Topic: 78 CB750 Slow jets

[lucky]

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.

Sorry to hear that.
Can we ask what changes you made?

1978 model CB750.
stock air box.
Exhaust 4 into 1.
#120 main jets.
 idle jet??? #40"S??.
needle no shims.
accelerator pump and nozzles working.


If you took the float bowls off to change the jets did you make sure that little O ring for the accelerator pump on the edge of the float bowl was still in place?

Also if the change you made was to #40 idle jets and you say it will not run at all, did you open up the mixture screws a little??
What about with the choke?

What the results were is IMPORTANT information.

[TwoTired]

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.

Next time you get a hangnail, you should just cut off that whole finger, so it can't ever happen again.

You should sell or give the bike (since it is a piece of sh!t) to someone that has the patience to find the proper faults an fix them.  I'll give you my address for delivery purposes, if you insist.

Or, you might check the gas tank for fuel quantity.

I've never known a bike that cared one way or the other about a frustrated mechanic.  I've seen plenty of people able to make a bad situation, worse.

[Caso]

You're correct lucky. 120 mains, 40 slows. New o rings and washers in the air screw. My accel pumps were working great. Now they're not. How can they be working great one day then not work at all the next?

And two tired. Trust me, I have the patience. Im a BMW mechanic and deal with bs all the time. Its just irritating when I have to tear apart the carbs again. This will be the 5th time in a couple days. And it was a joke.. I will get this bike running.

EDIT: Nothing has been changed from yesterday. I had the bike running, Left it alone, went camping, came back and now it doesnt run.

[TwoTired]

Darn.  I was soooo ready for a new project....


 

[lucky]

Darn.  I was soooo ready for a new project....


 

TWO TIRED... " Did you check the gas" You want to get your !#@$!$!@$ kicked don't you?  LOL...lol

I thought of that too,but I was not going to say that knowing how frustrated he was. I have been very frustrated too at times. I just did not want to admit it.


I have taken my CB750K carbs off at least 10 times but I am really learning a lot.

The last project I had was tuning a Honda VF1000R V4 engine with altered exhaust and intakes.
Try dealing with a V4.
Two of the carbs are down draft, and two are side draft. Two are jetted differently than the other two from the factory.

In the end, it got larger idle jets and TWO .020 shims on all of the needles.
It started right and ran right.

[lucky]

Caso...You ARE on the verge of some very important info for all of the forum owners with 1977-78 CB750's with altered exhausts.

So we are all pulling for you. Right on with your bad self.

[Caso]

UPDATE: I'm still in the garage but posting from my iPhone. I've figured it out and I'm pretty happy. Had a clogged fuel filter. Replaced that and the bike runs and rides great. My set up is as Followed.

Bike is stock besides a 4-1 open header. No baffle no nothing. OPEN.
#40 slow jet
#120 main jet
Factory needle settings.
Air screws out one turn.

I think this setup would be ideal for anyone running an open 4-1 exhaust with maybe minor adjusting depending on your elevation.

[lucky]

A great piece of information people!
It took forum member Caso a lot of work but now we all can benefit.
NOTE: The CB750 comes standard with a #110 main jet and a #35 idle jet.
List below are the changes Caso made.

A 1978 CB750K.
Bike is stock besides a 4-1 open header. No baffle no nothing. OPEN.
#40 slow jet
#120 main jet
Factory needle settings.
Mixture screws out one turn.
Accelerator pump and nozzles working.
Clean fuel filter.


I am glad that you found the problem keeping your bike from running Caso!
So I will install those #40 idle jets in my bike also and they should work well.

BTW Caso can we ask what type of fuel filter you are using? Thanks -LUCKY

[Caso]

I went to my local auto parts store and found some clear 1/4 inch fuel hose and a generic 1/4 inch small fuel filter. Was like 8 bucks, but the best 8 bucks I've spent in a while. I highly reccommend clear fuel hoses for everyone. Ill go out and take a picture real fast.

[lucky]

I went to my local auto parts store and found some clear 1/4 inch fuel hose and a generic 1/4 inch small fuel filter. Was like 8 bucks, but the best 8 bucks I've spent in a while. I highly reccommend clear fuel hoses for everyone. Ill go out and take a picture real fast.

Yes... I put clear fuel hoses on my accelerator pump nozzle distribution lines and it is very helpful.  Also i put them on my float bowl drain lines.

I do want to see the type of fuel filter you purchased.

[Caso]

Sorry for the qaulity, taken with my iPhone.


And heres the bike so far... Anyone wanna buy this tank?

