Topic: Carb conversion Happiness!!

[lucky]

Today I was finishing up my  conversion on my 1978 CB750.
I installed my 1969 CB750 carbs onto the 1978 engine.

WOW!!!! What a difference!!!

Revs clean and tight and crisp. Good throttle response- good idle .

I used the stock 1978 carb boots and connected them direct to the carbs.
That was big because the boots on the 78 are shorter to accomodate the air box.
There was a .060 difference in the carb spigot and rubber boot, but they did tighten up properly.
When I got the carbs the mount plate was missing so I adapted a 1972 mount plate to the 1969 carbs.

The person on Ebay that sold them to me did not tell me that he put #38mm idle jets in them. They are supposed to be #40mm idle jets.
Cost me more dough. When you buy carbs on Ebay be sure to ask about jet sizes or some sellers will just unload all the jets on you they do not want.

It had #120 main jets.

I have a 4 into 1 exhaust.
I have the stock 1978 air box with stock filter and I used the 1969 rubber velocity stacks.
When mated to the air box there was a gap at the top of each air box hole between the velocity stack and the air box which I filled in with stick on foam.

It idled poorly with out the air box of course.
Then with the air box and 1/4 turn less (richer) than stock on the mixture screws it worked perfect. I synced the carbs two different times until they were perfect.

What is really nice is now I can change jets with the carbs on the bike AND I can change the needle clip position with the carbs on the bike with ease!!

All that is left is test ride and check plug readings.

Click on photo for larger version.

[spiritof67]

With due respect, the later model (K1-K4) carburetors never were better than the KO models. The '77-'78 models with the accelerator pumps are a different story. But they aren't better carbs. Just heavier, more complicated and with a much heavier throttle pull.

I used to love to put my K1-K4 riding friends on my KO and watch them lurch up the street - finally riding a CB that reacted to throttle opening. Good luck with this swap!

Spirit

[lucky]

With due respect, the later model (K1-K4) carburetors never were better than the KO models. The '77-'78 models with the accelerator pumps are a different story. But they aren't better carbs. Just heavier, more complicated and with a much heavier throttle pull.

I used to love to put my K1-K4 riding friends on my KO and watch them lurch up the street - finally riding a CB that reacted to throttle opening. Good luck with this swap!

Spirit

I have some 1972 (K2) carbs. Do you think those are better? Just asking.

Getting to the slide needles is still a major operation though.

[iron_worker]

Very cool that you can pretty much make any adjustment to these carbs without removing them from the bike.

I would have thought an accel pump would be a nice feature to have assuming it's working as it should.

IW

[Roach Carver]

If it were me and its not, I would get the right rubbers for those carbs. They are very easy to get and not terribly expensive. Plus I am thinking the later model rubber intakes are hard to get. If thats the case, It might be nice to sell them to someone going the other way or just store them away in case you change your mind later.

[lucky]

Very cool that you can pretty much make any adjustment to these carbs without removing them from the bike.

I would have thought an accel pump would be a nice feature to have assuming it's working as it should.

IW

The HONDA CR racing carbs do not have accelerator pumps.

[lucky]

If it were me and its not, I would get the right rubbers for those carbs. They are very easy to get and not terribly expensive. Plus I am thinking the later model rubber intakes are hard to get. If thats the case, It might be nice to sell them to someone going the other way or just store them away in case you change your mind later.

I have brand new stock rubbers but they are longer. I also do not know if they fit the 1978 cylinder head.
They arrived AFTER I put the carbs on the 1978 boots.

Also since the stock rubbers are longer it would not leave much space between the carbs and the air box.


I suppose i could take EVERYTHING apart and see it it will work but WOW what a lot of work!

Take it all apart.
Put on the new boots and all the parts.
Take it all apart if it will not work.
Put back all of the stuff I have on now.


If I do not sell the motorcycle then this winter I could do that when is very cold.

The whole project is not very appealing.

BTW new stock 1969 rubber boots were $94.00!!!!

But I know you may be right?

[Roach Carver]

The head side is the same. airbox location/distance I am not sure about.

[MCRider]

Very cool that you can pretty much make any adjustment to these carbs without removing them from the bike.

I would have thought an accel pump would be a nice feature to have assuming it's working as it should.

IW

I was led to think that the accel pump was an add on to overcome the lean condition that was built into the carbs for emission control purposes. Not for performance though that's what it would seem.

OCICBW

[lucky]

The head side is the same. airbox location/distance I am not sure about.

Thanks Roach Carver.

[lucky]

Very cool that you can pretty much make any adjustment to these carbs without removing them from the bike.

