'86 VFR 750 cylinder head replacement

I shall preface this thread by saying I don't know if this is possible:

Originally the '86-87 model VFR 750 had smaller valves and the exit angle of the ports is different to the later model with the concequence of less mid range torque and a different exhaust system - there are no doubt changes to the cams and carburettors, specifics of which I am unclear.
Just recently I aquired a complete '89 motor with water in the bores and is seized but could make a good top end replacement for my '86 bike which is currently in pieces due also to water damage in one bore that I am getting re-sleeved so half the work is already completed anyway
Has anyone out there done this and is it as straight forward as it seems, apart from the obvious need to change the exhaust system to suit? It's probable that the piston scallops are also too small for the larger valves and require modification - I can pull the sieze engine apart and measure all this but if someone has already tried this I may save myself a lot of time.
This is a long term project so I do have time to do the research and the '86 was a Japanese domestic market bike originally that was seriously de-rated in engine performance and now seems the ideal time to get the best out of this old girl.
Any knowledge appreciated

 

Benefits vs cost I don't think is there. Nothing wrong with a good 86 motor.

That being said there is value in trying something and seeing it thru. Just know that the "satisfaction" value is gonna have to likely make up for any kind perceived performance advantage and associated costs in time and money.

Just my $.02

Cheers!

 

Thanks for the feedback, prompted me to do my own comparison so pulled the heads off the sized motor for a look:

Turns out the block was only mildly siezed and a bit of WD40 soon had it freed up and turning so have left that for another day
The heads themselves are identical even down to the valve size, angle and port angle so a bit of a conundrum here
The '86 model was built December '86 for their domestic market and came to Australia 15 years later, we would call that an '87 model
Engine number two is a 4th month '89 built and certified for the Australian market and clearly one would expect the related modifications
So - why has the '87 model got the same valves etc? (I have no method to check cam profiles - perhaps they are different?)
There is a small change evident in the way the rocker arms are located but it is cosmetic and would not have effected performance that I can tell
Was it the case that by the time '87 model was built that Honda had already changed the valves etc - perhaps even only on their domestic product?
Japanese domestic production at the time was performance limited - some say in the carburettors - others claim all sorts (I only know reduced output)

So there you have it - I will leave the original engine with its original heads, but will check the cams with an engine reconditioner for advice
The '86 enging is very low kms -28,000 (17,500miles) and still looks pristine inside, hone marks still in the bores so I shall re-sleeve the damaged bore and restore the bike to her former glory when present project bike is completed

 

If you look at various parts suppliers such as https://www.cmsnl.com/honda-vfr750_model323/ you should be able to identify the exact part number for each item for the two given years and sales locations.

That will tell you if the cams, valves, springs etc changed between your two factory engine builds. Whilst it won't tell you what the changes mean at the level of the reprofile of a lobe, it will tell you if the components are meant to be different.

It will also tell you if someone else has already modified one of those engines, should any unique parts for the later engine be found on the earlier engine.

One other thing you could try re cross-checking the cams: If the profiles are different, then the distance around the cam surface will almost certainly be different.

Whilst its certainly possible to re-profile to the same effective surface length using shorter but fatter profiles, it would be very unusual to achieve a perfect match. So its a good bet that if the distance around the same cam on each engine is different, then they are probably running a different profile. A piece of wire and a fine marker pen should be enough to find out, at least to within a whole mm.

 

The inlet ports are 36mm on the FJ/K engine as are the carbs

 

Those are all good ideas but one snag here - the Japanese domestic market machines are "unrecognisable" by any method I can come up with - and Honda agents world wide to my understanding will not quote parts for them. Their engine and VIN numbers are not listed in any list I ever found making identfication impossible.
Perhaps Honda distributors in Japan know the answers but I'm not that desperate and can wing it for most parts I need- or buy second hand where one can see and compare items prior to purchase.
Just one of the hassels of restoring old bikes

 

There is an old saying "the more we learn, the less we know" and I think this is one of those occasions:

After responding to the last comments from others I decided to try and find the article that referred to the change to larger carbs and valves etc that I recall reading many months ago (a long time for an old man)
Found one "clipping" I had saved from somewhere that stated the engine differences between '86 and '89 included larger carbs and valves - but I had failed to save the source reference of this information (a "bad" on my part)
Then I found a copy off the internet of a genuine 1986 Honda workshop service manual with much detail on that year bike......... Here it says the '86 carbs are 34.5mm KEIHIN VD BOB series
My Japanese domestic 1986 model has 34.5mm KEIHIN VD BOA series (built 12/86)
My Australian licenced 1989 prodution 36.0mm KEIHIN VD JOB series (built 12/88)
My AU licenced production engine also 36.0mm KEIHIN VD JOB series (built 04/89 - a recently aquirred seized engine still in frame )

This set me to looking now at the inlet / outlet bore sizes in the two different year models ('86 - '89)
Something I had overlooked previously and yes - they are different by perhaps 1.5mm -surprise, surprise
But the valves remain doggedly the same over both year models

So what have I learned - buggered if I know, and I have still not looked at the cams though I expect a difference
I am looking to see where they got this increase in mid range torque - was it just a carb and port upgrade?
As a footnote to the exhaust port angles - they certainly appear the same arcoss all heads, contary to my previous reading and comments elsewhere on this forum - something I must now correct!
Just wish I could find that article so I could blame someone else