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Help needed: XR650L/600R de-compressor compatibility

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Dear All,

I’m presently modifying an NX650 (same as an XR650L) engine with a 1987 XR600R cylinder head. I’ve run into a bit of a problem with the de-compressor arrangement.

The camshaft I’m using is a Megacycle 148 x9 with no provision for a de-compressor there. So, I’m left with the mechanical alternative, i.e. the little trigger-and-wire activated shaft on the right side of the valve cover that pushes the right-side exhaust valve.

Now, the XR600R, so far as I know, did have a trigger-action de-compressor (but I may be wrong), however the corresponding opening on the valve cover is blind. The rocker directly inside it does have the little “ear” to be pushed.

The original cover off the NX (XRL) does have the arrangement, so I’m planning to remove the de-compressor shaft from it and fit it to the XR600 head. To do this, I’ll drill open the blind port on the XR600 cover. It sounds pretty straight forward, but I noticed that the outer part of the XR600 cover, the housing where the decomp shaft + seal were supposed to fit, is a little smaller than the NX decomp shaft + seal. If they are different, there is nothing telling me that the center of the hole (and therefore the decomp shaft itself) will line up properly with the rocker inside. This in turn would mean that there is a risk the de-compressor shaft may not fit after all (and I’ll end up with a drilled [i.e. holed] valve cover) or it may not work properly and/or interfere with the movement of the rocker.

Being silly enough, I haven’t taken any photos of my covers. I attach some off an XR500 cover with indicative comments on it.

XR500RValveCoverAAA.jpg

XR500RValveCoverBBB.jpg

XR500RValveCoverCCC.jpg

Has anyone come across this situation before?

Any feedback, comments etc highly appreciated.

Thanks in advance

Manos

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back in the winter of 02 i hopped up my pig with a .40 JE 10.5 and a hot cam with no provision for the decompresser. that was 11000 miles ago still have the origional starter and battery in the bike. i will replace the battery in the spring.

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back in the winter of 02 i hopped up my pig with a .40 JE 10.5 and a hot cam with no provision for the decompresser. that was 11000 miles ago still have the origional starter and battery in the bike. i will replace the battery in the spring.

Thanks bigdawg41,

That is to say I don’t have to worry, the bike will start up even with high compression and without a de-compressor. Is that what you mean?

I use the same piston as you are but with a modified 1987 XR600R cylinder head. I have more compression than you (I’m at about 11:1). Also, I don’t want to use the original battery. I plan to use a very small and light battery, something like this http://www.thumpertalk.com/forum/showthread.php?t=845336&highlight= and the release of compression will be of great big massive help.

Thanks again

Manos

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If the NX/XRL cover has the manual decompressor shaft, why don't you just use that cover, and replace the rockers with the ones from the XR600 cover?

(The rockers and cams are the only things amongst these parts that differs when comparing a automatic and manual decompressor camshaft setup)

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You could get a cover from a single carb xrr, it's a direct swap.

Drilling your xrl cover could be done, but I don't think anyone has done it.

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Thanks all for your replies, however the valve cover off the original NX/XRL head is seriously knackered where the cam sits so I can’t use it. It is scrap.

Anyway, I can’t see why it can’t be done by drilling the cover so I’ll go ahead. If I damage it I’ll just have to buy a new one. But I rather not, so I thought I’d ask here. If anyone has done this please tell.

I do find it strange though that this hole is blind.

Thanks again

M.

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Ok, then I see.

I'm sure it can be done, but I wouldn't use anything else but a stable proper pillar drill machine. Apart from drills you'll also need some reamers to get a nice round hole with good surface so that the decompressor axis will seal and be properly guided, (bad English?).

Also, you'll have to ream up to fit the seal as well as drill to fit the locking pin.

Good luck!

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I apologize, but I'm confused. Your using an 87 head (dual carb) and the cover doesn't have decomp? I ask because I've got a cover from a dual carb (85 i think) motor with decomp that I don't need and would sell cheap.

PM me if your interested.

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torchevo and uranys, thanks to both of you for the feedback.

torchevo what you said is what I had in mind. Of course I'll try to get the best surface finish I can.

uranys, I think I am more confused than you are. Yes, I have a dual carb head ('87) and the decompressor hole is blinded. It's not welded shut, it was cast blind. Does this mean that the 1987 XR600 had no manual decompressor?

You have PM

Thanks again

Manos

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shit i was confused at the first post :worthy:, read throught it twice to get the jist of it :cheers:. i see your problem, i dont know the answer, and i wish you luck and will be watching thread on your progress :banghead:

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I think all years came with manual release.

The only covers I have seen that don't have it are the XRL or NX covers, but these wouldn't fit because of the cam lobe spacing differences (or your using a single carb cam).

Are you sure it's from an XR600?

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I looked at a bunch of parts fiches this morning, the only head covers that do not have the manual decompressor shaft fitted are the XR650L and the NX650. Somehow, someone has fitted one of those onto your '87 head. :banghead:

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That’s odd. The valve cover off my NX head does have the manual decomp. The cover off my dual-carb XR600 head is blind (actually, all fittings are there, only the hole is blind).

I’m guessing that the previous owner of the NX had the cover replaced at some point, as creeky suggested (:banghead:) and the valve cover I bought together with my XR head did not belong to the same bike. Come to think of it, I’m lucky I have at least one cover with the fittings...!

