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Dave Blackburn

ENGINE EXPERTS, Low comp & Head

18 posts in this topic

Hi,

I got my 87 xl600r a few weeks ago. Starts easy, runs fine, but it seems to lack power.

The gearing is 13/43 and in second gear the front end wont come up at all. It seems to have less power than my Suzuki dr400

I tried some compression tests and the best I could get was 130psi. What is normal?

So, out with the engine.

6000 miles on the bike and the engine looks great. I can still see the cross hatch marks on the cylinder wall.

Re-lapp the valves and spec everything. Adjust the valves and all is good.

Wait, it looks like someone changed the head once. The head has the expantion chamber, but NO expantion valve. There is a place in the casting for it, but its not there??? The only decompression I have is the valve lifter??? My service manual and this site seems to say this is wrong??

I opened up the expantion chamber on the side of the head and its shiney new inside.

Do I have the wrong head????? Is this causing the low compression??

The piston only goes to within about .170" from the top of the cylinder. Is that right??

I did a leakdown after I put the engine back together and it was 5-8% @ 100psi. but still low compression.

I rolled the bike down a hill and let the clutch out in fifth and I could only get to 120psi or so.

Thanks for your help. I am very good at rebuilding engines etc. so it is nothing I have overlooked. Please answer specifically about the head and piston/deck height c/r.

Thanks,

Dave:applause:

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Hi, You may have a XR650L or NX650 head on there. They have bigger combustion chamber domes for their 100mm bores (and no 5th poppet valve decomp).

The 0.170 deck height you mentioned sounds about right - its massive as standard. The problem is, you could shorten the cylinder, but then you will need to refabricate the top engine mount and slot the cam sprocket to maintain the correct valve timing (as the cam chain tensioner accounts for the extra chain slack- it pushes the chain further in and hence retards the cam timing).

Its one of those little jobs that leads into many to get the best outcome!!

Visit wiseco for a higher domed piston. ...easiest solution!

Hope this helps

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OK, we're talking about an '87 XL600R correct? So I'm guessing your rocker cover doesn't have the decompressor arm assembly on the intake side of it that attached to the kickstarter via cable on the earlier models? Is the head drilled between the decompressor valve pocket and the decompression chamber? If it is, there's your lack of compression. Also, if the valve pocket is there with no valve in it, that will lower your compression all by itself. Either way your engine will be considerably down on power.

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The rocker cover has decompression via kick start and handlebars. Valve lift decom, not subchamber. There is no hole for the valve, just a spot on the casting where the valve would go.

So what do you think would be best. New piston or head. I have a flat piston now, if I get a new wiesco one does any one have a resource to figure out c/r with this head?

Thanks,

Dave

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What sort of power should I expect from this bike, 87 xl600r??

13/43 gearing and can't raise the wheel in second without poping the suspention and pulling hard.

Thanks,

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What sort of power should I expect from this bike, 87 xl600r??

13/43 gearing and can't raise the wheel in second without poping the suspention and pulling hard.

Thanks,

It should be better than that. I consider the dual carb XL600 engine to be about the strongest of the 600/650 line. A little less torque than an XR600, but more top end. If you have the engine apart, check the ring end gap.

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Compression height doesn't sound right. Maybe it's a 650l piston. Wiseco has 3 100mm pistons listed with different deck heights for each motor xl600, xr600 and xr650l.

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after reading your post again I realized you have an XR600 head with probably a bigger chamber for the longer stroke of the XR motor. Get a domed piston from Wiseco specifically for an XL600 in the highest comp ratio you can get and the large chamber will lower it somewhat.

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I would go with a JE piston insted of the weisco.I have heard that wisco is a poor design.Just what I have heard from many people.

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I would go with a JE piston insted of the weisco.I have heard that wisco is a poor design.Just what I have heard from many people.

+1 on this.

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Personally I like the "new" weisco.Years ago they were crap! ring locators would fall out when the dome got a little hot.Thier metal formula wasn't right they would seize all the time.But for the past 15years I havn't seen one go bad ,by design. (They don't like valves being dropped on them...)

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Thanks for the replies.

OK, so it is a 87 XL600r with 7100 miles.

I have been through the whole bike so far and cleaning the carbs helped a bit.

It just barely lifted the front by itself in second the other day. This is with 14/43 gearing. I did another comp test and got 170psi after lots of kicking, so I am feeling a bit better about that.

The head has been replaced at some point, and I am still confused about the sub-chamber. My factory manual says it is suposed to have a decompression valve??

I still think it is lacking a bit in power, but I have never rode another xl or xr 600 before.

Any suggestions? Thanks, Dave

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Thanks for the replies.

OK, so it is a 87 XL600r with 7100 miles.

I have been through the whole bike so far and cleaning the carbs helped a bit.

It just barely lifted the front by itself in second the other day. This is with 14/43 gearing. I did another comp test and got 170psi after lots of kicking, so I am feeling a bit better about that.

The head has been replaced at some point, and I am still confused about the sub-chamber. My factory manual says it is suposed to have a decompression valve??

I still think it is lacking a bit in power, but I have never rode another xl or xr 600 before.

Any suggestions? Thanks, Dave

your 14/43 gearing may be a bit tall(for pulling power 2nd wheelies). your compression test says you've got good compression.

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The Wiseco piston problems were years ago with 2 smoke pistons. Other failures on 2 or 4 strokes can usually be attributed to shops boring the cylinders to tight for a forged piston which requires more cold clearance. It also seems you have the later XR600 head which only opens an exhaust valve for decomp even though the chamber is still cast in the head so dont worry about that either. Also White Bros did an article years ago about rejetting the XL. If I remember right they pulled the snorkel Raised the nonadjustable needles with .020" washers and went up 1 size on the main jets. There was also a pilot jet change but I can't recall that. I did it all to my 86 XL and it helped alot but it still wasn't as fast as my old XL540. Good Luck!

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Ive had no problem racing with wiseco 's and my friend runs GSX drag bikes with them and says theyre very strong (but heavy). High silicon content too for durability.

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