Out of my depth

[Moooools]

Alright...
I have been messing with my CB125t for a while and have just confused myself with what is happening. It just seems to be doing weird things. I know(think) it is running rich and so have dropped the needles down, still not enough, so I bought some jets to hopefully fix it.

Would anybody be able to a: Give me jetting advice in general, b: Be able to help me physically with a rejet?

Any help would be much appriciated,
Cheers
Max

[richban]

Have you looked for air leaks in the manifold / carb. I had an air leak in my manifold and it was impossible to tune. Get it going on a fast idle and squirt some petrol on the manifold and see if the idle changes. A leak normally would make it lean but you never know. My 5 cents

[hmurphy]

Hey, check these out:

http://www.4strokes.com/tech/4sjetting.asp
http://www.iwt.com.au/mikunicarb.htm

On the first link, try to match up your problems with the solutions down the bottom of the page.

If you think your bike is running too rich then what you're saying is that it is bogging while trying to rev it out? With my new carb I was running a main jet that was a tad too small (on my FXR150) and it was also bogging with full throttle because it was too lean. It basically wasn't able to clean out at higher revs properly.

I have got a much bigger jet in the bike now and it has made large improvements but is still a little flat up top meaning it's still a tad lean.

Hope those links help you

[F5 Dave]

without a doubt read up on the net, there will be heaps of jetting articles for four strokes.

Just be aware that on a CB you will not have CV carbs like on most roadbikes, so make sure you aren't reading CV articles, try dirtbikes.

Biggest thing is (once you have the choke not jammed on or waggling in the breeze) is to consider the problem only by throttle position, not revs.

Not revs.

By throttle position.


Repeat every time you think of a symptom.



Therefore if you have a problem on full throttle you will need to adjust the mains. try bigger first in small steps, if it gets worse then smaller.

presumably you have ditched the airbox? that should make it quite lean to start with.

[jasonu]

Have you looked for air leaks in the manifold / carb. I had an air leak in my manifold and it was impossible to tune. Get it going on a fast idle and squirt some petrol on the manifold and see if the idle changes. A leak normally would make it lean but you never know. My 5 cents

Dangerous to go squirting gas on a hot motor, use ether or engine start instead.

[F5 Dave]

or some brake-cleaner, possibly flyspray. good old ozone depleting ones used to work a treat as a flame thrower for bored teenagers.

Be careful with engine start sprayed into an airbox area though if trying to start something. these dirty 4 bangers tend to backfire every so often which could be a fire risk. don't ask how I know.

[Moooools]

Thanks for all of the suggestions. It appeared to me that it was running very rich. It was starting very easily, smoking, but wouldn't rev up or handle any load. Sparks were being blackened. So I decided to do the biggest jet change I could and see what the results were.

I went from an 88 to a 68 (in both carbs). Put new spark plugs in, and the results were confusing.
It started easily, slightly less smoke, but would not rev passed closed throttle much at all. When it did rev it would be erratic or would not come back down from high revs. I thought this was a throttle cable issue but even with jiggling them or pushing on the parts where it could pull open, there was no effect on the high revs.

And then just to add to my confusion I decided to pull the sparks out and found this:

The clean one is from the right and the dirty from the left. The other picture is the bike that is messing me around.

[gatch]

Is the right cylinder even firing ?

[Moooools]

I think it is. It sounds exactly the same as it did before and has the same idle speed.

[richban]

The clean one is from the right and the dirty from the left. The other picture is the bike that is messing me around.

When it was going better were you riding it down the road and chopping the throttle and all that. Its almost impossible to tune the a carb with out it with out the engine under load. The only thing that you should be able to do is the pilot / idle setting but that is usually last on the list. An airbox might be a good idea if you have one.

[richban]

The black one looks way rich but if it hasn't been plug chopped proper then its hard to know. Try putting your hand over one of the carbs intakes and giving it a rev see if you are able to get it revving with moving your hand around. Also try pulling on the choke when it is warm. See what happens. Black one could be fowled. Chuck a new one in.

[F5 Dave]

OK NOW I AM GOING TO SHOUT BECAUSE YOU ARE NOT LISTENING ARE YOU?

. . . .

Biggest thing is (once you have the choke not jammed on or waggling in the breeze) is to consider the problem only by throttle position, not revs.

Not revs.

By throttle position.


Repeat every time you think of a symptom.



Therefore if you have a problem on full throttle you will need to adjust the mains. try bigger first in small steps, if it gets worse then smaller.

presumably you have ditched the airbox? that should make it quite lean to start with.

so clearly changing the mains will have little effect on how it starts or off close throttle.

Will it?

No it won't.

Also making big changes will likely confuse you more than learn you up.

[jasonu]

OK NOW I AM GOING TO SHOUT BECAUSE YOU ARE NOT LISTENING ARE YOU?

Also making big changes will likely confuse you more than learn you up.

I disagree. Small changes can often be hard to notice by the seat of your pants tuning but mostly can be seen if you were tuning on a dyno. A big change is more likely to tell you if your change is in the right direction or not.
If your bike had a proper (2 stroke) race engine I would say your change has made it way lean and to go back to say a 78 or so MJ. Then consider playing with the needle height, make only one change at a time.
Don't know if this will apply to your cam'n'valve Harley motor though...

[FROSTY:Me]

My suggestion to you is to go right back to basics. You have confused the buggery out of yaself and the bikes probably not too happy either (engine wise)
So I'd say nows the time to reset a baseline.
Throw a new set of plugs in her. Put the airbox back on. Reset the jets and mixtures back to factory and see how the bike runs. This will set you a baseline to make changes from.
only ever do ONE change at a time otherwise you'll have no idea what has been effective and what hasn't.

[glegge]

did it ever run OK before you altered anything?

if not, perhaps check the carb float levels, if they are grosely out they may give you some issues.

just some more thoughts, but I agree, go back to base line, jets, floats, pilot settings (idle mixture) etc etc.. and see how it goes.

any mods inside engine? cam etc?