New exhaust, what would you expect to change jet wise?

[Blue_SV650S]

I have put a new can on the WR250F, as it comes with a long header, it is more like a 3/4 system. The old one was a different shape/length and had a pea shooter end.

Now I know this is a bit of a piece of string question, but what would you expect to change jet wise?

I relaise the only proper way to do this is to stick it on a dyno, but I am happy to play ... just want to keep it 'safe' for the little motor ...

anyhoo, it had a 170main, I have put a 175 in there (fairly arbitrary size swap) ... how much would you expect to have to upjet for a new zorst??

My main cocerin is that if I whack the throttle wide from low revvs, the bike bogs down - presumed it was lean .. but wondering if rich as when I put the choke on it doesn't prevent it and rolling off the throttle it then picks up??

To be honest I am not sure the jetting has ever been right ... going to have a gander at the manual to see what stock it ....

I guess this post is really just airing my thoughts ...

At the moment, the bike has a massive flat spot low 1/3 revs - it then takesoff .... as I am whacking the throttle wide and it is NOT a CV carb (I don't think) ... I am only considering the main jet at the mo ...

I might just put the standard one back on
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The purpose of life is to fight maturity.

[Kickstart]

Hi

Presume it is a CV carb?

Either way if it is bogging down at half throttle, etc, from being lean then lift the needle a touch.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Blue_SV650S]

[Kickstart]

Hi

Well, you shouldn't be using full throttle at low revs . Full throttle at low revs and the carb is going to barely work at all.

I presume it has an accelerator pump setup. In which case see if you can adjust the stroke of that.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Blue_SV650S]

[Kickstart]

Hi

Main jet should only really be relevant on full throttle which you should only be using at high revs. Most of the time it will be the needle that is important.

Jetting is important on a 4 stroke. Probably not as prone to sudden seizure from running lean as a 2 stroke. Still possible to burn a hole in a piston from running lean. And quite easy to lose a load of power from getting the jetting wrong.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Blue_SV650S]

[Blue_SV650S]

Ok, ... tried playing with the accelerator pump settings, tried the O-ring mod ... did make differences, but not really 100% happy with the results ...

So, had the carb fully apart inc the accelerator pump - no muck or water in there as it turns out.

Adjusted the needle height raiseing the needle up 1 clip (moved physical clip down a position - raising needle)).

Still wasn't happy with results ..

The bike was loud, it lacked low down grunt/had big flatspot and then went totally larey when it came on cam/pipe ...

I mostly wanted a new pipe to try and get some more low down grunt, perhaps to liberate a bit more top at the same time (why not) and to make the bike look less ghey .. it didn't really achieve any of these .. so I have taken it back off again and thrown it to one side in disgust!

Attached is the standard can ghey monstrosity I was trying to get rid of ... with the little bendy over bit at the back, it is V-Ghey

https://www.bikechatforums.com/files/wr_original_can.jpg
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The purpose of life is to fight maturity.

[stinkwheel]

When I fitted a stage 1 dynojet kit to my bike to suit the louder exhaust and more free flowing air filter there was a realtively small increase in jet size.

The throttle needle profile was radically different though.

If you can afford to splurge on an exhaust because you don't like the look of the old one. You can afford a stage 1 kit.

They cost £75, which is a lot. But they've done the R&D to answer the question that has not been answered here. Just keep telling yourself that's what you're paying for, not a plastic box with a jet, a throttle needle and two washers in it.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[ms51ves3]

[Blue_SV650S]

[Ariel Badger]

Blasting your exhaust at the rear tyre helps to keep the tread clear of mud.
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[Kickstart]

[Blue_SV650S]

[Kickstart]

Hi

The needle is probably picked from a box marked "Dynojet" .

The idea of the dynojet kit needles is that they shaped for what is expected in the engine in the state of tune that it is aimed at (hence various different stages of kit), with needles having grooves so that they can be moved (unlike most standard needles).

Sure if you take it to a dyno shop they might have a box of hundreds of different needles for your carb (but unlikely), but by the time you have paid them to change the needle half a dozen times until they get one that works well (and probably similar in profile to the dynojet one) it has probably cost even more.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[stinkwheel]

The advice i got from my local dyno place was to fit the stage 1 kit and if I wasn't happy, pop it down for fine tuning.

He said that the first thing they'd do is fit the needles and springs from one anyway then work from there. On my bike that's a good half hours labour saved straight away, as well as getting the best online price for the jet kit.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.