ORIGINAL: JDW

Flyboy Dave

It is you who should read the thread!!!

My carb also had about 40% restriction on the spraybar tube entry. Even with 70 % restriction the hole would still be plenty large enough to allow WOT fuel flow. This is simply NOT the problem.

The engines I have sorted could all be LEANED several turns on the main needle once the idle needle was correct. Yes, you read it right! - LEANING the idle mixture enabled the main needle to be LEANED heaps and all the problems that LOOKED like muffler pressure, fuel flow restrictions etc magically went away WITHOUT drilling, filing, sealing or otherwise modifying anything else at all.

Changing to a muffler with less back pressure (Tower) is also not a problem IF the idle needle is set correctly, so it's not a muffler pressure problem. Since there is VERY little flow from the muffler to the tank I'm sure a hole 25% of the standard nipple ID would be tons adequate. Remember that the pressure in the tank is NOT dependent on the nipple hole size UNLESS the flow rate is so high that there is a head loss through the nipple. With the flow rates we are talking about that is just NOT an issue. Remember that it is air we are talking about here. If fuel can get through a standard nipple without significant head loss at the flow rates we are talking about then you dont need much of a hole for air from the muffler to do the same!

The reason you have to get the idle needle correct is to have the geometry of the carb correct at WOT. Leaning the IDLE needle has no direct effect effect on WOT mixture BUT it does have a VERY dramatic effect on fuel draw - i.e the engine will only 'suck' correctly if the gap between the idle needle and the spray bar tube is correct.

As I said before, don't take my word for it, try it and see for yourself as others have.