So close to finish but stumbling with RPM's

[Honda750Cafe]

Hey guys!

I have a 1979 CB750F that I'm converting it to a cafe racer.  Below is the work I've done on the engine so far:

1)Rebuilt carbs (factory carb rebuilt kits)
2)removed airbox and now running K&N pods.
3)replace exhaust with a vince & hines 4 to 1 pipes.
4)replace spark plugs with KNG Iridium plugs.
5)replace faulty ignition coil wire.
6)synced the carbs.

The bike starts and idles like a champ but when I push 5K rpm the bike just sputters no matter what gear I'm in.  I'm adjusted the jets from the 2.5 turn out to about 4.5 and did see some improvement but still losing power in the mid-band.  I would like to know what you guys think I should try next to resolve this issue.  Thank you in advance for your help.
 

[TOOLS1]

Very nice bike. First, the thing you keep turning/adjusting is not a jet. It's the fuel screw, and will not help your high RPM problem. Second, The really bad news is these bikes have CV carbs, and are next to impossible to make work with pod air filters. These carbs require a lot of vacuum which is produced by the factory air box, and pods just do not supply it. If you put the stock airbox back on, it should run right again.
TOOLS

[Honda750Cafe]

TOOLS1...thanks for the response.  So with the higher RPM issue you don't feel going with a larger main jet will help that issue?  The last thing I want to do is drop 800 large into a new set of carbs after spending a few weeks in cleaning the ones I have.  I still have the factory airbox but that just kills the look of the bike.

[TOOLS1]

Your problem is so common. At least once a month we have someone trying to run pods on a DOHC with CV carbs. Sometimes they claim to have jetted around the problem, but they are just trying to make themselves believe they fixed it. However they always come back with the same problem, or end up putting the stock airbox back on. I have heard that the "Dynojet kit (which requires some extensive modifications) will make the run almost as good as the stock airbox. I do have one tip for you though if you are dead set on running the pods. I always stuff pods with breathable foam. This helps with recreating the lost vacuum.
TOOLS

[Honda750Cafe]

Tools do you have any thoughts on the Mikuni carbs?  Or do you have another recommendation for after marker carbs?

[TOOLS1]

Sudco sells a set already set up for your bike. Kind of pricy but worth it in my opinion. I have never used them on a CB 750, but have used them on twin cylinder bikes. The only problem with them is that they are jetted way too rich. Also I recommend using the smallest diameter you can get for your application. Like 22-26 mm. The 32 mm is pretty big for a stock 750 cc engine.
TOOLS
P.S. There is a guy on the forum here right now converting his bike to the 77-78 keyhole carbs. You might want to check it out.

[Honda750Cafe]

TOOLS....thoughts on the below post?

http://honda-cb750-s.456789.n3.nabble.com/I-GOT-PODS-TO-WORK-td4039223.html

I think I'm going to give this a run.

[TOOLS1]

Good luck.
TOOLS

[Honda750Cafe]

Tools--

I wanted to give you an update since you took time to give me advise on this forum.  I contacted K&N and asked for their recommendation for jets.   They sent me the attached pdf which was a great starting point and I simply increased there jets recommendation by 10% because of my stage 2 exhaust.  I'm happy to report that the bike is screaming using 98/125 for my jets.  There is no tape on the pods and I'm using the factory carbs that I rebuilt.  Again thank you for your time. 17077_inst.pdf

[Lucky 1]

Internet size photo.

You cannot just change the main jets when you add pods. It will not work.

The main jets do not control throttle from 1/4 - 3/4 open.
Idle is controlled by idle jet and mixture screw.
It needs a slightly larger idle jet.

That is why you had to turn the mixture screw out so far is to get the bike to idle.


You will have to change the needle position in the slide.
Otherwise it will hesitate when giving it quick throttle.


The reason Honda went to vacuum operated carbs was to make the carbs work at different altitudes
with out adjustments or jet changes.

Extreme example:
If you lived in Tucson Arizona 2000'ft above sea level and drove to the top of Mt Lemon (8000"ft)
most Hondas at that time would not idle very long because the mixture would be so rich with conventional carbs. When the vacuum operated carbs came along you COULD ride to the top of Mt. Lemon and the bike would still run correctly.

You can make the bike run with pods. The trouble is that owners on this forum keep making a jet change(main jet only) and when they get poor results they quit and give up.

You may have the main jets correctly sized now if at WOT throttle the engine will keep on pulling and not hit a flat spot. . Now you just need to work on the idle jet and midrange controlled by the slide needle position.

So lets say you have a stock idle jet size#40. I would increase those to #42's and see if it will bring down the idle and make the mixture screw only about 1.5 turns out.

But if it was me I would want to get the mid range fixed first because 90% of your riding is between 1/4 and 3/4 throttle.


Try raising the needles one step. Usually by lowering the clip one notch.
I don't remember the set up on the vacuum slides and how the needle adjusts .
So you will have to work that out.
I would try raising them all about .030 thousandths to start. You may have to use a little washer/shim under the needle. You can get some .020 thousnadths brass ones at the hardware store but measure them with calipers because some can be .020 thousandths and others are 024. You can sand them to equal thickness with a finger nail sanding board.

You know you have it right when you give it a quick burst of throttle and their is NO hesitation at at all.  
One day one of these forum members is going to get this right by not quitting
on it and solve it for all the others.

Oh..One more thing... Only one change at a time and really be with that for a few days to make sure if it was better or worse.
I know you will have to take the carbs off or change jets but that is the price to heaven.
I spent a lot of time working on velocity stacks and pods for the 1978 CB750 and took the carbs off at least 10 times.
I even went too far at one point trying #44 idle jets. But then i realized that #42's were as big as you can go.
You may have to go too far to know that you have gone far enough.