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Author Topic: Cb750k1 airbox  (Read 237 times)

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Offline Remcod

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Cb750k1 airbox
« on: December 02, 2025, 04:58:50 AM »
Im going to pickup bits of a cb750 k1. How do i recognise a k1 airbox? I believe the air intake is bigger, but what is the size? Any other id marks?

Thanks,
Remco

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Offline lash

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Re: Cb750k1 airbox
« Reply #1 on: December 02, 2025, 05:04:23 AM »

Same airbox from 71 to at least 76
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Offline newday777

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Re: Cb750k1 airbox
« Reply #2 on: December 02, 2025, 05:24:10 AM »
Quote from: lash on December 02, 2025, 05:04:23 AM

Same airbox from 71 to at least 76
71 K1 airbox has bigger air inflow openings than the K2-K6 have(more restriction for the different cam)
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Stu
Honda Parts manager in the mid 1970s Nashua Honda
My current rides
1975 K5 Planet Blue my summer ride, it was a friend's bike I worked with at the Honda shop in 76, lots of fun to be on it again
1976 K6 Anteres Red rebuilding project, was originally my brother's that I set up from the crate, it'll breath again soon!
Project 750s, 1 K2, 4 K6, 1 K8, 1 F1, 1 F3
2008 GL1800 my daily ride and cross country runner

Prior bikes....
1972 Suzuki GT380 I had charge of it for a year in 1973 while my friend was deployed and learned to love street riding....
New CB450 K7 after my friend returned...
New CB750 K5 Planet Blue, demise by ex cousin in law at 9,000 miles...
New CB750 K6 Anteres Red, to replace the totaled K5, I sold this K6 at 45k in 1983, I had heavily modified it, many great memories on it and have missed it greatly.....
1983 GL1100A, 1999 GL1500 SE, 1999 GL1500A

Offline Remcod

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Re: Cb750k1 airbox
« Reply #3 on: December 02, 2025, 05:39:16 AM »
Quote from: newday777 on December 02, 2025, 05:24:10 AM
Quote from: lash on December 02, 2025, 05:04:23 AM

Same airbox from 71 to at least 76
71 K1 airbox has bigger air inflow openings than the K2-K6 have(more restriction for the different cam)
Yes, but what size is the opening?

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Offline newday777

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Re: Cb750k1 airbox
« Reply #4 on: December 02, 2025, 09:41:06 AM »
Quote from: Remcod on December 02, 2025, 05:39:16 AM
Quote from: newday777 on December 02, 2025, 05:24:10 AM
Quote from: lash on December 02, 2025, 05:04:23 AM

Same airbox from 71 to at least 76
71 K1 airbox has bigger air inflow openings than the K2-K6 have(more restriction for the different cam)
Yes, but what size is the opening?

I was saying that to Lash.

I can't find my K1 airbox at the moment to measure it. All I could find is a K2-K6
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Stu
Honda Parts manager in the mid 1970s Nashua Honda
My current rides
1975 K5 Planet Blue my summer ride, it was a friend's bike I worked with at the Honda shop in 76, lots of fun to be on it again
1976 K6 Anteres Red rebuilding project, was originally my brother's that I set up from the crate, it'll breath again soon!
Project 750s, 1 K2, 4 K6, 1 K8, 1 F1, 1 F3
2008 GL1800 my daily ride and cross country runner

Prior bikes....
1972 Suzuki GT380 I had charge of it for a year in 1973 while my friend was deployed and learned to love street riding....
New CB450 K7 after my friend returned...
New CB750 K5 Planet Blue, demise by ex cousin in law at 9,000 miles...
New CB750 K6 Anteres Red, to replace the totaled K5, I sold this K6 at 45k in 1983, I had heavily modified it, many great memories on it and have missed it greatly.....
1983 GL1100A, 1999 GL1500 SE, 1999 GL1500A

Offline Oly

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Re: Cb750k1 airbox
« Reply #5 on: December 02, 2025, 01:14:05 PM »
The two openings measure 3.97 " X 0.50 ".
One of the openings has a flared Lip opening making it look wider but the internal throat of the openings are the same.
These pictures are off my CB750 K1 manufactured 8/70 and is original equipment.
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1971 CB750 K1
1967 BSA 441 Victor
1966 Yamaha 250cc YDS3c Big Bear Scrambler

Offline HondaMan

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Re: Cb750k1 airbox
« Reply #6 on: December 02, 2025, 05:10:18 PM »
The inner 'wall' of the slot openings in the K1 airboxes are lower than in the later ones by about 1cm. This 'closed upward' in the K2 versions to be only as wide as the curved outer section.

You can (and I often have) lower the inside 'wall' of the opening to be that same 1cm lower opening. It just takes a sharp blade. Be aware that if you're riding behind a big fairing (like a Vetter) this will increase engine noise slightly above 3000 RPM, though.
« Last Edit: December 02, 2025, 05:11:56 PM by HondaMan »
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See SOHC4shop.com for info about the gadgets I make for these bikes.

The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
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Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book
Link to My CB500/CB550 Book: https://www.lulu.com/search?sortBy=RELEVANCE&page=1&q=my+cb550+book&pageSize=10&adult_audience_rating=00
Link to website: https://sohc4shop.com/  (Note: no longer at www.SOHC4shop.com, moved off WWW. in 2024).

Offline Remcod

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Re: Cb750k1 airbox
« Reply #7 on: December 02, 2025, 11:13:00 PM »
Thanks all, very helpful!

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Offline BenelliSEI

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Re: Cb750k1 airbox
« Reply #8 on: December 03, 2025, 06:31:09 AM »
Great….. Learned something today!
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Offline Rookster

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Re: Cb750k1 airbox
« Reply #9 on: December 05, 2025, 12:07:53 PM »
The K1 airbox also does not have the embossed lettering CB750F on the side.

Scott
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