84 XJ900: should lap valves for 20psi compression difference

Hi all,
I've been a regular lurker on this site and have learned lots thanks to all.
I have been restoring an 84 XJ900 and when compression testing I found that No.1 cylinder was about 120 psi and all the rest were 140-145. I did the "oil in cylinder test" and no real change. So I think the cylinder is losing pressure in the valves. Looking down through the spark plug holes, with the ehaust valve held open with a cable tie I see that this valve does not have the clean contact ring that the other valves have.
20 psi is the recommended maximum differnce in compression between cylinders.
Does anyone have any guidelines or rules of thumb in regards to whether I will need the valve seat recut or whether that should be able to be fixed just by lapping. I have read that valves can actually be lapped too much (?), putting too wide a contact on the valve face to the seat.

Any advice guys ?

 

Good question.

First, a couple of questions about the history of the bike:

Was it a "runner" when you got it, or had it been sitting?

Has it been run much prior to the compression test?

What were the valve clearances like when you started?

And here's why I ask:

The "wet" test result and the fact that you can visually tell the valve wasn't seating would, under normal circumstances, make the answer to your question a resounding yes.

However, when a motor is at rest, one or more valves is going to be open. If that was a valve that was propped open for 20-some odd years, it may have oxidized enough to have lost it's "shine." If that's the case, you could see the "clean ring" reappear after a few hours of running.

Or not. Which is why I asked about the clearances.

The way to determine whether or not a valve seat needs to be re-cut or just lapped is determined by inspecting the condition of the seat and its "lappability" after you take it apart; and by the results of the intial "lap."

Yes, valves can be lapped "too much" if the lapper doesn't know his craft. Reconditioning valves and their seats is not difficult, but it does require patience, a steady hand, and understanding what it is you're trying to accomplish, ie: the width of the seating "ring" and its location, both on the head of the valve and the seat. When the lapping process begins, if it becomes immediately obvious that the location of the "shiny ring" (the seal) is too far up or down the seat, or in the wrong place on the valve, then the seat is lightly cut to correct whichever angle is throwing it off.

Honestly? Anytime I've been working on a head and ran into a situation where the seat needed to be re-cut, that's where I folded and took it to a machine shop.

You can't do any real damage over-lapping that can't be corrected by re-cutting the seat. You CAN ruin a head by re-cutting a seat incorrectly. The valve seats in the XJ are three angle compound cut, not for the faint of heart.

So my rule is: lapping = DIY; seat cutting = machine shop.

However, if you want nice documentation of the whole process, it's covered very well in the factory books. PM me with your email address and I'll scan those pages out of the 650 book. While some of the actual dimensions are likely going to differ, the process itself is the same and very well explained.

 

+1 ^
If you just corrected a "too tight" valve I'd ride it and see if it improved.

If it's a warped valve, it will be obvious rolling it on a piece of glass.

 

^This.

Give it a good run regardless and recheck.

 

Thanks for the replies all, in particular bigfitz52.
The compression has been low on that cylinder I think since I got that motor into the bike, (after a lovely little old lady drove her car into the side of my bike as I was travelling along causing the crankcase of the old motor to be cracked, so a second hand motor was installed).
About six months later a shop bike mechanic compression tested it as part of a rough running problem and found it to be low in No.1 cylinder.
I've checked valve clearances, reshimmed to spec. and ridden the bike for a few hours: compression is still low in No. 1 cylinder.

So my plan now is to take the head off, clean up and inspect valves and seats, lap all the valves, get seats recut if necessary and install new valve seals.
Maybe polish the ports a bit with a Dremel while it's all apart ? (is that a good idea ? Or not ?)

I was about to balance the carbs but didn't really want to set all cylinders down to the performance of a poor sealing No. 1 cylinder, which will only get worse over time.
Thanks again for your help guys, this bike was just about to go to the wreckers till I found this site....

 

Sounds like a plan.

I wouldn't polish the ports; however the castings are often a bit sloppy and it wouldn't hurt to remove any "webs" or ridges of casting flash that are sticking up into the airflow.

 

JEEZ !! 8O How'd your leg survive THAT??

Another place to clean-up ports - there usually are cut edges where the iron valve seats are inserted into the head. Blend the edges away BEFORE taking the head in for work.

 

glad you found it

 

"JEEZ !! How'd your leg survive THAT??"

Nearly broken, was black and blue from ankle to thigh. It's an ill wind that blows no good though, her insurance wrote the bike off, I bought it back for a song and ended up finding XJBikes!

So, a quick whiz over any sharp edges with the Dremel grinder, after the valves are out.

Thanks again for the help guys!

 

When I got around to doing the Head on my 900, ... I found the Intake Ports didn't need much work on them at all.

There was one Casting Imperfection in an Exhaust Port that needed smoothing-out.
Since I bought the Dremel and Flex Wand to attack that tiny ridge that Exhaust was accumulating on, ... I did as much of the Exhaust Ports as I could reach with the Dremel, ... rendering them pretty nice and Ported for an old man.

This was at 34,000.

I Lapped all the Valves.
Had the Guides "Knurled"
Replaced all the Valve Seals.

The whole shebang got De-carbonized and smoothed-out.

The Polishing Piston Crowns will spark an argument.
Some say its a Performance gain and a preventative measure alleviating future Carbon build-up.

Others say its a detriment to cooling.

If you consider yourself a Rock-'n-roll Hot-Rodder,... go for it.
You won't melt a Piston unless your Mixtures are way out-of-whack.

 

Yep the flexi hosed Dremel what a handy tool. Probably will tart up the outside of the head with that as well once the work is done.
Sounds like intake valves and valve guides are usually relatively safe areas on these motors from what I've read.

What is "knurling the valve guides" Rick ? What does that involve ?

 

A Guide knurling tool is run through the guide, it creates a spiral groove.
The groove acts like a furrow and raises the metal on each side. This effectively reduces the inside diameter of the guide. Then, a Guide reamer can be used to resize the guide back to its original dimensions.
The grooves also help to control and seal oil better than a smooth bore guide.
This will allows employing somewhat tighter guide-to-stem clearances.
It rejuvenates the Guide when the wear is minimal and saves the time and expense of having New Guides pressed-in.

Severely worn Guided should be replaced.

 

I would NOT "knurl" a valve guide in a high-RPM motor like an XJ.

Rick, you're betraying your age. Knurling valve guides was something that was common in the sixties (and sometimes quite effective with the soft bronze guides in British sports car motors.)

I'll bet you couldn't even find a modern machine shop that knows what it means (unless they have an old fart on staff.)

It's a lost art, like "grinding" valves. (Most modern valves cannot be ground.)

 

When I sent the 900 Head out, ... they told me that they Knurled a few of the Guides.

Maybe I paid for some "Air."
It's possible.

But, it did quit using Oil.

 

I'll have to see if the valve guide clearance is excessive when I get the head off. I'm actually not expecting it to be too bad.
If they are badly worn I will probably just put my spare head on: I have a spare 851cc (93) motor which should have the same head as the '84 891cc motor (?)
If they're both badly worn: does anyone do bronze valve guide liners ? (Chacal ?)