75 CB750 wont idle with rebuilt carbs & no airbox

[harsh94blu]

Hello Forum,

Im having an issue with my 75 cb750. Its in the shop but the mechanic says he stumped. He cant get it to idle. It does start and run but the min you let go of the accelerator it starts to die and it dies.

The carbs have been rebuilt and i don't know much of the terminology but here is what my mechanic says he did.

First all gaskets and jets have been replaced. he says the stock carbs come with 100 jets. he has tried 103, 115 and now 125 jets and he says it has improved but still dies.

the carbs do not have an air box, we are doing straight individual air filters for each carb.

The bike was also fitted with a 4 -2 -1 header.

He says because of the straight filter and headers the bike needs more fuel, but he hasnt been able to level it correctly.

Does anybody have any ideas. Im converting this bike into a cafe racer and this has been an on going problem now for over a month. If anyone here has had this issue or has recommendations on jet sizes, leveling etc.. your help is greatly appreciated.

Thank you for your help.
Max

[shinyribs]

I dont want to second guess your mechanics skill level,but it could just be a plugged or dirty idle jet. There could be something off with the tune also. Increasing the main jet will allow a slight change in the idle mixture,but minimal. I have a similar bike with pods and open exhaust. I run 125 main jets and 42 pilot jets. The idle mixture screws are out 1 1/4 turn,whereas the factory setting is 1 turn +/- 1/8 turn. The pilot jets and the idle mixture are key when tuning for non-stock intake/exhaust. The pilot jet is the circuit used to take you from idle to part throttle/cruise where the main jet starts to take over. More than likely tuning the idle mixture will cure your idling problem,but if you still have stock pilot jets then the bike will be lean down low. This will cause the bike to crackle and pop on decel and even some at idle.

I'd check the idle mixture setting first. Set it 1 1/2 turns for starters. It might be too much,but it will tell you if it is making a change on the bike or not.

The pilot jets will eventually have to be checked and maybe changed. But they are easy to get to without having to pull the carbs. Just pop the bowls off.

[TOOLS1]

It sure sounds like the idle circuit is plugged, to me.
TOOLS

[Lucky 1]

Of course the "mechanic" is stumped.

The shops these days cannot fix anything unless you can plug a laptop into it.

[Lucky 1]

I can help you .
Tell us the serial numbers on the carbs.

Shiny ribs previous post will get you very close.
You will need those #42 pilot jets.

BTW Tell that mechanic to start reading this website and SOHC to learn about CB750 carbs.

[harsh94blu]

Thanks alot for your help guys... I did talk to him and told him what you guys suggested. He said he was going to get a a 44 slow jet. Excuse my ignorance, but is a slow jet the same as a pilot jet. He said the bike is running 125/40 but was thinking of going 42 or 44? Could this be the fix that you are mentioning?

He said he dipped the carbs in acid and cleaned them but he doesnt think the pilot jets are clogged. The bike does crakle a bit when it decel i thought that was just cause of the 4 - 2- 1 header. It does rev up smooth though, if i could only get to idle it would be at home...

Another question... Does anyone think this could be a spark or coil issue?

Thanks for all your help. I will keep the forum posted on what happens.

[harsh94blu]

Lucky, i will ask him for the serials... but what would you need that for?

the mechanic is not a CB 750 mechanic... he admited that these were a challange, he knows carbs but this was out of his expertise... Hopefully he will get with all your suggestions...

[shinyribs]

It would be very hard to find a mechanic that was an expert on every model and brand of motorcycle.It would also be hard to find a mechanic that specializes in CB750's. There just aren't that many of these bikes roaming the streets to justify that. That said, nobody should be too hard on your mechanic,Im sure he's doing his best.


Yes,the slow jet and the pilot jet are the same thing. A free flowing exhaust will crackle some.Mine does still,but very little. If I give it just a touch of throttle it quits. A 44 jet might be a touch too big,but not enough to really hurt anything. I have considered getting 44's to see if my last little touch of crackle on decel goes away...but I'm lazy. Good luck!

[Lucky 1]

Slow jet or pilot jet are the same thing.

The #44 idle jet will be too large.
The #40 will probably not work with pods. You WILL need the #42's

Quote: "he doesnt think the pilot jets are clogged" Very lame. He needs to actually take them out and look through them and blow them out with compressed air.

Spark issues need to be checked before carbs are worked on.

[Lucky 1]

I have the 4 into 2 into 1 exhaust now also.

It has the best ground clearance.

[Lucky 1]

Need to know if the carbs are 7A models or 657A's.
The needle clip setting was different .

Honda Man seems to know much more about these carbs.

