Topic: 75F engine in a 78K frame?

[yitzac1990]

I tried searching and couldn't really find a direct answer.  will a 1975 CB750F motor fit on my 1978 CB750K bike?  What differences, if any, are there?  Im not too worried about differences in power or internal "stuff", just as long as it will bolt up and run/ride, i will be happy for now.

[shifty5150]

From my one experience with this the case bolts in fine but the chains are different (530/630) so you have to deal with the rear sprocket or use a K rear wheel assy. unless someone makes a conversion front sprocket (they mount differently and can't be swapped).  That's as far as I got, bike is still sitting so I'm curious as well to her what others chime in.

[shifty5150]

.....and I believe you mean K series engine into an F frame.   

[yitzac1990]

no I have a 1978 cb750K bike now, and found a 1975 F engine on CL.  I was also wondering about the chain/sprockets.  my bike (K) now has 530 chain setup.  are you saying the output shafts between a 75F and 78K engine are different?  I got the JT sprocket that fits right on the 78K shaft for 530 chain.

P.S. - the reason I ask is because my engine is knocking, so id like to try replacing the engine, instead of rebuilding, since I dont have that much time/money/space right now.

[shifty5150]

Ahh, ok I've got a K engine going in an F frame and in my scenario the sprocket from the F engine has an odd two bolt mount and the K engine has a single center bolt mount that holds the gear against a  step on the output shaft.  Yours may be entirely different.

[yitzac1990]

I know what you're talking about.  When I was researching chains/sprockets, I think the sprocket I got will fit not only the K engine, but im not 100% sure.  However, something like that doesnt worry me as much as if the engine physically fits and will work with the 78K electronics (points system, oil pressure sender, neutral switch, etc.) as well.

[yitzac1990]

well according to JT's website, the sprocket that I got will fit the 75 cb750F, so that's good.

[yitzac1990]

oh also, will 1978 PD carbs work on the 1975 engine?  Ive got pod filters, 4-1 exhaust, and 115 mains and 38 slow jets.  Would this be alright on a 1975 F engine?  or am I going to need totally different carb setup??

[700504]

You can use the 78 carbs as long as you use the 78 rubber manifolds.

[Jerry Rxman Griffin aka MuthaF'er]

Yes it will "fit" with a little work. The 78 engine has a 10mm offset at the output shaft for it's double row bearing that has to be dealt with all the way back while the 75 F engine does not. Everything else is fine. I did the reverse and converted the 78 engine to a 530 chain in a 75 F frame. Had to update the front sprocket, rear sprocket carrier and swing arm to widen my drive line by that 10mm. You MAY be able to get by with just an earlier sprocket carrier and fabricate a spacer that is 10mm wider.

Converting from the 630 to 530 required a front sprocket with 1 shoulder not 2 like the stock 75 F 530 sprocket has.

[yitzac1990]

Jerry, could you be more specific or point me to some pictures of what you mean?  what do you mean a double row bearing?  where would that fabricated 10mm spacer go?  what do you mean a sprocket with 1 shoulder?  i looked up some images of 75 and 78 stock front sprockets, and they look almost exactly the same except for the 2 holes for the locking plate on the 75.

This is the engine that I am looking at getting to put in my 78K frame:
http://nashville.craigslist.org/mpo/5741030270.html

[Patrick]

You have to use the sprocket carrier, axle and rear axle spacer that comes with the engine you plan to use. Any SOHC 750 engine will mount into any SOHC 750 frame. The chain will not ride straight unless you use all the rear wheel components appropriate to the engine, not the frame. I've done it both ways - early engine in late from *K2 engine in F3 frame) and late engine in early frame.

And yes, the carbs will work fine with the carb holders appropriate to the chosen carbs, not the frame.

Patrick

[yitzac1990]

so...the 1975 F engine actually brings the front sprocket in closer to the center of the bike, am I understanding that correctly?  and I would just need to find the sprocket carrier and correct length left side spacer?  Why would I need a new axle also?

or, hypothetically speaking, if i could space out the front sprocket the correct distance (10mm?), could that work?

[yitzac1990]

I was on Cycle X website and they actually have an offset front sprocket, that pushes it out the correct amount.  I assume I could just use this, and that would eliminate the need to replace any of the rear drive items?

[madmtnmotors]

I was on Cycle X website and they actually have an offset front sprocket, that pushes it out the correct amount.  I assume I could just use this, and that would eliminate the need to replace any of the rear drive items?

That is correct. You would need to use your front sprocket cover off of the 78' K motor. The later sprocket cover has the extra 10mm clearance. If you use the 75' F sprocket cover with the offset front sprocket, then the chain will "self clearance" the 75' sprocket cover for you... 

[yitzac1990]

haha, gotcha, good info!!

[jamesw]

http://forums.sohc4.net/index.php/topic,155902.msg1786238.html#msg1786238

I had a similar issue with my first k2.

Scroll down in the above thread to see Jerry's picture of a shouldered front sprocket.