Topic: What makes the early 750K moters faster than the later?

[grumpy]

I frequently read about how the K1 motor is considered the fastest of the bunch.
Why?

I know there were changes in the pipes, carb jetting and factory needle settings, and gearing but what changed in the motor?

[evanphi]

There's lots of this sort of stuff in the Thoughts of the Hondaman thread.

http://forums.sohc4.net/index.php?topic=7401.0


Maybe HondaMan will chime in...

[Davez134]

All that stuff you mentioned changes air/fuel through the engine, effecting power. There were also timing and advance changes as well. I don't think there was anything internal to the motors that changed anything. (I have heard that early cams were different, but have not had the chance to measure one from a confirmed early motor side by side) Most of the changes were probably done for either emissions or longevity of the motors.

[70CB750]

From reading the book, Honda was forced to choke the engine down for better mileage and EPA regulations.

[madmtnmotors]

Urban myth. Everyone knows the 78's (both F and K) were the fastest!      

[Wobbly]

The CB750 F2/F3 models are the most powerful (73 vs. 67 hp) SOHC CB750 Fours--and the fastest. The 77/78 K7, however,  would do 0-100mph in 11,2 seconds (Motorrad Magazine, Germany, 1978) but its V-max was lower than some of the earlier models (below 200kmph). madmtnmotors is thus right--but not with the windshield installed on his F model. The V-max would only be reached lying down on the tank. Sitting upright, could make as much difference as 10mph.  Not that any of this matters in the States, but it did on the German Autobahn when it was time to find out who has the largest... Anyhow, my 2010 BMW probably has a higher V-max in second gear (6th gear transmission) than my K7 in fifth gear--with me on it, anyhow.

[ekpent]

  Anybody remember the first oil embargo and long gas lines in the 70's. Going from a 120 jet down to an eventual 105 helped to save gas. Earlier HM300 pipes with a real baffle had less chambers and less restrictive than the later K2 and on HM341's. The K1 engines liked to rev. I have heard there are differences in the cam as well. Also pre Goldwing period a lot of 750's were turning into baggers with Vetters and Honda noticed that and also tuned the engines for more low end torque and less top end kick.
   The 71's are fine running bikes, my personnel favorite.

[madmtnmotors]

 

My observation is based solely on the highly refined and world renowned "seat of the pants" dyno. I've never driven anything older than a 78' CB750 so it must be true... dammit!     

[Don R]

 The ports were hand finished on the early bikes, I read that the head castings got better on the late bikes. They allowed the mid year engines to make less hp and saved production time/money while doing it.

[754]

+1 on the hand finishing, and the cams were a little hotter..
 I dont think. Any 77/78 factory pipe (don't know if you can even pull the baffle like pulling a bolt out on early ones) would make the hp of HM 300,s with baffles out.
 So those things plus gearing..did add up

I dont really care about listed hp..more 1/4 mile times or seat of the pants feel..

[Desert-SOHC]

My sandcast was frikin fast, WAY faster than either one of my 78's, but it was pro-tuned with a 4 into 1.

The fastest thing I had was a 74 CB450 DOHC with a full race HRC engine, redline was 11,600 and it idled at almost 2K. At about 6K it kinda took on a Banshee scream and shot to redline like a rocket, you pretty much shifted it as fast as you could till 4th. I used to be able to eat KZ650's for breakfast and you could haze the rear tire at will in 2nd and 3rd, it was a REALLY fun bike to ride.

[Wobbly]

but what changed in the motor?

http://www.satanicmechanic.org/enginemods.shtml

[MoMo]

From reading the book, Honda was forced to choke the engine down for better mileage and EPA regulations.

That is my recollection also.   Honda would not make anything slower unless forced too ...Larry

[seanbarney41]

Hondamans book talks about all of the above with technical details and photos...and also explains how to reverse all of the changes, for free, cheap, and only using oem parts.

[Davez134]

Urban myth. Everyone knows the 78's (both F and K) were the fastest!      

Unless it's a 72 with 836, cam, JMR ported head



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[Don R]

 The older I get, the faster I was.

[kmb69]

My sandcast was frikin fast, WAY faster than either one of my 78's, but it was pro-tuned with a 4 into 1.
.........

+1. The earliest Sandcasts with the "Lotus Roots" pipes, which were stock by the way, were the fastest of them all. Even though the 78's made a little more power, they were heavier than the very first '69-'70 750's. Early K0's would scream too. Like somebody said earlier, EPA regs, and busted chains made Honda pull back.

[Wobbly]

The older I get, the faster I was.

Thank you for making my day. How true.

[seanbarney41]

uhhh, guys are forgetting one other thing...K0 came with 16 tooth front sprocket...I feel like that was the bulk of the butt dyno boost and is available for any year 750

[754]

16/45.   Seems pretty close to 17/48 I think..

[atm cycles]

Quote from: Don R on July 02, 2015, 05:32:42 PM
The older I get, the faster I was.

Thank you for making my day. How true.

Ain't it the truth!!!!

[70CB750]

The older I get, the faster I was.

My CZ250 2 stroke was the fastest of them  all, or was I just younger? 

[David B]

After riding my 750 with a k5 motor & then jumping my my buddy's k1 the difference in power is noticeable. Even sitting next to each other at an idle you can tell a difference in the engines. My k5 idles & purrs quietly, careful not to offend anyone. His k1 has a bit of a growl and you can you can hear the hop in the cam.

[kmb69]

The older I get, the faster I was.

That has been Bear's signature on this forum for quite some time.
http://forums.sohc4.net/index.php?action=profile;u=7543
I love it and use it regular in real life. Credit to Bear first, I think.

[MoMo]

The older I get, the faster I was.

My CZ250 2 stroke was the fastest of them  all, or was I just younger? 

Appropriate avatar Prokop .  The CZ I had accelerated faster than most every other bike I've owned...Larry