Burning oil

sbtm

XS650 Enthusiast
Messages
36
Reaction score
0
Points
6
Location
Denver, CO
I have an 80 xs650 thats been mostly sitting for at least 15 years. When I bought it 5 years ago, it started and smoked out of the right side. Recently a friend and I have been trying to piece it together for him to ride.

I did a compression test today, the left was 140 cold, 120 hot. The right (the smoker) was 130 cold and 120 hot. I am suspecting valve seals at this point. I went ahead and pulled the engine and head and jugs.

The right piston skirt had some scoring, the cylinder is smooth. The right jug (outside) and piston arm look like they have been cooked... they are pretty dark. I also gapped the piston/cyl clearance... its at least .007in on each cyl. I suppose I could check the rings to see if theyre oversized or not (they looked in really good shape), i believe the pistons are stock.

So, since I have decent compression does that eliminate bad rings/pistons? I am going to replace the valve stem seals since there is just so much oil.

Any thoughts?
 
Have you done a leak down test?

In a healthy motor, compression should build up rather quickly. Low compression reading on the first stroke, followed by very gradually increasing compression on successive strokes, indicated worn piston rings. A low compression reading on the first stroke, which doesn't build up during successive strokes, indicated leaking valves. Heavy carbon deposits on the valves could also cause lower than average reading as well as a possibly higher than average reading. Record the highest reading that the gauge stops at after cranking for a bit.
If the readings are much below normal (standard) readings, try and add some oil, about a teaspoon's worth, directly in the the spark plug hole and repeat the test.
If the compression increases significantly after the oil is added, the pistion rings are definitely worn. If the compression doesn't increase significantly, the leakage is occuring at the valves.
 
I have an 80 xs650 thats been mostly sitting for at least 15 years. When I bought it 5 years ago, it started and smoked out of the right side. Recently a friend and I have been trying to piece it together for him to ride.

I did a compression test today, the left was 140 cold, 120 hot. The right (the smoker) was 130 cold and 120 hot. I am suspecting valve seals at this point. I went ahead and pulled the engine and head and jugs.

The right piston skirt had some scoring, the cylinder is smooth. The right jug (outside) and piston arm look like they have been cooked... they are pretty dark. I also gapped the piston/cyl clearance... its at least .007in on each cyl. I suppose I could check the rings to see if theyre oversized or not (they looked in really good shape), i believe the pistons are stock.

So, since I have decent compression does that eliminate bad rings/pistons? I am going to replace the valve stem seals since there is just so much oil.

Any thoughts?

The first thing you need to do, is to read the Yamaha Service Manual. Go to www.biker.net, and you will find the service manual. You will be able to find the section that deals with measuring the engine internals for wear.

Did you measure the piston/cylinder clearance at the right location? If you measured 0.007", then you will have to go up to the first over size pistons and then have the cylinders bored to match the new pistons.
 
Since you have it apart this far, it would be a good dea to replace the valve stem seals no matter what. They are cheap and easily installed.

When you say the right cylinder and piston arm (I take this to mean the connecting rod) are cooked, is it possible to post a picture? Also pictures of the pistons would help.

If the rings are sealing well, you will generally have a dark band above the upper compression ring, you may have a few small dark streaks on the parts of the piston below the oil ring. That's because the rings are doing their job of keeping the combustion gases where they are supposed to be. If the piston has a lot of dark discoloration below the ring grooves, then the rings are not sealing, that's called 'blowby'.

Piston in pretty decent shape:
Piston_rings.jpg


Piston showing blowby residue:

4064814720_ccf4286b49_o.jpg


Like retiredgentleman says, make sure you have measured the piston to bore clearance at the right spot. Look for a 'ridge' at the top of the cylinder also. You'll have to remove carbon to see any real ridge in the metal. If there is ~any!~ ridge at all, you need to bore the cylinders and go 1st oversize. Otherwise, that ridge could break new rings and cause tremendous damage.

0900c1528027a156.jpg


These engines were made to much tighter tolerances than the average small block Chevy (where you could generally check engine clearances with a wooden ruler and still get it to run) so I would avoid the olde tyme 'ridge reamer' so called 'fix'. If there is no ridge, the bores check out as stock and a new piston puts the bore to piston clearance where it should be, then you can probably just 'flex hone' the cylinders and use new pistons. But honestly I don't think you will be that lucky.
 
I measured the clearance with the piston at the bottom of its stroke, and its actually up to .010", does that mean i should go for 2nd oversize? The compression test started low (30 psi first kick) then did gradually build. I guess i dont need anymore convincing on a rebuild.

Im located in Denver, CO. Anyone recommend a shop for the boring? Ive called a couple motorcycle shops, who wont tell me what machine shop they use, they just want me to drop the cylinders off for them to send to "some undisclosed location".

I took some pictures, not very good though, the flash and lense on my phone are broken (about the same state of repair as my motorcycle). And if the links are broken, ill have to fix them tomorrow.

Thanks

IMG_20111202_095545.jpg

IMG_20111202_095555.jpg

IMG_20111202_095614.jpg

IMG_20111202_095622.jpg

IMG_20111202_095632.jpg
 
Ive called a couple motorcycle shops, who wont tell me what machine shop they use, they just want me to drop the cylinders off for them to send to "some undisclosed location".
I'd ask about machinists at some parts counters. Pretty soon you'll get an idea of who's good. And maybe who to avoid too.
 
Those dark streaks on the right piston skirt look like seizure marks to me. There's also some grooving in that cylinder which says the same thing. I would not try to reuse that set of pistons. You need to get a machinist to mike the stuff out for you and get an idea of just how much of an overbore will be nneeded to clean that up. I'm thinking it might need an .040 overbore (second O/S, IIRC).

A city the size of Denver has got to have some machine shops. I agree with xjwmx, ask at a few auto parts stores. NAPA stores generally will have one central machine shop in a given city, BTW.
 
I've been making calls all day (I'm at work). I was quoted 65 per cylinder for the bore, up to 1mm over (4th over?) By two shops. Sound about average?
 
That's pretty close. Remember, they will have to make probably 2-3 passes through each cylinder (plus honing to final size) so it takes more time and is more involved than it looks.

Be sure to get some references and make sure of what OS pistons/rings to order. There's few calls I like less than the one that contains the words 'hey, we have a little problem here...'

While you have it down and are doing the valve stem seals, you might as well lap the valves in. It's easy and can save some future aggravation. I'd suggest getting the brass head nut washers, too. Mike's XS sells them, along with other suppliers.
 
Bore sizes increase by .25mm or about .010" on the 650. 4th oversize would be a 1mm or .040" increase. Original bore size was right around 75mm. There is a 3 digit number stenciled on the side of one of the cylinder sleeves that stick out of the bottom of the cylinders. This is the fractional part of the original bore size as in 75.XXXmm. There is a 3 digit number stamped on the top of each piston. This is the fractional part of the original piston diameter as in 74.XXXmm.
 
Back
Top