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Butta

Help! First time with RFVC head.....

12 posts in this topic

Looking into my new (used of course) '96 XR600R.....planning on putting in a new topend to freshen it. Checking the valves before I tear it apart and they were ALL too tight, which doesn't surprise me because of who I bought it from (old guy, only owned it a month, fix and flip type deal). Anyway, I got it cheap enough and I'm not afraid of what he's done to it.

I'm trying to get my mind around how this head works. I've become intimately familiar with YZF heads, and understand their workings with the decompression and all. Now, looking at this one, I'm not sure of it yet. I did a compression test on it: 5,000' elevation, 30 degrees, throttle full open, kick kick kick. Only showed ~50 psi. The book says 70 +/- 14.2 (not sure what it should be for 5k, but that seems about right) IF the decompression is effected. What does this mean? Without the decompression effected, it should be upwards of 200 psi, and I can't imagine the thing would even start with that much of a compression deficit (it did start and move under its own power).

Thanks in advance for the help!!!

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Yeah, I am going to check the compression once again with the valves adj properly, and back the RH exhaust adj out so that the decomp won't be effected.....I think that's right.....

??

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The decomp is on the cam and pushes down on the right side exhaust valve. You can "turn off" the automatic decomp by loosening the right exhaust valve adjuster several turns so the automatic system doesn't push it far enough to open the valve.

In my experience the bore and piston are the last thing to go. On my XR600 with a zillion hours and last year the rings were completely shot with and end gap of about .175 when it's supposed to be .020. However, the piston and bore hardly showed any wear. New rings and a hone job and she was good to go. If you have the proper tools you can easily measure these parts. The valves are a different story. In my experience they go before the rings. I had to have a few valves and seats replaced about 3 years ago. The oversize seats are not an OEM part, but can be made by knowledgeable shops. If you do some compression tests with the adjuster loosened you will be able to tell whats leaking.

If you take the jug off, take a look into the crankcase and see what the trans gears look like. Some wear isn't too much to worry about, but heavy pitting on the gear faces is.

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That's right. You'll never get the correct compression reading without backing that rocker way off. Bruce

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Thank you very much! That's the answer I was searching for. I was wondering if these were auto decomps, and now I'm not as concerned as I was. I'm still tearing into it, for reliability sake, but sounds like it should be fairly benign.

On a side note, this is my "other" 600. I bought this one first, two days before I found my fully farkled 600 that I intend to keep. SO, this one I'm tearing into now is a practice run for my main bike, and will be eventually sold in the spring.

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If it doesnt have a auto decomp it will have a cable operated decomp actuation lever on the RHS of the cam cover that can be backed off (much handier for compression testing in that respect)

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Well, got the jug pulled off and a quick inspection of the piston and cylinder shows a good amount of blow by past the rings, and signs of possible piston scoring on the skirt (might explain the sound I heard when it ran)....So, I'll look into honing the cylinder, slapping a new piston set in and go from there. Transmission gears look good, no noticeable wear to speak of. Cam looks darn near new, no signs of scoring or wear on the lobes or the journals.

Should run like a raped ape in comparison!!

?- These are Nikasil plated, right? Can you hone Nikasil? Any different process compared to an auto engine?

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A small number of the xr's run a plated barrel but most are an iron liner.

If yours is plated then you need to measure the bore and see if it needs a replate

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you may want to look at replacing the timing chain , tensioner and check the guides/sprockets, etc...

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Well, got the jug pulled off and a quick inspection of the piston and cylinder shows a good amount of blow by past the rings, and signs of possible piston scoring on the skirt (might explain the sound I heard when it ran)....So, I'll look into honing the cylinder, slapping a new piston set in and go from there. Transmission gears look good, no noticeable wear to speak of. Cam looks darn near new, no signs of scoring or wear on the lobes or the journals.

Should run like a raped ape in comparison!!

?- These are Nikasil plated, right? Can you hone Nikasil? Any different process compared to an auto engine?

My old '96 XR600 was iron. I think the only Nikasil plated ones were '89 and '90? Hopefully someone who knows for sure will chime in. Bruce

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