Author Topic: CB750A Performance Mods  (Read 23686 times)

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Offline stevematic

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Re: CB750A Performance Mods
« Reply #50 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 #51 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 #52 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 #53 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 #54 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."

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

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Offline stikman

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Re: CB750A Performance Mods
« Reply #55 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.
1977 CB750A "High Horse": 836cc, mild porting, custom cam, K5 roundtops, v-stacks...
1973 CB750K D5 plunger frame chopper currently building..

Offline HondaMan

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Re: CB750A Performance Mods
« Reply #56 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

Link to My CB750 Book: https://www.lulu.com/search?adult_audience_rating=00&page=1&pageSize=10&q=my+cb750+book

Link to website: www.SOHC4shop.com

Offline Yamahawk

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Re: CB750A Performance Mods
« Reply #57 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
1971 CB750K1 (newest bike), 1996 Royal Enfield 500 Bullet (therapy bike), 1981 Yamaha XV920RH, 2006 Kawasaki Concours (retirement bike), 1975 Yamaha RD350 (race bike), 1989 Honda VTR250 Interceptor (race bike), 1986 Kawasaki EX250 Ninja (race bike in progress), 1985 Honda Elite CH250, 1973 Yamaha GT1 80cc, 1974 Yamaha DT360 project bike.

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Offline stikman

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Re: CB750A Performance Mods
« Reply #58 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.
1977 CB750A "High Horse": 836cc, mild porting, custom cam, K5 roundtops, v-stacks...
1973 CB750K D5 plunger frame chopper currently building..

Offline tscott

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Re: CB750A Performance Mods
« Reply #59 on: November 26, 2019, 08:18:52 am »
Purchasing a K cam right now that doesn't have the chain drive (sprocket) attached... that should be fine right? I can use the chain drive (sprocket) off my A cam and put it on my new K cam correct?

Also, I see some jetting options for the K roundtops, but I took JWeeks advice and went with the Seca/Maxim/XJ650 CV carbs (32mm vs. the 28mm K and 24mm A carbs). I can't find any info on what kind of main jet rework this may need... anybody have any experience with the S/M/XJ650 carbs and jetting? (Will email Jon again also)

Currently planning to put a K cam in but may run the new carbs without the cam swap to begin.
Any advice would be welcome.

Thanks,
Toran
« Last Edit: November 26, 2019, 09:03:10 am by tscott »