Honda CB250 (1976) jetting.

[Justinas]

Hello guys,
again asking for help. In a very childish and inexperienced decision I have given my bike pod filters and straight exhaust. As this is the only CB250 in my country, nobody told me that that's not very smart for your first project. And now I need to rejet the carbs, because frame modifications doesn't allow me to go back to original setup.


First question: Maybe anyone has a similar setup with pods and straight exhaust on their CB250 with Keihin 744B carbs and could give me correct jet sizes for such situation?
From various web tutorials I got and idea, that a good starting point would be to increase main jets by 5, pilots by 2,5 and also raise the needle by one step. The needle on 744B doesn't have steps like ones on CB360, so I'm guessing I will just have to use a shim.


Second question: Does anyone know a jet supplier who ships to Europe and deals in jets for old Honda/Keihin?
Couldn't find anyone. And ir my home country of Lithuania everyone just stares strangely at me when I tell them I need to to such updates for such a strange bike:] (vintage bikes here mean URAL, Dnepr, IZ and Jawa)


I would be very grateful for any help!

 

[mydlyfkryzis]

I have a 76 CB360t, and the needles aren't stepped on that either.

These engines are not happy with pods and straight exhausts.

Not only do the fuel jets nee massaging, but also the air jets. On some of these, the air jets are push in, not threaded.

Depending on which pods you used, they may need smaller jets. A lot of the smaller pods like the Emgo have less air flow then the stock air filters.

The best path right now is to leave the jets alone, and try running the engine. then check the plugs to determine if you are too rich or too lean.

Also, the CB250, except for the cylinder bore and jetting itself, are identical to the CB360t motor and frame. You shouldn't have any more issues finding parts then a CB360 owner.

The jets themselves are not all that unusual. I am sure there are suppliers in Europe. Someone else can probably point you in the right direction for that.

 

[Justinas]

Thanks for the answer! I noticed that Dime City Cycles ship worldwide and have some main jets from 360 which should fit. So I'll try to play with these.


Just a question - does push in and threaded jets are interchangeable? (this may sound as a very silly question:/)


Cause my main jets are standard threaded ones (attached).
But all the main jets for CB360 on sale in picture looks like push in ones:

 

[crazypj]

Push in jets are in 68~73 CB350 twin, 500f/550f
Screw in doesn't work
You'll probably need 105 or bigger
I've had several CJ250's but they were using 745 carbs, same as CB360 (which surprised me a bit, I thought they would be different/smaller)

 

[mydlyfkryzis]

With 744 carbs. I believe ALL the fuel jets are threaded. The air jets (Under the diaphragm for instance) are likely push in. I have 754a carbs on mine, so they are probably slightly different.

My carbs do look like the bottom picture of the fuel bowl.

There are at least 5 different carbs for the 250/360 bikes. 744, 745, 747, 754, and 759 that I am aware of, and they have A and B designations, which I assume are slightly different jets, or left and right configuration maybe.

Sirius Consolidated has a variety of jets available too.


I wouldn't re-jet until you run the bike....see if it really needs it. I ran foam type filters and a 2 into 1 exhaust without changing jets. It ran slightly lean, mostly popping on decel, bit otherwise was fine.

 

[Justinas]

Thank you guys!

 

[Bert Jan]

i have a tuned cb200 twin wich runs really great on 100 main jets. I would think that would be good for a 250 aswell. CMSNL.com has all the parts for your bike and they ship worldwide. davidsilverspares (UK) has a lot too.

Or, if you feel lucky, buy the right mm drill and diy. you can clamp the drill in a vice and just use your hand to turn the jet over it. goes waay easy. Again, not recommending this thru a proffesional sight, but it saved my ass couple o times

 

[crazypj]

Hi Bert,
Problem with 250 and 360, the 3 jet CV carbs need a lot more work than the slide carbs on 200.
As you already have 110 secondary main jet, try it out before doing anything.
Unless you run open pipes you shouldn't need more than 110 (I run 112.5 in my 378cc motor, but, it has a long exhaust system)

 

[Justinas]

Hey guys,
found a problem. Can't seem to find primary main jet and slow jet for Keihin 744b nowhere. The internet is full of secondary main jets in any specs you want, but somehow these two are nowhere to be found... Are they not as important? Or is there a way to compensate for they default size? I'm to scared to modify them myself, as in case of my failure, I wouldn't be able to find a replacement...

 

[crazypj]

You don't need to change either of them, work fine to 4,500 rpm, then the secondary mains start operating
I've made a couple of #70 primaries but it didn't change anything, smaller won't be any use.
I've tried 40, 45 pilots, just made bike run rich (on 378cc motor though)
Pilots come in repair kits for 360 carbs so are easy to get hold of, I could send you a couple for price of postage, have several from doing carb re-builds (only #35's, but you can drill them out if you want tp experiment)
I don't think anyone I've done carbs for will mind?

 

[Justinas]

You don't need to change either of them, work fine to 4,500 rpm, then the secondary mains start operating
I've made a couple of #70 primaries but it didn't change anything, smaller won't be any use.
I've tried 40, 45 pilots, just made bike run rich (on 378cc motor though)
Pilots come in repair kits for 360 carbs so are easy to get hold of, I could send you a couple for price of postage, have several from doing carb re-builds (only #35's, but you can drill them out if you want tp experiment)
I don't think anyone I've done carbs for will mind?

Thanks crazypj! Will try to only update secondary jet at first then and results will tell where to go then.