View Full Version : ATT: Four-stroke tuning gurus!
t3mp0r4ry nzr
1st April 2009, 07:45
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
1st April 2009, 20:57
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
2nd April 2009, 06:50
Hi Jeff,
runs mikuni's. according to the web they run 100's standard.
TripleZee Dyno
2nd April 2009, 07:14
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
9th April 2009, 22:03
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
20th April 2009, 19:29
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
20th April 2009, 21:15
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
20th April 2009, 21:15
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
20th April 2009, 22:35
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
21st April 2009, 09:14
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
21st April 2009, 13:15
http://www.kiwibiker.co.nz/forums/showthread.php?t=25560
kiwi cowboy
21st April 2009, 20:20
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
25th June 2009, 20:54
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
1st July 2009, 20:00
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
1st July 2009, 20:26
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.
t3mp0r4ry nzr
2nd July 2009, 08:30
Ocean1: thats gold!
cowboy: a talk to the local engineering supplies store (bay engineers), and I they got them in for me. about $3 each.
it goes without saying that you should take the jets out a little at a time. do all your homework, and then some more, wait, have a think about it, then do it.
kiwi cowboy
2nd July 2009, 20:57
Ocean1: thats gold!
cowboy: a talk to the local engineering supplies store (bay engineers), and I they got them in for me. about $3 each.
it goes without saying that you should take the jets out a little at a time. do all your homework, and then some more, wait, have a think about it, then do it.
finaly found some bits small enough as i couldnt find any between 1mm and 1.05mm but got put onto a company in dunners that said they didnt have any metric bits between that ether but then i mentioned the number system and bingo they had number 60#[1.016mm] and 59#[1.041mm] so they will do for a start and yes will think twice cut once cheers
Ocean1
2nd July 2009, 22:20
finaly found some bits small enough
Word of advice, if you haven't got a baby air drill you should buy a pin chuck.
It's way easy to break drills that size, especially whan you're loading up just the outer tips like this. Takes a few minutes by hand, but you've got far better feel, much safer.
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