GS400 tuning issues

[TrikerJoe]

Hello!
I have a 1982 GS400L/Z (see build log in my signature) with K&N filters and a Mac 2 into 1 exhaust. Carbs rebuilt, cleaned and vacuum balanced. Valves adjusted, intake boots ok, o-rings are new, timing adjusted.

I had my idle tuned in pretty close and added a couple of spacers to my needle to get my midrange closer to where it should be so I could test out my high RPM range.
My high RPM range was very lean, so I changed from a 117.5 main to a 122.5 main and now the bike will not idle without the choke pulled out a bit.
I've tried playing with the air/fuel mix a bit in both directions but it just doesn't want to idle correctly and dies out when I try to give it gas or if I let the choke all the way in.
How does that main jet affect the idle so much? I thought they were different circuits? What should I change to get the idle back so that I can get the bike to operating temp so I can test my high RPMs again? Any advice is welcome.
Thanks!
Joe

[TrikerJoe]

Should this be in the Carb Forum?

[pete]

Hey Joe, on the CV carbs the mixture screws affect all circuits to a certain extent I believe, and I believe the mains may even do the same. I'm not a guru, just passing what I've learnt along the way and have seen with my own carbs.

As I have increased my mains I have had to lean out the mixture screws and also I've had to fiddle with my idle.

With a free flowing custom 2 into 1 pipe on my 450 plus K&N pods I have my needles as rich as they will go and have 137.5 mains in there. I think I need bigger mains but my needles feel pretty good. I still have the stock pilots.

At the moment, I'm still too rich with my mixture screws too and I'm still getting things dialled in.

[TrikerJoe]

With a free flowing custom 2 into 1 pipe on my 450 plus K&N pods I have my needles as rich as they will go and have 137.5 mains in there. I think I need bigger mains but my needles feel pretty good. I still have the stock pilots.
.

Pete, What size mains did you start with? Just wondering how much bigger i'll have to go. I started with 117.5 mains.
Thanks for the advice.

[pete]

The stock ones in the 450 are 117.5.

For me, I could really feel the difference when going richer. Too lean gave me hesitation and stumbling at low throttle openings and the transition from closed to open throttle was terrible.

Right now, I feel like I'm good up to 3/4 throttle but then I still feel I'm missing something at WOT which is why I need to try 142.5's.

[Steve]

You say that you have "tried playing with the air/fuel mix a bit in both directions", but don't mention what your mixture screw settings are.

Try starting with them about 3 full turns out from lightly-seated. The richer mixture will act a bit like a "choke" and help the bike run until it warms up, then you can tune them back to where they are happiest, which might be in the 2 to 2 1/2 turn range.

.

[TrikerJoe]

You say that you have "tried playing with the air/fuel mix a bit in both directions", but don't mention what your mixture screw settings are.

Try starting with them about 3 full turns out from lightly-seated. The richer mixture will act a bit like a "choke" and help the bike run until it warms up, then you can tune them back to where they are happiest, which might be in the 2 to 2 1/2 turn range.

.

The last little bit i've just been trying to get it to idle after changing the mains from 117.5 to 122.5. It was idling ok before that. I've varied the air/fuel from 2 to 3 1/2 turns out in 1/2 turn increments but it just won't idle unless i have the choke at least a little bit pulled out.

[TrikerJoe]

The stock ones in the 450 are 117.5.

For me, I could really feel the difference when going richer. Too lean gave me hesitation and stumbling at low throttle openings and the transition from closed to open throttle was terrible.

Right now, I feel like I'm good up to 3/4 throttle but then I still feel I'm missing something at WOT which is why I need to try 142.5's.

Oh wow, so I'm probably still WAY off on my mains... I don't have any jets that big right now, largest I ordered was the 125. So I'm guessing that i'm just wasting my time until I get the mains a lot closer??? Only problem is getting an engine up to operating temp to test the high RPMs when you can't get it to idle and it stalls every time I give it gas... it's really weird, the only thing i changed from the last time it idled well were the mains 2 sizes up... well that and playing with the idle screw to see if i could get it to idle...

[pete]

What I'm finding at the moment is that the richer I make the needles and mains, the richer it makes the idle circuit. Every time I fiddle with them, I need to go back and lean out the mixture screws and adjust the idle.

Koolaid Kid did an excellent write up on getting CV carbs mixture screws adjusted correctly too.

http://www.thegsresources.com/_forum...d.php?t=199096

The way I've done my tuning so far is purely by feel but once I get my mains where I feel they're right I'll go back and give this procedure a better go.

