Carb Choices - Derbi Senda

[HD]

I have asked this couple of times with no definate response.

Basically if I ride my bike flat out it revs cleanly, accelerates well. If, however, I pull away normally, it gets caught up at low revs then gets into the power band then gets caught up again up the top. I constantly use the clutch to get it out of this crackle/shitty bit.

I thought this was maybe normal as it is a two stroke with a decent exhaust and I remember my motocross bike being like that but not so. Ridden a few other peoples bikes and they rev smoothly all the way through.

I must say that I have one of those shitty 1/4 turn quick reaction throttles. No I didn't buy it

So what do you think is the cause?

Also, the bike never needs choke to start, not even in that cold weather we had.

One person said maybe down jet as it is flooding when pulling away normally?

I think the main jet is somewhere around 75 (think its 78) and clip is on the needle at either 2nd or 3rd.

Like I said it has one of those shitty throttles on and a metrakit exhaust. Standard airbox with air filter.






Copied that out from another thread. Basically all of the symptoms are the same. Rode a mates bike yesterday and makes me want to change my jets again. Cleaned out his carb for him too (well the float bowl and jets, not confident with the shitty throttle cable attachment on these) and realised how easy it would be to change the main jet.

Should I just go ahead and bring the main jet down a bit? But the thing is, it isn't at throttle openings, it's at rev ranges.

What you reckon? Just drop down a couple and see if that works? Or is there something else I could try.

Two reasons why I want to do it.

Selling it next month so I don't want the buyer to be scared off by a seriously minor issue.

I'd like to ride it when it runs smooth.



Cheers (again for I think the third time )


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Rusty '02 Vito Camper + CBR600F3

[Avora]

Check the colour of the spark plug. Sounds like it might not have been rejetted after the exhaust was added, which means it'll be running lean. The mixture screw is just for idle, and wont have any effect once you turn the throttle. If it idles, leave it alone. But you need to check the plug, buy some spares, I usually dont ride without a spare.
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Kawasaki AR125 B8 - Getting ready for a summer time soon!

[HD]

[prawny1]

You need to clean your carb athen set the mixture screw to it's default position and make sure the choke is fully closing when you turn it off.
If the bike still plays up it would be time to look for air leaks and the like,

Once your confident that your not taking in to much air you can move on the tweaking the idle mixture, you will need to set the engine at a sensible idle speed (approx 1-1.3 krpm) then with the pilot screw you will need to turn it in until you are about to stall then turn it out untill you reach your highest idle speed counting how many turns you did then set the screw in the middle of these two points.

If you are having to set the screw at more than about 3-4 turns out in total you need the next sized up pilot jet,

Or you could get a color tune sparkplug thing to set the mixture by looking at the color of the flame front in side the cylinder. I have heard a few people say they are easy to use and get you pretty close to ideal with them.

As far as changing the carb for a different one goes I wouldn't bother unless It is a standard fitment on other years and therefore jetted right for your perticular bike.

Don't get tempted to pull one off a scooter or something as they will have different taper needles and emulsion tubes etc for different fuel delivery characteristics.
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hellkat wrote:

Some people are only alive cos its against the law to kill them.

[temeluchus]

No oil should really be collecting on the spark plug-sounds like its over oiling. The combustion chamber doesn't need lubrication like the bottom end does.
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Some shite cruiser. Now with guns and FREEDOM!

[HD]

Checked the spark plug the other day and it was black. I took a photo but the flash made it seem browner so I ain't gonna put it up. It was the morning after I had been out and it was dry. Is that because it had dried up or it had been burnt as the thread was still wet with oil?

Also I have been thinking. I have read a few places that you should upjet by 10-12%. So my standard main jet was 71. So surely I should put it at about 80 or so? (as a sports exhaust would put it higher?) I think I have either a 75 or 78 in atm, recommended by PJME (whom I bought my exhaust from). My bike has also been rebored so that would add on another couple of jet sizes, right? So theoretically, I should be looking at anywhere from 80-85?

So a black spark plug means too rich? And my main jet is possibly too low? If I upjetted would it make this problems disappear?

Another theory is that this shitty throttle is sticking in too much fuel causing it to flood when riding slowly? But in the high revs it can burn the fuel as the throttle is working properly?

Need some help guys as it is pissing me off

I mean I don't mind riding it flat out but I want this bike to last me til December without another rebuild. Also it's nice to have the smoothness in traffic etc. And when riding past/behind police I either keep the revs low and go super slow, or ride normal and sound fast and get pulled for a loud exhaust?
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Rusty '02 Vito Camper + CBR600F3

[Avora]

If your going to make the main jet bigger, then its going to have more fuel, considering its already rich then you need to go down. Dont go by what people tell you...you need to do it yourself, so buy a set of jets, and a few spark plugs. And Google Plug Chops. Do that until it is perfect browny chocolate colour. And itll run like a dream wide open. Then play about with the needle clip position to get rid of any mid throttle issues. Job done. Until of course there is a massive change in temperatures, or something else changes. This is the only way to effectively tune any engine that is to use full potential.
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Kawasaki AR125 B8 - Getting ready for a summer time soon!

[HD]

[Avora]

Too much fuel is rich. Too much air is lean. If its just the mid throttle then open the top of the carb, the slide will some out with a spring. You need to get the slide off the cable. And dont drop the needle. On that there will be a tiny clip, really really dont drop that. Move it up the needle to lean off, but experiment with it till its perfect. Itll be frustrating the first couple times you do it, but soon it will be a regular thing that'll be done in seconds.
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Kawasaki AR125 B8 - Getting ready for a summer time soon!

[HD]

[HD]

Moved the clip up one and it is pretty much perfect. In the manual it says it should be on the fourth notch. It was on the 3rd so we moved it up to 2nd and now it runs well and the spark plug is browny.

Thanks Avora!
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Rusty '02 Vito Camper + CBR600F3

[Avora]

No probs
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Kawasaki AR125 B8 - Getting ready for a summer time soon!