GS400 tuning issues

[TrikerJoe]

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.

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.

Mekanix,

The drilling of the slide is something I hadn't stumbled across yet, thanks. I checked out the DJ kit and they use a 5/32 drill bit to drill out the vacuum slides but apparently they have an insert that they put in there. Do you have any idea what size that hole ends up being? i'm envisioning the larger hole increasing the vacuum pressure above the diaphragms, drawing them up faster and easier? I'm not sure why an insert is required, couldn't the hole size just be increased? I'm thinking I could try drilling it out, see if I can attain WOT and then work with spring length/float height/needle position to work out my midrange?

I should also note that all my testing is being done on the center stand as it is snowing ATM and riding season is still a month or so away here.

[Guest]

If your carbs are the same as mine, good luck finding a DJ stage 3 kit...

Even though it's a great idea, there's only one I've ever seen which is the E3305 kit and appears to be in Europe only, not the US.

I checked with my local reseller of Dynojet stuff and they'll want about $AU180 for the kit and I'd need to wait 6 to 8 weeks so that's waaaaaaay too expensive for this little black duck.

If you do have the same carbs as the '83 and onwards 450's, then there is a jet kit available for that but I don't recall if it's a stage 1 or 3 kit...

The guy I've been talking to on the Kat forum about this stuff has said a 3.5mm hole for the slides, but I haven't done any further research on it. He was the only who told me about the slide springs but I did some more Googling before going down that path.

There's no mention of an insert or anything either, not sure what that's about.

I'm definitely considering drilling the vacuum hole now as I've been bitten by the bug... the bike is soooooo much more responsive and keen to go since the needle and slide spring changes.

[TrikerJoe]

The guy I've been talking to on the Kat forum about this stuff has said a 3.5mm hole for the slides, but I haven't done any further research on it. He was the only who told me about the slide springs but I did some more Googling before going down that path.

I'm definitely considering drilling the vacuum hole now as I've been bitten by the bug... the bike is soooooo much more responsive and keen to go since the needle and slide spring changes.

I saw the part in your build thread about the spring changes (very cool), but I must admit, I haven't gotten through the whole build. It's pretty daunting for a slow reader like myself. What did you end up with for needle settings? Do you think I might have to change my needle with the changes I've done with mine? That way I can have an idea of what parts I need to order so I can start tuning it once the roads are cleared off here.

I think the bulk of my tuning may have to wait a month or so in order to get some real world tests going on under load. In the meantime, I can concentrate on getting it running a bit better on the stand (possibly drilling it to get the mid-range closer) and road-legal by then. I still have a lot to sort out by spring. My main goal is a long road trip in early August.

[Guest]

Hard to tell anything while its on the stand. But it should rev to the moon easily.

I've been finding allot of info on the 500's upping to 140-150 mains and 40 pilot and then shimming the needle up 4mm. That alone is what they claimed worked best without using a kit.

The carbs are a little different than yours Pete. Its just in the bowls and how they designed in there.

[Guest]

Yup, saw the different carbs over in the projects thread and realised it's a GSX... not quite the same

[TrikerJoe]

Hard to tell anything while its on the stand. But it should rev to the mood easily.

I've been finding allot of info on the 500's upping to 140-150 mains and 40 pilot and then shimming the needle up 4mm. That alone is what they claimed worked best without using a kit.

The carbs are a little different than yours Pete. Its just in the bowls and how they designed in there.

Thanks very much for the numbers. I've ordered every second size main up to 150 so that I can tune it up. Hopefully I'll be able to get the tune a bit closer on the main stand until I can get it out on the road for proper tuning.

Until then, should I start a new thread on drilling the vacuum slides to find out what size hole I need? I'm still getting used to the etiquette side of the forums...

Thanks!!!

[Guest]

Etiquette here is pretty forgiving. And its still in the realm of performance/tuning anyways.

