What is the difference? CB750K vs CB750F

**moderncafer** · 03-19-2016, 03:23 PM

Alright guys be nice....lol. I am new to the cafe seen but not new to engineering and design of motorcycles. Wondering if some can school a bit about the Honda 1977 CB750. I just recently picked up a 1977 CB750K bike and 1978 CB750F frame. Wondering what the difference is between the K version and the F version. Are the motors the same? Its obviously a SOHC motor. I am also assuming the serious runs from 1976-1978 right?

**Flogger** · 03-20-2016, 10:17 AM

Use this site to read about basic info and specs... The F is the Super sport version... Some things are the same and some things are not...

http://www.motorcyclespecs.co.za/mod...k7%20%2077.htm

Look at pix too, you will see body differences not mentioned in articles... Super sports tended to have painted rear molded fenders where as most older 750s had chrome fenders... Wheel spokes were another common difference vs mag wheels... Turn signals, instruments, and gauges would vary. Back then the super sports were more expensive but today to a collector it is the opposite... I prefer all the chrome, the spokes, the vintage motorcycle look... If you are cutting it up, it won't matter...

**moderncafer** · 03-20-2016, 11:41 AM

So the motors are all the same?

**dirtdigger** · 03-20-2016, 11:48 AM

Externally for the most part yes they are the same, internally no, the F has upgrades and is more powerful.

**moderncafer** · 03-20-2016, 12:07 PM

Upgrades such as?

**dirtdigger** · 03-20-2016, 12:19 PM

different head with bigger chamber and valves, different camshaft, different pistons to match bigger chamber, different gear ratios internally, different output shaft, the list goes on.

**Flogger** · 03-20-2016, 01:39 PM

I think all these SOHC bikes were around 38 HP but if you want a real HP difference, the DOHC ones about doubled it to the 75 or 80 range...

**dirtdigger** · 03-20-2016, 01:51 PM

The dohc made about 10 to 12 hp more...not double the hp 67 compared to 79 hp at the crank Although later sohc K motors made a couple less and the late sohc F motors made a couple more.

**Flogger** · 03-20-2016, 02:37 PM

35 to 38, doubled is 75 to 85 that are published numbers, not mine... and approx or "about"... Just reading published specs on websites, Honda brochures, manuals, etc, ... again, not mine... It is a well known fact the DOHCs are a lot faster and more powerful than the SOHCs...

**dirtdigger** · 03-20-2016, 03:48 PM

Not a lot faster. again published numbers from honda sohc 67 at crank, dohc 79 at the crank. If you have sohc making 35 hp even at the wheels you better rebuild that sucker because its shot. Stock motors will put out low mid 50 hp all day long at the wheel with a healthy motor and a good tune, get a KO and you will see about 60hp at the wheel. Yes you always loose horsepower comparing crank and wheel horsepower. Seen more than one stock sohc bike rear wheel dyno hp in low 50hp range. No matter how you look at the dohc does not make double the hp of the sohc motor in stock form, aint gonna happen unless the sohc is running on 3 cylinders. Either way the original post asked about differences between the K and the F not whether the dohc had more horsepower then the sohc.

**Flogger** · 03-20-2016, 05:07 PM

Originally Posted by dirtdigger

Either way the original post asked about...

Yep... and I gave an opinion answering his questions and used figurative speech, along with accepted documentation, and you wish to split hairs and precisely analyze to the tee... Folks that like SOHCs better do and will... Folks that like DOHCs better do and will...

**dirtdigger** · 03-20-2016, 05:32 PM

I am not splitting hairs saying a dohc makes double the horsepower is absurd. Look at the honda "factory" spec and you will see you are wrong. 12 more hp is hardly double. I know sohc motors they have never made 35 to 38 hp, think you better check your facts with some actual facts, because your accepted documentation is incorrect.

**amc49** · 03-20-2016, 10:44 PM

DOHC not that much faster, the SOHC back in the day often claimed 68 at the crank. A CB550 made about 50+ at the crank. 35-38 is around a 400-450 cc. power level. My 400 Kaw triple made 38. A SOHC 750 that made only that has something seriously wrong with it. The ATX one probably dropped a few hp to be able to pull down low from dead idle because of the torque convertor and only two speeds, they had much smaller carbs to do it.

**Flogger** · 03-21-2016, 08:53 AM

Originally Posted by amc49

DOHC not that much faster

That's subjective... published specs have any representative 76-78 SOHC top speed of 123.2, for the DOHC the top speed is 127.2... The quarter mile time and speed for the SOHC is 13.5 sec and 100.9 MPH... Quarter mile for the DOHC is nearly a full sec faster at 12.4 sec and reaching 106.9 MPH... If the SOHC was a better bike, Honda would have kept making them that way...The crowd praising the SOHC is for just older, more nostalgia... or the cafe/bobber/customizing group- because a SOHC is more easily customized...

