Freestyle Mikuni SBN drilling out returns

[ventec]

I'm looking into drilling out the returns on my set of AM Mikuni 46mm SBN carbs, but not really finding any answers why it is a benefit or what drilling the returns out really does. Any info would help as well as you opinions.

 

[Matt_E]

The return passage has a built-in restrictor that serves to raise the fuel pressure inside the carb to something the jetting can work with. Drilling them out and not putting restrictors somewhere else (in the return fuel line) will result in low fuel pressure and you'd have to up your jetting quite a bit. It wouldn't run right even with fatter jetting.
The only reason to drill them out and use an external restrictor is to fine-tune the fuel pressure with differently sized restrictors than what is found stock.

 

[ventec]

??

is there any major improvement in power when tuning a set of carbs that have more fuel pressure, by having a restrictor in the return line and the carb drilled out. compared to not having it drilled out ?

 

[Matt_E]

That I don't know.

 

[munki63]

xmetal drilled out my 46's and ive got a #70 jet in the return line... just an fyi not really any advice but maybe you could start there

 

[kevbo]

Drilling the return is like cutting out the entire venturi. Bad idea but a lot of people do it.

 

[easyrhino]

I heard a shared restrictor will synchronize fuel pressure in a twin carb setup.
My question would be:
is a fuel pump issue easier to detect in a shared restritor setup?
does it help with cold starts, preventing one carb from going lean causing hi revs during warmup on the trailer?

 

[kevbo]

I would rather install a single pump with individual restrictors. This way there is no chance of reverse flow and air would always purge quickly through the return line. Two pumps and single return restrictor can cause problems if one of the pumps are weaker than the other. Flow through the weaker carb will slow or even reverse making it difficult to quickly purge air. This would not be a problem for throttle blippers but could be for long full throttle operation.

 

[tilleryrat]

newbie quick question on this topic, I just purchased a set of sbn 44's and x-scream recommended the same, the t with 70 jet restrictor and told me to drill out the returns on my carbs with a 3/16 bit. My question is that it looks like the return outer dia is not much more than 3/16 itself. I feel like I'm going to be awful thin on the return. anyone with experience doing this?

 

[2XLR8]

Drilling the returns and running a larger restricter like a 70 will drop the fuel pump pressure at low rpm. Once rpms begin to go up the fuel pressure will over come the larger restricter.

 

[tilleryrat]

so being a newbie I thought they were talking about drilling out the return nipple on the carbs, but apparently it is the return hole in the carb itself. Does anyone have a pic or description on how to do this?

 

[Crab]

I would rather install a single pump with individual restrictors. This way there is no chance of reverse flow and air would always purge quickly through the return line. Two pumps and single return restrictor can cause problems if one of the pumps are weaker than the other. Flow through the weaker carb will slow or even reverse making it difficult to quickly purge air. This would not be a problem for throttle blippers but could be for long full throttle operation.

good to know, I did have issues with a set of oem 44's last year.

 

[tilleryrat]

any carb experts to help with this?