Topic: 750four engine removal

[750Dave]

Hi, I'm new to the site, i saw a posting of a CB750 with the top part of the frame cutout obviously so you don't have to remove the engine, does anybody know if this is common practice and won't hurt the structural side of things, sure looks easier than removing the motor.

[Gordon]

There was a member here a while back (Ilbikes) who used to sell frame kits to do this, but he hasn't sold them in several years, as far as I know.  I get the occasional PM about them because his name is also Gordon. 

If it's done right it can be a good work-around to pulling the engine without compromising structural integrity, but to do it right you have to pull the engine, so the question becomes how many more times are you planning on removing your engine? 

[754]

 Someof the kits are weld in, and tend to be heavy.
I was making a much lighter aluminum version that needed no welding. Will start making them again soon ,
 Work has started on them.
Some folks cut them and never put anything in, but the frame will likely flex. Despite that  we have a member with a turbo that runs without the tubes.

[750Dave]

Thanx for the replies guys very helpful. if I decicded to cut the frame was gunna replace the frame when finished, thought about it that it would be a less time consuming job than to pull the motor, and then if there happened to be a next time would be less a hassle if the conversion was in place..thanx again.

[SOHC4 Cafe Racer Fan]

On my first 750, I thought that I'd never need to take apart the top end of my motor more than once. Hah!  After pulling the motor the 3rd time, I belatedly installed a Gordon's frame kit.  Frank aka 754 has a weldless kit that you can do yourself easily.  Unless this is a exacting restoration, do it.

[FuZZie]

Gordon still has them listed on his site:
http://www.mcssl.com/store/rrr-tool-solutions/honda-cb750

[750Dave]

Thanx Cafe Racer Fan and Fuzzie, more great info, will definately look into that.

[Stev-o]

I have 754's weldless frame kit on my K3, you'd never know it's there unless you remove the fuel tank.  The frame had been cut by the PO so I had no choice.

It is really not that difficult to pull the motor but with the frame kit, a lot less parts have to be pulled to get the top end off.

[750Dave]

Thanx for that Steve-o, must be more common than I thought this frame cutting, my '73 CB750 has some oil weeps from the head, won't be doing the repair just yet, think I'll be doing the frame cut to make things a lot quicker and easier when the time comes..Ciao for now...

[750Dave]

sorry bout the spelling mistake Stev-o..

[Sam Green Racing]

750 Dave, the other option is to get a CB750A frame.

[chewbacca5000]

I have a c hook and engine hoist.  It pops right out without injuring your back.  No struggle engine removal.

[750Dave]

I was checking the timing on my K3 750 by the static method and the 1-4 timing plate had to go a long way advanced to get it right, checked 2-3 also, bike idles better now, my 28yr old son is a mechanic and was watching me and said seeing as 1-4 fire together and 2-3 fire together why did I check 2-3 as once 1-4 is ok 2-3 will be fine when I thought about I think he's correct, after all 2-3 fire after 1-4, so they can't be wrong, what's the theory behind checking both? ...750Dave.

[madmtnmotors]

I was checking the timing on my K3 750 by the static method and the 1-4 timing plate had to go a long way advanced to get it right, checked 2-3 also, bike idles better now, my 28yr old son is a mechanic and was watching me and said seeing as 1-4 fire together and 2-3 fire together why did I check 2-3 as once 1-4 is ok 2-3 will be fine when I thought about I think he's correct, after all 2-3 fire after 1-4, so they can't be wrong, what's the theory behind checking both? ...750Dave.

He either needs to go back to school or read the timing procedure more closely, since the 2-3 points set are adjusted independently of the 1-4 points... i.e. once the 1-4 points are set the 2-3 points can very well be off. The 2-3 points are mounted on a "sub-plate" that is adjusted independently of the main points plate. 

[750Dave]

We adjusted the points like your spose to, but seeing as though CB750's fire two slugs at once, it shouldn't be necessary to check the timing on 2-3 when you've timed 1-4 as they can't do anything else but fire after the other two have fired, not like a car where every cylinder fires separately, or is it because they fire two at once..I don't quite understand it all, anyway I did the timing the way the book says..

[madmtnmotors]

They fire as pairs. 1/4 fires as a pair, and 2/3 fire as a pair. The 1/4 points are timed by rotating the primary/main points plate and then locked down. Then you move to the 2/3 points and adjustments are made by moving the 2/3 sub plate (NOT the primary/main points plate). The points for 1/4 and the points for 2/3 are adjusted independently, therefore adjusting one does not mean that the other is "automatically correct". The 1/4 must be adjusted first because the main plate adjustment affects both the 1/4 and 2/3. Once the 1/4 points are set (and the primary/main points plate is locked down) then the 2/3 points are adjusted by moving the 2/3 sub-plate without disturbing either the primary/main points plate or the 1/4 points. Are you not using the 2/3 points sub plate to adjust the 2/3 points? The 2/3 points are mounted on a separate plate that mounts on top of the primary/main points plate. The 2/3 points are adjusted independently of the primary/main points plate. If you are rotating the primary/main points plate to adjust the 2/3 points then you are doing it wrong.

EZPZ 

[750Dave]

Yes we adjusted both sets of points as the book says, then I did the timing statically, then checked with a timing light, is a fair bit advanced to get the timing right, bike is idling better than before though, bought this bike off a seller who has lied about what has been done to the bike, just relining the inside of the fuel tank at present among other things he lied about doing..thankyou for the help..750Dave.

[rickmoore24]

The C-Hook is a great tool but for top end stuff Franks (754) kit is a great frame mod. Rick.

[750Dave]

Took my engine out of the frame yesterday after being told when I took it for a tuneup it needs a topend rebuild, I'm in Newcastle NSW Australia, was thinking maybe a rebuilt exchange engine maybe the way to go, so if anybody knows if this is a possibility could you let me know..750Dave..

[madmtnmotors]

I am not aware of an exchange program for these old CB750's where you ship the old engine as a core and receive a rebuilt engine. CycleX comes close in that they will sell rebuilt parts (head, cylinders, transmissions, etc.) and you will get a discount if you provide a useable core, but I am not aware of any exchange program for an entire engine. You might find a used engine in better condition and try running that one while you rebuild your current engine. Member Terry in Australia may be able to help by recommending reputable shop(s) "down under". 

Was the mechanic/shop that gave you the diagnosis a trusted and reputable shop that is knowledgeable about these old engines? What diagnostic results did they provide that supports their conclusion?

Hope you are able to get it sorted.

I did the timing statically, then checked with a timing light, is a fair bit advanced to get the timing right

I don't understand what this means. "A fair bit advanced to get the timing right"? Are your points not firing on the "F" mark at idle? Are they firing at the advance marks at 2500 rpm (full advance)?

[750Dave]

Thankyou madmtnmotors for the info I'll contact Terry....cheers... 750Dave...