Topic: 1972 CB750K2 (start from scratch) project. Cases back together

[Davez134]

For what it's worth Dave, I think it sounds KILLER without the baffles. As long as you have enough backpressure, which I would imagine it does, my vote is to leave 'em out. Awesome build. Well done! Next time I head out that way for SEMA my brother and I should put the Cb750s in the trailer and ride with you!

Bring em out! I'd love to go for a ride

[Davez134]

A couple things came up at once last week, so I had to order some parts. Small things, but annoying. First I heard air sucking in from one of carb boots. They were all new honda ones, but the culprit one had a bunch of surface cracks and looked "old." Replaced it. Second, the black/white wire from 2/3 coil came un-soldered from coil body during a ride, fixed that (cut out instantly and I feared the worst, glad it was easy) finally fine-tuned the jetting. Rafi, I know you were asking for a ballpark for your next build since motor will be similar...122 mains with this setup is perfection! Sad thing is I got all that done this morning, including carb r/r in 45 minutes!

[edwardmorris]

How does that 122 main work? 120 main + needle position? Or did you drill it out?

You've got me concerned about the boots, I've got new Honda ones waiting to go on as well. Did you happen to use any silicone lube when installing them? I can't recall where but I read it helps with installing AND preserves the rubber a bit better....

[Davez134]

I noticed I was a bit lean at wot, close but little hesitation that went away as I slightly closed throttle, then bike accelerated. While ordering the new honda boot from 4into1, (they are close to me, in Cali) I saw they had 122 keihin jets in stock so I bought some. I had already tried 125 and was a bit rich. Needle is in 4th position from top, idle jet is 40.

As far as boots go, all were good except that one just looked "dry." Also you do not need to crank those clamps down, just snug. I think I may have had em too tight, contributing to leak on that boot. I did use silicon lube when installing, super easy.


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[edwardmorris]

Good to know, thanks for sharing!

[Davez134]

Noticed a drop of oil under the bike today. After some investigation, unfortunately I found that the crankshaft seal is the source. This sucks. One question I have is...somehow I did not get the seal flush all the way around. Hard to explain but shows in pictures.


You can see bottom right, it sits in more, and top left it is flush with outside of case. Anyone have a pic of how their non leaking seal sits? I almost want to try and drive it in more, but not sure if that will make leak worse.



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[edwardmorris]

Drats! I've gotta check mine again. I used permatex on that as well as the final drive shaft seal just for added insurance. Did you install yours dry? I'm curious if it can be changed out without needing to split cases. Or there's always the hammer and drift fix (DONT do it).

[Davez134]

I used hondabond around it, but somehow seems to have got misaligned from square in its spot.

[flybox1]

Because this seal has a lip around it, cases do have to come apart to replace it   
You could remove the advancer shaft, clean the area really well, and seal the outer edge of it with some permatex or the like.

[Davez134]

Yeah, I know cases have to come apart to replace. That's why i saw so upset when I saw this. The cases are not coming apart. See in the pictures how it's not sitting square? I don't have any close up pics that I took of the seat where the seal goes. I kinda want to give it a few taps to see if I could square it up in there. But I'm not sure what it looks like behind there, and I don't want to drive that seal back if there is space behind. Not sure if that makes sense?

[madmtnmotors]

Yeah, I know cases have to come apart to replace. That's why i saw so upset when I saw this. The cases are not coming apart. See in the pictures how it's not sitting square? I don't have any close up pics that I took of the seat where the seal goes. I kinda want to give it a few taps to see if I could square it up in there. But I'm not sure what it looks like behind there, and I don't want to drive that seal back if there is space behind. Not sure if that makes sense?

I don't think "driving it in" will solve the issue. From flybox's picture, it looks like the seal should finish flush. It looks like one side of your seal is already pushed back too far. Damn!

I do remember a write up by someone that replaced this seal without splitting the cases. IIRC, the process went something like:

>Remove the old seal, making sure to get any remnants that might get left "in the groove".
>The outer edge of the opening in the cases needs to be beveled (kind of like the bottom of the cylinder sleeves to accommodate the piston rings). This provides a "ramp" for the lip of the seal to follow, instead of getting sheared of by the square edge of the engine casting. This would likely be easiest to do with a Dremel tool and the appropriate cutting tip. Pack the hole first to keep shavings out. Maybe sand it really smooth with some super fine sandpaper to finish it off?
>The seal was pressed in slowly using the advancer shaft with a nut and an appropriate stack of washers/shims/spacers. Don't recall the specific application of lube and or sealer. I imagine this would work best if liberally lubed with maybe some "Hondabond" or equivalent in the groove?

