Topic: RE: CB750s To Rebuild Or Not To Rebuild?

[chewbacca5000]

Just wondering how many of you rebuild every single CB750 engine you come across?  I got one that I has smoke, but that may have been due to gas in the oil from a bent float.  There is no sign of an oil or mess except near the engine sprocket.  It's a 1972 CB750.

Would you rebuild this engine or not?

[HondaMan]

Just wondering how many of you rebuild every single CB750 engine you come across?  I got one that I has smoke, but that may have been due to gas in the oil from a bent float.  There is no sign of an oil or mess except near the engine sprocket.  It's a 1972 CB750.

Would you rebuild this engine or not?

In a heartbeat. K2 is the design zenith, IMHO.

[chewbacca5000]

Just wondering how many of you rebuild every single CB750 engine you come across?  I got one that I has smoke, but that may have been due to gas in the oil from a bent float.  There is no sign of an oil or mess except near the engine sprocket.  It's a 1972 CB750.

Would you rebuild this engine or not?

In a heartbeat. K2 is the design zenith, IMHO.

Thanks Hondaman!  It's my favorite as well.  Turning 40 September 1 and want to ride this bad boy.  Decisions, decisions.  If only I had unlimited cash and time. 

Have you ever seen one without the head gasket leak?  Seems like this is the only dry one I have found yet.

[Don R]

Not enough information for us to decide if it needs a rebuild. Do you know how many miles? Was it maintained previously? You could do a normal tune-up and oil change and see how it goes. Compression test?  Check the sump?
 No, I don't rebuild every single engine I come across. It would be better to freshen it up, personal choice.  K2 is a fine machine.

[ncstatecamp]

Just wondering how many of you rebuild every single CB750 engine you come across?  I got one that I has smoke, but that may have been due to gas in the oil from a bent float.  There is no sign of an oil or mess except near the engine sprocket.  It's a 1972 CB750.

Would you rebuild this engine or not?

In a heartbeat. K2 is the design zenith, IMHO.

why do you say it is the zenith?

[lrutt]

pretty tough motors, If it had oil leaks at the head then yes. If no, I'd exhaust all other options before cracking the motor. You can do valve seals without pulling the head. Run some MMO through it to loose the rings up and give it a chance. My 76 and 78 have never been cracked open, and I'll likely keep it that way till I'm worm food.

[madmtnmotors]

I would need more than just a little smoke to warrant a rebuild. For a smoking engine I would check the plugs for excessively rich condition. If the fuel delivery proves OK then I would replace the valve seals and see if that resolves the issue. If it's not smoking too bad, I might just ride it and live with it till it got worse or went away (stuck ring?).

Of course, I've always leaned towards the frugal side... but never to the point of compromising safety.

[chewbacca5000]

A little background I got this bike about a year ago with 25,000 miles and really hasn't run since the 1980s.  There is no oil leaks runs pretty strong, but did have the smoking issue probablly due to gas in the oil from a bent float.

I like the idea of boring out to 2nd over to regain lost horsepower, but I am not sure I will be getting anything.  It goes pretty good the way it is.

Would a leakdown tester give me a definite answer?

[dhall57]

If it ain't broke don't fix it change oil, complete tune up and see how it does. Don't assume just because bike is 40 years and has 25K on it you have to go into engine. Mr Honda built these bikes well and if the PO or owners took half way decent care of it 25k on the clock is nothing.

Good luck Kyle and happy 40th birthday.

[chewbacca5000]

If it ain't broke don't fix it change oil, complete tune up and see how it does. Don't assume just because bike is 40 years and has 25K on it you have to go into engine. Mr Honda built these bikes well and if the PO or owners took half way decent care of it 25k on the clock is nothing.

Good luck Kyle and happy 40th birthday.

Much thanks!  Looking at the budget the odds are stacked against me, but I like a good challenge.  I might have sell some stuff as a fundraiser of sorts.

[HondaMan]

Just wondering how many of you rebuild every single CB750 engine you come across?  I got one that I has smoke, but that may have been due to gas in the oil from a bent float.  There is no sign of an oil or mess except near the engine sprocket.  It's a 1972 CB750.

Would you rebuild this engine or not?

In a heartbeat. K2 is the design zenith, IMHO.

Thanks Hondaman!  It's my favorite as well.  Turning 40 September 1 and want to ride this bad boy.  Decisions, decisions.  If only I had unlimited cash and time. 

Have you ever seen one without the head gasket leak?  Seems like this is the only dry one I have found yet.

Oddly enough, the late K1 and the K2 were the least-leaky of all, until the F0/F1. Then they got leaky again. I suspect a lot of it has to do with the mistranslated Jinglish that made the dreaded "10w40 oil" show up in the Owner's Manuals as the preferred oil in the mid-1973 year. This oil raises the head temps about 20 degrees on hiway trips, cooking the head seals and shrinking the head gasket, hence the leaks.

Personally, I have never seen a leaking K2, nor an "old factory" K1. The "old guys" must have had a trick or two?

[HondaMan]

Just wondering how many of you rebuild every single CB750 engine you come across?  I got one that I has smoke, but that may have been due to gas in the oil from a bent float.  There is no sign of an oil or mess except near the engine sprocket.  It's a 1972 CB750.

Would you rebuild this engine or not?

In a heartbeat. K2 is the design zenith, IMHO.

why do you say it is the zenith?

