I'm running a set of 2001 GS500 carbs on my 82 GS450 and very happy with them, but I'm still confused about the interaction between the mid main jet and the needle jet/jet needle.
Being down under, we have adjustable needles with 5 positions for the E clip on the jet needle, so I can tune by lifting or dropping that but with these carbs I can also adjust the size of the mid main jet.
Sounds great that I have two areas of adjustment there, but does anyone know how the mid main fits in the bigger picture? Does it have more affect lower or higher in the slide's range or RPM's? Where does it feed fuel to, just extra in to the side of the needle jet?
I haven't been able to find any other info other than stating that it allows extra adjustability in the mid range, although it's likely I'm just searching for the wrong things.
I don't have a GS500 manual (because I don't have one) so I'm not sure if I'm missing something from there or not.
Any help or input greatly appreciated!
There is a lot of stuff about that on the forum however it's difficult to find. This sort of answers your questions http://www.iwt.com.au/mikunicarb.htm
Ive posted onthis a few times and someone worked with me to trace the fuel flow channels. Cant find it now.
The conculsion was that the mid main is incorrectly named (and is differently, and corrrectly named, in manuals where the carb isused on other vehicules (snowmobile or side-by-side if I remember correctly) and only comes into play when the choke is open.
If you change the "mid-main" yuo will find all kinds of wierd cold starting behaviour when using the choke, or not even needing the choke.
Thanks for the input guys, I've really struggled to find any info on this at all other than seeing jetting info where people have changed them.
I'm familiar with the diagram on IWT's site, doesn't really answer this though as it's just general theory which makes sense to me already, it's the action of the mid main and interaction with the other circuits that I'm confused about.
So its only impact is when the choke is open hey? Well that explains why I've never really understood the impact of changing it during previous tuning attempts, I've always tried to ascertain what different it makes at midrange RPM's!
Thanks for the clarification on that, turns out I have compression issues but once they're sorted I'll need to revisit the jetting so it's good to know this for then :thumb:
Just adding clarity here, when you have low compression, the carbs seem to misbehave a lot. I think it's due to insufficient vacuum to lift the slides the right way, so you never open the main jet. This probably explains just about all of your problems.
Sorry to hear about the compression.
Oh most definitely my current issues are low compression related! Never gonna work right with one cylinder at the service limit and the other 10 PSI below where it should be...
This is more to get an understanding of how best to tune with the mid main and needle once I get that issue sorted out.