Author Topic: CB750 tuning and miscellaneous  (Read 1395 times)

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Offline dammage

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CB750 tuning and miscellaneous
« on: January 08, 2012, 09:31:20 PM »
My first post on this forum - I inherited a 1970 CB750 basket case that I'm currently restoring and wouldn't mind some tech help.  I've lots of experience with motorcycles and car engines, but this is my first 4cyl. bike.  Just a cuppla quickies for now: The engine will be like new from the bottom up - it's been taken .025 over, I've installed a Webcam #41 grind cam, will be running a K&N filter and probably a lo-cost 4 into 2 exhaust.  Most of my riding will be done in the mountains between 2000 and 4000 ft. alt.  Any rejetting suggestions, or from anybody's experience will they be necessary at all?  Just trying to be pro-active before I put the carbs on.  Also, I've read that taking some weight off the rotor is a good mod (makes sense to me - it's a monster).  If I do this, then how much and from where (inside, outside)?  Any help is greatly appreciated.

Offline 754

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Re: CB750 tuning and miscellaneous
« Reply #1 on: January 08, 2012, 10:44:16 PM »
 Rotor is cut at the front, side, and back and taperd and sometimes drilled. Depends wether you run kick or e-start.. I can cut it if you need it done.

 Where in BC, are you? can yiou give us approx engine number, just curious there are at least 5 KO nengines here in Ktown..
Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline Jerry Rxman Griffin aka MuthaF'er

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Re: CB750 tuning and miscellaneous
« Reply #2 on: January 08, 2012, 10:45:30 PM »
With your elevation which is only minor, that may negate the mild cam jetting increase. The K & N stock type filter will give more air which could then require a 1 size bump in mains. The 4 into 2 could require 1 size increase also. Maybe not. In other words you really won't know until you start to play with it. You can always put it on a dyno and dial it in easily. I believe it came with 120 mains which could do the trick. Try them first and make sure the needles are in stock position. From there just do plug reads/throttle chops. You may have to adjust needles &/or mains but then again you may not. Without bigger pistons you may be able to get by with stock or slightly larger mains for stronger top end and no needle changes (which are a pain in the ass!) Bigger pistons would probably need more mid-range fuel too (needles). Cams and freeing flowing air are more top end (mains). Mains also overlap midrange needles. Carb dynamics suck. Thius is definitely a learning experience.
As of today 3/13/2012 my original owner 75 CB750F has made it through 3 wives, er EX-wives. Free at last.  ;-)

Offline dammage

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Re: CB750 tuning and miscellaneous
« Reply #3 on: January 09, 2012, 01:55:06 PM »
Thanks for the advice - I'm just kind of hoping it'll be fine as is.  I've had a lot of experience tuning Brit bikes and have found most of the time the recommended increase in mains (with the usual breathing improvements) aren't necessary.  Being above sea level probably makes the enrichening thing less of an issue.  The serial is 1036780 and the bike originally came out of Edmonton.  It belonged to an old neighbour and good friend of mine who died a cuppla years ago and his wife left it to me knowing I'd do something with it.  I'd always wanted a CB back in the day but never got around to it.  The neighbour (Mike) had bought it used and ridden it all over the planet until it "siezed" on him (by now we're probably into the 80's).  The bike sat in his garage for years until I agreed to look at it and see what it needed (now we're into the late 90's).  The cam had snapped clean in two at the chain sprocket (is this common?).  I offered to take it off his hands but Mike'd have nothing to do with it, so it sat again, in pieces, until his unfortunate departure to the big breaking yard in the sky two years ago.  The upside is that it was always stored in a garage, and it was still in the labelled Clamato cans that I put it in when we tore it down.  I brought it out here after the funeral and didn't get around to working on it 'till last fall.
I digress, though - back to the rotor.  Is the lightening thing a good idea?  I don't plan on drag racing it, but I'd heard that under hard use the cranks can snap due to the weight of the thing, but I don't recall knowing anybody that this happened to and I knew some guys who rode their CB's really hard. 
I live up in hillbilly land between Greenwood and Grand Forks, by the way.  It's good to know that there's a couple in the area.  I don't know anybody with a CB around here, at least that's on the road.

Offline 754

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Re: CB750 tuning and miscellaneous
« Reply #4 on: January 09, 2012, 07:53:55 PM »
 It will rev quicker with a lighter rotor, If you do a lot of hi-rpm shifts it s a good idea, it will be more responsive with some weight off..
 Cam problem indicates that oil supply to one set of cam journals got blocked.

 Boundary country will try to get there a few times this year. An old buddy is the Mayor of Grand Spoons near there.. and I know a girl that moved recently to thre Fort in Greenwood. Do you live near the top of the pass , or Phoenix turnoff? Last time we went thru there a buddy said they dumped a bike  at the top of the pass and someone living there helped them out getting the biker back on the road.. took a day or two.

 Glad to see another KO nearby... we got two motors here in the 44K range (9numbers apart!!),  a 20K, 16K, 15K and a set of cases from 10 K.. and a buddy in Cowtown has a 43K
Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline dammage

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Re: CB750 tuning and miscellaneous
« Reply #5 on: January 10, 2012, 09:04:03 PM »
I'm just past the Phoenix turn-off.  We all know the mayor and I'm aquainted with the guy that built Fort Apache in Greenweird there.  It's easy to get to know people in a place like GF, and you know how the motorcycle community works - I'm sure we have mutual aquaintances.  I remember a bike crash a year or two back - there's a real good head lives right near there who's into older big Kawis, or something (older BMW 2002 cars as well).  He's probably the one who helped your buddy out.  Real nice guy - we met and chatted for quite a bit last spring.

Ya, top end's first thing to go usually if there's an oiling issue.  I got a different head and rebuilt it before I owned the bike.  I want to keep the electric start, so I assume I'd be more limited in what I could take off of the rotor.  I've got a buddy with a lathe - what would you want to turn it down?  I'm up in Kelowna occassionally for this and that.

Offline 754

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Re: CB750 tuning and miscellaneous
« Reply #6 on: January 12, 2012, 08:37:09 AM »
 That bike spill happened about 20 years back, the guy that helped lives 1/2 way between the towns.

 Up till 72 had the heaviest rotors, after that they were a bit lighter. Probably 40 bux to turn one down, or can sell a later one, if you want to keep the original.

 Is the Ko fairly stock or modified..?
Maker of the WELDLESS 750 Frame Kit
dodogas99@gmail.com
Kelowna B.C.       Canada

My next bike will be a ..ANFOB.....

It's All part of the ADVENTURE...

73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline dammage

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Re: CB750 tuning and miscellaneous
« Reply #7 on: January 12, 2012, 11:16:10 AM »
The bike was totally stock except it had been taken .025 over, has later carbs with the mechanical linkage (I still have the originals with the spaghetti cables) and it had a 4 into 1 (Mike had dropped it and totalled half the original pipes).