Topic: 78 CB750 Slow jets

[Caso]

Anyone know where I can pick a set of these up at? I took them out to clean them and noticed one was slightly bent. Would this be an issue? These are the push in style jets. Thanks!

[brewsky]

This is the only place I know of:
http://www.siriusconinc.com/pro-detail.php?pid=&product_id=1107

[lucky]

It is common to see that bottom portion of the slow jet slightly bent.
It is soft brass and if you drill a hole in a metal block you can put that jet in it and push on it with your fingers and bend it back. DO NOT tap on it with anything because it would get out of round.

BTW did you try those #40 idle jets yet?
I am going to go from my #42's  to #40s because it still smells rich but it runs really good.
Thanks...LUCKY

[BobbyR]

If it seals at bottom you are probably OK. There is bent and there is BENT.

[lucky]

YES...photo is always good if you have time.

Hope we are not talking 45º bend LOL...lol

[Caso]

Haha, No not 45°, but it is noticeable. Was just wondering if this could have an effect on why my bike is not runnining right. A flick of the throttle and it dies. I've clean these things 3 times! I just wanna ride it.. 

[lucky]

Haha, No not 45°, but it is noticeable. Was just wondering if this could have an effect on why my bike is not runnining right. A flick of the throttle and it dies. I've clean these things 3 times! I just wanna ride it.. 

No that is not the problem. I have used the slightly bent slow jets without a problem.

" a flick of the throttle and it dies"= lean condition.

What size slow jet?

Year?
750?
Intake system?
Exhaust system?
Main jets?
Needles?
mixture screws?

I am working on a chart and putting all this data on it for the forum members.

[lucky]

Put a small pair of vice grips on it and pull while twisting.

You will scratch it but don't let that worry you.

[Caso]

78 CB750k, Stock airbox, open 4-1 header, #35 slow, 120 main, havent been messing around with the air screw. But I'm positive the accel pump is working.

[Caso]

I can ease into the throttle and get it to rev. But a flick and it dies.

[lucky]

78 CB750k, Stock airbox, open 4-1 header, #35 slow, 120 main, havent been messing around with the air screw. But I'm positive the accel pump is working.

No wonder it dies. The stock slow jet is a #35 WON'T WORK.
Put in a #40 and that should do it. about 1 full turn out on the mixture screw.
Maybe a touch more 1/4 turn.

Don't just "be positive" about the accelerator pump. Actually CHECK it.
The answer is only Yes it works OR no it does not work.
It either is or isn't.

With the engine off. Air filters off.
Float bowls full of gas, look into the carbs with a bright LED flashlight and turn the throttle a couple times. Make sure you see a squirt of gas from ALL of them.
Not just one.   -LUCKY

[Caso]

Thanks Lucky. Where can I pick up #40 slow jets? I will triple check the accel pumps again.

[Caso]

Nevermind, Found them! I will report back after I check my accel pump and install the #40 slows. Thanks guys.

[lucky]

Thanks Lucky. Where can I pick up #40 slow jets? I will triple check the accel pumps again.

Sirius Consolodated has the push in type jets.
Jets R us
Parts-N-M0re.

[Caso]

Alrighty, So I got the #40 slow jets in from Sirius Consolidated. Put them in, as well as checked my accel pumps AGAIN (they are working). Turned the air screws out 1 and a half turns and.... SHE RUNS!! Although once I get on my bike to ride, it only runs good for about the first 5 minutes. It wants to stall if I give it anywhere from 1/4 to 1/2 throttle then it will just eventually just stall out. I'd wait a minute, start it up and drive it some more for a another 30 seconds before stalling again. Any suggestions?

So I'm concluding the bike runs awesome when cold. No choke, its like 80 degrees right now. But once the bike gets up to temp it wants to stall out.

Edit: My initial problem is solved. Before, the bike would stall with a little bit of throttle. That is now fixed.

EDIT: Okay I think I'm a half turn out to far on my air screw. I swear I read 1 and a half turns. O well, I will update tommorrow...

[Imago]

78 CB750k, Stock airbox, open 4-1 header, #35 slow, 120 main, havent been messing around with the air screw. But I'm positive the accel pump is working.