Read this. HONDA MAN
Honda's recommendation on the "sooty 657A carburetor calibration" was to first try dropping the needles
 one notch. If that wasn't enough, to go to the 110 jet was the next step.

In actual use, dropping the needles has more effect on daily riding than the smaller jet, since you're riding
 most of the time in the lower 1/4 throttle range where the combined too-rich idle and too-rich needle are
 causing the issue. Thus, dropping 5 on the mainjet will only drop it about 0.5% until the slide clears the
wasp-waist in the throat: then it goes to about 2.5%. Dropping the needles drops it a little over 1% per notch.



The early K1 pipes (HM300) had no baffles inside. The later ones had one baffle near the upsweep corner(s)
 on each pipe, which increased the backpressure enough to make the 115/notch4 calibration too rich, and
 the 'unofficial advice' was issued via the factory reps' word-of-mouth. BTW: the 115 was the original jet in
the 657A carbs.


Some other things to watch for on the 657A: the air screw seats were thinner than the later carbs. So,
 turning the air screws in to seat, then out 1 turn, slowly widens that seat. Today, you may find that you
 are better off with about 7/8 turn instead. This seat got about 0.5mm thickerin the 657B, among other things.


Also, the 657A carbs often had 25mm float levels, and this level was set from the tiny flat pedestal
 that is right alongside the floats: not by using the notch on the sides of the carb body. If you attempt
to meausre a 657A carb that is properly set from those side notches, it will appear to be set at 24mm,
which leads some folks to adjust it. Don't...


The 657A and the original 4-cable K0 carbs both suffered very soft brass tangs on those floats that
were also made from brass. Check for a tiny dimple in the float tang of each one, and polish it off
with either a needle file or such, then reset the float level. This little dimple makes the float valves
 stick and occasionally overflow the carbs for no apparent reason. The 657B until K3 was also
 famous for this, but had plastic floats.


LUCKY -REMEMBER with your 4 into 2 into 1 some of this info may or may not apply.
Also the mixture screw is VERY sensitive to adjustment 1/8 turns makes a difference!!!

[Trionda]

I stumbled upon this thread after trying to research my friend's CB750.
I have to say--this forum looks like a really decent place full of resourceful people who want to help each other out.  That can't be said for a lot of forums.  

A little history:
The bike is super clean for a rider, and although it seems to have been well cared for, it's definitely a bitza, which makes it hard for me to know base figures for dialing it in.  My friend brought his CB750 to me 9 months after buying it from a bike shop that "specialized" in customs and odd builds.  Although their work seems to be adequate, they clearly never addressed many of the components of the bike that will need attention soon.  When he last rode it, he was complaining about a lean stumble off idle, and lack of power on top.  

The skinny on the bike:
It's a 1972 CB with a 1977K engine and 1978 carbs with accelerator pump (all carbs have a stamp just above the float bowl that reads 41BAOK7.  The A has a box around it, and the 7 at the end is stamped crooked).  
The bike has 4-into-2  Mac pipes, and Bikemaster pods.  The main jets are 110s, and I have not checked the idle screw locations or the pilot jets.  The plugs are a very light tan color, which verifies his concern over a lean condition.  The engine has either been rebuilt by someone careful, or has very low miles and is really clean.  The carbs have not been messed up by a jackleg mechanic, and look nearly new.  The valves are clean as a pin.
I have no idea whether it's been fitted with a lopey cam, but I will assume the engine internals are bone stock.

I am new to these carbs, but have plenty of experience with Keihin CVKs from the 1990s and on, and wrenching on everything from bicycles to big rigs.
These carbs look very solid, and I'm interested in learning more about them.

Since I do not have access to a dyno, or a decent place to do "true" plug chops, if you can offer any baseline suggestions as to initial jetting and idle screw configuration, I'd be grateful.

[TOOLS1]

First I would make sure that the accelerator pump is working. You should see gas being squirted from the four nozzles at the rear of the carbs. Then I would start with a 120 main, and drill the pilot out to 0.45mm. That should get you close. You might need to shim the needle, there is a thread here by Lucky1 that shows how to shim them. You can shim up to 1.5mm but after that you would need adjustable needles out of a 77K, or F carb. Also all the A models used the same needle, and are adjustable. Also you should start your own build thread. It would be more organized, and easier to answer questions pertaining to your project.
TOOLS

[Trionda]

Thanks for all the good information.  I really appreciate it.  
The 1972 tank has been recently painted and epoxy coated inside, and I have a new petcock for the fuel, (I'm busy hunting down a 2-into-1 fuel hose barb), and will have this bike fired up before too long.
Again--much appreciated.
Regards-