Topic: Are the 1980's 750K DOHC really any better than the early 1970's 750K SOHC?

[kyle750]

I'm not sure how to define better but for the sake of this discussion lets try to stick to handling and performance. This 1980's 750K  just showed up for sale locally at a very good price.  I really like the old style 4 into 4 exhaust but also like the more modern styling and comstar wheels.

[dusterdude]

Probably,but not as cool

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[bryanj]

If its an early one there was an important mod that needed to be done, dont like standing , carbs as finicky as the late pd type

[kyle750]

Looking at hp the 1980 K is listed at 77 - a big increase from the 1970s K

It is my understanding that the early 1970s CB750s K0 and K1 were listed at 67 hp but over time and with more restrictive exhausts and carbs the hp dropped down to 57 hp on later models.  Is this correct?

[kyle750]

My Thai friend Kannika has been in touch with the owner.  Lots of new information on the bike that we are sorting now and currently checking the vehicle ID and engine number.  Bike only has 12,000 km on the odometer (most likely original as there is only 1 previous owner who rarely rode the bike).  Bike is running with lots of new parts including new tires and brakes and new brake fluid and oil. 

[kyle750]

If its an early one there was an important mod that needed to be done, dont like standing , carbs as finicky as the late pd type

Thanks mate.  I'm reading about the carbs now. 

The idle (pilot) jets are pressed into the carburetor bodies rather than screwed in. This makes them extremely difficult to remove for thorough cleaning. If the jets become clogged the bike might experience difficult cold starts and stalling at idle or poor performance below 3,000 RPM.

[crashmaster]

Probably,but not as cool

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This

[kyle750]

Probably,but not as cool

100% agree that the 70s SOHC K's look the coolest . . . but what about handling and performance???

[MauiK3]

I may be confusing bikes and years but I thought there was a top end valve related problem with a few.

[kyle750]

I may be confusing bikes and years but I thought there was a top end valve related problem with a few.

The bike is from 1978 and I believe is a first year Honda CB750 KZ

[kyle750]

The 1971 Honda CB750K1 weighs approximately 498 lbs (226 kg) dry, while the 1978 Honda CB750K (KZ) weighs around 509 lbs (231 kg) to 513 lbs (233 kg) dry. The 1978 KZ model is roughly 11–15 lbs (5–7 kg) heavier due to its transition from a single overhead cam (SOHC) to a double overhead cam (DOHC) engine.

[denward17]

Was KZ only a European model? (or ROW)

[kyle750]

Was KZ only a European model? (or ROW)

Yes I believe only sold in Europe and not USA.  Still trying to determine if it was also sold in Japan.  Bike was imported and first registered in Thailand in 1994.

[Kelly E]

We've resurrected two DOHC bikes, a 79' CB 750F and a 82' CB 900F. We have a 83' CB 1100F still to do. My brother had a 79 CB 750K but he didn't really like how it handled compared to the 900F he had before.
The F models are nice bikes but a bit tall for me. I really liked the 82' CB 900F. It was smooth, fast and comfortable, I called it the gentleman's express. We ended up installing a Delkevic exhaust when the stock system died and it sounded awesome.

[PeWe]

The fun  part with the SOHC were all aftermarket parts !!
Possible to modify it to a racer, chopper or a cool look to the chopper style with longer fork tubes, fatter rear 16"wheel, banana seat, higher bars, loud 4-1 or drag pipes etc etc. Body kits....

Bolt off, bolt on. Possible to return to the stock style later on.

Loads of go fast parts which we still can enjoy.

I have seen the DOHC CB750-900 in pristine shape sell for very little the last 15 years.
Maybe the later  CB900 F2 bol d'or has gone for better prices.

I remember CB900 owners complaints in the early 80's about how slow they were.
Suzuki GS1000, followed by GSX1100, Kawasaki Z1000J and R were something different.

Honda  CB1100R and  Honda CB1100 F -1983 looked good.

Many Kawasaki and Suzukis in my town were updated with first Russ Collins parts followed by Vance & Hines.  We had and still have a good tuner that provided parts and jobs for racers,  dragracing and street.