No wonder it dies. The stock slow jet is a #35 WON'T WORK.
Put in a #40 and that should do it. about 1 full turn out on the mixture screw.
Maybe a touch more 1/4 turn.

Don't just "be positive" about the accelerator pump. Actually CHECK it.
The answer is only Yes it works OR no it does not work.
It either is or isn't.

With the engine off. Air filters off.
Float bowls full of gas, look into the carbs with a bright LED flashlight and turn the throttle a couple times. Make sure you see a squirt of gas from ALL of them.
Not just one.   -LUCKY

Why won't the stock #35 jet work? I just cleaned them in my '78 CB750F and it seems to be working fairly well.

[lucky]

78 CB750k, Stock airbox, open 4-1 header, #35 slow, 120 main, havent been messing around with the air screw. But I'm positive the accel pump is working.

No wonder it dies. The stock slow jet is a #35 WON'T WORK.
Put in a #40 and that should do it. about 1 full turn out on the mixture screw.
Maybe a touch more 1/4 turn.

Don't just "be positive" about the accelerator pump. Actually CHECK it.
The answer is only Yes it works OR no it does not work.
It either is or isn't.

With the engine off. Air filters off.
Float bowls full of gas, look into the carbs with a bright LED flashlight and turn the throttle a couple times. Make sure you see a squirt of gas from ALL of them.
Not just one.   -LUCKY

Why won't the stock #35 jet work? I just cleaned them in my '78 CB750F and it seems to be working fairly well.

Is that your real name???
Look up IMAGO image.

Anyway the #35 would work if every part of the engine is stock.
Stock airbox, stock exhaust, stock needle setting etc.,. Then the stock jets will work.

We were not talking about a completely stock bike.

[lucky]

Alrighty, So I got the #40 slow jets in from Sirius Consolidated. Put them in, as well as checked my accel pumps AGAIN (they are working). Turned the air screws out 1 and a half turns and.... SHE RUNS!! Although once I get on my bike to ride, it only runs good for about the first 5 minutes. It wants to stall if I give it anywhere from 1/4 to 1/2 throttle then it will just eventually just stall out. I'd wait a minute, start it up and drive it some more for a another 30 seconds before stalling again. Any suggestions?

So I'm concluding the bike runs awesome when cold. No choke, its like 80 degrees right now. But once the bike gets up to temp it wants to stall out.

Edit: My initial problem is solved. Before, the bike would stall with a little bit of throttle. That is now fixed.

EDIT: Okay I think I'm a half turn out to far on my air screw. I swear I read 1 and a half turns. O well, I will update tommorrow...

My 1978 is set up the same basically.
78 CB750k, Stock airbox, open 4-1 header, #35 slow, 120 main, havent been messing around with the air screw. But I'm positive the accel pump is working.

Except mine has a baffle type muffler.
Does yours have a muffler at all?

With a #42 idle jet it runs best with 3/4 turns out.
With the #40 idle jet I an guessing it will need 1-1/4 turns out or 1-1/2 turns if you can get away with it.

Please report back as soon as possible. I am very interested in your results.

I am wondering if the crossover between 1/4 and 3/4 throttle will be smooth enough with that #40 idle jet.

[Caso]

Okay, just got back in from the garage. Turned the air screws out 1 turn. Seems tad bit better. Didnt ride it only but a minute. (roads were wet and no front fender). But once I pulled it back in the garage, it stalled. Waited a second, started it up and it idled fine. Im hearing a slight gurgle and what sounds like very slight back firing in the exhaust at an idle from time to time...

[lucky]

Okay, just got back in from the garage. Turned the air screws out 1 turn. Seems tad bit better. Didnt ride it only but a minute. (roads were wet and no front fender). But once I pulled it back in the garage, it stalled. Waited a second, started it up and it idled fine. Im hearing a slight gurgle and what sounds like very slight back firing in the exhaust at an idle from time to time...

Did the bike accelerate smoothly?? from 1/4 throttle upward??
I would turn all of the mixture screws out + 1/4 turn. Adjust the mixture screws when the engine is off and it is a cold engine. A lot easier on your hands.


Keep me informed thanks.

