kz 650, garrett turbo, need jetting help

I have a 1978 kz 650, it has the original smoothbore mikuni carbs on it and I have installed a garrett turbo on it. 2 questions, what size were those stock mikunis? I'm having problems getting the jetting right, it starts boosting around 5,000 and is boosting hard by 7,000. Blw through setup. Once good boost is hitting it it falls flat on it's face and usually dies. Any thoughts?

trx250r wrote:

I have a 1978 kz 650, it has the original smoothbore mikuni carbs on it and I have installed a garrett turbo on it. 2 questions, what size were those stock mikunis? I'm having problems getting the jetting right, it starts boosting around 5,000 and is boosting hard by 7,000. Blw through setup. Once good boost is hitting it it falls flat on it's face and usually dies. Any thoughts?

vm24
not smoothbores

are your floatbowls pressurized?

Any pics of the install?

The vm24ss on the 650s came with either 102.5 mains or 97.5 mains both with 15 pilots depending upon year and if pumper or not.

Well see I'm learning stuff already. I'd say they are, I just built a manifold that sits where the air box used to be, the turbo boosts directly into that manifold so I'd assume they are being pressurized, unless I'm missing something or am missunderstanding what your asking, which is entirely possible. This is my first attempt at a turbo on a bike. I'm used to working with turbos but normally on 7000 plus cubic inch v-12 engines with 10 inch pistons running on natural gas, so a little out of my rhelm here.

Yes,24mm.
I've allways wanted a turbo on mine.
My roomate had an allmost new one off a GS1000 dragbike.He sold the ENTIRE,complete bike for $800.I kid you not.
He did this while i was at work.He never mentioned selling it so cheap.The only thing wrong with the bike...a blown head gasket.

Jetting was hard enough with a pipe and pods,i wouldn't know where to start with a turbo.:blink:

Let us know how she runs.

they had the 102.5 mains in them. Pumper? As in they made these carbs with accelerator pumps? If so, not a pumper that I can tell.

Kind of a crude install but I wanted it to really hang out there and demand attention. Also I did the whole thing on the cheap, I've got less than 200 bucks in the whole system, including the turbo! This is the only pic I have at this time, I'll try to get some more and of better quality soon.

Yes some of them did have accelerator pumps.

Interesting, but no mine doesn't seem to have them. For a starting point and because I had an extra set of 102.5 mains laying around we started by drilling them out with a 1/16 drill bit, I thought this would be entirely to rich, but oddly enough it didn't seem to be and the bike actually seemed to still run just fine with them drilled that large and the turbo disconnected. Any ideas where to go from here? Are there any carb mods I need to make other than re jetting? Is a blow through set up even possible with these carbs? Any info would be greatly appreciated.

Wow, dude you've got a set of balls to just reach out and try this. I salute you.

In a normally aspirated motor the fuel flows from the float bowl through the mains, the needle jet where it mixes with air from the air bleeds, and up into the venturi. It does this not because of a vacuum in the venturi but because the pressure in the venturi is lower then the pressure in the float bowl which is vented to the outside world.

Float bowl pressure = Atmospheric pressure < venturi pressure.

Carbs 101 I know. But I told you that to tell you this. If you start adding boost to the venturi (blow through style) then suddenly there is more pressure in the venturi then in the float bowls which would make the fuel flow in the wrong direction.

I could be wrong though. I've never turbocharged anything so what do I know. It just seems to make sense to me.

On a side note. Did you warm the motor all the way up with the drilled out jets?? Did you go full throttle and take it for a decent rundown?? What size turbo is that??

Ive got 3 Yamaha XJ650 Turbo’s (2 in pieces, 1 mint), got a set of carbs set aside to put on a project I have…

Heres an extract from the Yamaha service manual explaining the necessary type of carburettors needed and why:

Pressure-sealed Carburettors:
The carburettor is a fully enclosed type BS30; float chamber ventilation is opened to the surge tank, because the float chamber must always be under boost pressure to supply sufficient fuel to the venturi.
These carburettors have been specifically developed for use with a turbocharger. As they are subjected to high intake pressure created by the turbocharger, they too must be pressurised to work effectively. To achieve this, the throttle valve and starter plunger have o-rings to prevent pressure leaks, and to ensure they do not leek, special passages deliver pressurised air to complete the seal. Air will leak from the throttle shaft and stater o-rings, but this is normal. Should however, fuel begin to leak from these passages, the carburettor must be serviced immediately…..