I knew one guy that rebuilt his GS1000-78 with lots of handling improving mods to handle the bike long after the speedo had passed 200kmh.  Avoid high speed wheelies etc
It must have been the turbo that initiated that.   

Honda CB1100R -1983  have very similar power curve and rear wheel power as my CB750 with 970cc
Same rpm's and HP but 5-6 Nm more in torque.

That recipe  available back in the 70's too.

[kyle750]

@Kelly E Thanks for sharing the photo of your CB900F - Beautiful Bike    I really like the stealthy look of the blacked out engine, exhaust, front forks, and comstars and also like the small fairing in the front. Bike looks Fast!

[kyle750]

Here is a short 18 sec video of the bike running

https://www.youtube.com/shorts/pt5E21XrofY

Honda CB750 KZ 1978 RC01

- first inline four cylinder Honda with DOHC
- no contact points - transitorized electronic ignition system (Hooray . . .  Hallelujah)
- dual front disc brakes
- 4 into 4 exhaust (some got 4 into 2)
- comstar alloy wheels (some got wire spoke)

First shown at the 1978 IFMA in Cologne the new CB 750 was presented by Honda as the successor to the CB 750 four for Europe and available for sale later that year. With a new DOHC engine design and numerous other new features it was presented as "what happens when you keep winning races".

[kyle750]

The fun  part with the SOHC were all aftermarket parts !!

It looks like the Honda CB750 KZ RC01 was only made for 2 years in 1978 and 1979 so finding parts for the bike would be challenging and most likely very expensive.

[kyle750]

Honda CB750 KZ 1978 RC01 was offered in Europe and Japan

The Honda CB750K (often referred to as the KZ or K-series with RC01 engine) was sold in Japan. It was launched domestically in December 1978 to celebrate the 10-year anniversary of the original CB750 Four. The 1978 model (RC01) featured a 748cc DOHC 4-valve engine, Comstar wheels, and was styled as a touring model with four mufflers.

USA 1978 Model: The U.S. was still selling the CB750 K8, which was the final version of the original SOHC (Single Overhead Cam) series. The RC01-based DOHC models did not arrive in the USA until the 1979 model year, where it was sold simply as the CB750K (often referred to as the KA model).

[kyle750]

I may be confusing bikes and years but I thought there was a top end valve related problem with a few.

You are correct.  Mahalo Brother!  Valve Problems: Early DOHC models had issues with soft valve stems that could mushroom or valve keepers/retainers failing, though many were fixed under warranty.

Late Edit:  Reminds me of the problems Honda had with the early Sandcast CB750s with chain failure and crankcase damage that was fixed by Honda under warranty

[beemerbum]

Seems Honda had no choice but to go with the DOHC for marketing reasons. Kawasaki and Suzuki would claim that two are always better than one.  To some, the twin  cam motor looked more "high performance"

[kyle750]

More photos of the bike taken in May 2025

[jlh3rd]

"USA 1978 Model: The U.S. was still selling the CB750 K8, which was the final version of the original SOHC (Single Overhead Cam) series. The RC01-based DOHC models did not arrive in the USA until the 1979 model year, where it was sold simply as the CB750K (often referred to as the KA model)."


there was also a CB 750C custom in the U.S.
Engine code RC01" was used through 1983 in the U.S. The vin (now a real vin) starts with JH2R....1981+
No reference to "KA" in the identification guide, maybe somewhere else.

[MauiK3]

I still think I was aware of some valve problems or something with the early ones.

[kyle750]

It looks like the Honda CB750 KZ RC01 was only made for 2 years in 1978 and 1979 so finding parts for the bike would be challenging and most likely very expensive.

The bike looks complete and I can't see any parts missing or in poor condition that might need replacing.  But just to check availability I did a quick ebay search and there are parts available mostly from Germany and the UK (no surprise there).  There was even one seller in Germany who clearly was a professional dismantler of used bikes with many parts for sale.  Unfortunately not cheap like used CB750 parts in the USA

Late Edit:  I just noticed the tall ape hanger handle bars on the chopped middle red ZX