Topic: 78 CB750 SUPERSPORT

[cheapride]

Hey guys,

My bike has 28,000 miles and I am thinking of freshening up the engine.  What are things to address in the head?  Anything recommendations will be great.  The bike smokes a little bit now and seems to be running fine.  I just need to fix the valve cover gasket and the head gasket so I thought I might upgrade.


Thanks,
Brent

[Old Scrambler]

so a seach on the regular page..........look for egged-out valve guides. Ape (sponsor) has very nice replacements.  If your leaking a little oil it may be the pads under the cam-tower.........

[Flying J]

How much do you want to spend? F head have problems, mine has worked fine but if im going to spend the money to rebuild im going to go the extra mile to make sure my valves dont drop into my motor.
APE has good stuff
Cycle x has lots of parts and so does dynoman.

[cheapride]

I am thinking about $1,000 or so.  I might be dreaming though.  I though of a seal kit and cycle x has a f2 rebuild kit for $349.  Is this the right way or just replace the springs, retainers, and valve guides.  I would have to have some one do the head work because that would be out of my league.  Any tips or just save up more and do the whole works.


Thanks,
Brent

[MoMo]

If it is running well and the only problems are valve cover and head gasket leaks you can do that for the price of a gasket kit, under a 100.  To take it to the next level could easily add a thousand-new rings with honing, valve guides which now would require a machine shop to recut the seats which usually is needed when the guide is replaced, possibly new studs, new cam chain and guides.  IMHO, if the oil leak and smoking aren't all that bad, keep on riding while changing the oil more frequently and adjusting valves, cam chain and timing  per normal schedule...Larry

[Flying J]

Thats what i would do cheap ride. Im going to do the 349 kit but i have a set of 836 pistons so im going to have to drop those in and while im that deep im going to have to replace the chains and guides. So it all starts adding up. Thats why i keep riding it. If it blows it blows. I have spare parts to fix it.
In the end you just need to figure out how deep you are comfortable going.

[cheapride]

Sounds good guys.  I will just deal with it for now and start saving more money.  A full rebuild would be nice some day.  I keep on looking at bikes on this site and there are some very nice ones.  Whats the average cost for a rebuild?  Would $3000 be more realistic.


Brent

[Flying J]

Well with an F motor you have some options. The head has issues, and that is fixed with that 349 kit but then you come to the issue of pistons. Hard to impossible to find pistons. Cycle x has them now but for that price you can almost get a bigbore kit for the F. I got lucky and found a set cheap so im going to stick with the F head and cylinder. Your other option, and the way i was going to go was to swap over to the K head and cylinder. Then you dont have to worry so much about the valves droping and you can save that $349. Also with the K pistons are readily available. But the K is not as "high performance" as the F. What to do? Well buy a set of those Bigbore pistons on ebay for $100 and now you have a 836 K. Better then a 750F. When you start adding up the parts to make a F motor bomb proof it makes more sense to swap over to a K. And even more so with those $100 pistons.

[cheapride]

Can you just change the head and cylinders or should I start looking for another complete engine?

Brent
 

[Skunk Stripe]

Well your smoking is likely not caused by gaskets. The F3 was known for valves going bad in a short time and that is most likely what is happening to you.
I would PM mike Reick about having your head cleaned up by him and maybe ported at the same time, or contact Ken at cyclex about it.
Get prices from them for refreshing things what they suggest.

[Flying J]

No, you have to change the jugs and the head together. Do a search, there are a lot  of threads on it.

[Skunk Stripe]

Smoking though is not going to be caused by gaskets, it is caused by oil leaking past rings or valves or excess fuel. Plus it is not mandatory to even remove the jugs, it is common practice to just do them together. If you have a good base gasket, you can just do the head and then you do not even have to worry about honing or re-ringing. It is a time and work saver, in the long run, to do them together.

[Flying J]

Were talking about different things.  Weather you do just the head or head and jugs is a decision on how much you want to rebuild/ spend. What I'm talking about is swaping the head from a k motor. You have to swap the jugs also. The jugs are not the same.

[Skunk Stripe]

Ah then yes you would need to swap both, my misunderstanding.

[Freaky1]

All I'm going to say is do your research, get many opinions, make a plan and stick to it. Starting without a good plan or changing your mind midstream can wind up costing you a boatload of cash. This forum is a unbelievable vault of information, use it.

[BlackMax]

Sounds good guys.  I will just deal with it for now and start saving more money.  A full rebuild would be nice some day.  I keep on looking at bikes on this site and there are some very nice ones.  Whats the average cost for a rebuild?  Would $3000 be more realistic.


