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:eek:. 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?:nono:



. . . .

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?:nono:

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]

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?

[F5 Dave]

he's just fired an engine of probably unknown parentage in a different frame. Made a non std exhaust & presumably no airbox & surprise it doesn't run properly. An airbox would be nice to try if you have one & it still fits, -it is a good suggestion so you can get a decent start to fault find other issues. Perhaps one leaking float, or the choke on, or another problem.

presumably the smoke is black on that side (rich), not blue (oil residue)?

But consider if rich' means at idle, it could be lean at full throttle which is a different cct & thus different jets. This richness (if real) could be due to a fault as described & jet swapping won't help until fixed.


And Jason, How dare you disagree with me bitch? Dog big black up you get a.

[Moooools]

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


Originally Posted by F5 Dave
. . . .

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.

I AM GOING TO SHOUT RIGHT BACK EVEN THOUGH THIS IS NOT ACTUALLY SHOUTING IT IS JUST WRITING WITH CAPITAL LETTERS.

I AM REALLY CONFUSED.

I marked out the throttle position the throttle tube and it was just to hard for me to work out what was happening. I now have come to the realization that how it idles has nothing to do with it.

I will try and give a description in terms of throttle position:

Before: Would not bog at any position. WOT would still not bog. However at 1/2 throttle or greater, any load would lead to the engine bogging.

After 20 decrease in jet: Would not get to 1/2 throttle at all. Engine would die when throttle opened.

I never had an airbox as the engine came from a wreckers. It is on pod filters but I know that they are crap.
It is also not on stock exhausts as it is an engine transplant. Going back to the baseline isn't really an option. Nothing has been altered except the needle height and main jet.

I always knew it wouldn't run well for a start, but the whole thing just confuses the shit outta me!

[White trash]

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





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.

Lol. You're funny as fuck man. I just sprayed my keyboard with Steinlager Pure and it's all sticky.

Cockhead.

[F5 Dave]

Lol. You're funny as fuck man. I just sprayed my keyboard with Steinlager Pure and it's all sticky.

Cockhead.
Better beer than the usual reason you pervert:nono:

[F5 Dave]

. . . .

Before: Would not bog at any position. WOT would still not bog. However at 1/2 throttle or greater, any load would lead to the engine bogging.

After 20 decrease in jet: Would not get to 1/2 throttle at all. Engine would die when throttle opened.
. . .

ahh, so there is more to the story than first ventured.

well there is bog & bog. one is like rich burrble bog & one is lean bog, sort of dies, but can get more bitchy. can be hard to tell.

but with a huge change in mains you could have gone from too rich to too lean or viceversa & missed the midpoint.

But in this case it was too lean & you've made it waay too lean. small to medium steps richer than 88 to see if it gets better.

But with the plug fouling thing you want to clear that up. go baqck to where it ran & sort that out before proceeding. My be just a duff plug, a duff coil/cap, or that carb, or that cylinder. swap out where you can.


obviously full testing has to be done under load. What will run just ok in neutral will be too lean for under load & needs to be richer.

it makes sense that you will need to go up a mainjet size or so for a std engine with the airbox removed. other jets may need a tweek with adjuster or a pilot size up, or may get away with it.

[speedpro]

OR . . . . you could be pissing in the wind with a weak ignition. Stock standard 125/2 ignitions are crap. Instant horsepower is to be had with an electronic version, say from an FZR250, plus suddenly it will start easier and on both cylinders. Tried and tested on a dyno, not a theory.

[Buckets4Me]

OR . . . . you could be pissing in the wind with a weak ignition. Stock standard 125/2 ignitions are crap. Instant horsepower is to be had with an electronic version, say from an FZR250, plus suddenly it will start easier and on both cylinders. Tried and tested on a dyno, not a theory.

