Mikuni BSR 36 + K&N Ru2922 on short stroke 750.

[Losiu]

The question I have is ... Where to start? There is a motorcycle powered by a short stroke 750, it is powered by Mikuni bsr36, pilot jets 15, main 117.5, needles in the middle position. At the very beginning, I used original airbox tubes with a sponge and a net as filters. It worked, let's say poorly, and it looked even worse I decided to buy k&n ru2922. I poured them with nicely dedicated oil, with considerable effort I pressed into place (water frame), I made synchronization and composition of the mixture. Time for testing.... And then there's the drama. That's not going to go. There is no power, the engine does not reach revolutions above 5k. It clearly lacks fuel (too little vacuum?). What to change to get started? Needles up? What about jet? Pilot? Main? In the pictures we have a patient

[Buzuki]

Long time ago I have the same problem with my 750 short stroke when I installed K&N RU2922 racing air filters , by changing standard #112,5 main jets with bigger #135 solved the problem , #37,5 pilot jets remained the same , all this on the standard Mikuni BST36SS carbs ,

btw , be very careful with that dedicated original K&N filter oil ,  that oil in contact with any rubber parts will have very negative effect causing rubber hardening .

[Losiu]

Thanks for the hint, I have where to start. I also thank you for the oil warning

[Captain Chaos]

The RU2922's i bought new, were pre-oiled from the factory. 

[Losiu]

My ones seemed too "dry" to me

[Oilyspanner]

If it idles and does gentle cruise the pilot is okayish, but likely needs more fuel.  5000 revs is needle area and it's the easiest to adjust - try 4th and 5th position and see if this helps. It's likely you need a different needle profile to get it spot on. Buzuki will be right with the m.jet size, you always have to up the size many sizes when you ditch the airbox with CV carbs. Main jets only start working when the motor is really spinning, there isn't enough of a pressure drop to lift the slides until high revs are reached - regardless of where the throttle is.

The joy of carbs !

[Gixer1460]

15 hours ago, Captain Chaos said:

The RU2922's i bought new, were pre-oiled from the factory. 

This  . . . 

Is about right out of the packaging and correct for use. Pouring more oil on them is couter productive, less air flow and richer running! I've run them virtually black with no difference in air flow or AFR on a wideband. K&N oil is good to use on foam filters also!

[Losiu]

9 hours ago, Oilyspanner said:

If it idles and does gentle cruise the pilot is okayish, but likely needs more fuel.  5000 revs is needle area and it's the easiest to adjust - try 4th and 5th position and see if this helps. It's likely you need a different needle profile to get it spot on. Buzuki will be right with the m.jet size, you always have to up the size many sizes when you ditch the airbox with CV carbs. Main jets only start working when the motor is really spinning, there isn't enough of a pressure drop to lift the slides until high revs are reached - regardless of where the throttle is.

The joy of carbs !

Tomorrow morning, when I come back from work and take my child to kindergarten, instead of getting enough sleep, I will go to work in the garage

[Losiu]

6 hours ago, Gixer1460 said:

This  . . . 

Is about right out of the packaging and correct for use. Pouring more oil on them is couter productive, less air flow and richer running! I've run them virtually black with no difference in air flow or AFR on a wideband. K&N oil is good to use on foam filters also!

Brand new. They really seemed dry to me, but maybe that's all I thought. However, the test firing before splashing with oil was, even in the garage did not want to freely reach high revs, behaved / behaves like a motorcycle when the fuel runs out and you have to switch to reserve

[Losiu]

I quickly picked up the needles for the last groove and put on the main jet 120. It's a little better, it suffocates at 6000/min On Friday pilot jets 30 and main jets 135 should come. However, we will wait for the effect, because we have The Gustav Havel's 300 Curves this weekend and I intend to drink a Czech beer there in the city 

[Gixer1460]

That rpm is bang on Needle territory and if you are max rich its going to need different profile - hence why DJ kits came into existance!

[Losiu]

As for needles, have you tried using those from bst 36 in bsr 36?

They seem to have such a more DJ profile;)

I admit that I compared them physically (those with bst and with bsr),

so I do not even know how with the length and diameter.

[Losiu]

I'll tell you how things are. Pilot jet 30, main jet 135, needle on top. To be honest, with the short stroke 750 coming from teapot, I don't expect more.

PS: I physically compared bst and bsr needles. I don't see the possibility of swapping

[Losiu]

Small update.
I came home from work in the morning and planned to trim the hedge, but it rained until noon, so I decided to tune the carburetors. At the very top of the revs, it seemed to be a bit low on fuel. I put on 140 main jets and left the rest as last time. Test drive - yes, now it's very cool