Brent
[/quote

I have a 78 SS too.  First thing you want to do (well, you don't have to, but prob should, especially since it's winter, and you're looking to spend a couple bucks anyway   is to pull the head off, and send out to APE in Rosaville, CA.  Tell them to rebuild it.   Heads on SS motors are designed to make bigger HP.  The tradeoff is that they also eat valve guides.  Mine were about twice the size of the stem.  As for the bottom end, they are pretty bullet proof.  Once the heads are proven good/rebuilt, I'd pull the bottom off, and take a look at the oil intake screen on the oil pump.  30 years of crap being dropped into the engine and gasket sealer.....you'd be surprised.  I was. 

From there, it's all aesthetics (and carb tuning...can't forget that). 

[cheapride]

Well with an F motor you have some options. The head has issues, and that is fixed with that 349 kit but then you come to the issue of pistons. Hard to impossible to find pistons. Cycle x has them now but for that price you can almost get a bigbore kit for the F. I got lucky and found a set cheap so im going to stick with the F head and cylinder. Your other option, and the way i was going to go was to swap over to the K head and cylinder. Then you dont have to worry so much about the valves droping and you can save that $349. Also with the K pistons are readily available. But the K is not as "high performance" as the F. What to do? Well buy a set of those Bigbore pistons on ebay for $100 and now you have a 836 K. Better then a 750F. When you start adding up the parts to make a F motor bomb proof it makes more sense to swap over to a K. And even more so with those $100 pistons.

If I was looking for the pistons, jugs, and head off of a K what year would they have to be off of.  What years of engines are a direct switch? 

I was also reading about just getting the pistons redone for standard rings.  Has anyone done this?  Do I have to even worry about these as long as they have good compression?

I would like to keep the F head if I can. 

Brent

[cheapride]

[/quote

I have a 78 SS too.  First thing you want to do (well, you don't have to, but prob should, especially since it's winter, and you're looking to spend a couple bucks anyway   is to pull the head off, and send out to APE in Rosaville, CA.  Tell them to rebuild it.   Heads on SS motors are designed to make bigger HP.  The tradeoff is that they also eat valve guides.  Mine were about twice the size of the stem.  As for the bottom end, they are pretty bullet proof.  Once the heads are proven good/rebuilt, I'd pull the bottom off, and take a look at the oil intake screen on the oil pump.  30 years of crap being dropped into the engine and gasket sealer.....you'd be surprised.  I was. 

From there, it's all aesthetics (and carb tuning...can't forget that). 

The $649 doesn't include new springs or retainers.    This add $225 to the mix.  Would this be enough or would a port and polish make any difference?  Can you change the head studs w/o removing the jugs?

Brent


Brent

[TwoTired]

The pistons, rings and barrels ought to be good for 70K (or more) miles, provided the oil is changed regularly and it doesn't spend ALL the engine run time at red line where piston speed is the highest.  (This doesn't mean you should avoid use of the red line routinely and when appropriate.  Just don't expect a 24 Hrs of Lemans run regime.)

If you have valve guide wear, you will need new valve guides, the valve seats and faces reground, probably all new adjusters and the valve stems ground flat. Lash caps would complete the top overhaul.  That should get the head sound for the next 30K miles with the same barrels/pistons undisturbed.

You should, of course, check the pistons and barrels for wear limits, while you have the bank off to replace the base gasket.

I hate to mention this, as it has to do with oil.  But, synthetics have a higher breakdown temp.  So, keeping the guides well lubed ought to help delay the wear in that area while the engine is hot, while stop in summer traffic heat.  I like semi synthetic for this reason (if not full synthetic), as even expensive oil is cheaper and more convenient than a top overhaul.

What others have done is to use the barrels and pistons (the entire top end) of a K7 or K8 CB750 engine.  These don't have the guide wear issues of the F2 and F3.  And they bolt onto the top of the very stout lower end of the F2/F3 CB750 engine.

Cheers,

P.S. You might have a talk with Mrieck (PM) about F2/3 engine repairs.  He is not a fan of the F2/F3 head.  However, he knows what needs to be done to make them work again.

[madmtnmotors]

My 78' CB750F motor (currently mounted in my 78' CB750K pictured below) is pushing 60,000 miles and has been to the burnout pit at the Iron Horse in Ormond Beach each of the last three years. I pack a spare since I smoke it till it pops. The 30 year-old valve seals have been the only casualty so far... bike started smoking after the second annual burnout. The seals were so old they shattered when removed. New seals and no more smokin'. Considered a hone and re-ring of the cylinders but stock rings are so hard/expensive to find I would have gone with an overbore and new pistons, but then what about the bottom end?

I settled for valve seals and new head stud pocket "Coin" seals under the cam towers and called it a day since the motor still runs like a champ. 60,000 hard miles and still going strong. I had swapped in the "F" motor at a time when the "K" motor was running poorly. An engine swap was easier than a carb rebuild but the "F" motor ran poorly too so I ended up having to rebuild the carbs anyway. I'll put the "K" motor back in once the "F" chassis is roadworthy. Considering building up the "K" motor while it's out, especially since I can still ride while I build!