I would have thought an accel pump would be a nice feature to have assuming it's working as it should.

IW

I was led to think that the accel pump was an add on to overcome the lean condition that was built into the carbs for emission control purposes. Not for performance though that's what it would seem.

OCICBW

The problem is that even with the accelerator pump working it still is not smooth and the worst part is that if you give it a quick turn of the throttle, it takes  along time for the idle to come down. The accelerator pump is not consistent if you turn the throttle slow or fast you get different results etc.,.

It is a wild card.

[Stev-o]

Lucky - I do not have any of those issues you describe on my K8. And I always have a quick start up, throttle pull is light.
Spirit - maybe you need new cables?

Maybe I am just Lucky?!

[lucky]

Lucky - I do not have any of those issues you describe on my K8. And I always have a quick start up, throttle pull is light.
Spirit - maybe you need new cables?

Maybe I am just Lucky?!

I have brand new motion pro throttle cables.



 

[BobbyR]

Lucky - I do not have any of those issues you describe on my K8. And I always have a quick start up, throttle pull is light.
Spirit - maybe you need new cables?

Maybe I am just Lucky?!

Same here, no issues. You set up the pumper actuator properly, get some 120's in there, you get instant smooth power. Never saw the point of going backwards

[scottly]

The problem is that even with the accelerator pump working it still is not smooth and the worst part is that if you give it a quick turn of the throttle, it takes  along time for the idle to come down.

That doesn't sound like an accelerator pump issue at all.. An accelerator pump should only operate when the throttle is opened. A proper pump will dispense more, or less fuel, depending on how quickly the throttle is opened. The condition you are describing is commonly referred to as a hanging idle, and is usually due to a lean idle mixture, over compensated for with a high idle speed, and/or poor sync.

[lucky]

Lucky - I do not have any of those issues you describe on my K8. And I always have a quick start up, throttle pull is light.
Spirit - maybe you need new cables?

Maybe I am just Lucky?!

Same here, no issues. You set up the pumper actuator properly, get some 120's in there, you get instant smooth power. Never saw the point of going backwards

I was using 120 main jets.  I had no air leaks.
I covered all the bases. It was not that bad, but just not as perfect as I expect.

[phil71]

I'm really confused, aren't the PDs better?

[bollingball]

I'm really confused, aren't the PDs better?

Yes they are IMO I have not had any trouble with mine and others say the same.I don't know why Luck has had so many problems with his.
Quote from Lucky I was using 120 main jets.  I had no air leaks.
I covered all the bases. It was not that bad, but just not as perfect as I expect.
 You did not cover them all or they would be working fine. With all you have posted and data you have I can not for the life of me know what you are doing wrong or what part is bad and you have not found it. There are to many of us that are able to get them perfect.

 Quote from Lucky
The problem is that even with the accelerator pump working it still is not smooth and the worst part is that if you give it a quick turn of the throttle, it takes  along time for the idle to come down.
 That may be true for your bike. But not from one that is set up correctly
The accelerator pump is not consistent if you turn the throttle slow or fast you get different results etc.,.
 That is the way it should be. If you don't move the throttle very much then you are telling the bike you don't want to go fast so the pump does not let out as much fuel. If you open the throttle fast you are telling the pump you need more fuel so it squirt a lot more fuel.
From IW
I would have thought an accel pump would be a nice feature to have assuming it's working as it should.

You thought right

[trueblue]

Lucky just doesn't like PDs because they are harder to tune to pods

[phil71]

I'm trying a new kind of popcorn out right now. I'll let you know how it is

[andrewk]

Is it "cobble" flavored popcorn?  Egads.

[72 yellow]

I'm staying home today to see how this plays out. 

[Tews19]

At my desk now and have this waiting.
At least I know why LUcky hasn't posted a video of his exhaust as per requested another thread. Keep pluggin away Lucky. I want to hear the difference from a 750-550

[lucky]

I'm really confused, aren't the PDs better?

They get better 1/4 mile times I have been told via a magazine test but I like the 1969 carbs because they are very easy to adjust and make changes to them.
They weigh almost 1/2 the weight of the carbs with all of those lifter arms and shafts.

The needle on the slide is easily adjustable on the bike.
The jets are changeable on the bike.
The mixture screw is very easy to get to.
You can adjust the idle on each carb separately.
The idle screws are marked so you know when they start to open the slide although you can feel that too.
Only ONE throttle cable needed.
No choke cable needed, it has a flip up/down lever for the choke.
Idle jet unscrews unlike the 1978 idle jet.

[Roach Carver]

If you like it then thats all that really matters. If you decide you dont like them later, it just so happens that i do.