increasing compression of 650L

My 650L engine is in great shape at 3700 miles, but I am itching to start some mods (other than simple uncorking and bolt ons). It seems to me the most obvious way to increase power on the "L" would be to boost compression.

1. Is it possible to just use a thinner head gasket and/or base gasket?

2. Is there a difference in the gaskets 650L vs. 650R?

3. How much can the comp. be increased without needing race fuel?

4. Is an oil cooler required?

5. I would expect a decrease in high rpm power...but how much?

6. If a simple gasket switch won't work, I will probably mill the head. Will I need to re-machine the squish band?

Any and all input is appreciated!

Wow - where do I start ? The distance between the crank and the head on a overhead cam motor should not be messed with. It sounds like you are thinking in 2 stroke terms. Yes - the path to more horsepower with the 650L is more compression and the best way to do this is a new piston. You also have the option of bigger bore at the same time, you will need premium fuel with either choice. Higher compression will increase power everywhere, high and low rpm. The 650L and the 650R are different motors - one air cooled one water cooled.

Gaskets don't interchange. You need an oil cooler on this bike stock. The good news here is that with a few simple mods the 650L rips!

Nothing like compression to wake a thumper up. I am on my 3rd 650L, all have hi-comp. big bores. I also did valves, cam, big-fins, exhaust, carbs, etc. I jet slightly rich and use 93 octane pump fuel with no problem. I have an XR's only big bore kit that uses a 12/1 or 12.5/1 compression ratio. I have a Mikuni flat slide(Great Shape) that is set up for the larger airboot on a 650L I will sell for a good price if interested. $90.00... shipping paid. this has the cable, and I believe a Gunnar gasser throttle assembly. Great addition to hi-compression.

Best of luck. PM me with any questions... Wattner@aol.com

If you are itching to do it, you had probaly better go ahead and scratch.

1)The drz's shim with head gaskets. A 1.5-2 point compression gain between the E and S, but I don't know how that affects the chain or stroke on base assemblies. I've never heard of shimming the jug, but maybe the head on an L. Let me know.

2)yes

3)I oppted for 10.25:1 and have never had any trouble 91 octane. My parts guy says 12.25:1 on street fuel if jetted right, but I wanted safe gains.

4)Any mods to the L should be accompanied with a remote cooler or fins. I say that because a modified bike usually means it's going to be ridden harder. I've heard of several cases of people replacing tanks and burning motors up in single track due to losing the factory shrouds. That tells me they run real close to max temp at factory settings.

5)Expect a big gain. I also put in a high performance cam when I replaced my piston, but the increase all around was amazing. I used to max at 55-60 cruising with 14/48, now I cruise at 70-75 and pull past 100. It's not the same bike.

6)Squish band? Don't change the stroke and read about cams closely, but a simple "bolt on" install won't require any head work.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now