Topic: Valve reliability issues: CB750 F

[HondaMan]

Recently, I've read several posts where guys are having valve problems on CB750F and CB650. Here's a little history, and a fix for you, if it's not too late... 

One of the "refinements" that Honda made to the CB750 engine ends up causing valve problems in the K5/F/late K engines. This problem can be easily fixed
by retuning. The story goes like this:

In the beginning, the engine was volumetrically designed for a straight-thru exhaust pipe (it was actually a glass-pack in K0 and K1) and a very open intake tract (4 sq. in. intake holes). This meant that the exhaust valve is larger, relative to the intake valve, than most of today's engines. (Some people would say "the intake valve was smaller"...). Later bikes had more restrictive intakes and exhausts, but the engine never changed. Only the spark advance curve was modified to a slower curve to compensate, and the carbs were typically rejetted with every model change to try to cope. Close, but not as perfect as the K0/K1 tuning.

The original exhaust valve guides were longer, tapered to the top, and had no seals (picture below, after I take it, from my early K2 head, which is really a leftover K1 head from Honda's production scheme). This length improved the heat transfer from the valve to the head for cooling.

About the time of the K2, Kawasaki came out with the 900-4. Its oil change interval was 1500-2000 miles, even with a wet sump engine. Honda, not to be outdone on the "low maintenance" image, magically changed their specs from the original 900-1000 miles to 1500 miles by reprinting the manual. Their engineers scrambled to make this happen without hurting reliability, and they did it by switching the longer exhaust valve guides to the same type as the intakes, and added that valve seal from the intakes to prevent the exhaust gases from entering the oil via the crankcase. (Later, the parts microfiches were changed to reflect these parts from the beginning, it seems...  .)

This is where the problem started. The intake guides are lots shorter and the seal makes the valve heat up more, to boot. The answer that we, the roadracers of the early 1970s, came up with was to increase the exhaust valve's lifter clearance to .004"-.005" up to 8500 RPM, and .006" to 10,000 RPM (stock = .003"). I raced mine at 10,000 to 12,000 RPM, using .006". This extra clearance lets the valve sit on the seat slightly longer, transferring a bit more heat into the head. Yes, it loses some cam duration, but on the exhaust side this doesn't hurt you like it does on the intake side.

Long term, like on a street/touring bike (or a 35-year-old bike), this translates into exhaust guides that gall and stick the valve. Then, one day the rider grabs a big handful of RPM and the piston taps the slow-moving valve and this breaks the guide. Hence, the "F" model (and CB650-4 model) problem with exhaust valves burning and guides breaking.

That's what happened at Honda. To fix your "F" (or CB650) to prevent this, run .004" exhaust valve clearances. On the CB650, make it .005".

I've kept this .004" on my K2, and 112,000+ miles can't be wrong: the guides are still nearly perfect, despite an early chip on one from a missed-shift 14,000 RPM trip.

[Jerry Rxman Griffin aka MuthaF'er]

Hondaman,

Which F models 75/76 F0/F1 and/or 77/78 F2/F3.

[HondaMan]

All the "F" and the later "K", like K7/K8 were affected.
Also, the "custom" series, but most often the CB650 in that group.

[maksuttt]

The original exhaust valve guides were longer, tapered to the top, and had no seals (picture below, after I take it, from my early K2 head, which is really a leftover K1 head from Honda's production scheme). This length improved the heat transfer from the valve to the head for cooling.

Just tried to find picture... Maybe it's too late I am collecting information about top end repair prior winter and interested in pictures too.

[eurban]

Hondaman- Not sure I agree with you completely on this one . . .The F2 and F3 (77 and 78 Fs) engines used larger diameter valves for better breathing and stiffer valve springs to handle both the heavier valves and higher RPMs at which these motors were still making power.  These heavier and stiffer parts add to the loads and wear on the valve train, particularly the valve guides.   To the best of my knowledge the primary culprits for F2 and F3 valve train reliablity issues were these changes that Honda made to hot up the SOHC engine.  I haven't really heard of  or experienced excessive valve train related issues on the Ks or the F0,F1s.  . . . An exception might be that Honda also used different keepers and matching valves on the F3s and K8s (and perhaps some of the 1977 bikes) which may or may not be more trouble prone. The parts themselves seem to be lighter weight which I presume was the idea . . . .

[oldfart]

Well, I usually just lurk, 'cause I hate like hell to argue with people.  Not the arguing per se, I just think it ought to be done face to face.  Yes, the black (F2, F3) engine had a valve guide
problem.  I was a trained Honda dealer tech at the time, and shortly after, I
worked on the warranty team at the factory in Los Angeles.  Honda never spelled it out in so many words, but here's my take on the situation.  For what it is worth.  The problem is not valve size, valve guide type or material, but rather, valve angle.  Honda essentially copied an early Chevy trick by making room for bigger valves in a given chamber by canting them.   Unfortunately, the change in angle increases stress to the valve guide.  The exhaust in particular, which routinely wear out in just 4,000 to 6,000 miles.  This same problemalso occured on the earliest CB650s.
Both of these engines had their valve angles changed from earlier designs.  Since the rocker arm positions
weren't changed, the heads of the valves were tilted away from each other while
the stems remained in place so the established valve gear would still operate
them.  (The 650 was based on the less-powerful 550, the 750F2and 750F3 were
revised for more power old style 750s). 

