Cb125 jet sources. Help!

I

InkandIron

New Member
Hey there, I have a 1975 cb125s with a stock carb (as far as I know). I'm in the middle of a build and plan to run a pod filter and possibly change exhaust at some point. I've searched for jets for this bike with not much luck. I searched jets-r-us and found the slow jets but no mains, and also searched bike bandit with no luck. Anybody know a good source to find these jets?

Second of all, I have always heard that for each modification you should bump up 1 jet size. This has worked for me in the past. Is this what I should expect with this as well?

Which jets should I change? Slow? Main? Both?

Any help would be great! Thanks guys!


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I'd leave the jets alone until after the modification and then figure out if you need them or not. Pre-optimization is the root of many evils.
 
Sonreir said:
I'd leave the jets alone until after the modification and then figure out if you need them or not. Pre-optimization is the root of many evils.
Click to expand...

That was basically my plan, just preparing for the worst case scenario to keep the build moving along! Thanks for the Input! I appreciate the response!


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Good information to know.
I'm totally ok with keeping the stock exhaust. It's in great shape. Unfortunately I'm beyond the point of going back to an airbox. It's been removed and a smaller, tidier battery box was installed. I thought about fabing up a longer intake tube to mount the filter to the end and try and mimic the length of the original intake... What about velocity stack? I know, it's a tiny tiny bike, so that might be a bit overkill, but I would like it to run ok... Any suggestions or help would be Great...
If the cb125's DO hate pods finding a decent source for jets would be even more important, to try and tune it. Thanks!


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Check Sirius for jets.
With enough jets and time you should be able to make the power loss by going to a pod or stack as little as possible, but will still be a loss.
 
Maybe I should have clarified it. I'm using a uni foam filter, not a wire mesh pod filter...


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jungalist said:
wouldn't putting a uni foam filter on the bike be closest to stock airfilter?
Click to expand...

From my experiences and what I've read, it's more about intake length rather than filter element type.
 
surffly said:
Check Sirius for jets.
With enough jets and time you should be able to make the power loss by going to a pod or stack as little as possible, but will still be a loss.
Click to expand...

Sirius didn't carry the main jets similar to other places. Also maybe I'm missing something, or maybe you could explain. I realize this is a SMALL bike, but does it somehow defy the common thought that more air and more fuel creates more power? Of course a more free flowing Intake might reduce low end torque which might be a bigger deal on this tiny thing.


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Well the first thing is assuming that the stock air ox was some how restrictive or otherwise the weak point in the system. This might be true for an 80s K car, but overall is not the case for Japanese motorcycles.

The idea in any mod should be to identify what is holding the system back, and make it better.
To often things are done on assumptions or for looks.

Let's work in round numbers, and make a few of those dreaded assumptions.
The cb125 makes what? 12.5hp? Maybe a little more, maybe a little less right?
So in round numbers that is 100hp per liter of engine. This is also done with a naturally aspirated engine. That is a fairly impressive feat in the automotive world, even today.

Now the assumption that more air and fuel make more power is right, but only to a point.

Gas needs about 15 parts air to one part fuel to run well, again round numbers.
But if you hot a garden hose and a air compressor there is a point that the engine can not swallow the mixture and use it.

The wire mesh junk that DCC and others sell is MORE restrictive then most filters, not less.
Uni is fairly good, though.

The volume of the air box helps the engine have a supply of clean air for the engine to pull from.

Also there is more to an engine then just being an "air pump"
Valve volume, engine rpm and other factors are important, maybe even more important on a small engine.

At best the changes move the power around in the RPM range and make the bike worse on the road.
 
surffly said:
Well the first thing is assuming that the stock air ox was some how restrictive or otherwise the weak point in the system. This might be true for an 80s K car, but overall is not the case for Japanese motorcycles.

The idea in any mod should be to identify what is holding the system back, and make it better.
To often things are done on assumptions or for looks.

Let's work in round numbers, and make a few of those dreaded assumptions.
The cb125 makes what? 12.5hp? Maybe a little more, maybe a little less right?
So in round numbers that is 100hp per liter of engine. This is also done with a naturally aspirated engine. That is a fairly impressive feat in the automotive world, even today.

Now the assumption that more air and fuel make more power is right, but only to a point.

Gas needs about 15 parts air to one part fuel to run well, again round numbers.
But if you hot a garden hose and a air compressor there is a point that the engine can not swallow the mixture and use it.

The wire mesh junk that DCC and others sell is MORE restrictive then most filters, not less.
Uni is fairly good, though.

The volume of the air box helps the engine have a supply of clean air for the engine to pull from.

Also there is more to an engine then just being an "air pump"
Valve volume, engine rpm and other factors are important, maybe even more important on a small engine.

At best the changes move the power around in the RPM range and make the bike worse on the road.
Click to expand...

Thanks man, you make some good points. Like I said, I've tossed the old airbox, and I will totally admit I made that decision purely on looks. My train of thought, is if I'm gonna build a tiny bike like this, if it's gonna move slow, it's gonna look cool! But I will do everything to minimize the loss in performance where I can... So back to my original question, any other good sources for jets? Seems as the the slow jets are abundantly available and the mains are nowhere to be found... Anybody have some they wanna sell? Other sites? Thanks!



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from my experience with my 125T i've used the stock idle jet and went from 85 to 90 with the main jet.

but get it running with stock jets first
 
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