GSXR 750 carb tuning - any input?

So my GSXR750 quad that I've been building is pretty much done just in time for our annual trip to the Idaho St Anthony dunes. Having some jetting issues. GSXR was an 1989 that was complete but rough. Never really did ride it, just ran it in my garage. Got it cheap. So now it runs pig rich below about 4k rpm. Above that it clears out and pulls pretty well. Plugs are black and sooty, and actually fouled one of them. Took the mains out to inspect and it looks like they are Dynojet 142 and the needles look like dynojet as well. But that jet size corresponds to a Mikuni 132.5. Stock was 112.5. And I'm at 3500' elevation. And it's stock other than some pod filters. Yeah that could be why it's rich. Ordered a few different sizes of main jets, looks like I'll be doing some tuning instead of just duning.

Taking my KX250 along too, as at least it's sorted.

Has anybody on here wrenched on one of the older oil cooled gixxers? Any reason why the PO would have had such huge jets in it? He was sort of clueless in general, so maybe figured more gas = MORE POWAH!!

check out maximum-suzuki.com. lots of info on the Bandit/katanas (same engine) with lots of tuning tips

doc_speeder wrote: He was sort of clueless in general, so maybe figured more gas = MORE POWAH!!

Maybe, the guy probably had a brodozer to roll coal with.

What throttle setting does it run good at not rpm but % of throttle? Also are theses diaphragm stock carbs? If so check your pod filters your likely closing off the vent side of the carb body. pull off one pod at the top of the carb mouth you'll see a slot shaped like a bean. if this gets blocked in any way the slide can do strange things.

In reply to 44Dwarf:

The crappy running is really more rev related than throttle related. At very low throttle % it is a little bit better but still poor.

These are the stock diaphragm carbs but I'm not 100% sure that they're really original to this bike or maybe they were pulled off an 1100? That is one theory I have because the jetting is close to stock for an 1100 of the same year. They did both have the 36mm Mikuni CV carbs if my research is correct.

I haven't ridden it enough to diagnose. I bought 3 different sets of mains to swap around this weekend. I'll ride it first with the jets that are in it to see how it pulls up top. If it's really clean and strong from like 7k - redline then I'll look elsewhere. Needles are already all the way down but if it's got 1100 emulsion tubes and needles in it I'll need to swap those out too.

Good point about the slide vents. I'm running Uni filters. The flange seems plenty deep to not be blocking these off so I think that's ok.

The motor runs really nice with no load on it. Just acts up under load. I'll be able to tell more when I get some more seat time on it.

Being that it's pretty much a dune only quad, I can likely just avoid the bad spots for the most part until I get it dialed in. Not like I'm creeping along in traffic. I'll just rev n roost.

Since it's rough down In the rpm range, check the pilot jets for size, cleanliness, etc.

Those typically affect the first 1/4 to 1/3 of throttle.

Assuming some sort of slide carb. Try dropping the needle height a click. Also check float height carefully. Did the swap clock the motor any?

Kenny has a valid point if the motor is angled say down at the front( as if it where in the bike) it may throw off the float height adjustments. also are you using a fuel pump? if so you need to regulate it down to 2-3 psi of change to 2.5 or 2.0 float needle and seats. float needle and seats come on 3.0 for gravity feed all the way down to 1.5 for fuel pump feed but 1.5 generally were only on small bore not high rev as they don't flow much.