Pretty sure it was this forum that I read that post...

[Davez134]

Thanks for the reply. Yeah, that post was here, I've read through it. I'm going to try and lightly tap it square in there and go from there. Not really going to drive it in, just even it out. Is is a Very slight leak. I'll report back on it in a hundred miles or so.

[Davez134]

30 mile ride, dry in there. Fingers crossed, but squaring it up seemed to do it.

[Davez134]

So simple things make me happy. Calling this fixed! For anyone with a new and leaking crank seal, make sure it sits in there square! Was seeing oil in here every time after opening up. 50 plus miles and all dry



Damn you engine, as much as you try, you will not leak on me!!!!!


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[SOHC4 Cafe Racer Fan]

Hey Dave,

What's left or is there anything left to button up?  Or are you busy riding it like you stole it? 

Great bike!

[Davez134]

Hey Dave,

What's left or is there anything left to button up?  Or are you busy riding it like you stole it? 

Great bike!

Nothing left, just riding it. Mornings only now though, getting in to the 100's temps here now.

[SOHC4 Cafe Racer Fan]

Hey Dave,

What's left or is there anything left to button up?  Or are you busy riding it like you stole it? 

Great bike!

Nothing left, just riding it. Mornings only now though, getting in to the 100's temps here now.

Oooof! Vegas baby, Vegas.

[Davez134]

So, I am going to embarrass myself to save someone else any problems/headache in the future. The only leak I have from this motor is from the crankshaft oil seal. I thought I fixed it by lightly tapping it in and squaring it up in its place but leak came back even worse when I rode it hard last week. Now I know why. Look at this picture, I installed the damn thing backwards!!!

The metal ring is supposed to face inward! The oil seal I got from cycleX did not have a pronounced lip on one side to make it obvious, and I had never used a seal with a metal ring in it before. I know the "open end" of a seal should generally face toward where the oil is. I now realize, when the metal ring is removed that this is the open end.
I ordered some universal 30x42x8 crankshaft oil seal without a lip, cleaned area, applied hondabond lightly to surface and drove it in slowly and evenly with a piece of PVC and dead blow hammer. The right direction this time.


Hope this works. So far so good.


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[SOHC4 Cafe Racer Fan]

Good on you for pointing it out so that even one person doesn't do the same.

[edwardmorris]

I used the OE seal and it had that lip, so I feel better about installing, but not sure enough, so I'll pull the advancer and check closely.

[Retro Rocket]

So, I am going to embarrass myself to save someone else any problems/headache in the future. The only leak I have from this motor is from the crankshaft oil seal. I thought I fixed it by lightly tapping it in and squaring it up in its place but leak came back even worse when I rode it hard last week. Now I know why. Look at this picture, I installed the damn thing backwards!!!

The metal ring is supposed to face inward! The oil seal I got from cycleX did not have a pronounced lip on one side to make it obvious, and I had never used a seal with a metal ring in it before. I know the "open end" of a seal should generally face toward where the oil is. I now realize, when the metal ring is removed that this is the open end.
I ordered some universal 30x42x8 crankshaft oil seal without a lip, cleaned area, applied hondabond lightly to surface and drove it in slowly and evenly with a piece of PVC and dead blow hammer. The right direction this time.


Hope this works. So far so good.


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Dave, in this order mate, seal first, beer second....

[Ravie]

I used a small amount of aviation permatex on the outside of mine.   Hell no to leaks! Lol.

We will find out in a week or so.

[Davez134]

Dave, in this order mate, seal first, beer second....

I thought I worked better after a couple. Apparently not!

[Retro Rocket]

Dave, in this order mate, seal first, beer second....

I thought I worked better after a couple. Apparently not!

At least you've got it sorted out now....

[Davez134]

We'll see. I did take it out right after driving in new seal, ran it pretty good, 20 miles or so, 80-90 mph, rpm's up, no leaks yet. I was going to put some more miles on it this week, but I just separated my shoulder and fractured part of scapula. No riding for a bit. At least it wont leak just sitting there!