1977 Yamaha XS750D

Well that wasn't very promising. 60 psi across all 3 cylinders

Even added a bit of oil to check if it was the rings like the manual suggests and nothing changed. I don't think I really need a leak down tester with results like this. I'll do another test tomorrow after I get the bike real hot to see if that drastically changes the results at all. If not looks like I'll be pulling the head off.

This engine rebuild has been a real P.I.T.A
 
I dont recall and forgive my laziness for browsing via phone, but what all did you do to the head? We're valves lapped, seals and guides replaced, and vacuum tested?
 
You'll be able to visually confirm if its valve related, if they are all oily down the stems, they leak there. Overhauled the top end on an xs1100 last year, valve stems were oily, seals leaked, smoked like a fisherman.
Edit: thats more probably pistons yea
 
From what I can remember the machine shop replaced one valve that was bent, 3 angle valve job on all, and probably replaced the valve seals. Guides I'm not sure, I'll have to make a call tomorrow to see if they have it all on file.

If it's pistons/cylinders then I'll be going with a big bore kit, if it's head related I'll take it back to the machine shop since I have a two year warranty

Ohh yeah and the clutch has now decided to start slipping. clutch springs and discs are all new, and the mechanism adjuster is set basically perfect. If i turn the screw counter clockwise any more the bike will just barely creep in 1st gear with clutch lever pulled in.
 
clay69camaro said:
From what I can remember the machine shop replaced one valve that was bent, 3 angle valve job on all, and probably replaced the valve seals. Guides I'm not sure, I'll have to make a call tomorrow to see if they have it all on file.

If it's pistons/cylinders then I'll be going with a big bore kit, if it's head related I'll take it back to the machine shop since I have a two year warranty

Ohh yeah and the clutch has now decided to start slipping. clutch springs and discs are all new, and the mechanism adjuster is set basically perfect. If i turn the screw counter clockwise any more the bike will just barely creep in 1st gear with clutch lever pulled in.
Click to expand...
Did you soak the plates in correct oil prior to installation? And do you use the right engine oil?
 
datadavid said:
Did you soak the plates in correct oil prior to installation? And do you use the right engine oil?
Click to expand...

I soaked them in the same oil I used for the break-in, which was lucas 30W break-in oil. Now it's filled up with 20w50 conventional yamalube. The engine has about 250 miles on it now and it slipped under pretty light load multiple times today.
 
Thats really weird.. mine slipped a few times around the ton at full throttle.. probably 30 years old plates. But now they grip enough to pop wheelies.
Must be adjustment. Screw the bottom one in a bit more. Did the trick for me.
 
I think I'm just gonna pop off the right side cover and take a look at the clutch, do the second gear fix on the left side, and order up a big bore kit 836cc. While the head is off I'll inspect/replace valve seals get it all cleaned up and do a tiny bit of port work, not rework the head a whole lot but just smooth out all the casting marks. Unless I convince myself to build a home made flow bench and go at it with the port work

Also I could not stop thinking about chopping the hoop off last night and re-welding it back on at an angle to give myself more clearance. Right now the tire will rub the hoop on big bumps and I really want to mount my license place centered in the back. Kind of like the bike spirit of the seventies "S7" and re-work the mounts for the seat with tabs welded to the frame instead of big spacers mounted to the electronics tray.
 
I went through the same thing on my xs750....made the hoop and then decided I needed clearance and cut it off and re-welded it at an upswept angle. it worked out really nice for the tail light and plate too.

1008689_1459895157557947_1696849207_o.jpg


1507127_1459895164224613_1056166377_n.jpg


tail light I used was from DCC, made the bracket from some sheet metal and bent it up to hold the plate.
 
Got the hoop done today and got the seat roughed in





The seat has more of a point in the tip and the hoop is really round, not too sure how much I like that.

 
Talked to my buddy today about my poor compression also the machine shop.

Machine shop is a little baffled on my oil consumption, they are interested in leak down test results. They also told me there wasn't a parts charge for valve seals, but there's no way they would disassemble the head and do all the valve work only to reinstall old seals.

Buddy who is a moto guzzi mechanic asked if I held my throttle wide open for the compression test. I did not, seems I haven't done a compression test in a while so I forgot that little step. So plan is to warm it up Sunday/Monday and give it another test, and borrow his leak down tester sometime this week to get those results. I'll be glad to find the leak and get it all plugged up so I can enjoy the bike.
 
Any trick fuse box's anyone has experience with? I'm really not happy with all the wiring mess. I wanted it all to be very clean, but I feel like a made a bit of a mess on some things.

Going to re-do the wiring with a cleaner fuse box and smaller connectors. I previously bought some that have all water tight connections and stuff, but it's kind of overkill so I want to convert those plugs to something a little more stream lined.

Re-doing all this stuff is still cheaper than paying someone to do all the work haha
 
I've gotten lucky that we have a giant Pep Boys auto parts store here with an entire aisle about 40' long that's just wiring stuff, got a nice little 4-pot fuse box that has a clear plastic cap that clips onto it for like $12.

their website unfortunately sucks and I can't find it to link you.
 
Tore the top end off last night. Bike was just smoking so bad and sputtering and all that. I pulled the rings off pistons put them in the cylinders and measured the gap. They were massive, way beyond the maximum limit listed in the book. So taking the cylinders and pistons to the machine shop for some accurate measurements.

New rings and gaskets ordered, possibly will send out the pistons for some anti-friction coating on the skirts from swain tech.

I'll follow up the progress as it comes. I got a new bike to work on so I'm trying to finish this things so I can start the next.
 
Hopefully the cylinders wont turn out out of spec, otherwise i recommend getting 850 cylinders and going for the 896 kit! Super happy with mine.
 
Cylinders are still in spec, well at least the bottoms are, the tops are a little loose but I don't think they are too loose. I'll be sending the Pistons out to swain tech for a piston skirt coating. Take up .002" and snug the Pistons in there a tad. If I have to I'll buy oversize rings and file them down to spec. I'll check the new rings I just bought tonight for proper end gap.


Sent from my iPhone using Tapatalk
 
Cool stuff that coating, i also heard about thermally isolating the crowns ceramically, not that these engines would require it (when used sensibly..) .
 
I finally shipped the pistons off last week. Decided to get the skirt coating and the crown coating, the crown coating was only like $12 or $20 more so I opted for it.

Getting hitched in a few weeks so the engine will get back together late in October or early November.
 
You must log in or register to reply here.

DTT Light or Dark

  • DTT Light
    • www.jadusmotorcycleparts.com
    www.lostapostle.ca/
    www.sparckmoto.com

    Spread the Love

    Back
    Top Bottom