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Author Topic: CB750A Performance Mods  (Read 19009 times)

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Offline 122JG

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Re: CB750A Performance Mods
« Reply #50 on: December 18, 2015, 12:51:02 pm »
i applaud your incredible patience and tolerance in this thread Jon . :) extremely ZEN.

agreed   ;D

thanks to everyone...

Offline Tintop48

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Re: CB750A Performance Mods
« Reply #51 on: January 01, 2016, 08:23:07 pm »
Hello to this forum, especially this topic. I own a 77a which I installed a k1 engine in a few years ago,fun but am getting up in years and would like to re-install the a-engine. My concern is the Carb issue.finding Carb boots are impossible for the smaller a-carbs.I read earlier to use k boots with k carbs,but the spigots on the a-head are smaller.can someone explain how to do this. Thanks

Offline Medyo Bastos

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Re: CB750A Performance Mods
« Reply #52 on: January 02, 2016, 05:24:03 am »
the spigots are the same tintop.  you can use k model carbs with k model boots just fine.  what will you be doing with your k motor?  where are you located?

Offline Jerry Rxman Griffin aka MuthaF'er

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Re: CB750A Performance Mods
« Reply #53 on: January 02, 2016, 10:31:07 am »
I'm thinking all the A's used the -392 head from the 75/76 CB750F. Therefore the spigots are the early head size spigots.
As of today 3/13/2012 my original owner 75 CB750F has made it through 3 wives, er EX-wives. Free at last.  ;-)

Offline Tintop48

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Re: CB750A Performance Mods
« Reply #54 on: January 02, 2016, 11:40:33 am »
Thanks for the replies,according to my Honda manual the a motor uses 24mm carbs and the k motor uses 28mm carbs. I guess ill have to Mc gyver it because I want to stay with the less complicated carbs. Yes I'm going to get rid of the k1 reside in kalifornistan and the engine weighs 225+- so its a bit## to ship

Offline kmb69

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Re: CB750A Performance Mods
« Reply #55 on: January 02, 2016, 11:52:59 am »
1. Thanks for the replies,according to my Honda manual the a motor uses 24mm carbs and the k motor uses 28mm carbs.
2. I guess ill have to Mc gyver it because I want to stay with the less complicated carbs.
......
1. Carb sizes, 24mm and 28mm, are correct. But the A head has the same spigots as the K head and will easily mount the K1 carbs.
2. The K1 carbs are definitely less complicated IMO. No need to Mc Gyver, stock K1 carb boots will work.


Offline Medyo Bastos

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Re: CB750A Performance Mods
« Reply #56 on: January 02, 2016, 02:17:50 pm »
Thanks for the replies,according to my Honda manual the a motor uses 24mm carbs and the k motor uses 28mm carbs. I guess ill have to Mc gyver it because I want to stay with the less complicated carbs. Yes I'm going to get rid of the k1 reside in kalifornistan and the engine weighs 225+- so its a bit## to ship

specific boots are what adapts the different carbs to the head.  you need matching boots for matching carbs.  i prefer the f2 carbs on matics...

Offline Yoshimatic

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Re: CB750A Performance Mods
« Reply #57 on: January 06, 2016, 01:09:48 am »
I have K1 carbs on my 77 magic and it goes like hell. Mainly because of the 836 kit, F2 cam, race valve springs and Yoshi pipe though.
1969 GL175 twin
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Offline jweeks

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Re: CB750A Performance Mods
« Reply #58 on: April 13, 2016, 05:00:39 pm »


First pass on Vic Force's record holding Hondamatic twin. Perfect reaction time. Any Hondamatic can be modified to run better on the street or the track.   ;D
« Last Edit: April 13, 2016, 05:03:16 pm by jweeks »

Offline kac3177

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Re: CB750A Performance Mods
« Reply #59 on: April 13, 2016, 06:45:14 pm »
Any tips for tuning the carbs? I am doing basically the same exact set up. Any initial setting recommendations? First time usin the round tops. They are jetted to 120.

     I used pd41a's on my 76' jetted to 115, very fincky and cold blooded, that motor only has an aftermarket cam, otherwise stock. Once warmed up it runs good, definite change, lot more top end however I wish it ran a little better when cold.
kac3177

Offline dave the welder

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Re: CB750A Performance Mods
« Reply #60 on: April 16, 2016, 05:21:09 am »
kac read back through the older post several of us posted how we set up the carbs
don't buy it build it

Offline stevematic

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Re: CB750A Performance Mods
« Reply #61 on: November 28, 2016, 05:36:46 am »
This is my first time posting on this site. After recently swapping K carbs onto my A, I would like to contribute. When changing carbs, not only do you need the same boots, you also need the same cables. K carbs require K throttle cables. Most suppliers list this as a difference between years (70-76 vs 77-78)

Offline TurboD

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Re: CB750A Performance Mods
« Reply #62 on: November 28, 2016, 06:11:56 am »
Yes you will need to use the earlier K boots and cable(s).

