- Joined
- Apr 22, 2020
- Messages
- 3,157
I have not played with modifying intake manifolds on a Commando, so as far as I know what I am babbling about in this post only applies to the motor mounted straight up in the frame.
Cutting custom length boots out of fuel filler hose for connecting spigot carburetors to a spigot intakes has the potential to present cold start problems. It sure did for me. I don't know about the rest of the world though. The completely flat surface in the I.D. of the fuel filler hose boot creates a dropped gap, and in the case where one is going from a larger carburetor to a smaller intake, a gap and a bump for fuel and air to get past. I'm of the opinion that startup fuel pools in that gap and has a hard time getting to the cylinders until the gap is filled with fuel. Even true with an accelerator pump carburetors like the Keihin FCRs. I don't think the gap is particularly good for smooth low speed performance either.
I had trouble cold starting my motor with a longer intake boots that created a 5/16" gap between the intake manifold and carburetor. It took too much kicking and too much fuel to get fuel into the cylinders. I closed that gap up, and obviously things got better with the improved flow path. Ideally, the use of a Mikuni style boot designed to fill that gap works better. I did not use the Mikuni solution, because I was planning to use longer boots initially. I did consider cutting some tubing to fill the gap, but didn't have any tubing that was the right thickness. In the end, the shorter manifold boots gave me more room to get the air filters on and off. Anyway, below is a sketch showing what I'm talking about.
I did radius the mouth of the smaller diameter intake manifolds out to 35mm to sort of match up with the FCR spigots. The radius is steeper than I'd like, but it is better than a square edge.
Cutting custom length boots out of fuel filler hose for connecting spigot carburetors to a spigot intakes has the potential to present cold start problems. It sure did for me. I don't know about the rest of the world though. The completely flat surface in the I.D. of the fuel filler hose boot creates a dropped gap, and in the case where one is going from a larger carburetor to a smaller intake, a gap and a bump for fuel and air to get past. I'm of the opinion that startup fuel pools in that gap and has a hard time getting to the cylinders until the gap is filled with fuel. Even true with an accelerator pump carburetors like the Keihin FCRs. I don't think the gap is particularly good for smooth low speed performance either.
I had trouble cold starting my motor with a longer intake boots that created a 5/16" gap between the intake manifold and carburetor. It took too much kicking and too much fuel to get fuel into the cylinders. I closed that gap up, and obviously things got better with the improved flow path. Ideally, the use of a Mikuni style boot designed to fill that gap works better. I did not use the Mikuni solution, because I was planning to use longer boots initially. I did consider cutting some tubing to fill the gap, but didn't have any tubing that was the right thickness. In the end, the shorter manifold boots gave me more room to get the air filters on and off. Anyway, below is a sketch showing what I'm talking about.
I did radius the mouth of the smaller diameter intake manifolds out to 35mm to sort of match up with the FCR spigots. The radius is steeper than I'd like, but it is better than a square edge.
