Jump to content

77 XS750 Top End Rebuild Help


Recommended Posts

Hi guys! So I posted recently through my top end rebuild and I'm damn near done. I've run into some snags though and could use some help. I called all of my local shops, repair shops, muffler shops, restoration shops, and even some friends but I'm on my own. Shop were all full or not in "Vintage Season"... Anyways, let me start by listing what I have done and then where I need help! Thanks in advance!

**In all steps, I'm using my Clymer manual as my guidance.

-Pulled Carbs and disassembled.
-Chem dipped jets, bowls (note: passages are in the bottom of the bowl as well as jets.), and just got a good cleaning through the carbs. (3 mikunis)
-Replaced air/fuel mixture screw x3 (PO tightened up and bent the tips.)
-Replace needle jet and retainer in middle carb as it was different than the other two and the plastic retainer was broken. (Odd contraption)
-Reassembled carbs and replaced the "manifold boots"? that connect it to engine. Added inline fuel filter so as to be able to easily see fuel quality. (Note has a relatively high capacity in filter and installed correctly. Not thinking it has any impacting effect on any relative issue.)
-Measured pistons and cylinders (all in spec and intact)
-Replaced all Piston Rings (no gaps lined up all to spec)
-De-glazed/honed cylinder walls.
-Oiled rings.
-Oil rinse pan.
-Put cylinder back over pistons with new gasket.
-Cleaned the area underneath where the seat of the valves are.
-Replaced Intake shims to fit spec (Exhaust shims were still within spec) (Likely PO work, as this was as far as anybody had been into this bike as I could tell when disassembling.)
-Put Cylinder head back on with new gasket.
-Tightend cylinder head bolts to spec. 25 up top 14 on the two below.
-Helicoiled center spark plug hole (PO cross threaded)
-Reinstalled cams with lube in requested areas. (No marks were as described in any manual as far as placing them back in a certain way, there was a dot of red/orange paint on the cog of each as well, however, pretty sure I've got em in right, shim clearances are all within spec now.)
-Re installed new master link.
-Remasterlinked the cam chain.
-New gasket on cylinder head and fastend down cylinder head cover.
-Replaced exhaust studs and nuts. Honestly question that system of studs flanges nuts and retainers and how they efficiently do their job...but it was 77..we've come a long ways.
-Reinstalled exhaust.
-Clamped one loose muffler to the exhaust pipe. (PO had it hanging slipped on). Improved running sound.
-No fire/fuel button turn over x3-5 times to shift some oil around.
-Full crank.


Ran better than before for a bit. Had a little white smoke off the rip that settled. Idled at about 1100-1200 rpm. Died out after a minute. Fastend all 3 air/fuel mixture screws and then backed em up 2.5 turns as manual said. Cranked and idled low. Slightly tightend idle adjustment screw. String of bad cranks. Popping while running. Loosened #1 air/fuel mixture screw 1/4 turn. Crank and runs just right. After a minute dies out. Notice 3 slightly watery oil drips from front left of engine and stop.

Now here's where I need help.

-Oil leak. At front left jug/head area...will include picture.
-Carb adjustment syncing? (Never done it) Ordered the tool I'd need to go through that process but any tips would be good.
-Outside of that any tips on the tuning of the carbs? I find that with how it's set now it'll run for a bit then slowly die out. Also if I throttle to a certain point there's no reaction to engaging the throttle any more past that point.

I figured the collective knowlege and experience would give me the boost I need to wrap this project up. I've never worked on an engine. The most extensive work I've done with engines is removing the intake manifold off an 05 titan to replace the starter. I've grown up with vehicles that I've needed to tinker with but usually lighter things like carb work. Any tips help! Appreciate you guys! If a picture is needed for reference just tell me and I'll get it!


This was the first crank after everything. My phone doesn't record audio but I got it nonetheless. Doing another crank with laptop video and will post here in a few!















Link to comment
Share on other sites


OK! Added video of secondish crank (some adjustments before I recorded). Actually running pretty good! Gonna let it sit for a second and go again to see if I have same result! Video with sound!

Did not get the same result twice, unfortunatley. Wouldn't hold it's idle w/ or w/o choke.

Link to comment
Share on other sites

You've got one oily plug, was that before or after the rebuild?

Link to comment
Share on other sites

3 hours ago, smitty_jc7 said:

That was before! 

And are they all the same now?

We could really do with a video of what it's doing WITH sound... lol

  • Like 1
Link to comment
Share on other sites

9 hours ago, Snakebite68 said:

And are they all the same now?

