Topic: Valves : How bad is it? '78 CB750F3 SS

[hammerspace]

Sorry its an F3 so the exhaust guides are shot---they are over 90% of the time. They MUST  be fitted and sized by a competant Honda type shop.

Yeah I'm trying to find out if there's anyone here where I live that works on old Honda's or has experience with them. If I can't, I think I'll just be shipping it to CycleX

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[hammerspace]

If the bike has been sitting for a long time (years) and the engine not run, then compression can be low due to cylinder wall rust, rings being stuck,  You would be burning oil as well until the rings re-seat.   Since you have the engine out and head off you can measure the cylinder wear. You may be able to get away with just a re hone of the walls and new rings.  I would remove the valve springs and take the valves out.  Check for cam and rocker wear, valve-valve guide wear and do a light lapping to look for seat pitting.  Be very careful to keep each valve and rocker labeled for proper orientation so things go back in the same location!  If all is good and within spec, you can reassemble and use new valve seals and piston rings. 

-P.

The cylinder rings were moving freely on the pistons. I'll of course be measuring cylinder wall wear and piston rings wear next week. 

This is what the cylinder walls and pistons look like.

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[hammerspace]

I am not an engine rebuilder. 
To test for valve leaks.   As long as you have not removed any valves you can pore a liquid, maybe pure alcohol, and fill the area in the head.  Of course put the spark plugs back in.  and watch for leaks. 

If you find no leaks in the valves.  It must be the piston rings that are stuck.

Thank you, I'll try that today.

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[Jerry Rxman Griffin aka MuthaF'er]

How tight were the cylinder stud nuts? Looks to me like there was varying degrees of sealing around the cylinders. Might be a good idea to put a set of HD studs in there to prevent stud stretch. 

[bryanj]

Looks to me like bores, pistons and rings will run again OK

[PeWe]

Perfect reason for 836 and refurbished head.
I guess the primary chains and cam chain with tensioners need to be replaced too.

I do not know any cheap pistons for F2/F3.
  cruzinimage.net has $120 sets oversize for K0-K6, K7- F1. 
The pistons for K7-K8 will get a little less compression, might work.
https://www.cruzinimage.net/2017/08/17/77-78-honda-cb750k-k7-k8-0-5mm-over-size-pistons-set/

Maybe someone has tested?


My first costly upgrade of my K6 started with a bad stock bore when bike had done around 60.000km many years ago.
Ended up in 836, ported head with bigger in valves (F2), moderate cam.
Lightened alternator all chains and all bearings replaced.....

[hammerspace]

Just did a valve leak test and only cylinder #1 intake valve has a leak.

[hammerspace]

How tight were the cylinder stud nuts? Looks to me like there was varying degrees of sealing around the cylinders. Might be a good idea to put a set of HD studs in there to prevent stud stretch.

I think you're probably right. Some nuts were easier to loosen than others. And one nut in the first cylinders spark plug well was bigger compared to the others. There was more drag on the threads while unscrewing

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[hammerspace]

Perfect reason for 836 and refurbished head.
I guess the primary chains and cam chain with tensioners need to be replaced too.

I do not know any cheap pistons for F2/F3.
  cruzinimage.net has $120 sets oversize for K0-K6, K7- F1. 
The pistons for K7-K8 will get a little less compression, might work.
https://www.cruzinimage.net/2017/08/17/77-78-honda-cb750k-k7-k8-0-5mm-over-size-pistons-set/

Maybe someone has tested?


My first costly upgrade of my K6 started with a bad stock bore when bike had done around 60.000km many years ago.
Ended up in 836, ported head with bigger in valves (F2), moderate cam.
Lightened alternator all chains and all bearings replaced.....

I think I'll hold off on the big expenses till I've saved up a little more

For now I just want to get it up and running on a budget and get familiar with the insides of the engine and overall functioning.

I have bigger plans for the future. I've been building a 3D model of a cafe racer based off a 750 even before I bought the bike haha

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[PeWe]

Restore an engine cost some. +40 years have dried those parts that are not worn.
Also possible to have a side project if you can find a spare engine very cheap.

[754]

Just for grins puts a piston in and try a  2 and 3 thou feeler guage.
 I guess you don't want to take tgevrings off though if you want to try to reuse them.

