Topic: CB750A Performance Mods

[stevematic]

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)

[TurboD]

Yes you will need to use the earlier K boots and cable(s).

[HondaMan]

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.

[fireman1073]

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

[HondaMan]

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.

[stikman]

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.

[HondaMan]

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.

[Yamahawk]

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

[stikman]

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.

[tscott]

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