Motorcycle Mikuni carbs on 3 short intake pipes

[Racer-X-]

I've done independent carbs before on different engines.  One carb per cylinder.  That works, although it can get a little weird with tuning because of the pulsed nature of the draw through the carbs.  You generally need a carb that seems bigger than optimal because your CFM all happens in pulses that are only 25% of the time.  I've actually made the intake pipe set and done all the fabrication and set up on this for some 4 cylinder engines.

 

If you want to do 3 carbs on flat head 6 cylinder, you're either going to need to have a common plenum design, or you're going to have to get the carbs to be on opposite cylinders in the firing order.  Cylinders 1 and 6 would share a single carb.  Cylinders 2 and 5 would share a carb, and cylinders 3 and 4 would share a carb on this particular engine.  It can be done, and if you're careful (and really OCD), you can even set it up with equal length pipes to the intake ports. Usually for this, the carbs end up side by side, with the one for the end cylinders closest to the engine, and the one for the two center cylinders furthest from the engines.   I haven't done anything with a 6 cylinder and this type of setup, but I have worked with this setup with another different 4 cylinder engine using two single carbs.   You still have some pulsed vacuum, and your carbs still need to be bigger than the raw numbers would indicate, because the carbs are flowing only 50% of the time.

Edited November 20, 2021 by Racer-X-

[Sniper]

Problem is that our flathead sixes run a common intake port.  1 and 2, 3 and 4, 5 and 6 share the same port in the block.

 

I have a Thickstun dual carb intake that I have fitted up dual Weber 42mm EFI throttle bodies.  But is has a common plenum design.

 

http://www.yourolddad.com/tbi-adapters

[RCrombie]

5 hours ago, Racer-X- said:

I've done independent carbs before on different engines.  One carb per cylinder.  That works, although it can get a little weird with tuning because of the pulsed nature of the draw through the carbs.  You generally need a carb that seems bigger than optimal because your CFM all happens in pulses that are only 25% of the time.  I've actually made the intake pipe set and done all the fabrication and set up on this for some 4 cylinder engines.

 

If you want to do 3 carbs on flat head 6 cylinder, you're either going to need to have a common plenum design, or you're going to have to get the carbs to be on opposite cylinders in the firing order.  Cylinders 1 and 6 would share a single carb.  Cylinders 2 and 5 would share a carb, and cylinders 3 and 4 would share a carb on this particular engine.  It can be done, and if you're careful (and really OCD), you can even set it up with equal length pipes to the intake ports. Usually for this, the carbs end up side by side, with the one for the end cylinders closest to the engine, and the one for the two center cylinders furthest from the engines.   I haven't done anything with a 6 cylinder and this type of setup, but I have worked with this setup with another different 4 cylinder engine using two single carbs.   You still have some pulsed vacuum, and your carbs still need to be bigger than the raw numbers would indicate, because the carbs are flowing only 50% of the time.

 

Triple Carbs are very common on straight sixes, such as those found on Datsun L series motors. No problems with one throat per cylinder, although you have to make sure that each carb is balanced first, and that the opening of the throttles are synchronized. Otherwise you get poor off idle performance and a shaky motor in low rpms. 

For the 2.4 litre six cylinders, carb throat sizes in the range of 40-45mm were used depending on how how the motor was built. 

I built several engines running DCOE webers, and they work well, but were rather finicky. They didn’t play so nicely with modern fuels, and requires a systematic approach to tuning them as there are idle jets, main jets, emulsion tubes and air correctors that all have a theoretical impact somewhere in the power band. They would require a balance tube for things like the vacuum advance, but otherwise it wasn’t necessary for a smooth running engine.

On the Dodge sixes with the shared intake ports, three single barrel side drafts like Moose has would work well. Maybe improvements could be made on the intake manifold to shorten the length of the runners, depending on available space.

I don’t know if it has been mentioned yet, but six cylinders sound GREAT with individual carbs. Adds at least 20hp to the butt dyno.

Anyway, I spent a lot of time tuning triple carbs and so I can ramble about them enough to bore most people...