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Tuning A Keihin Carb

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Hi everyone

I seem to remember there have been posts regarding your Keihin set up, but can't seem to find the one I need.

For your information, I have a 300 TXT Pro, flywheel weight, spacer on top of cylinder and slow action throttle. All that has really calmed it down, but since putting the spacer on the cylinder head I haven't managed to get the carb running right. I've got a JJH needle, 125 other jet and have just reduced the 45 jet to a 40 as it was very rich and i'd turned the air/mixture screw to only 1/2 a turn from fully in.

The bike pulls ok low down ... struggles and splutters mid way then pulls really well at mid to high revs. So when I'm trail riding between sections or wanting mid revs to roll up through stuff I'm never sure of the engine will splutter or suddenly kick into life once I get past half throttle.

I have a GJH needle which I think will help to give a more progressive spread of power (I think Peter B said the JJH was for the 'splat' boys, and I'm definitely not one of them) but I can't imagine it sorting out the poor mid range.

Any tips on how to tune my carb? I haven't checked the plug, but it was rich with the 45 jet, and now with the 40 I think it will be a little lean.

Any advice appreciated.



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I've a similar spec 300 & have tried a few calming mods too.

I fitted a raga head & tried the low comp insert. This gave spluttery carburation, so the std insert went back in & clean fuelling returned.

It seems that too low a compression will mess the carburation up.

I tried a reed block spacer next. Spacer is from early Pros or TXT, moves the reed block out 5mm or so.

This (with Boyesen reeds) gives a smooth response & clean running.

I tried the Keihin & the Dellorto flatside - both worked very well, but both were too crisp for this pilot.

I've leaned the jetting a little on the std Dellorto.

I've another mod in the pipeline. I'll let you know if this works later.

Edited by motomerlin

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I also had a lot of trouble with the Keihin carb on my old 09 300 pro, it ran way to rich on the standard jetting 125 main, 45 pilot, JJH needle with the clip in the middle, 'Factory 'Kev' at GasGas UK told me this jetting was right (More lies from GasGas UK as usual), but it also created Knocking, Gasgas UK told me to fit a 3 slide in the carb to stop this but didn't do a thing to it, the mixture on my bike was that rich it just started to die on me like it was running out of fuel & kept having to rev it to keep it running but it eventually died & couldn't get it going again, took it back to the van, had a look at the plug & it was very black & oily which is a sign of being to rich. (I seriously can't understand why GasGas UK said the jetting was right)

I tried running it on a 34 pilot jet which the mixture wasn't as rich but was still knocking.

GasGas UK will not tell you anything that helps, so just keep trying with it yourself.

I also went with mine to a Dyno-tester to see what was wrong with it, he said the Jetting was set up wrong also.

give it a go on the 34 pilot jet & see how it runs. mine always ran good on the 34 & never really spluttered but did on the 45 pilot it just had a knock.

This is a big problem with most Gassers maybe they haved worked on it on the 2010 models.

I'm so glad i have a Beta now!!

Keep trying with it anyway & see where you get, Good Luck!



Edited by GasGasben

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Hi Ian,

I run a 122 main ject and 45 pilot with a JJH needle. The mixture screw is an air screw so you're actually making it richer when you screw it in. Changing the main jet for a smaller one mainly affects larger throttle openings, but it does have an effect at the lower end too and personally I'd try a 122 or 120 before changing needles. This advice is based on a typical English climate, mind you! Good luck, Dave

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