Engine tuning and the influence of needle taper and shape
 

I was contemplating my latest r/c challenge, tuning my Webra, and started thinking about how each of the engine speed ranges are tuned. It became clear that the taper and shape of the low speed needle maps the entire engine operating range *sort of*.

If the low speed needle has a straight taper, and is engaged into the spray bar at all throttle settings, it will define the low, mid, and high speed jetting. The high speed needle, in this case, will only be able to make the high speed setting more lean but not richer.

If the low speed needle has a double taper or single taper with a curved end, it can define the low and mid speed jetting but will disengage the spray bar at high speed and allow the high speed needle to define the jetting. This is what we want.

One last factor is the chamfer at the end of the spray bar. This can have an effect on high speed jetting depending on its position relative to the end of the low speed needle.

Of course, all this assumes that the throttle barrel has a lead so that the low speed needle is pulled out of the spray bar as the throttle is opened.

So, on my Webra, it seems as though the low speed needle has a single taper with a blunt squared off end. When the low speed jetting is set for optimum idle, the low speed needle is still in the spray bar at full throttle and the high speed jetting is made too lean. The high speed needle has no effect. If I curved the end of the low speed needle, I believe I could have a good low speed jetting and have a responsive high speed needle.

Any thoughts. Am I totally off track. :stupid:

Carbs
 

Planes, I have only been successful modifying one carb, the one on my Webra .91. I carefully slanted the end of the spray bar, maybe 10 degrees, and either I lucked out or just did the right thing but it cleaned up the midrange. I slanted it in towards the bottom of the carb, all other carbs I've fooled with got worse. By the way the Irvine Jet Stream carb for the Irvine .72 fits the Webra .91 and 120 perfectly, same venturi dia. and neck dia. Irvines are the best carbs in our hobby.

Engine tuning and the influence of needle taper and shape
 

I chamfered and rounded the end of my low speed needle. It allowed me to make the low speed leaner without adversely effecting the top end. The engine ran great all day. I guess I got lucky too.

Engine tuning and the influence of needle taper and shape
 

Hallo guys!
Why are you bothering!!!! The Webra engines and their carbs are one of the markets best ...! They work in th esame manner that all others do...by setting the highspeed needle first then....setting the lowspeed needle!!!
The idle needle is regulatin fuel....all fuel if screwd all in ,but that you should not do!
The highspeed needle is also regulatin fuel...all fuel if screwed in!!
The secret to good engine behaviour is to open the idle needle enough,(what that is).
On the Webra .40 -. 50 that means that the idle needle should be approximately 1mm from the orrifice at full throttle opening.
This is the basic set-up!
Then full power is set with the highspeed needle. Highspeed needle regulates the main distribution of fuel to the engine, the idle needle just restricts the fuel getting in the carb at midrange and idle.
Its that easy!

Regards!
Jan K
Sweden