Offline HondaMan

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Re: CB750A Performance Mods
« Reply #63 on: August 30, 2017, 07:34:14 pm »
I'm thinking all the A's used the -392 head from the 75/76 CB750F. Therefore the spigots are the early head size spigots.

You're right about the Casting, Jerry, but they did almost no port work, probably due to the smaller carbs being present. The heads can pick up LOTS of flow with some simple port work on the "A" bikes I have done. This, plus 836 (CruisinImage) pistons, a nice K cam, and think about swapping in one of the spark advancers from either the F1-2 or K7-8, or maybe a K4-5 with slightly tighter advancer springs, will brighten these up. The stock advancer is the "333" model from the CB350F, but with heavier springs.
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Offline fireman1073

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Re: CB750A Performance Mods
« Reply #64 on: September 22, 2017, 02:27:45 pm »
i have a 76a motor and a 75k and the pistons and heads appear identical

the 750a has less compression?

is this combustion chamber size or pistons because i plan to use the 750a top end (pistons, jug and head) on a 750k I'm building with a k cam

the cams are different from my rough measurements as the k has more lift and/or duration correct?



Steve
« Last Edit: June 18, 2019, 05:21:27 pm by raymond10078 »

Offline HondaMan

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Re: CB750A Performance Mods
« Reply #65 on: October 01, 2017, 08:08:13 pm »
i have a 76a motor and a 75k and the pistons and heads appear identical

the 750a has less compression?

is this combustion chamber size or pistons because i plan to use the 750a top end (pistons, jug and head) on a 750k I'm building with a k cam

the cams are different from my rough measurements as the k has more lift and/or duration correct?


someone local has a 78 750a with title, stored indoors and presently not running asking 800 need gone asap so maybe cheaper

here you go---> https://southcoast.craigslist.org/mcy/d/1978-honda-750/6313442841.html


Steve

The "A" has the -392- (bigger chamber) head, but with -300- type pistons, and about 20 degrees less cam duration. This makes compression about 8.8:1 or so.
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.
Hondaman's creed: "Bikers are family. Treat them accordingly."

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Offline stikman

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Re: CB750A Performance Mods
« Reply #66 on: October 14, 2017, 04:36:16 pm »
The "A" has the -392- (bigger chamber) head, but with -300- type pistons, and about 20 degrees less cam duration. This makes compression about 8.8:1 or so.

The A cam also has about .015" less lift on both in and exh than a mid 70s K cam.  And I believe that's even at the lobe not the valve.
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Offline HondaMan

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Re: CB750A Performance Mods
« Reply #67 on: October 14, 2017, 07:20:09 pm »
The "A" has the -392- (bigger chamber) head, but with -300- type pistons, and about 20 degrees less cam duration. This makes compression about 8.8:1 or so.

The A cam also has about .015" less lift on both in and exh than a mid 70s K cam.  And I believe that's even at the lobe not the valve.

Yeah, that's about what I measured, too. A good K cam or F cam can quickly perk it up. Since it only has the two speeds, I'd lean toward the F cams for hiway use, the K cams for in-town perking-up work.
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.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

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Link to website: www.SOHC4shop.com

Offline Yamahawk

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Re: CB750A Performance Mods
« Reply #68 on: March 31, 2018, 07:45:26 pm »
A fellow brought in a set of CB750A carbs to the shop today, and they were small compared to the CB750 PD series... I believe they were PD44's? The slides were about the same as the CB550 PD 46 carbs, and I wasn't sure if the PD44's were the stock CB750A carbs. They did have the accelerator pump, and a bracket which might have had the idle solenoid on it at one time, but was bare and flopping around.
Charlie
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Offline stikman

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Re: CB750A Performance Mods
« Reply #69 on: April 11, 2018, 10:36:15 am »
A fellow brought in a set of CB750A carbs to the shop today, and they were small compared to the CB750 PD series... I believe they were PD44's? The slides were about the same as the CB550 PD 46 carbs, and I wasn't sure if the PD44's were the stock CB750A carbs. They did have the accelerator pump, and a bracket which might have had the idle solenoid on it at one time, but was bare and flopping around.
Charlie

Yep, you're corrrect.  PD44's are stock CB750A carbs.
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Offline Don R

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Re: CB750A Performance Mods
« Reply #70 on: June 16, 2019, 10:05:34 pm »
 I'm wondering if the oil pump rotors are anything like the K rotors? Doubtful, but if so a set of Mirko's rotors might pick up pressures a bit.
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Offline kmb69

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Re: CB750A Performance Mods
« Reply #71 on: June 17, 2019, 06:39:19 pm »
I'm wondering if the oil pump rotors are anything like the K rotors? Doubtful, but if so a set of Mirko's rotors might pick up pressures a bit.

Nope, they're much larger diameter rotors like in the DOHC's.

Offline Don R

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Re: CB750A Performance Mods
« Reply #72 on: June 18, 2019, 06:58:24 am »
 OK thanks.
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