[hammerspace]

Picking up from where I left off - as expected, exhaust valve guides are worn. I'll be buying new exhaust valve guides, a set of valves and valve springs. The intake valve guides are in good condition, so would it matter if I ran new valves in them?

Should I go for Dynoman, APE or CycleX parts?

Also what's the best place to send the Camshaft and rocker arms for refurbishing? There's pitting on a couple camshaft lobes and rocker arm pad surfaces (pics attached)

I managed to find a Honda mechanic who'll be installing the new guides and cleaning up the seats.

[PeWe]

Replace rocker if chrome is flaking off or have deep marks where in contact with cam lobe.
There are good used ones on eBay.

Why replace springs? You will use a hotter cam?

[hammerspace]

Replace rocker if chrome is flaking off or have deep marks where in contact with cam lobe.
There are good used ones on eBay.

Why replace springs? You will use a hotter cam?

I plan on reusing the stock cam (after getting it resurfaced hopefully). The mechanic that's doing the valve guides for me suggested that I get a set of new springs while I'm at it.

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[Jerry Rxman Griffin aka MuthaF'er]

Something to keep in mind, the F2/F3 utilized stronger springs

[jlh3rd]

Sorry its an F3 so the exhaust guides are shot---they are over 90% of the time. They MUST  be fitted and sized by a competant Honda type shop.

Yeah I'm trying to find out if there's anyone here where I live that works on old Honda's or has experience with them. If I can't, I think I'll just be shipping it to CycleX

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curious what part of northern Louisiana as Im from shreveport originally. G&C honda was the first honda dealership around before the others, who are gone. This doesn't mean that G&C  can handle your rebuild however. We all know good old techs are hard to find. Just though id mention this if it helps....

[hammerspace]

Something to keep in mind, the F2/F3 utilized stronger springs

Thanks Jerry, I'll keep that in mind. If the measurements are within spec I need not worry about it then?

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[hammerspace]

Sorry its an F3 so the exhaust guides are shot---they are over 90% of the time. They MUST  be fitted and sized by a competant Honda type shop.

Yeah I'm trying to find out if there's anyone here where I live that works on old Honda's or has experience with them. If I can't, I think I'll just be shipping it to CycleX

Sent from my Mi A2 using Tapatalk

curious what part of northern Louisiana as Im from shreveport originally. G&C honda was the first honda dealership around before the others, who are gone. This doesn't mean that G&C  can handle your rebuild however. We all know good old techs are hard to find. Just though id mention this if it helps....

Shreveport here! I spoke to G&C and they said they couldn't help and that the nearest place I could find someone was Lafayette. Called a few other shops to no avail.

The "mechanic" helping me is a enthusiast himself who races smaller Honda's and has been doing it since the 80's so he's my best bet.

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[70CB750]

My Red has 78F engine and I done it all - valves, guides etc.   Your mechanic will need to recut valve seats, it can not be avoided if he replaces guides.  I am sure he knows it, just saying. 

For Red I upgraded my valve retainers and springs with a set from CycleX.  They also have some info on that:

http://www.cyclexchange.net/Honda%20cb750%20(%2077-78%20)%20%20F%202%20Section%20Page.htm


Also it would not be a bad idea to replace tappet screws.

[hammerspace]

My Red has 78F engine and I done it all - valves, guides etc.   Your mechanic will need to recut valve seats, it can not be avoided if he replaces guides.  I am sure he knows it, just saying. 

For Red I upgraded my valve retainers and springs with a set from CycleX.  They also have some info on that:

http://www.cyclexchange.net/Honda%20cb750%20(%2077-78%20)%20%20F%202%20Section%20Page.htm


Also it would not be a bad idea to replace tappet screws.

Thank you! Appreciate the help.

He'll be recutting the seats, yes.

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[HondaMan]

Before you bolt the head back down, contact me for some special O-rings for those oil journals at the back of the cylinders. Otherwise it WILL leak oil after all your work is done...!

[HondaMan]

Don't get new springs, but DO get CycleX's special valve retainers for this engine: that is REAL important.
A step-bore should be enough to clean it all up: I'd suggest the piston kits from CruisinImage. I just got [another] one from him for the F2/3 engine. You must use that piston, as they have a domed crown. Contact me for more details if needed.