Personal observation, mostly. The late K1 "old factory" and the K2 bikes were the ones that made the high-mileage legendary, while the stunning change from previous bikes to the typical K0 (80k miles) made a lot of press in the day. I have met more 100k+ K2 and F0 bikes than any others in the series.

The K0 made the most power, while the K2 had the lowest maintenance of all, based on my personal notes.

[ncstatecamp]

Just wondering how many of you rebuild every single CB750 engine you come across?  I got one that I has smoke, but that may have been due to gas in the oil from a bent float.  There is no sign of an oil or mess except near the engine sprocket.  It's a 1972 CB750.

Would you rebuild this engine or not?

In a heartbeat. K2 is the design zenith, IMHO.

why do you say it is the zenith?

Personal observation, mostly. The late K1 "old factory" and the K2 bikes were the ones that made the high-mileage legendary, while the stunning change from previous bikes to the typical K0 (80k miles) made a lot of press in the day. I have met more 100k+ K2 and F0 bikes than any others in the series.

The K0 made the most power, while the K2 had the lowest maintenance of all, based on my personal notes.

Awesome, well this just made my day as that's what I picked up. Any advice on a k4 motor?

[bytio]

Hondaman, do you mean that the 10w-40 oil is something you would not recommend, if not which one would you think is better?

[Thegreatgreg14]

20w-50 should be correct.

[Old Scrambler]

Building the motor? If it turns over it should be saved.  If it is mildly stuck but no water in the crank-bearings.....probably.  If its a broken sandcast, YES.  Only the very worst of the worst should be discarded. I have saved motors that sat in the ice and snow for years with open intakes and exhaust. Out of about 20, the 2 CB750 motors I have parted-out came to me with broken top and bottom cases and a broken cam, and pistons that were rusted to the cylinder and a rusty crank.....

[chewbacca5000]

Building the motor? If it turns over it should be saved.  If it is mildly stuck but no water in the crank-bearings.....probably.  If its a broken sandcast, YES.  Only the very worst of the worst should be discarded. I have saved motors that sat in the ice and snow for years with open intakes and exhaust. Out of about 20, the 2 CB750 motors I have parted-out came to me with broken top and bottom cases and a broken cam, and pistons that were rusted to the cylinder and a rusty crank.....

This motor is in pretty good shape.  My motivation is to make sure everything is tip top.  I ran into a problem on my 73 after the top end rebuild where 4th gear will no hold.  It still runs strong, but I can take it above 5000 in 4th gear without a trip to redline.  I want avoid that here.

It is tough in my opinion to evalutate lower ends.  With the extra power of the .5 and the serdi valve job it may have been too much.  Could also be bad bearings as Hondaman suggested to me.

The motor really looks good, but hasn't been run in 20 plus years.  The chances of at least one open valve causing rust is high.

The 73 leaked like a sive but with .005 resurfacing and hd studs it is leak free chain oiller aside.

[bikerbart]

wow,hondaman.I have a F0 in my 72K project with the 72 motor half apart which I was thinking of putting the 836 kit in .Good to know,thanks.

[Greggo]

I just bought five motors...they all turn over, and they're all getting rebuilt...just not tomorrow

[chewbacca5000]

What do you guys think of race cut gears?  Is it worth it to have the dog ears cut to draw the gears into each other?  If so who is your preferred vendor for this service fast by gast, cycle x, or APE.  These are the three I know of.

[Stev-o]

If you don't plan on racing I don't think you can justify the cost. 

Is this bike a keeper, BTW?

[chewbacca5000]

If you don't plan on racing I don't think you can justify the cost. 

Is this bike a keeper, BTW?

Yes I don't ever plan on selling this one.  I don't race, but I don't want it to slip out of gear either.

[chewbacca5000]

Ok I decided to rebuild as can be see here http://forums.sohc4.net/index.php?topic=111377.0 and everything looks pretty good so far.  How can you tell if the main shaft and counter shaft bearings are good?

Thanks!

Never mind according to Hondaman's book you need to look for symptoms when the bike is running.

Now I got to find a source for these.  Anyone order them on the internet?  Really don't want to waste a bunch of time trying to locate a bearing supply house.

[HondaMan]

What do you guys think of race cut gears?  Is it worth it to have the dog ears cut to draw the gears into each other?  If so who is your preferred vendor for this service fast by gast, cycle x, or APE.  These are the three I know of.

If the dogs and holes are nice and square, there's not much need. If they have 100k+ miles on them, they might need the help. If they were abused and show chips and roundness, consider having it done as to ensure a long life, at least for gears 1,2 and 3.

Recently there have been 2 posts about K1 bikes slipping out of 4th gear. The only thing I can think of that can do this is either a bent shift fork or pin, or the upper (C) shift fork not engaged to the mainshaft (which can happen when reassembling those cases!). I know there is/was a spacer used between the right side bearing and the mainshaft, and it came in 2 thicknesses (or none), but this and its Service Notes appeared in 1975 with the F0 engine. Before that, I don't remember seeing much of it. I don't have a single engine here with one of these spacers (K0/K1/K2/K3/K4 engines all), either.

[HondaMan]

Hondaman, do you mean that the 10w-40 oil is something you would not recommend, if not which one would you think is better?

20w-50 should be correct.

What he said. And, make sure it is low detergent type, like Castrol 4T Motorcycle Oil or one of the mineral-based oils like Bel-ray. It won't be the cheapest oil, but there's a good reason why [not].