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Bike bog on throttle, lean or rich?


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Hi all,

Thanks for the help previously, much appreciated. 

I have recently acquired a Yam TW125 for some green-lane fun.  

Bike starts fine and seems to idle OK too. When building revs very slowly the bike seems to reach max with no issue. When opening the throttle fast the bike bogs down and dies, unless I let off immediately. 

The plug was quite sooty, I thought it must be too rich. All of my research however tells me that my symptoms are that of a lean bike and I should adjust float hight.

I have been experimenting swapping multiple sizes of pilot and main jet. Just swapped to a larger main, 120, and noticed fuel coming from exhaust, must be too rich. 

The base main jet I started with was 100, now I want to reduce to 90 ish, does this sound small? I know it is dependent on airflow but standard jets all seem to be 100+? The air flow on the bike should be good as it has an aftermarket exhaust and filter.

Ps. Float height has been adjusted in both directions, neither seem to help, and definitely do not cure the issue. 

So, does bogging sound like a rich bikes problem?

Is sub 100 main jet size small?

Anything I'm missing?

 

Thanks in advance 👍🏻

 

Edited by DJYAM
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Hanging revs normally indicates a weak mix or an air leak. Could be both. Does it have a cv carb, could be a diaphragm (spelling?) issue if that's the case.

Is it a new plug.

I don't think I have ever known the float height be an issue unless a bike has had a carb rebuild. It either floods or works, once its set unless something fails, or some silly bugger fiddles with it. 

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I've checked for for air leaks, all ok. 

Slide keihin carb. Any place to get jets that isn't China? Delivery is shocking at the moment!

New ISH plug. 

Previous owner was that silly bugger unfortunately, hence why the carb isn't standard 🥴

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Can anybody advise on jet sizes?

All kits seem to start at 100 for the main, does going smaller than that seem like an odd thing to do? Could the carb just be too big for the bike?

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check timing if its adjustable first.

check 2 stroke oil as too much weakens the mix

you need a jet kit, allens or NRP, you need to ensure mains set first for WOT, do plug chops to check mix.

I assume the carb is set up and cleaned throughout?

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It's not 2T.

I have a bunch of jets already, just not below 100 as stated. 

I assume a 100 jet in one carb would provide a different mixture to the same jet in another, due to air flow and carb design?

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4 hours ago, DJYAM said:

It's not 2T.

I have a bunch of jets already, just not below 100 as stated. 

I assume a 100 jet in one carb would provide a different mixture to the same jet in another, due to air flow and carb design?

I'm suprised its finicky then. 
I'd still check timing, fuel height, valve clearance . before you start on different jets.

BTW fuelling the top end still applies, get that right (over 3k) there' usually a transition around there from pilot 

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So...

I tried a much smaller main jet, no better. 

So the next step was a much bigger idle jet, bogging was reduced by a good amount! Still slightly bogging, but much better. The problem now is, the bike is running way to rich, white exhaust smoke, excess fuel in carb-engine manifold, black plug. So what now, because reducing jets just seems to create bog? Is the carb just not suited to the bike?

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are the carbs clean inside, ie all the small jet holes unclogged?

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The idle jet is now 55, and the main 140. This is running way too rich, but seems to run better with less bog. 

So why can't I reduce bog with smaller jets, seeing as it is now running rich?

Changing needle height didn't seem to have much of an effect at all. 

Float height? I've tried a few different ranges and not much difference noticed. 

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13 hours ago, DJYAM said:

Yes... As I said, tried multiple sizes of each jet, all clean. 

i didn't mean the jets themselves. more the carb body internals where the jets fit into

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20 hours ago, drewpy said:

i didn't mean the jets themselves. more the carb body internals where the jets fit into

Yep +1, I recently cleaned a set of vmax carbs for a friend. He said they were fine as was but as he was rebuilding the bike anyway I got him to let me have them for a few days.

The rustycrap that had built up inside was surprising for a bike running ok. Especially when you start using compressed air to blow through, a couple of jets were blocked and needed a copper brush (strands of electrical flex) to clear them. Its  running far better now.

These are jets big enough to feed a 1200, just think how easy a 125 carb will clog.

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It's definitely clean, I have been through it multiple times. 

This still doesn't explain the rich running, as surely if a jet was blocked, it would run ok with slightly bigger jets, to get the same amount of fuel, not huge jets. 

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3 hours ago, DJYAM said:

It's definitely clean, I have been through it multiple times. 

This still doesn't explain the rich running, as surely if a jet was blocked, it would run ok with slightly bigger jets, to get the same amount of fuel, not huge jets. 

but there's air bleed jets surely? if they are blocked, then it'll always be rich

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At the end of the day you are arguing that everything is ok, then saying it isn't.

Your engine is running rich, that is down to any number of things. Problem is you say they are all fine and its a mysterious problem. Its not, its a machine. It obey's the laws of physics.

That means if it has a bright properly time spark, a clean air filter then there is a problem in your carb. Simple.

Fit the factory jets and give it another run, something is wrong, random guesses don't help. We are pretty good most times but we only have the information you give and our combined experience. 

We are both currently concentrating on the carb at the moment. 

It is possible for there to be crap in internal air ways, could even be floaty bits in the carb that get sucked into a jet, then drop away. Some Chinese crappy jets can be as much as 30% off on calibration, which is unhelpful at best. 

We are not trying to wind you up, just trying to help you understand a carb meters air as much as fuel. If the amount of air moving is wrong you will never get the jetting sorted, if that is the issue.

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I am not arguing anything. 

I am simply responding to suggestions that I have already confirmed are not issues. 

No need to be sarcastic. It's been suggested about cleaning jets multiple times, which I have done. Checking for leaks, which I have done. All mentioned in the original post. 

I have asked multiple questions regarding float height etc with no direct response, just more nonsense. Answers about 2T and jet kits aren't helpful for obvious reasons. 

The ONLY thing I can think that is suspect, is there is a carb balancing 'port' on the engine side of the carb, which to my knowledge needs to be sealed, and is. With a fuel line and some glue, 100% sealed. I am not aware of any other 'air bleeds' that need to be open/closed. 

I do understand the physics of a carb, I have spent a long time making adjustments to them in the past. The questions I've asked about being too rich, but only running in this state haven't been answered. 

Howcome it runs relatively well when rich, but not at a 'normal' mixture? This doesn't suggest that there is a fuel blockage. 

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