Uranys, I’m using a double carb cam. Everything from the top cylinder gasket and up is dual carb (but the valve cover, apparently!!!:cheers:)

Manos

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I'm gonna kick myself... I never compared the covers from the single and dual carb models to see if they were the same.

IMG_1288.jpg

They look to be the same with different rockers, so you could use an XRL cover with a dual carb cam by swapping out the rockers and sub rockers as evident by your situation.

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I'm gonna kick myself... I never compared the covers from the single and dual carb models to see if they were the same.

IMG_1288.jpg

They look to be the same with different rockers, so you could use an XRL cover with a dual carb cam by swapping out the rockers and sub rockers as evident by your situation.

Interesting, sounds like one could theoretically use an XRL motor/ head with a dual carb cam/rockers/cover in order to get a manual decomp...:banghead:

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Interesting, sounds like one could theoretically use an XRL motor/ head with a dual carb cam/rockers/cover in order to get a manual decomp...:banghead:

I don't know if I got what you said right...

It depends on what bike you start with. Unless mistaken, all valve covers from all big (>500cc) XL/XR/XRL/NX are interchangeable. The rockers differentiate between single and dual carb models. So, pick the valve cover you need (with/without the manual decomp) fit the rockers for your type of head and fit the lot on the engine. It seems to be that simple.

...Then again, if it were that simple (and I knew earlier) I wouldn't have come to this point, trying to modify things to fit. But that's part of the fun with project bikes, innit?

Manos

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I don't know if I got what you said right...

It depends on what bike you start with. Unless mistaken, all valve covers from all big (>500cc) XL/XR/XRL/NX are interchangeable. The rockers differentiate between single and dual carb models. So, pick the valve cover you need (with/without the manual decomp) fit the rockers for your type of head and fit the lot on the engine. It seems to be that simple.

...Then again, if it were that simple (and I knew earlier) I wouldn't have come to this point, trying to modify things to fit. But that's part of the fun with project bikes, innit?

Manos

:banghead: Exactly, let us know how the project turns out. It's all about enjoying the build.

I have an xrl big bore with no decomp at all right now, so i'm thinking this may be an easy way to add one. Off to Ebay I go...:cheers:

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:banghead: Exactly, let us know how the project turns out. It's all about enjoying the build.

I have an xrl big bore with no decomp at all right now, so i'm thinking this may be an easy way to add one. Off to Ebay I go...:cheers:

Well, yes and no.

My predicament started when I considered replacing the battery with an ultra light (and ultra small) unit – I’ve posted separately requesting feedback on this.

These small batteries are not as strong for as long as the original items and since I’ve gone on the wild side of compression I’d like to help the battery turn the engine as much as I can.

The addition of a decompressor does this (that’s the “yes” part).

The bad news is that my engine only had the auto type that is incorporated on the camshaft (and which to this day I can’t figure out how it works!). The original head, valves and cam were scrapped for a twin carb head and rather wild aftermarket cam without the auto decomp arrangement. Had I wanted to keep the original battery, I wouldn’t have worried too much about the high compression. But I hadn’t. So I worry.

Fitting the manual decompressor won’t be that much of a hassle, I don’t think. But the manual decomp is only controlled... manually i.e. by your finger. Here comes the “no” part: Nothing prevents you from keeping the valve open for a fraction of a second too long while turning the engine by the electric starter. And this is a recipe for a damaged valve and/or piston and/or head etc. I reckon I’d have two options here:

a) De-comp off, turn the engine until it can’t turn any more due to high compression (if it can, it will start so I’m home and dry). Leave the starter button alone and press the decomp to release the compressed air. De-comp off, press the starter button. Hopefully it should work.

:worthy: I have measured about 4mm of clearance between the exhaust valve and the piston at TDC. I need to work out a relation between the rotation of the de-compressor shaft and the travel of the valve. If necessary grind a bit of the shaft off to ensure that at no instance the decompressor drops the valve more than, say 2-3mm. If I manage this, then I will be able to decomp the engine during starting without worrying about hitting a valve.

It is a headache but I agree most of the enjoyment is solving little problems like this (and frankly, if the engine ever manages to start up and idle I’ll be a happy man...!).

Manos

Edited by Manos

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So... did we reach agreement that the castings for the early/dual carb XR/XL covers and the NX/XRL covers are the same? The differences being that the XR and XL dual carb covers have different rockers and, of course that all but the NX/XRL covers have the manual decomp.

Bottom line being that one could use an early XL or XR600R cover on an XR650L if the rockers were swapped. Or am I missing something?

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Stock, there shouldn't be any chance of a valve hitting the piston. The XL and XR 600s that came with the manual decomp can be kicked over while holding the decomp lever to clear the cylinder if the engine becomes flooded. If it wasn't fool proof, they wouldn't have produced such a potentionaly catastrophic mechanism.

Not all covers have the same mechanism for holding the cam chain tensioner shaft in place, those heads that use a pin instead of a bolt to hold the tensioner shaft in place have a protrusion on the head cover that ensures the pin doesn't back out.

New covers and all the workings for the manual decomp for the late XR600s are still available new from Honda. Probally a $350 cost for the total parts needed. IMO better than scraping around for near used up parts at half the cost.

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