I remember mucking around with my first ever bucket to be
lots of fiddling with the points and mucking around with sandpaper
it would run nice on 2 then drop back to 1 cylinder
I sold that and got a 2 stroke instead

I think speedpro may have the answer for you find someone down welly way to help and change the ignition or at least help you set up the points properly

[Moooools]

The ignition is actually CDI. So there shouldn't be too much of an issue there.

I have ordered some more jets so that I can test richening the mixture as well. I also will end up with a fair range of jets to pick from so whichever way I need to jet it I should be able to optimize it.

Anyway for now I will wait for those to come through and avoid confusing myself anymore that I need to be.

Might make myself a spark plug cleaner in the mean time as between this and my two stoke I am going through far to many.

[Henk]

From what I can tell from the wiring diagram with the switch on, Black and Red are conected to allow the electrons to flow and with the switch off Green and Black/White are connected to kill the ignition.
I'd try
1. Hard wire black and red together to get the system livened up
2. Hook Green and Black/White up through a kill switch so you have some way of turning the thing off.

Then kick it over with the plug out to see how things are going, you should be able to turn the spark on and off nicely. Try it with the plug out since if I'm wrong you may end up with a bike you can start but can't stop.

As for the airbox, they run a different set of rules in Chch so if you leave it off you may find yourself kicked up into a class where that isn't nearly enough in the way of modifications to make you competitive, I'm not sure if they run the modified and stock bikes at the same time, if so no big deal, if not you may find yourself getting seriously demoralised and if the speed differences are big enough quite a hazard.

[Buckets4Me]

The ignition is actually CDI. So there shouldn't be too much of an issue there.


makes life easier

as for sparkplugs EBAY and get them by the box :)

[SS90]

Better beer than the usual reason you pervert:nono:

Now that is what I call "owned":niceone:

[jasonu]

[quote=f5 dave;


and jason, how dare you disagree with me bitch? Dog big black up you get a.[/quote]

cobblers!!!!!

[Moooools]

From what I can tell from the wiring diagram with the switch on, Black and Red are conected to allow the electrons to flow and with the switch off Green and Black/White are connected to kill the ignition.
I'd try
1. Hard wire black and red together to get the system livened up
2. Hook Green and Black/White up through a kill switch so you have some way of turning the thing off.

Then kick it over with the plug out to see how things are going, you should be able to turn the spark on and off nicely. Try it with the plug out since if I'm wrong you may end up with a bike you can start but can't stop.

As for the airbox, they run a different set of rules in Chch so if you leave it off you may find yourself kicked up into a class where that isn't nearly enough in the way of modifications to make you competitive, I'm not sure if they run the modified and stock bikes at the same time, if so no big deal, if not you may find yourself getting seriously demoralised and if the speed differences are big enough quite a hazard.

I think this was meant for the noob question thread?

[Henk]

Right. Sorry. I'll paste it across. Blame it on incipient senile dementia.

[Moooools]

I think that I do have an oil leak in the engine. There is still a bit of blue smoke coming out after the top end rebuild.

New rings cost $80, new cylinders, pistons, rings and gaskets cost $180. I think I know which one I'm going for.
Hopefully those will arrive soon and then I can be sure of everything I do.

[Henk]

How much smoke is it? A few of the CB125Ts at Mt Welly blow a bit of smoke, usually when getting on the gas after a slow corner, and they don't seem to blow up that often.

[Moooools]

Quite a bit.
And it drips black oil from the exhausts...

[speedpro]

That's amazing for one of them. My brothers one did at least 5 Ohakea GPs and every bucket race inbetween and it still ran fine.

[SS90]

Quite a bit.
And it drips black oil from the exhausts...

Erm, it drips oil from the exhausts, and you have been pissing about with jets?

:blink::facepalm:

I think you may have answered you own question with that one!

[Moooools]

That was a discovery I made recently. There was a bit of oil under the bike and so I investigated.

[SS90]

That was a discovery I made recently. There was a bit of oil under the bike and so I investigated.

Oh right.

Fair to say there is a bloody good chance your fault lays there.

I am impressed with the price of the new complete top end.....