Now the really interesting part.  As a Honda tech, I was told
about a Delta change to the guide.  (A Delta change means a secret change to the
part without the part number changing).  I was shown by a Honda factory service
rep, that when ordering the replacement guides (whose part number hadn't changed
remember), I had to check the guide with a drill bit for size (I don't remember
the drill bit size).  That is, the drill was used  to differentiate between old and new style guides, not to drill out the guide.  The modified
guide had a slightly larger hole because it was so much harder it couldn't be reamed (all Asian motorcycle valve guides are designed to be
reamed slightly to size after installation). 

Four questions
remain.  One -- Could your bike have already had the guides replaced?  Certainly,
it could have, if you are not the original owner.  Two -- Are there bikes out
there with this problem still?  Undoubtedly.  Three -- Are there original guides
out there still on dealer's shelves?  Not very likely.  Today's dealer doesn't
hang on to parts more than a few years at most, and usually no more than 6 months
if he can help it.  (That's why they never have what you want!  :-)  )  So, nearly 30 years later now, all the replacement guides should be the good ones.  Finally
(four), how to tell if you have the problem?  It isn't always noticeable to the
average rider, especially if his engine is making cam chain and primary chain
noise on top of valve noise.  The last 78 model I did (in 1992), I originally took apart for a leaking
head gasket, and decided to check the guides just in case.  They were bad.  Pissed off my shop manager, but the customer was happy I found it.  A carefully
tuned engine might be quiet enough for the sound of bad valve guides to be
evident.  Symptoms are noise and smoking.  The noise all the time, the smoking
mostly on deceleration.  The wear will be more on the exhaust (in fact Honda
warrantied only exhaust guides -- I was on the factory warranty team) than on the intake, and the valves will be
rattling around in the guides like a pencil in a Coke bottle.  My recommendation? 
Pry on an exhaust valve stem (the spring will make it harder to visualize, but
you should be able to see it) the next time you do a valve adjustment.  Try to
push the stem laterally.  If you detect any movement at all, you probably have
the problem.  Don't let the wiggly spring give you a false reading.  Concentrate
on the valve stem.  You might even set up a dial indicator on the stem, although
the movement if it moves will be 0.100" or more.

[upperlake04]

great post OF

 

rattling around in the guides like a pencil in a Coke bottle.

my F3 intake guides were pretty consistent around .002th (.04mm) slop, the exhausts were sloppy, the worst at .020th (.51mm)

The exhaust in particular, which routinely wear out in just 4,000 to 6,000 miles.

I replaced the exhaust guides with Kibblewhites, very beautifully made - I hope you mean the original guides...... please say I'm good for more miles than that

[nickjtc]

Very interesting stuff. I wonder how many other parts have been Delta changed by Honda over the years....?

[oldfart]

Very interesting stuff. I wonder how many other parts have been Delta changed by Honda over the years....?

A lot of parts, but usually a Delta change is what it is called when the part is new, and this is only an insider's term.  I work for another OEM now and we call it PI, which stands for product improvement.  :-)  Long after the fact, which is now of course, Honda is still changing parts because like any manufacturer they find they have better parts on newer models that will retrofit back, so they do, retrofit them back in the parts lists. 

[oldfart]

great post OF

 

rattling around in the guides like a pencil in a Coke bottle.

my F3 intake guides were pretty consistent around .002th (.04mm) slop, the exhausts were sloppy, the worst at .020th (.51mm)

The exhaust in particular, which routinely wear out in just 4,000 to 6,000 miles.

I replaced the exhaust guides with Kibblewhites, very beautifully made - I hope you mean the original guides...... please say I'm good for more miles than that

I'm no engineer, but I would guess the geometry change in the valve train that cause this whole problem took place mainly on the exhaust side (only need to move one valve to make more room...), hence their guides' accelerated wear.  As for Kibblewhite, I agree.  Good quality.  But remember that better guides doesn't necessarily change what made the originals wear out.  Honda substituted harder guides, so they must have decided that would work.  I would say you have a better than average chance.  But if those Kibblewhite (or PM) guides are bronze, which racing guides often are, then...I don't know.  I don't think they're as hard as the original even.  Keep an eye on those exhaust valves is all I can say.  :-)

[HondaMan]

Well, OF, that helps explain some of the oddities I've seen in some "F" bikes. I just thought they were after-marketed by the owners with something like Action Fours or RC parts to get those different valves.

It's nice to put a name to the process: "delta" parts. Like the original K0 spark advancers that suddenly dropped from 40 degree advance angle to 34 degree, later to 26 degrees in the K2 and later, all having the same part number. Among other parts...   

[oldfart]

For those of you really interested in this topic, here is a treat just for you.  The following pix are of my contact logs.  I was once a warranty analyst at American Honda.  These pages (different years) show the authorizations I gave to authorized dealers to repair situations that were out of warranty.  Note the entries for the CB750F2 and again for the CB650Z.  You may want to copy the images out and then zoom in on them in a paint program such as Photoshop, so you can see the detail.   

[nickjtc]

For those of you really interested in this topic, here is a treat just for you.  The following pix are of my contact logs.  I was once a warranty analyst at American Honda.  These pages (different years) show the authorizations I gave to authorized dealers to repair situations that were out of warranty.  Note the entries for the CB750F2 and again for the CB650Z.

Fascinating.

[scondon]

"Cb750 f2, 6000 miles, smoking badly, customer Irate"    

    Good to see ya posting Mike.

[oldfart]

Thanks...  :-)

[kghost]

Now I remember why I threw my F2 engine in the corner of the shop after I robbed the B pack outa the clutch