**amc49** · 03-21-2016, 09:32 PM

You're picking extremes, there are DOHC that won't break 13.0s just like that and the early SOHCs could hit high 12's when tuned correctly. I put the real world difference at no more than about a half second or so. Top speed is a waste of time, change the final drive and both will run 130 mph, you picked theoretical top speeds assuming the DOHC will hit 9500 rpm, they often won't since the CV carbs limit full open. AS well, by the time the DOHC rolled around most of the bike magazines used Peewee Gleason to do 1/4 mile testing as he weighed 120 lbs. and a good drag race launcher. Most faster DOHC times were set by him like many other fast bike times of that era. Look at average CB750F times, almost impossible to get below 12.50 real world. BTDT. I had fits getting to 12.80s-12.70s on mine at 170 lbs. and that same 38 hp. Kaw 400 did 13.70s after I ported it. I hit 13.20s-13.10s on my friends' brand new Kaw 650-4. Later header and carb work and sprocket hit 12.50 on the 750F. Magazine times are often unobtanium, they are not real.

You also have no idea how we dog the DOHC for unfulfilled dreams that Honda could have made simple easy cheap changes to make them MUCH more powerful. The stupid curves in the cylinder head to accommodate close to SOHC carb centers are worth close to 25 hp., when the engine is tapped all the way out. The engine absolutely refuses to breathe above about 150 hp. regardless of WHAT you do to it unlike other engines which just keep on going higher. Direct lift carbs like SOHC has if picked correct size just for a stock engine are a solid 10 hp over the CVs. And the rod design right out of the SOHC severely limited them as well did the cam chain adjusters. The DOHC is one of those weird engines that is pretty much bulletproof in dead stock form but press on it at all for more performance and it fails in several ways, any one of which usually leaves one with a basket of exploded engine parts. Why the few who tried to race them choked while the big Kawasakis and Suzukis utterly destroyed them. Those two designs were MUCH more robust. Honda while roadracing found that out the hard way, blowing engine after engine, there were races where every single rider went out early due to blown engines. One of the major rules to Freddie Spencer was not to rev them over 10K, or slightly over stock rpm, they 'blow up there' his tuner emphasized to him.

Hey, for all the trumpets of 'new age design' the CBX had the same issues, as did the later 1100XX Blackbird, Honda is known for making flimsy hi-perf bikes...........

**Flogger** · 03-21-2016, 10:15 PM

Interesting thoughts but one note, I just used factory numbers for all... and we all know what that means... I do concede that folks like customizing the SOHC because it is easier to work on... and then you can blow away a stock DOHC... I am a stock guy and don't like customizing since I go for restoration... The rest is history why Honda did as they did... But manufacturers don't do the racing for money, it is to support the sales and Honda has always made great street bikes for the consumer. These bikes and the CXs, Goldwings, etc., that we all seek are 35 or more years old... Everyone describes all these motors as indestructible. I don't see near as many other makes of old bikes out there...

P. S. Not only do all bikes possibly miss making some of the claimed specs, but almost all riders could take any perfect, new, flawless bike out to a track- and not achieve the published specs too.... takes practice and experience and perfection on the part of the rider too... perfect riding, shifting, etc and at some scary speeds and accelerations.

**amc49** · 03-22-2016, 06:29 PM

Obviously you never saw the 24 hr. CB900F racetrack test did you? At WFO top speed for 24 hrs.

The CX were recalled for oil system issues that tore some of them up. Goldwings were destroyed when Fram oil filters came apart in them. The V-30 500cc. V-4 was recalled for broken crankshafts. Goldwings used the same carbs that gave so many people fits on DOHCs. The SOHC 750 had issues with the double chain inside the cases wearing goofy to break and shatter cases. Hot rodders commonly tore the engines up due to it, one reason why the later ones were detuned somewhat.

**Flogger** · 03-22-2016, 08:11 PM

Wow, don't sound like you like many Honda's... The CX oil issue was actually operator error running too low of an RPM for too long... Goldwings set the standards for big bikes, still making them after 40 years. Every bike, make and model has dealt with some recalls and some production changes/modifications... These bikes reputations and longevity speaks for themselves... I have a 20 year old Goldwing with 87,000 miles on it... Had a CX back in the day and still have a 33 year old CX (GL650)... The 750 was built from 1969 till 2003 for 34 years... If I search for a vintage Honda to buy, I find hundreds available and still in the road... The same search for any other make only turns up 2, 3, maybe 5 bikes...