First thing is to get the main right for the best top end pull at wot and not running lean. Then you can play with the rest of the ranges.

[TrikerJoe]

I've been able to get out on the road for some test runs!
Here is where I am at now.

140 main jet
Jet needle at position 5
Stock pilot jet
1.5 turns out on the AF screw
Vacuum slide hole drilled to 3.14mm from 2.82mm
Vacuum slide springs swapped from 111/112mm to 107/108mm springs from a spare set of gs400TX carbs.

After drilling the vacuum slide hole and putting in the shorter springs, I am able to get through my midrange to WOT to do some drag style plug chops and those chops are coming up with white plugs, so I am guessing I need to go to a larger main jet?

That is what I will do next and report back.
Thanks for your interest!

[Guest]

White plugs is definitely lean, so keep going

When you say needle at position 5, is that the position furthest or closest to the tip?

If that's the furthest position, then you likely have it far too lean as it will lower the needle.

[TrikerJoe]

White plugs is definitely lean, so keep going

When you say needle at position 5, is that the position furthest or closest to the tip?

If that's the furthest position, then you likely have it far too lean as it will lower the needle.

Position 5 in this case is the one closest to the tip, furthest from the end and closest to the middle if that makes it more clear! The setting that pulls the jet needle the furthest out of the needle jet.

[Guest]

Ok cool, that's the richest setting... just thought I'd best make sure

Oh, and one thing GregT mentioned earlier in the other thread about a flat spot in throttle response after drilling...

I'm starting to see what he's saying now, and yes I guess that's probably what I have now.

I can just crack the throttle up to say 1/4 and get off the line like I used to pretty much.

However, if I crack the throttle a bit more than that, probably 1/2 or more... it's a world of difference and she hauls in comparison.

So yes, I think the drilled vacuum holes does give a bit of a flat spot in the throttle range... but it's not a bad thing at all

[TrikerJoe]

Ok, here we go again!

So I opened up my carbs once more to swap out to my 150 mains and decided, just for kicks that I would double check my float height.
Prepare to laugh at me... the float heights were around 25.5 mm (spec is 22.4).
I must have measured from the wrong spot last time as I hadn't done as much reading on the subject as I have now, and this has been corrected to 22.3mm on each float, measured from the gasket mating surface rather than the lip around that mating surface.
The difference is, as I'm sure you will all already know, quite astounding. I can now run around at different throttle settings and everything feels much much better.
It's a little cold out today (-6C this morning), already went for one test ride this evening. Seems to be some color in the plugs but I will have to do some more plug chops to be sure. WOT is feeling pretty darn good!!!
I'll let you all know when I find out more!

[Guest]

Excellent! Pays to double check these things, good find!

[GregT]

Ok cool, that's the richest setting... just thought I'd best make sure

Oh, and one thing GregT mentioned earlier in the other thread about a flat spot in throttle response after drilling...

I'm starting to see what he's saying now, and yes I guess that's probably what I have now.

I can just crack the throttle up to say 1/4 and get off the line like I used to pretty much.

However, if I crack the throttle a bit more than that, probably 1/2 or more... it's a world of difference and she hauls in comparison.

So yes, I think the drilled vacuum holes does give a bit of a flat spot in the throttle range... but it's not a bad thing at all

Wrong way round - a flat spot is a LACK of response - without drilling you had a flat spot where the vacuum wasn't enough to lift the slides and airflow stalled. Drilled, the system works as intended and slide lift matches the engines needs.

[Guest]

Wrong way round - a flat spot is a LACK of response - without drilling you had a flat spot where the vacuum wasn't enough to lift the slides and airflow stalled. Drilled, the system works as intended and slide lift matches the engines needs.

Actually Greg, I'd meant to come back to this and say I think I'm mistaking response at low rev's with a flat spot

After doing the drilling I just kept opening the throttle because it was so fun... but of course anything below about 5K is just not the same as above say 5.5K... therefore my misinterpreted flat spot...