Metal Backplate For .049

Hi New
No it isn't from castor buildup. I'm talking of very clean backplates now. Some have a huge outlet hole 1mm or larger dumping into the venturi inlet while others have nothing much more than a pin hole size?

Thanks
Ice

I do not think the size hole for the fuel really matters as long as it allows the flow of the metered fuel amount into the venturi. Smaller holes will need more NV turns to open for the best setting and larger holes the other way around.

Drilling the venturi inlet may help the engine boost its output a bit, but - especially with non-tanked engines where the fuel is drawn from a distance rather than from the integral tank sitting around the venturi - the suction capability of the engine may be compromised.

I think the guys that designed and built these little marvels over many years probably experimented enough to find the right venturi combinations for best performance and reliability.  I know some here try to ring out as many RPM's as possible, many to astounding success, but at what price.  That said, there are manufacturing variances.  

Lot's of holes in these little things, most can fill up quickly with castor goo if left unclean, IMO the biggest impediment to performance.




I tend just to clean them up and let nature take it's course.  Both vent/filler tubes were totally filled with castor.  Took Simple Green and fine wire to clean them out.





Anymore after running I stick a Remoil straw down the NV hole and give it a squirt after loosening the four case screws and let it drain.  Cleans it right out.



Do it before it turns to this:

As far as the needle orifice is concerned, as long as you can get the full range of Lean to Rich, then there's nothing to be gained. An enlarged needle orifice may make it more finicky to lock in a good setting.
Rusty