915 superbike on the cheap

[Brodie]

Very nice work on the fairing, I am not usually a fan but that blends in quite well.

 

[rockcitycafe]

took it out for a spin, had to fix the o ring on the oil tank, and the battery went dead, will keep debugging as I go. otherwise pretty fun

 

[Maritime]

Nice work Sean. That will be one hell of a fun scoot when the 915 goes into it. Love the tank, seat and Fairing.

 

[rockcitycafe]

some pics of putting together a sandbent 4-1 for it

 

[crazypj]

Normally you'll have the 'outer' pipes 'on top' and 'inner's' underneath, that way the lengths are closer to equal

 

[rockcitycafe]

Yup, noticed that, funny how you get punchy after a few minutes running a 1" rosebud full blast indoors... If it was for a customer, I'd be getting more tube, but since its mine, I'm just going to chop them up a bit, shouldn't look bad

 

[rockcitycafe]

Been riding the bitch, she hasn't killed me yet, getting used to the different feel of the bike, still turns really quick and is stable, just feels a hell of a lot different than my stock 750

 

[Rich Ard]

Do you have a curb weight figured out? I'm curious to see what it tips the scales at.

 

[rockcitycafe]

not sure, I'll get a couple bathroom scales sometime, it's pretty much a stock frame, but my guess is the front and rear swap took at least 15lbs off, the tank and seat maybe another 10, I'm thinking of doing the rear portion of the frame in aluminum so it supports the shocks and bolts to the main steel frame, that might shave 4-5

 

[canyoncarver]

Get a few nice outside pics in the light. I'd love to see more of this bike.

 

[rockcitycafe]

Got these installed the other day, now for a few days of tuning

 

[crazypj]

GSXR carbs?
I'll see if I can get William to drop his over and see what specs are (on CB750)

 

[rockcitycafe]

yeah, katana 750 '91, specs aren't going to be close though, the 4-1 pipe I bent for it made the stock carbs run stupid lean (still lean at 120 mains), so I'm going to get some mik jets and do some more testing

 

[crazypj]

GSX750F, I have a lot of specs on Katana's, the SACS motor is probably my favourite engine ever
I call it the small block Chevy of the motorcycle world, it was in production in one form or another from 1985 until 2007
One 'good' thing about it, the bore spacing and main bearing journals are the same from 600 to 1127cc
I'm almost certain the 1172 and 1376 are just bigger crankpins with longer stroke
In theory, you could, with very little modification have a 1300cc 145bhp Katana '600' (yay for can-o tuna ;D )
I'll probably build an 1145 when I'm done with 360's and XS650's

 

[rockcitycafe]

so finally got the carbs on, flipped the boots from left to right and upside down so the float bowls are level, seems to run okay, but rich cruising and extremely lean at idle and decel, 32.5 pilot, 127.5 main... looked at the idle circuit and it looks like it goes (air jet - pilot - transition holes - idle screw) with the main feeding the pilot. so wondering if I should be looking at a smaller air jet, or larger pilot? it seems like going to a larger pilot would make the rich cruising even worse?

 

[crazypj]

127.5 should be very close, 32.5 also.
May need slightly bigger pilot jets (35~37.5 maybe even 40~42.5) and needles with different taper ('fatter' at cruise position to lean off)
Have you had vacuum gauges on it?
Could be the 915 and stock cam is pulling much higher vacuum?
Normally, you'll have between 12~28cm/Hg somewhere 15~20cm/Hg is 'average'
If it's higher, you should be drawing more fuel so pilot will be too small

 

[rockcitycafe]

the 915 is still on the bench, it's stock F motor in the bike to limit the number of things I'm fighting at once, I want to get the geometry and suspension right before I put something scary in it. I'll take a look at pilots and needles

 

[rockcitycafe]

so I've been tuning and run into a bit of a wall. I've been able to get the idle good and stable, but coming right off idle on the throttle winds up being too rich, cruising is too rich, and wot is pretty good. I kept bringing the pilot down and now the idle is way too lean, and unstable, coming off idle on the throttle is better, but still a bit rich, cruising is still too rich and wot is good... I don't really want to have to change the needles, but am I pretty much at that point? They're dropped as low as they go.

 

[Sonreir]

What size needle jet are you running and what's the throttle cutaway size?

 

[crazypj]

I wouldn't mess with it too much as you may have to do it all over with the 915 motor?