We could really do with a video of what it's doing WITH sound... lol

There we go, think i got it now...had to keep trimming and trimming as the forum wouldnt let me insert

9 hours ago, Snakebite68 said:

And are they all the same now?

We could really do with a video of what it's doing WITH sound... lol

I also want to mention for that vid. I went through about 15 minutes of turn overs no cranks...all close...to this....then it was back to turn over no crank. This was the best is the point im making...not the norm

Link to comment
Share on other sites

Took my carbs off today and did a bench sync. Like I said first rebuild...got caught up in all of the other work I was doing that I completley overlooked a simple step. I've bench synched em now and im grabbing some new sparks so I can read the current situation. but hopefully that gives a much better outcome. Will replace prior video with new if good outcome.

Link to comment
Share on other sites

Okie dokie! That was a much better outcome. So I used 2 small identical bits, and I synchronized #1 and #2, then #3 and #2 treating #2 as master. Then I set the idle adjustment screw till it made contact. I've recently discovered thesre are Mikuni B34 Carbs, pretty sure on that. Anyways, remounted them and cranked her back up. Good news, she runs. That being said it was running pretty good so I decided I'd do a leak test and let it run for a second. I noticed after about 30s it idled down to about that 1100 mark from 1300-1500 upon start. I was spraying a soap/water mixture and when I got on to the right side...where the original leak was...between cylinder and cylinder head (gasket has been replaced now with new)...the spray killed the engine. UH-OH...so I check my bolts over their with my torque wrench and all but the far edge one was on spec.  I cranked up again and it was just running straight up on the rpms so I hit the kill switch. Did that a couple times...Soooo I decided to tighten up that one bolt a bit but it seemed like it was on the edge of not having a hold on the threads anymore so I stopped. ( Torue was not giving me the click either. Although it originally did when I first reassambled. So do I have a carb issue? A gasket? A bolt? Point me to the right direction first! lol

Link to comment
Share on other sites

No vaccum leak found with starter fluid at boots or head gaskets or anywhere else i checked. I did find a missing cap on the LH carbs vaccum port but I capped that up. I have it running around the 2000-2100 mark now. But it's oddly random. It kinda jumps up on it's own and settles back down. I gave it time to observe...and it did that twice while I had it running. 2000....3500....2000....3500. It got to a point actually where I could start to cut back on the choke and dial it back to that 1100 mark but too much and it killed the engine. Kinda wondering if that means I should just adjust my idle adjust screw or synch adjustment? Another video?

Link to comment
Share on other sites

Ok... I'm spitballing here, but...


When you had the head off, did you:

1/ Re-seat/lap the valves?

2/ Change the valve stem oil seals?

3/ Change the valve springs?


Is it possible you have:

1/ Messed up a piston ring?
2/ Tightened either end of the crank too tight?


Looking at your pictures, is that a rusty rod on the middle piston?
If it is, have you had the bottom end of the engine open?

Link to comment
Share on other sites

@Snakebite68 I feel pretty good about the rings. As for the crank, if I'm thinking correctly, I did not mess with. I also did no valve work. I was talking to somebody today about that and kind of regret it since I had it apart. However, I found a leak. It is confirmed. I'm running y'all through it with this one, sorry. I believe the cylinder bolt that was questionable, wouldn't quite torque like the rest is the culprit. The leak is at the upper cylinder head gasket there in line with that bolt. What I am going to try to do, as it is the #10 Cylinder bolt (10 up top-2 below)(2 of 10 up top are bolts rest are crown nuts), is remove all cylinder head bolts, as well as any pre-requisites if any, then I'll helicoil that #10 internal thread hole, I'll use my tap-die set to clean up that bolt, I have a potential replacement on the way in case. Then simply re-insert/re-torque those cylinder head bolts as manual describes? Can you confirm that that is a potential solution to the current issue?

I did replace, into spec, the valve adjustment shims to allow proper valve seating. On a water test I found no leaks as well. Still, not proud of not inspecting them further like I did everything else. 

Link to comment
Share on other sites

Whether that would cause the problems you've been having, I don't know, but if nothing else, it will stop the leak which is one fewer problems you have to deal with. It's possible that the effects you are seeing are a result of several things...


I've never done it personally, but lapping your valves looks straight forward enough. Might be worth doing while you have it open again.

Also while you have it open... Get an engineers straight edge and make sure the head is completely flat! No point putting it back together again if it isn't.

  • Like 1
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Who's Online (See full list)

    There are no registered users currently online

  • Recently Browsing

    No registered users viewing this page.

  • Create New...