Bigger jets?

[yorkie_chris]

Hmmmm. IMO if you start chasing low and mid settings before you get the mains right you'll chase your tail... a lot.

[Sid Squid]

You can't judge jetting from what happens on a shut throttle - it will tell you nothing, popping on the over-run doesn't say anything about jetting, it may indicate an exhaust leak, but it can simply be a very 'open' exhaust.

[dirtydog]

Ok fair enough. Ill be leaving the jets standard

My 1st a that had the akra system also had a desnorkled filter, I had dyno jet kit fitted and set up on a dyno and tbh it didn't feel a lot different

[yorkie_chris]

The desnorkeled filter would make a big difference.

Originally Posted by yorkie_chris

Hmmmm. IMO if you start chasing low and mid settings before you get the mains right you'll chase your tail... a lot.

No, the idle circuit is always operating. If you lean it out, you lean out the mid-range and main circuit too. Ditto with the mid-range, it's still in play almost until full throttle.
There are good explanations of carb operation, with graphs and charts showing which circuits are operating when. If you don't believe me (and of course you don't know I've been tuning carbs like this for nearly 20 years. I first learned it when I put Keihin CR Specials on my '81 Honda CB900F, and Keihin was kind enough to send me a tuning manual for those old honkers.), then please, look it up on the net. The bottom line is, idle first, mid-range next, main jets last.

Originally Posted by Sid Squid

You can't judge jetting from what happens on a shut throttle - it will tell you nothing, popping on the over-run doesn't say anything about jetting, it may indicate an exhaust leak, but it can simply be a very 'open' exhaust.

Quite right. My 650 is on the rich side everywhere, (medium brown on the plugs) and it often pops nicely with a shut throttle.

[yorkie_chris]

Originally Posted by wideguy

No, the idle circuit is always operating. If you lean it out, you lean out the mid-range and main circuit too. Ditto with the mid-range, it's still in play almost until full throttle.
There are good explanations of carb operation, with graphs and charts showing which circuits are operating when. If you don't believe me (and of course you don't know I've been tuning carbs like this for nearly 20 years. I first learned it when I put Keihin CR Specials on my '81 Honda CB900F, and Keihin was kind enough to send me a tuning manual for those old honkers.), then please, look it up on the net. The bottom line is, idle first, mid-range next, main jets last.

I've read similar and am familiar with the various circuits and how they overlap.

On CV carb, point of low pressure moves as slider opens, carb transitions off idle circuit onto needle/mains.

I don't think you will notice any difference if idle was all over place but mains were ok. Case in point how many bikes do you see with loads of sh*te in the idle passageways that run fine when you get revved up.

Originally Posted by yorkie_chris

I've read similar and am familiar with the various circuits and how they overlap.

On CV carb, point of low pressure moves as slider opens, carb transitions off idle circuit onto needle/mains.

I don't think you will notice any difference if idle was all over place but mains were ok. Case in point how many bikes do you see with loads of sh*te in the idle passageways that run fine when you get revved up.

Doesn't really matter whether the slide is lifted by a cable or a vacuum, the same things happen with the jets.
Changing the amount of fuel delivered by the idle circuit has an effect throughout the range of operation. That's why you need to start there. Changing mid-range jets and needles, or changing the height of the needles has an effect on most of the main circuit. That's why you need to go there next, and leave the mains for last.
I guess it all depends on how well you want your bike to run. The effects of changing the idle circuit after you've changed the main jets might be small, but it also might be significant. Some of that depends on how you've jetted the mains. If you've set the mains right at the edge, either rich or lean, changing the idle mixture might just make it run poorly, or it might make it run better. Who knows? If it makes it run poorly, what do you do next? Go back and change the mains again?
All I know is, I learned from the carb manufacturer how to properly tune carbs. It not only makes sense, it works.

[jambo]

I'll chip in, I'm quite sure dirtydog was clear on the conditions it was popping on but for some reason I'd obviously decided it was doing it with some throttle too, must learn to listen better.

With regards to the order to set jets up I'd start by saying I've never really had to re-jet so could be way off but I'd be going with YC's method from what I know. Yes the idle jets will have an affect on fuelling higher up the rev range but it'll make smaller and smaller differences. The pilots start at something like a 17.5 jet, if you changed that out to a 15 or a 19 that's quite a small change by the time you're comparing it to mains that might start at a 140, and quite easily go up to a 150+. I'd make the biggest changes first and then add the finesse.

Again, I've not got a whole lot of experience doing this in practice, but that'd be my thought and I'm happy to be disagreed with by those with experience.

Jambo

[yorkie_chris]

Originally Posted by wideguy

Doesn't really matter whether the slide is lifted by a cable or a vacuum, the same things happen with the jets.
Changing the amount of fuel delivered by the idle circuit has an effect throughout the range of operation. That's why you need to start there. Changing mid-range jets and needles, or changing the height of the needles has an effect on most of the main circuit. That's why you need to go there next, and leave the mains for last.
I guess it all depends on how well you want your bike to run. The effects of changing the idle circuit after you've changed the main jets might be small, but it also might be significant. Some of that depends on how you've jetted the mains. If you've set the mains right at the edge, either rich or lean, changing the idle mixture might just make it run poorly, or it might make it run better. Who knows? If it makes it run poorly, what do you do next? Go back and change the mains again?
All I know is, I learned from the carb manufacturer how to properly tune carbs. It not only makes sense, it works.

I tend to use the idle screws to get it to idle nicely. I use highest vacuum method. This seems to work and gives nice initial pickup.

Take a common example with SV, desnorkel filter. You start at 137.5, you need 160 main.

Nothing you can do to that will make it right in the mid range or top end, and when you do change mains to correct ones (if you tried to chase midrange) the mids will be slobbery rich and you'll need to go back down with the needle.

Simple reason being fuel to midrange has to flow through main jet first.



When I get the SV back on the road I might do some p*ssing about. How about doing a couple of standing mile speed tests with a "right" idle screw setting and then wind it in 2 turns and see what difference it makes?
Or putting stock filter in (instead of BMC race RS) and seeing if it still ticks over the same.