Bigger jets?

[dirtydog]

Seeing as I'm going to be having my carbs apart again this week, is it worth going up a jet size?

At the moment its got a bmc air filter, akrapovic system with a cut down can and some tank risers on it.
It pops quite a bit on overrun, which jambo pointed out could be that its running lean.

So is it worth upping the jets from the standard size?

[Bibio]

how does it feel when its running and what colour are the plug insulators.

[dirtydog]

Feels ok when it is running properly (sticking float/needle issues at the mo hence taking them apart again). Changed the spark plugs a few months back but didn't look at the colour of them. Just concerned about all the popping on overrun and the fact it should have more air going through but I've not upped the fuel

[Bibio]

well from what i know popping is an air leak in the exhaust system or unburnt fuel.

if you can crack the throttle open and it revs freely and the plugs are a nice tan/biscuit colour then all is well. if the plugs are white then bigger main jets.

the man to ask though is Y_C.

[wideguy]

What the plugs look like depends on what the bike was doing when you shut it off. If it was idling, which is almost always what it's doing before shutdown, the plug color will tell you about idle mixture and only idle mixture.
If you shut it down at half throttle, you'll learn about the mid-range jets and needles (plus the idle mixture, since the idle circuit is still working). Since most normal operation is at part throttle settings, you need to have your idle mixture right, then check the mid-range and adjust/shim the needles up if needed. Only after these two are correct does it make any sense to check the wide open throttle color by shutting down after a brief full throttle run. Then you'll know if you need bigger main jets, or not.

[yorkie_chris]

Depends which BMC filter

[dirtydog]

Depends which BMC filter

I think its the bmc road filter, its not got the massive hole in the top of it

[yorkie_chris]

In that case I would stick with stock jets unless you are having a specific problem that sounds lean. Did it run ok on that setup and jetting before?

Popping and farting on the overrun is more likely to be an exhaust leak that does no harm at all. Popping and a different sort of banging on the overrun is more likely an intake leak.

[dirtydog]

Yeah it ran ok with that set up on the previous sv, this one hasn't done many miles with this set up.

[wideguy]

You can try to tune carbs by sound and feel and what works in some other engine or by somebody's recommendation of what worked for them, or you can spend lots of money on dyno time, hoping the dyno operator knows what he's doing, or, you can do what everybody did before dyno's, and tune by plug color.
It's tedious, especially when the plugs aren't easy to get to, and the idle mixture screws are not easily accessible, but, it works as good as a dyno tune, and it doesn't cost much at all, mostly time.

Start with the idle circuits. When they're right, do half throttle chops and adjust the needles until the mid-range is right, then do full throttle chops and change main jets if necessary until the carbs are right throughout the entire range and your engine runs sweet and clean.

[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.

[wideguy]

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.

[wideguy]

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]

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.

[wideguy]

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]

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.

[dirtydog]

I think what I'm going to do is get it running properly and then worry about jets. Then I can see exactly when its popping, might even desnorkel my bmc road filter while I'm at it

[dirtydog]

Oh and it was popping on an open throttle today when testing with the carbtune

[wideguy]

You are of course free to tune your carbs in any order that you wish.
Removing the main jets entirely won't change the idle mixture. Removing the needles as well still won't change the idle mixture. This is because the idle circuit is the only circuit operating at closed throttle.
In similar fashion, removing the main jets won't affect the mid-range, since fuel flowing through the main jets goes next to the needle/jet combination, where it's flow is reduced the necessary amount to provide proper part throttle mixture. It's pretty simple, and seems quite clear. When the needle and it's complementary jet are controlling the mixture, all it needs is for the main jet to provide an adequate amount of fuel for that throttle setting. To the needle and it's orifice, there is no such thing as too large of a main jet. At 1/2 throttle, you aren't using nearly all the fuel provided by a 137.5 main jet, and the needle/jet will still meter the same amount of fuel if you change the main to a 160.

Start with the idle adjustment, because it will affect both mid-range and full throttle operation.
Then adjust the mid-range.
Lastly, change main jets as required.

[dirtydog]

Here's the filter I have
http://i181.photobucket.com/albums/x241/dirtydog88/null_zpsa1bc9bf1.jpg