Topic: '78 K 23,000miles and valve guides are shot?

[surveywaters]

As stated I have a 1978 CB750K with 23,000 on it. The bike sat then was ridden, now I am going up a size on the pistons be cause of blow buy. I'm also replacing the cam chain and tensioners. I was going to just lap the valves in but when I slid the first in to lap it, I notice alot of slop in the valve stem to guide clearance. I then checked the other valves and they are all pretty loose, not need to use a dial indicator. Is this common on these engines? Is there a higher quality replacement for the guides and it that all that's typically needed?  I'll have to take the valves to my machinist to measure them for tolerance. Any tips on where to source guides and maybe even vales?

Thanks
Jon

[kghost]

Yeah it was a problem.

Hondaman has done a write up on this.

Sorry you'll have to use the search function

[Hasenkopf]

had same problems with mine, had to rebuild top end.

[surveywaters]

Yeah it was a problem.

Hondaman has done a write up on this.

Sorry you'll have to use the search function

Wow, if only I'd thought of that sooner, and maybe even the FAQ also. Man this whole intranets thing sure is hard....

[kghost]

Yeah it was a problem.

Hondaman has done a write up on this.

Sorry you'll have to use the search function

Wow, if only I'd thought of that sooner, and maybe even the FAQ also. Man this whole intranets thing sure is hard....

Lol damn interwebby.

Wasn't giving you the blow off.....just Hondaman has a lot more technical expertise than I.

A quick search should turn up his post...and he gives lots of detail.

If you can't find it PM him

[surveywaters]

Most everything I am finding on the search is for the F model; I'll try harder to figure out how to search for spcific users. I did read Hondaman's new build thread and he mentioned Honda switching to cast guides. I really just wanted verification that what I am describing wiggle wise is as bad as I think it is. I found a set of alloy bronze guides from APE and oredered them. The valves look as though they are not worn, I don't have a micrometer to check them with though. What else typically needs replaced. As in are there any other known issues? I don't ride at redline so I hope to continue using the stock springs.

[70CB750]

It could be just guides that have to be replaced, but the valve seats will need to be recut, changing guides is too much for just lapping.

[surveywaters]

It could be just guides that have to be replaced, but the valve seats will need to be recut, changing guides is too much for just lapping.

Thanks, I plan to let my machinist have it to install the valves and redo the seats. I sure hope this is the last of my expenses....

[toytuff]

It could be just guides that have to be replaced, but the valve seats will need to be recut, changing guides is too much for just lapping.

Thanks, I plan to let my machinist have it to install the valves and redo the seats. I sure hope this is the last of my expenses....

Probably not. 

tt

[kghost]

Pm mark aka Hondaman and find out what he recommends.

There's an answer I just don't remember it lol

[surveywaters]

Thanks guys, I managed to find some NOS on Dusty Moto; staying with the '78. They'll be heading this way today.

[lrutt]

hard to believe you have to go 1 size up on the pistons at 23k. Also hard to believe the K has worn guides at that stage. I think I'd question the miles. Hows the primary chain??

[Duanob]

It could be just guides that have to be replaced, but the valve seats will need to be recut, changing guides is too much for just lapping.

 I sure hope this is the last of my expenses....

Silly boy! Sounds like you're just getting started!

have you priced out a cam chain tensioner? If you can find an NOS one they start at $200 and go up from there.

Also remember there is no difference in motors from 74 through 78 or the K and the F models as far as the 550 stuff you're working on. Same guides, valves etc. The difference will be in the more modern cam cover. And remember when installing the cam cover you have to do it right (use rubber bands) or all your work will be ruined, which could be the cause of the excessive wear in the valve guides from the PO.

[kghost]

Also remember there is no difference in motors from 74 through 78 or the K and the F models as far as the 550 stuff you're working on. Same guides, valves etc. The difference will be in the more modern cam cover. And remember when installing the cam cover you have to do it right (use rubber bands) or all your work will be ruined, which could be the cause of the excessive wear in the valve guides from the PO.

Huh? He has a 750..........

[flybox1]

Duane...did you stop at A Green Cure on your way to work? 

[Duanob]

kghost is correct I must have posted this before my morning coffee. I am a 550 kinda guy! Here I thought i was onto something. 

[surveywaters]

I've already got my new cam chain tensioner set, the rubber was like bakelite. I have good reason to believe the mileage is right, but we can't ever be certain when the speedo is so easily replaced. I believe it sat for a while then got fired up and run with out good prep work. The rings were worn which allowed blow by. I know alot of you guys will just replace rings, but I have never had good luck with this aproach and pefer to go ahead with a fresh bore and new pistons. As for the guides, I don't have a dial indicator to say how bad out they were, but there was noticable play and carbon build/oil blow by in the exhaust.

[surveywaters]

oops, dumb post

DELETED

[HondaMan]

hard to believe you have to go 1 size up on the pistons at 23k. Also hard to believe the K has worn guides at that stage. I think I'd question the miles. Hows the primary chain??

I haven't checked the Primary Chain for wear yet, fingers crossed. Can the rubber roller on the tensioner be replaced through the oil pan opening?

The chains can be measured for wear: use a caliper to measure from the oil pan gasket surface to the tensioner. The chains are probably fine, they are tough. The rubber roller is hard from age by now, but can't be reached from here.

The 77/78 bikes suffer from their lean-burn, emission-controlled fuel mixtures, and from cast iron valve guides combined with good valve seals (no lube!). Cast iron guides need umbrella-type seals, not good, tight, cap seals. The bronze ones fare far better. The carb jetting should probably go up at least a 5 size, if not 10, to help cool it off next time around.

[kghost]

Thanks Mark.......

Cast vs bronze was the part I couldn't remember.

[bender01]

Hey Survey? Where are you? Im in Beaufort County! SC is a big state and I like to travel in it!

[surveywaters]

I'm in Cayce near the Columbia Airport. I get to Beaufort a good bit with work.   

Thanks HondaMan, I tried to delete that question before Kghost got on me for not using the search function  

I'll be pulling the oil pump today and will check it, but I'm sure it's rock solid just like the cam tensioners are. I don't want to split the case, that starts getting outside of my comfort zone.

[surveywaters]

I've looked through my manual for directions on replacing the primary tensioner. It is in the middle of instructions for a complete tear down. I have searched the sire for some tips or tutorials. Can I split the case with out dropping anything out of it, just to replace the tensioner? What needs to be removed before splitting? As it sits now, jugs-up are off.

[70CB750]

To replace the tensioner, the jugs have to come off, but you dont have to split the case.

[surveywaters]

I appreciate that 70CB750, but we were discussing the primary tensioner, not the cam. Fortunately it's now, for me atleast, a moot issue.  My tensioner is supple and complete, and the chain is in great shape measuring at 65.86mm. First thing I haven't had to replace....