ATT: Four-stroke tuning gurus!

[t3mp0r4ry nzr]

1990 gsxr400
standard apart from minimal foam air filter system. straight through pipe.

question: how big or larger to go on the main jet approximately. Or what is general rule of thumb when doing these common mods in regards to how mush to step up the main jet?

dunno what the main is at the mo. will crack open and see on the weekend. point is, how much bigger to go?

or alternatively, has anyone jetted this model for these mods and care to share what main jet they went with?

anything to use as a starting point would be good

[geoffm]

What carbs has it got? 2 x twin throat (as per 1987 Impulse) or 4 individual Mikunis? The Impulse ran 118 mains with no airbox and open pipe. They have a horrible carb that you can't buy jets for. The later ones should use standard Mikuni jets.

[t3mp0r4ry nzr]

Hi Jeff,
runs mikuni's. according to the web they run 100's standard.

[TripleZee Dyno]

1990 gsxr400
standard apart from minimal foam air filter system. straight through pipe.

question: how big or larger to go on the main jet approximately. Or what is general rule of thumb when doing these common mods in regards to how mush to step up the main jet?

dunno what the main is at the mo. will crack open and see on the weekend. point is, how much bigger to go?

or alternatively, has anyone jetted this model for these mods and care to share what main jet they went with?

anything to use as a starting point would be good

Borrow a piece of private road, the correct jet is the one that makes the bike go fastest.
Same technique to get the best needle position
Provided you do the testing in a consistent manner this is as good a way as any to get to the right settings.

Or take it to someone with a dyno, bit quicker and safer but will cost a few $$
cheers

[t3mp0r4ry nzr]

for those interested, www.400greybike.com(??) suggest around 112-114 to start for K&N and race pipe. as my airbox is pretty open and I dont want to risk running lean, will stick some 115's in next week and go from there.

geoffm, not to far off mate!

[kiwi cowboy]

What carbs has it got? 2 x twin throat (as per 1987 Impulse) or 4 individual Mikunis? The Impulse ran 118 mains with no airbox and open pipe. They have a horrible carb that you can't buy jets for. The later ones should use standard Mikuni jets.

hey geoffm the 118mains you talk about where did you get them as you say you cant buy them?. i have a gsxr400 same motor as the impulse and think its a bit lean

[geoffm]

hey geoffm the 118mains you talk about where did you get them as you say you cant buy them?. i have a gsxr400 same motor as the impulse and think its a bit lean

Daytona used to do a jet kit for them. They haven't been available for years - mine came from SME when they were in Marua Road - that will date it.
Geoff

[geoffm]

hey geoffm the 118mains you talk about where did you get them as you say you cant buy them?. i have a gsxr400 same motor as the impulse and think its a bit lean

Daytona used to do a jet kit for them. They haven't been available for years - mine came from SME when they were in Marua Road - that will date it.
What have you done - taking the air box off (or air filter out ) made a huge difference. Changing the muffler you might get away with raising the needles.
Geoff

[kiwi cowboy]

Daytona used to do a jet kit for them. They haven't been available for years - mine came from SME when they were in Marua Road - that will date it.
What have you done - taking the air box off (or air filter out ) made a huge difference. Changing the muffler you might get away with raising the needles.
Geoff

thanks for the reply mate and in answer
I still have the air box on and foam in it [a bit thinner than the original but close.
i have altered the muffler a bit too.
how do i raise the needles in these? have i got to put washers in or have the needles got a clip and nicks in the top of the needle? as i havent had the needles out before [have cleaned the rest though.
And one more question-the mains in the outside cylinders i notice are stamped 98 and the middle two are stamped 94 any ideas why anyone?

[t3mp0r4ry nzr]

mains? as in main jets?

for your jetting questions, if you need to go up main jet size, maybe look at drilling out your existing jets, if need be.

I ended up putting in 117.5's and lowered the needles. its lean down low, so need larger pilots.

[nadroj]

http://www.kiwibiker.co.nz/forums/sh...ad.php?t=25560

[kiwi cowboy]

mains? as in main jets?

for your jetting questions, if you need to go up main jet size, maybe look at drilling out your existing jets, if need be.

I ended up putting in 117.5's and lowered the needles. its lean down low, so need larger pilots.

so what size bit do you use to get a 117.5 main?

[t3mp0r4ry nzr]

so what size bit do you use to get a 117.5 main?

sorry for late reply.

I brought new main jets. 117.5 size. they where too big. so dilled out the standard 100's with a 1.07mm bit. so approx. a 107 main.

also drilled out the 47.5 pilot to 57 with a 0.57mm bit

[kiwi cowboy]

sorry for late reply.

I brought new main jets. 117.5 size. they where too big. so dilled out the standard 100's with a 1.07mm bit. so approx. a 107 main.

also drilled out the 47.5 pilot to 57 with a 0.57mm bit

hey mate were did you get the bits from or did you borrow some as i would like to try drilling out mine a bit more but cant find bits that go up in that smal increments.

[Ocean1]

hey mate were did you get the bits from or did you borrow some as i would like to try drilling out mine a bit more but cant find bits that go up in that smal increments.

Try number drills:

http://shopswarf.orconhosting.net.nz/drills.html

You need an engineering supply shop, as opposed to some of the toy shops you see around lately. Between number drills, letter drills, imperial and metric nominals there ain't too many holes in the range.