[lucky]

I sure would like to see the exhaust side of your bike.-LUCKY

I am sure you will sell your tank.
Telling the condition of the tank would help to sell it.
BTW I like the way you routed your fuel hose.
I think i will change mine now.

[Caso]

No dents on the tank. Paint is decent for being 34 years old, no where near perfect however. Slight rust inside. Will need de-rustified lol.

And for the exhuast, its just a 4-1 header without a muffler for right now.

[lucky]

No dents on the tank. Paint is decent for being 34 years old, no where near perfect however. Slight rust inside. Will need de-rustified lol.

And for the exhuast, its just a 4-1 header without a muffler for right now.

Thanks Caso!!

[cben750f0]

just found this site if it helps for slow and mains jets

http://www.jetsrus.com/a_jet_kit_street/honda_750_CB750_1969-1971.html

http://www.motorcyclecarbs.com/Keihin_Pilot_Jet_N424-24_C92.cfm
peace

[Caso]

just found this site if it helps for slow and mains jets

http://www.jetsrus.com/a_jet_kit_street/honda_750_CB750_1969-1971.html

http://www.motorcyclecarbs.com/Keihin_Pilot_Jet_N424-24_C92.cfm
peace

I was actually talking about the push in type slow jets. The 77-78 didnt have the threaded slow jets. But thanks anyway!

[bollingball]

just found this site if it helps for slow and mains jets

http://www.jetsrus.com/a_jet_kit_street/honda_750_CB750_1969-1971.html

http://www.motorcyclecarbs.com/Keihin_Pilot_Jet_N424-24_C92.cfm
peace

These will not work on 77-78 750. They do have good shim washers. Sirius is about the only place for the press ins.
Ken

[BobbyR]

Sorry for the qaulity, taken with my iPhone.


And heres the bike so far... Anyone wanna buy this tank?

I have a 78K and my petcock outlet is in the back. I notice you are using a clamp. Next time you swap lines and filter I would suggest that you go to a small engine shop and get some Tygon line. It is self clamping and very durable. It allowed me to put the filter in between the carbs. This allows me a shorter run and better downward angle.   

[bollingball]

Bobby is correct that is not the right petcock for a 78K 750.I use the tygon 5.5mm no need for a clamp. The stock intank filter works just fine why not use it no need for a inline.
Ken

[bollingball]

How does that yellow headlight look as the driver would view it at night and how many watts is it? Was it tinted or is it yellow glass? Reminds me of some old driving lights I had on a Goldwing.
Ken

[Caso]

Thanks for the suggestions. I love the yellow headlight. I actually bought some Krylon stained glass spray paint and just sprayed the factory headlight. Looks good at night. Plus yellow headlights offer better visibility in poor weather conditions and get peoples attention! And of course gives it that European racer look.

[Blitzburgh207]

I'm suprised that you needed to install larger slows but did not have to shim your needles? 

[lucky]

I'm suprised that you needed to install larger slows but did not have to shim your needles? 

Not sure who you are talking too? Was it Caso?

Blitzburgh the reason the idle jet can be changed without the needles being adjusted is that the 1997-1978 CB750's have a accelerator pump feature that comes into play to help with that transition.  The the crossover of idle jet into mid range territory was just enough to do the job. Also with a #120 main jet there is cross over there also.

That is why 3 of us (Caso,Imago,Lucky) were trying to go from a #42 idle jet to a #40 idle jet and we all wanted to know if the transition was going to be smooth through the mid range. We all had bikes with very similar set ups.

Hope this helps-LUCKY

[lucky]

IMAGO has jumped to another thread.
"HOW DO THESE LOOK?"

I asked him what he was doing with the jetting and he is working on his spark plugs now.

He replied and said this,

QUOTE :"Oh, and Lucky: still running the #35 stock slow jets.


LUCKY- The plugs look just a tiny bit lean so going to a #38 jet would help your problem.
#40 might be overdoing it.
But if it stumbles when quickly opening the throttle cannot be cured with mixture screw then the #38 idle jets would be the next logical step.

Like you said the muffler you have is more restrictive and so that is why a #40 idle jet may not be necessary.

[Blitzburgh207]

I run pods and open 4-2-1, which I'm sure have a far greater effect on 1/4 - 3/4 throttle, but I was just saying I'm suprised he would have to increase his slows that much but did not have to raise his needles. - but I also see he's back to stock...

I'm not saying he should, just struck me as odd, that's all.  Whatever the plugs say goes!!

Also, the accel pump is for quick twists of the throttle.  I'f you're holding your throttle at a constant, say, half, the accel pump is not "squirting" the whole time, just at first.  - that is my understanding... Hold your throttle there and watch the pump jet things.  I belive it is the needle position that controls your mixture in that range.