Big bore question

[martinfan30]

im looking at the 102mm overbore kit from xrs(671cc.). i am thinking i want to keep the stock cam only cause of the auto decompresor and a little leary due to some of the reliability issues ive read about aft. mkt. cams. how do you think this would would work, and i use a fcr pumper. thanks!?

[roadrunner81]

call mega cycle cams, they grind and hardface your original, my cam rocks.

[martinfan30]

call mega cycle cams, they grind and hardface your original, my cam rocks.

an

so did you do a high comp. piston and had them just harden the stock cam?

[Beyond The Pale]

I'm sure that a bunch of people will flame me for this but here it goes anyway.

The stock bike is very reliable. On the 600 3rd gear is a known weak part and the 650L has pretty much the same tranny. The mid eighties had the biggest issues. It was addressed, but I had to replace 3rd on my 1993 XR. The next issue is the entire valve train. It is usually the first non tranny portion of the engine to need attention.

Anything that increases the lift or increases the speed of valve opening will be adding loads to the valve train. The same goes for the transmission. More torque equals more load on the trans. It also means a lot more heat. An oil cooler might be a good idea.

It's all relative though. I'd bet that a bored, hi lift cam, and high compression pistoned XR650L will still outlast a "modern" MX four stroke like a CRF450.

[martinfan30]

So do you think just using a 10.5 to 1 overbore piston and stock valvetrain would still get me areliable powerplant?

[Beyond The Pale]

So do you think just using a 10.5 to 1 overbore piston and stock valvetrain would still get me areliable powerplant?

I think that it will. The higher compression adds heat so get an oil cooler of some kind. Like I said reliability is relative. Will it go 10,000 miles? I would think so. I have heard reports of a stock 650L going 50,000 miles or more. I would expect that a hot rodded 650L might need valve work long before that.

A lot of it depends on how you are going to be using it. Mellow riding or racing. The mid 90's HRC kitted XR600s used in the baja 1000 were ready for a rebuild at the end of the race. A few didn't even make it to the end before a gear blew. Mellow riding with just a few blasts of wide open and it will last a long time.

[Denn10]

Ive thought about that 10.5:1 piston myself i wouldnt hesitate to put that in and see how it runs and then maybe change the cam, there is a few posts on here with people using a hotcam stage 1 with that piston and love it.

[Billahjack]

im looking at the 102mm overbore kit from xrs(671cc.). i am thinking i want to keep the stock cam only cause of the auto decompresor and a little leary due to some of the reliability issues ive read about aft. mkt. cams. how do you think this would would work, and i use a fcr pumper. thanks!?

From your garage, it looks like you have 650R. You can buy an HRC cam which comes with the timing gear flange and the autodecompressor installed from Service Honda for around $350. Its expensive but I noticed that you have an FCR carb which is twice as much as an HRC cam.

I am running an HRC cam and Moriwaki full exhaust. My bike rips to 85 then still gets to 100 pretty easy on stock gearing. It will hit the rev limiter at around 103/104 MPH. At first it felt like my bike had a softer low rpm power output, but now I enjoy it. I can control the bike easier at low RPM and then rev it out for bigger power on top. Previously, while riding in dry conditions with a FMF pipe/header the bike would spin up the back tire barely cracking the throttle. Now I can modulate it a little better. I like my bike how it is right now and would not be able to effectively use more power except for street riding.

I am not sure how much a 10.5:1 piston will do for you. As I understand it, many aftermarket pistons are heavier and reduce your over-rev capabilities when compared to the stock piston. This could be bad if you go high compression with a high lift cam and rev your bike out by crashing valves into the piston. I am not aware of anyone doing that, but then again I haven't heard of many people putting in big bore kits for the 650R. If you go for a HRC piston, I read that you can mill off the bump in the center to make it run on pump gas. I would be inclined to get the HRC piston over another brand, but that is just my preference.

[martinfan30]

billahjack, that is actually an "L", but your advise is appreciated nonetheless!?

[Billahjack]

There is a new 600 or a 650L HRC air cooled cylinder on E-bay under the XR650R listings. Had 4 or 5 days left. The guys doesn' know what he has but he does show the part number on the e-bay listing.

[martinfan30]

There is a new 600 or a 650L HRC air cooled cylinder on E-bay under the XR650R listings. Had 4 or 5 days left. The guys doesn' know what he has but he does show the part number on the e-bay listing.

what would the HRC head have that the stock one doesnt?

[martinfan30]

There is a new 600 or a 650L HRC air cooled cylinder on E-bay under the XR650R listings. Had 4 or 5 days left. The guys doesn' know what he has but he does show the part number on the e-bay listing.

all i saw under that listing was an "R" cyl. head.

[Billahjack]

No it was a Cylinder, not a head. There are fins on the side of it in the picture. I think you use the stock head with an overbore cylinder/Piston for the 628 HRC kit.

I think I saw a post about detonation in hot weather with raised compression on the air cooled motors. Might want to check into that before going high compression.

Good luck and I hope your ride turns out well!

[roadrunner81]

Regrind with different cam specs, JE tri coated piston 11:1 standard bore and a bunch of other stuff, hopefully I wiil get around to dyno next week. My bike runs great, easy start and tons of power.

[martinfan30]

Regrind with different cam specs, JE tri coated piston 11:1 standard bore and a bunch of other stuff, hopefully I wiil get around to dyno next week. My bike runs great, easy start and tons of power.

pump gas? let us know the dyno results, cuz you may just sell me!

[Alex 400]

im looking at the 102mm overbore kit from xrs(671cc.). i am thinking i want to keep the stock cam only cause of the auto decompresor and a little leary due to some of the reliability issues ive read about aft. mkt. cams. how do you think this would would work, and i use a fcr pumper. thanks!?

call mega cycle cams, they grind and hardface your original, my cam rocks.

Is the xr600 cam and the xr650l cam the same? i have heard that the motors are similar, am i right? At anyrate the megacycle cams retain the auto decopressor mechanism.

[martinfan30]

Is the xr600 cam and the xr650l cam the same? i have heard that the motors are similar, am i right? At anyrate the megacycle cams retain the auto decopressor mechanism.

from what i understand, the 650L cam is milder.

[roadrunner81]

I thought we were talking about a xr650r but the same should apply, worth a a call at any rate. I went with a regrind because I did not want to run a billet steel cam.

[martinfan30]

I thought we were talking about a xr650r but the same should apply, worth a a call at any rate. I went with a regrind because I did not want to run a billet steel cam.

why is this?

[roadrunner81]

If your asking about the cam its because cast cams are a little softer than billet and tend to wear better against the rockers, more similar to original cam material.