problem. I have a line on a rear wheel too, so gearing changes should be easier that way also for street-oriented runs.

Any idea how big a CS sprocket the 600 will take? My 250 will only go as big as a 14...

Leo.

motometal · Apr 7, 2003

I think you could put at least a 15 on it if not bigger.

I bet you will be happy with that piston and porting! If you are worried about the heat, you may want one of those thermometer oil caps.

You may want to buy a good shop manual for the bike. I would have never guessed my bike would come apart in the frame if the Honda book hadn't said it would (but maybe 600s are different). I'm sure the manual would come in handy anyway.

XR600 big bore questions

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