EDIT:  Sorry I got you mixed up with Imago. Get confusing not having separate threads. But stay on this one.
Let us know when you change to a #40 idle jet.
The #35 is NOT going to work. I can tell you that positively.

[lucky]

You know if this works after you turn the mixture screws out a little, and it will rev when the throttle is turned quickly, and it accelerates smoothly from 1/4 on up throttle you are DONE!!!

And many people on this forum with this set up will have an easy time of it.

THE SET UP AGAIN:
Stock air box. stock style air filter.
open 4-1 header.
 #40 slow.
# 120 main.
 Mixture 1-1/4??
Accelerator pump and nozzles working.


If the #40 will not work then it will have
 to be a#42 with 1/2 to 3/4 turn open on the mixture.

We are about to find out from Caso.

[Imago]

78 CB750k, Stock airbox, open 4-1 header, #35 slow, 120 main, havent been messing around with the air screw. But I'm positive the accel pump is working.

No wonder it dies. The stock slow jet is a #35 WON'T WORK.
Put in a #40 and that should do it. about 1 full turn out on the mixture screw.
Maybe a touch more 1/4 turn.

Don't just "be positive" about the accelerator pump. Actually CHECK it.
The answer is only Yes it works OR no it does not work.
It either is or isn't.

With the engine off. Air filters off.
Float bowls full of gas, look into the carbs with a bright LED flashlight and turn the throttle a couple times. Make sure you see a squirt of gas from ALL of them.
Not just one.   -LUCKY

Why won't the stock #35 jet work? I just cleaned them in my '78 CB750F and it seems to be working fairly well.

Is that your real name???
Look up IMAGO image.

Anyway the #35 would work if every part of the engine is stock.
Stock airbox, stock exhaust, stock needle setting etc.,. Then the stock jets will work.

We were not talking about a completely stock bike.

My name is Andrew... the Imago name came from from an old CRT computer monitor I had years ago... it appealed to me because it said to me... "I'm a go...ing".

Anyway, my bike is stock except for the exhaust which is a baffled MAC, 4 into 1 so perhaps that's why the stock jets work fairly well. To be honest, you're likely onto something because even when warmed up, if I hit the throttle my bike bogs and takes a second to recover. Do you think that would improve with the size 40 jets? I have a manometer I got from the PO... maybe I should sync the carbs while I'm at it as well?

[lucky]

To help you get your bike running right I would need to know:

1978
CB750
type of intake?
Exhaust Mac 4 into 1
Idle jet #35
Mainjet # 120
Needle was not adjustable.
Mixture screw turns out?
Accelerator pump and nozzles working or not? EDIT:
They ARE working

We do know the bike is too lean at idle. because of the stock idle jet.

BTW... your imago image is when you meet the first mate in your life, you often look similar, and that is when you try to work out problems you could not work out with your parents. We all may go through it.

Basically waiting to find out if the #40 or #42 idle jet is going to be the best choice.

Three of us have our lives hanging in the balance...LOL..lol

Caso. Working it. #40's idle jets
Lucky. Waiting for #40 idle jets.
Imago. Needs a larger idle jet.

Caso is closest to finding out the answer.

[TwoTired]

Anyway, my bike is stock except for the exhaust which is a baffled MAC, 4 into 1 so perhaps that's why the stock jets work fairly well.

I agree.  Your bike should run fine with stock jetting, but you may have to find a better setting for the idle mixture screws.

... if I hit the throttle my bike bogs and takes a second to recover.

Almost certainly an accelerator pump issue with the PD pumper carbs.

When it recovers, does it burble before power, or snap back like it was just switched back on?

Do you think that would improve with the size 40 jets? I have a manometer I got from the PO... maybe I should sync the carbs while I'm at it as well?

Any issue with run performance is best served with doing the routine maintenance items first (3000 mile tune up check list), and that includes ensuring carb sync is within parameters.

I don't expect you'll need pilot jet replacement after you get the accel pump to work as it should, and do a proper tune up.

[Caso]

Okay now my bike isnt even running. It will start up for a about a split second and thats it. Im about to push these peice of sh*t into traffic. It was running decent the day before.