1978 cb750 carb swap

[shultz_1978]

Hi everyone

I don't know if this is the right place for this, But I have a 78 that I want to try to put 80 carbs on. Just wanted to know if there is anything I should know about. Like do I need the intake boots off the 80?

Thanks any for help

[Half-Caf]

By looking at your other post, I see you're running pods on that bike. If you like the pod setup, I definitely wouldn't change over to the cv carbs (like my '80 dohc has) -they are notoriously hard to tune for pods and are really designed to be used with the factory airbox because it supplies a necessary vacuum (hence the name Constant Vacuum carbs)

As far as the spacing, I think it might be slightly different, but don't quote me on that. I'm sure some folks such as sees, tools, and shiny will chime in, too.

Sorry to burst you bubble, but those are my 2 cents.  

[Half-Caf]

Oh, and welcome  Good to have you!

[TOOLS1]

Most would like to swap their CV carbs, for the keyhole carbs, you should already have. The spacing is diferent. However, you might be able to overcome it by trying different carb boots until they fit.
Welcome to the forum.
TOOLS

[shultz_1978]

Thanks for the reply. The reason I was think of switching carbs is 78 there is no low end adjustment to comp for the pod filters and drag pipes. I think I am going to try drilling out the fuel jet to a .045mm and see what happens. Thanks

[Re-run]

Um totally wrong unless I misunderstood you. The 78 carbs are awesomely adjustable. The fuel screws and accel pump allow for a lot of flexibility. Leave your 78 carbs on and make sure the accel pump is working properly. Turn out the fuel screw an extra turn and run it for a bit to see how lean it is.
Get the manual and read up on the 78 carbs. You have a main and idle jet. The idle is a press fit jet but the main is a screw in like any other. The fuel screw allows a large range of adjustment. The needle can be shimmed if the midrange is lean.

[shultz_1978]

I was told by a few bike shops that I am not getting the right fuel at idle and at low rpm's to much air. Should I drill out the jet with a .045mm? Witch would let more fuel in at idle. Then lean out with the screw. I have a set of 80 carbs but the rack is about  3/4" longer and the bore per carb is bigger. Thanks for the reply

[TOOLS1]

Have, you done any plug chops?
TOOLS

[shultz_1978]

No i have not.

[Re-run]

It is my experience with shops that they are stupid. Do not believe a shop, if the bike doesnt have a computer, they don't have a clue.
You have not said any of your symptoms nor have you done any troubleshooting that you have informed us of. Swapping carbs or drilling jets is not the answer until you know your real problem.
Your carbs have an idle fuel screw for a reason, you need to use it.

[shultz_1978]

I would have to say the same with bike shops. The reason I am messing with the carbs is at a low rpm the bike pops and idles ruff. It has new plugs wire and coils. If I chock it about 1/4 it will run nice. I a long with two shops have try adjusting the jet to no avail. I am just taking a sab in the dark. I know enough to get me in trouble and some times it works. lol Thanks again for any info

[Re-run]

A 1/4 choke barely cuts off any air. You need a plug chop after idle. If a 1/4 choke changes it, you just need to adjust that fuel screw.
I know people who have bored the motors and ported their heads and use the 78 carbs with 0 trouble.
People run pods with 4-1 exhausts and do not have to always rejet. So unless you are running a way open exhaust, pods, and have motor work done, the 78 carbs will be fine and will work better with pods than the 80 carbs will.

[shultz_1978]

Thanks it is run drag pipes. The bike I am talking about is the one I posted. It is punched to a 805. When you are running in the low rpm's it is not very smooth. It is idles kind of ruff and when you down shift it sounds like it is gargling out the pipe. cool But I would like to get it run better on the low rpm low meaning 2000 and under.

[TOOLS1]

Since, you say it runs better with the choke pulled out, that is telling me that the pilot jets are plugged. Again some history of the bike would help.  Did it run good before it was bored over, or the high performance parts were added? Has it sat, for any length of time?
TOOLS

[shultz_1978]

The motor work was done before I got it. I have the carbs off to try cleaning them and see what happens. The work was done in 2007 and it has 4900 miles on the rebuild now. But I do not know the history on the bike. I got it in 2010 with 4600 miles on it and it has ran like this since I have had it. The guy I got it for told me it is very cold blooded.

[Lucky 1]

I do not understand why you say the 1978 CB750 carbs are so adjustable.

If you know how to get the 77-78 carbs to run right with pods
 or stacks can you please post that information?

Not vague ideas, but actual facts. Idle jet ,main jet , slide needle shims, mixture screw.

Hundreds of owners would like to know.

So far I have not found ANYONE who knows how to do this, including SUDCO.

[Lucky 1]

Drilling the #35 (.013) thousandths idle jets to #45 (.017 thousandths)  idle jets is NOT going to get you where you want go. To get rid of the ugly big plastic air box. That idea comes from the Honda chopper website.

Here is the problem.

The stock air cleaner has an opening only 39% the size of the total area of the carbs openings
that is about 12.56 square inches. So the air box acts as a giant choke .

FIRST ATTEMPT
I tried the #45 idle jet with a 120 main jet and .058 thousands shims under the slide needle.
3/4 turn out on the mixture screw. Make sure the shims sit down in that bottom pocket inside the slide.
It ran real good but the spark plugs were black with soot. Good throttle response.
VIDEO of this first try.http://www.youtube.com/watch?v=Z-tqLPqS_rw
I did not do a road test but if you want to try this formula and do a road test,maybe this is the answer.
Just depends how the spark plugs hold up.
I am trying to remember why I did not do a road test, even though it ran good in the shop.
I think I was not happy about the black soot all over the spark plugs.

