A few things to check before you decide to use a quad motorbike carb setup.
Probably the most important thing to decide is what size and type carbs you are wanting to use.
1. CV or NON-CV
I chose the CV (Constant Velocity) style with flat slide because they are the most forgiving and cheaper than the non-CV carbs.
I can put my foot flat to the floor any time and the engine won't bog down.
Basically they are an advanced SU carb.
The non-CV carbs will give you better throttle response "IF" you know how to operate then correctly.
They can bog down if you give it too much throttle but, if you know what you are doing, they are the better choice.
The problem is the price. On average, they are between 2 to 4 times the price of the CV style.
2. Carby size.
Considering the standard carby for the 2.6L have a 30mm primary and 32mm secondary, quad 25mm carbies would be be a 25% increase in size, which would also be the same size as a 34-36 weber.
I initially chose 40mm angled inlet carbs, but, because I wasn't confident enough to make an angled manifold myself, I ended up going with the 38mm straight port carbs.
For a daily drive with a relatively standard to a mildly worked engine, quad 34mm to 40mm will work great and I would rate them to be better than a twin 40mm side-draft setup.
3. Room
Another thing to consider in your carbie choice is the room in the engine bay. If you don't have much room between the head and the strut, an angled intake may be helpful, so you can fit the air cleaners on.
The Manifold
I start with a standard manifold, cut the pipes off and then weld on some straight tube, spacing the ends the same as the carbies.
That way, there is no carby modification required.
Not pretty, but functional.
Do NOT use standard silicon tubing to join the carbs to the manifold, the petrol will wreck it very quickly.
The tubing I use is internally coated and fuel safe.
Just search EBAY, there are plenty of people selling fuel safe silicon tubing.
Basic CV carb setup
You will need a fuel pressure regulator that can regulate down to 0.5psi. These carbs are gravity fed on the motorbike so pushing anything more tha 1.5psi isn't a good thing.
I modified a standard holley carb fuel pressure regulator by compressing the internal spring a bit. This allowed me to be able to regulate the fuel pressure down to 0.25psi.
Based on my current setup of the quad 38mm Mikuni's off a 1995 Suzuki GSX-R750:
1. You shouldn't need to change either of the Idle jets (fuel and air blead). The idle AFR I'm currently running is around 17:1, without any issues.
2. Standard float height.
3. Main Fuel jet size 1.55mm.
4. Needle position will be determined by the wear on the "needle Jet", try the middle position first.
The only other thing to I'd recommend is to attach a cable for the choke. I only use it to start the engine on a cold morning. For the rest of the day, it isn't needed.
Because CV carbs don't have an acceleration fuel pump, pumping the accelerator when trying to start the car does nothing.
I'll post a few more pics in a few days. Just thought I'd better put a bit of blurb down while I'm in the mood.
Cheers.