I started by making my needle leaner as I thought it was too rich, but that made it worse. I ended up with it as rich as it would go before I got rid of all the hesitation and stumbling at lower throttle openings.

I found it best to just take it for a ride and get a feel for how it runs. At WOT is where I'm still missing something, or at least that's how it feels to me at the moment. I haven't managed to get my hands on bigger mains yet to try.

When tuning CV carbs you're supposed to do mains first, then needles, then idle circuit. I thought my mains were right which is why I focused on the needles, but now I think they're not...

[TrikerJoe]

What I'm finding at the moment is that the richer I make the needles and mains, the richer it makes the idle circuit. Every time I fiddle with them, I need to go back and lean out the mixture screws and adjust the idle.

Koolaid Kid did an excellent write up on getting CV carbs mixture screws adjusted correctly too.

http://www.thegsresources.com/_forum/showthread.php?t=199096

Oh, I see, I've been going the wrong way with my mix screw. That and I probably need much larger mains...
I will have to order some new jets (I don't live near anything resembling a decent bike shop) and do some more tuning.
Thanks so much for the help, I'll be back on here i'm sure to ask more questions!!!
Cheers.

[pete]

No worries. The way I've been going with my mains is up two sizes at a time, otherwise you'll find yourself buying a lot of jets you won't end up using...

Don't forget at some point you'll need to do plug chops or get a dyno run with AFR readings to confirm what you find.

When I attempt to do the final mixture screw tuning, I have a Gunson Colortune that I'll use to help me out there.

When I first tried it I couldn't get a lean mixture on the idle circuit... but at least now I understand why!

[TrikerJoe]

Don't forget at some point you'll need to do plug chops or get a dyno run with AFR readings to confirm what you find.

When I attempt to do the final mixture screw tuning, I have a Gunson Colortune that I'll use to help me out there.

Pardon my ignorance, but what are AFR readings?
And may I ask what you paid for the Colortune? I am interested in one...

[pete]

AFR = Air/Fuel ratio... the mixture of air and fuel that is provided by the carbs for the engine to burn. I'm not going to tell you the optimal number though as I can never remember

For the Colortune, go here:

http://www.carbtune.com/

I have the Carbtune Pro as well and got both as a Christmas present... both are excellent products although I as I said I haven't had the opportunity to put the Colortune to good use just yet.

[TrikerJoe]

AFR = Air/Fuel ratio... the mixture of air and fuel that is provided by the carbs for the engine to burn. I'm not going to tell you the optimal number though as I can never remember

For the Colortune, go here:

http://www.carbtune.com/

I have the Carbtune Pro as well and got both as a Christmas present... both are excellent products although I as I said I haven't had the opportunity to put the Colortune to good use just yet.

Thanks again Pete.
I made some adjustments to my carb using your advice and direction and it is now running and idling again. I'm going to have to order some larger mains.
Did you have to swap your needle or needle jet? if so what size(s) are you running now? I am hoping to get away with just some needle spacers but am not sure how realistic that is?
Sorry to keep going with the questions, just trying to make this all work.

[Mekanix]

Would a stage 3 dynojet kit for a 450 not work for this ? Same carbs once your into the 1982 and up models. Only difference I know of is the size of the jets. This is the way I'm going when I get there.

Something you might want to look at is float level as well. It has a drastic effect on low end and idle. + or - 2mm can change allot.

Having a larger main does affect the idle. I've tried going with larger mains and really they were way out there in the 150 range. At idle they definitely caused the thing to puke black smoke without any throttle.

The idle circuit gets its feed from the main jet so it makes sense that one thing would affect the other.

Running lean at WOT might not only be the main.

The DJ kit has you drill out the hole in the bottom of the slides. This will affect how much the slides will move up and how fast they react.

Same Idea as when Pete cut his springs down. It allowed the slides to move up easier.

I'm wondering if you were just not getting WOT because the slides were not moving all the way up ?

Without the airbox acting as a big restriction, there's not too much suction there to pull the slides up. That's the only reason I think the DJ kit is good, It adresses this first and then you get to play which main jet is best.

I know for a fact that you can run WOT on this bike and have the slides only going so high up and stoping. If the engine RPM isn't high enough, the carbs won't let you give it more fuel. But because you also can't get the suction you need to drive the slides up, your running really lean and then it seems like its the main jet.