SECOND ATTEMPT
Then I tried leaning it out. No good.
I used a #42 idle jet and .045 thousandths shims under the needle and 120 main jet.
1 turn out on the mixture screw.
I did do a road test and it would accelerate but the idle was terrible. Popping and uneven.
It ran terrible idle and would not rev up. Poor throttle response.

THIRD ATTEMPT
Then I tried just a #45 idle jet and no shims on the slide needle and 120 min jet.
mixture screw out 1 turn.
Would not idle right. poor throttle response.

By that time the spark plugs were messed up with carbon.
I put in new spark plugs. Still would not idle very well.

What these carbs may need is a slide needle with a different taper.

I am giving up and going back to the stock airbox which is like a choke that is on all the time.
It limits the air to 39% of carb openings all the time.

BTW no one on any website has a formula to make the 77-78 carbs work right without the air box.
SUDCO has no answer.


BTW the older bread box air filter housing might work if you can get the intake area to about 40%
of the total area of the carb opening area. The "breadbox" had a lot of holes drilled in it but you would have to count all the holes, calculate the area of one hole and multiply to get the answer.
The filter would need to be a restrictive paper filter like the OEM filter was.
It would still look better than the OEM air box.
Just my opinion.


A .013 stock idle jet#35m if 39% larger would be about 2.5 thousandths larger
Or a #42   but the #45mm seemed to work better. I only went out 1.5 turns on the idle mixture on the #42mm jet, but that was the best lean idle for that size jet. Of course the reason the #45mm probably worked better is if you add in the restriction of the OEM paper filter.

[shultz_1978]

 First I want to thank every one for the info. It is back up and running better then it has since I have had it sofar. It idles alot better and I don't need the choke to start it every time. I cleaned the carbs and used a #77 drill bit on the jet. Turned in the screw 1/2 a turn. I do not know what jets are in it they maybe stock. So I can not say this will work for every 78 with pods. Rode the bike around a little. It seams to be run good. Runs out the throttle nice. I will run it to work in the morning and see how it runs.


Thanks again for the info on the 80 to 78 carbs.

The only way to learn some thing is do it.

[Re-run]

Lucky, I know you frequent the other sohc4 site, but obviously you have not been around there long time. I was around there from years back. Now, I am going to tell you this once. Pods will never work better than the airbox will, it just will never happen, even if the motor is bored, for everyday use. You suffer from crosswinds and a host of other things. Honda designed these bikes to work with the stock airbox. Pods CAN be used and, with work, are able to work close to the level of the airbox. There is NO magic bullet for these carbs OR the earlier carbs. Why? Cause every bike is different not to mention a persons location. At sea level, settings will be different than at 1500 ft and that is different from 3000 feet. Therefore it is impossible to come up with 1 setting that will work for everyone. I do not get your desire to get exact settings when that is impossible to do since every bike is different. Why do you think honda gives BASE settings which are then tuned upon?

As for tuning the carbs, I also fail to see why this is hard to understand. It is widely known that the fuel screw is more flexible than the older air screw was. I tossed in a K&N in my box, didn't have to rejet. Ran with NO box or filters and did not have to rejet. I suppose if you are tuning for power, then sure, you might have to rejet. I tune for a balance and as such, I can knock on the door of 50 mpg at 75 mph, even on ethanol fuel and I have sufficient power to get my 220 pounds moving pretty darn quick.

And then you have the accel pump, what a lovely item that truly is. It allows you to have a leaner idle mix and then get a shot of raw fuel when you crack the throttle. This is why the 78 carbs are more responsive than the earlier 76 carbs are.

If you were to ask around, a number of people have bored to 836 and used the stock carbs with no troubles at all. Most of the time, this includes porting the head.

And finally, what benefit do the earlier carbs offer? An idle jet that uscrews? So what, you can get other press in jets, problem solved there. They are simpler? Only difference is no accel pump and really, the accel pump is not that difficult, it either works, or it doesn't.

Lots of people prefer the earlier carbs but I think most of that is because they do not understand the later carbs for whatever reason, I find this prevalent mostly with older riders. I have used both styles and I, by far, prefer the 78 carbs. Anything you can do with the early carbs, I can do with the 78 carbs. Anything.
I know how to tune these carbs and that is why my bike, even at 40 degrees and cold, will start with barely the touch of the starter.

And again, no 1 setting works for every bike and it never will, because no 2 bikes wear their internals exactly the same. If they did, you should be able to line up 10 bikes that used the same filters and fuel and oil and have the same mileage and compression readings should be identical. Guess what, won't happen.
But hey, you don't have to believe me. I am ok with that, after all, more carb parts for me.

[Lucky 1]

I agree with all that you say Rerun but let me get this straight. You say you took your 1978 CB750 with 78 carbs and put K&N filters on it and it ran fine. Did you change ANY of the jets??

OK...ok...I see the pod filters now in the photo you posted. Got it.



I understand now that the pods or stack will not run as good as the air box on this 1977-78 model.

I just want to know if YOU have a way to run K&N pods, and what you did to make that work? Thats all.

BTW...Stay away from that dealer.