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What is this head cover off of?

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So a buddy just brought his 86 xl600 in to my shop for a tune up, and it had a interesting top end configuration. Can anyone tell me what year valve cover this is off? It has a decomp release on the rear of the motor. And one also on the exhaust side...thoughts?ImageUploadedByThumper Talk1398353053.489104.jpgImageUploadedByThumper Talk1398353067.405041.jpg

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Hmm..If you look at the fiches it's supposed to be on all of them if Honda's correct,.

 

83 below,,

 

http://www.servicehondapsn.com/fiche_section_detail.asp?section=2539616&category=Motorcycles&make=Honda&year=1983&fveh=132489

 

87 below,,

 

http://www.servicehondapsn.com/fiche_section_detail.asp?section=2539620&category=Motorcycles&make=Honda&year=1987&fveh=132493

 

and all in between those years..I know my 86 had it but it was disconnected by me I think as was the other kick

start one,,,I use only the manual lever,,Other XL engines I had I can't recall whether it was there..

Edited by Horri

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The mysterious 5th valve. XL600s only had these. The decomp valve is notorious for leaking probably because of the hopelessly soft spring required for cable actuation. Best to remove the valve and weld up the port or lap it in real good and replace the valve spring with something far stiffer then convert to the XR style exhaust valve decomp as someone has already in that pic. The XL lever at the exhaust valve only had the upper cable position, XRs had both upper and lower

Edited by valvesrule
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 The decomp valve is notorious for leaking probably because.......

 

 

 

 

Can you provide proof of "notorious" leaking problems?

Edited by Onederer

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My 87xr has the front one only. My 83 xl has both and looks just like the head pictured above. The ones with the rear valve require a different gasket than the others.

What i find odd is that the actuation lever on that one is the one that accepts two cables (kick and manual). That lever would usually be on a head that doesnt have the rear valve i would think.

Also, without a cable on that rear one to hold it in the slacked position, wouldnt the pressure from the spring be applying pressure on the valve? Or does the lever hit the head and not allow that cam to rotate backwards that far? maybe not an issue but seems strange to be hooked up like that.

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Can you provide proof of "notorious" leaking problems?

What would you accept as "proof"? The fact that this guy has converted his to the exhaust decomp is plenty enough. I've seen more than a few myself. Craig Hansen(Hansen Race Technolgy), former VT and XL/XR engine builder for HRC during the dirt track wars of the 80s has personally told me they leak frequently into the decomp chamber. If you have oil, carbon or other residue in the decomp chamber you absolutely have a leaking 5th valve. And if you get a lower comnpression test reading than normal it usually is from the 5th valve since the chamber size more than doubles if this valve doesn't seal Edited by valvesrule

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My 87 head doesn't have the rear valve...no trace of welding it either. All the mg2 motor numbers match to. Weird:/

My 83 head has it, the 85 XL I had did NOT and I've seen 87s with and without. Not much rhyme or reason to which ones got it as far as I know. Since the cover is balck and the head silver we have no idea if there is even a 5th valve in the head. The cover is probably 83-84 and the head 87 XR not XL. Only the builder knows for sure.

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If I pull the rear comp release and turn the motor over, it relieves pressure. So it is working. And the head has been bead blasted so who knows what year is came off

Edited by Fivefive1
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If I pull the rear comp release and turn the motor over, it relieves pressure. So it is working. And the head has been bead blasted so who knows what year is came off

If pulling the rear decomp lever does indeed drop compression then the head is an XL600 probably 83-84 but also possibly up to 87. XR500s didn't have the 5th valve nor did any year XR600 as originally equipped. At least not US models.

Edited by valvesrule

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My 87 head doesn't have the rear valve...no trace of welding it either. All the mg2 motor numbers match to. Weird:/

All the dual carb heads XL AND XR  had the relief in the combustion chamber for the valve but only XLs actually were machined for and had it installed. It's completely unnecessary. The valve seats so lightly that a bit of carbon stuck in the seat will hold them open killing compression and eventually a possible burnt decomp valve can result or the seat can fall out from all the combustion heat rushing past it.. Easy starting but poor low speed response is a symptom. At higher rpm the port is so tiny that compression loss isn't such a big factor and power comes back. These 5th valves are another of Hondas dumber ideas like the reed valve XR500 head.

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 If you have oil, carbon or other residue in the decomp chamber you absolutely have a leaking 5th valve. And if you get a lower comnpression test reading than normal it usually is from the 5th valve since the chamber size more than doubles if this valve doesn't seal

 

Stuff is going to get into the chamber regardless because the decomp valve opens during the compression stroke which is charged with fuel. It is unrealistic to think that the chamber is going to be devoid of any buildup.

 

Low compression can be caused by several things and to suspect that it is "usually" the decomp valve is also unrealistic. If the decomp valve was suspected of leaking, a compression leak down test would rule that out right at the start because the decomp chamber is sealed; compression won't continue to leak if it was only the decomp valve. I wouldn't suggest that someone expect a leaky decomp valve from a normal compression test alone.

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The head cover is off of a "five-valve" XL600. But the dual cable provisions on the exhaust decompressor suggest that part of it is perhaps '85-86. However, since the top end parts are all mix-n-match, it's anyone's guess what's original. It's increasingly common these days to find '83-87 RFVCs with mixed top end combinations since some valvetrain parts have been NLA for a long time now.

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Stuff is going to get into the chamber regardless because the decomp valve opens during the compression stroke which is charged with fuel. It is unrealistic to think that the chamber is going to be devoid of any buildup.

 

Low compression can be caused by several things and to suspect that it is "usually" the decomp valve is also unrealistic. If the decomp valve was suspected of leaking, a compression leak down test would rule that out right at the start because the decomp chamber is sealed; compression won't continue to leak if it was only the decomp valve. I wouldn't suggest that someone expect a leaky decomp valve from a normal compression test alone.

Obviously there are other things that cause low compression. I in no way meant to imply that was the "only" possibility. The decomp valve adds other complications to the engine and is redundant. I have a 5 valve head on my XR500. The valve isn't leaking so far and I'm not removing it till it does but it is non functional just like the OP bike. You don't need it and they don't work as originally intended. They're extra dead weight and 20% more valves to leak and repair.

Edited by valvesrule

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