Throttle question

Jason_WI wrote:One possible reason is the ACE style tapered needle also blocks some of the fuel from entering the crankcase as it restricts within the fuel tank venturi.

That is out take as well but our throttle also restricts fuel flow somewhat in conjunction with the "venturi effect" (the choke tubes are tapered inside); otherwise the engines would run too rich with our throttles.

But then, the Ace version may restrict fuel flow to a higher degree.

Cox International wrote:As many of you know, we manufacture these throttles Cox International 049 Throttle Conversion that don't work to well on cylinders with SPI (Sub Port Induction).


Why is it that this version works well? Cox international 049 Bee Style Throttle

What I think is happening is that the Bee style throttle needle actually enters the venturi fuel orifice location,decreasing the area causing the lessened airflow volume to accelerate past the fuel orifice providing a better and richer fuel draw. At idle RPM, this richer fuel air mixture mixed with the SPI air is close enough to stoichiometric (i.e., proper air/fuel ratio mixture), so engine idles reliably.

The choke type throttle does not change the cross sectional area by the fuel orifice in the venturi. The decreased air velocity across the fuel orifice is of lower velocity versus the Bee style throttle, so draw is more lean. At idle RPM, the SPI air further leans this already leaner mixture, making it less able to support combustion.

Anyway, that is my take on what is possibly happening. Of interest is the OS Max .099 Pet used a choke style throttle above the normal venturi similar to the Sure Start choke tube throttle.

And interestingly enough, I hadn't seen your reply Bernie when I was composing my message, until after I sent it.

This back plate throttle rig looks an awful lot like one Ace R/C sold years ago. In an r/c plane one uses a short piece of firm fitting tubing over the needle and routed to a rod with a z-bend or quick link that is fastened to servo output arm. Then adjustments are made by sliding the needle and/or the rod inside the tubing to suit.

Cox International wrote:As many of you know, we manufacture these throttles https://coxengines.ca/throttle-conversion-for-cox-049-engine.html that don't work to well on cylinders with SPI (Sub Port Induction).

Why is it that this version works well? https://coxengines.ca/throttle-for-cox-049-engine-bee-style.html

I think that there is too much air in engine with SPI cylinder equipped with first type of throttle. Enlargening hole diameter of fuel inlet on backplate can solve the problem. In this way engine sucks more fuel, so air fuel mixture can be optimized. Although I don't have that kind of backplate and didn't try it.

Cox International wrote:As many of you know, we manufacture these throttles https://coxengines.ca/throttle-conversion-for-cox-049-engine.html that don't work to well on cylinders with SPI (Sub Port Induction).

Why is it that this version works well? https://coxengines.ca/throttle-for-cox-049-engine-bee-style.html

I think the 90 degree turn in your throttle assembly affects the air speed, pressure and even temp. All these affect the venture effect. You might want to experiment with a 180 degree intake assembly and see what happens. Just a thought.

In a lot of full scale, carbureted recip engine planes that had an induction intake that made a 90 degree bend right in front of the carb, the engines were especially prone to carburetor icing at lower power settings. I can't remember why, any more. But, if memory still serves me correctly, it involved a decrease in both air speed and pressure.

Now my brain is tired and hurts.

I think the good in this is that now I know the choke tube throttle doesn't work with SPI cylinder / piston sets. The tanked Bee style needle throttle through the rear venturi or the exhaust throttle ring work with SPI.

Back in the late 1970's, I was using Ace R/C's exhaust throttle ring for my SPI equipped Golden Bee on my then new Q-Tee. It worked like a charm.

If one plans to use the choke tube throttle, then it should only be used with the Sure Start non-SPI styled P/L sets or similar.

ticomareado wrote:This back plate throttle rig looks an awful lot like one Ace R/C sold years ago. In an r/c plane one uses a short piece of firm fitting tubing over the needle and routed to a rod with a z-bend or quick link that is fastened to servo output arm. Then adjustments are made by sliding the needle and/or the rod inside the tubing to suit.

Yes definitely We bought an ACE throttle on eBay, send it to a machine shop and said "copy it"

Don't believe the bend has much to do with this. Our throttle is an airflow restrictor; which also somewhat reduces fuel flow to some degree. However, SPI induces air on upstroke, mostly negating the airflow restriction of the throttle. They are "fighting" each other.

Our throttle does not work well with SPI and that is why we clearly indicated this on the listings. However, we sell many to buyers who use them on SPI engines but change the cylinder.

Thanks to everyone for the answers

Cox International wrote:

ticomareado wrote:This back plate throttle rig looks an awful lot like one Ace R/C sold years ago.

Yes definitely We bought an ACE throttle on eBay, send it to a machine shop and said "copy it"

Bernie, just examining your photos, I'd say your unit is definitely better quality than the original unit. I remembered close to 40 years ago that those by Ace were molded out of plastic. Although they worked, the machine shop that did the run for you did an excellent job, definitely top quality.

Shortening choke tube can be a problem solver. If air in choke tube flows in a short distance, then venturi effect will be greater in this way. (Edit: I think that this may be wrong.)

I think that enlargening fuel inlet is not practical for mass produced back plates, which I suggested. However I don't know whether it is a good idea or not.

If one thinks about it, with a non-SPI piston, fuel draw continues partially aided by the vacuum caused by the choke tube throttle to top dead center (TDC). With SPI, that fuel draw is interrupted by a gulp of air when piston skirt clears the bottom edge of the exhaust port. This interruption to fuel plus the extra air leans the mixture. I think that to fix would require a new redesigned backplate with throttling integrated into it for it to work.

Bill Atwood and Dale Kirn's redesign of the cylinder and piston to eliminate SPI with 2 bypasses and Tee Dee head increased the HP. I was impressed how well my late 1970's R/C Bee flew even though muffled with more power than a Babe Bee.

The latest iteration, the Sure Start cylinder has in addition a boost cut next to each bypass to widen it, further increasing power. For a choke tube throttled engine and a Tee Dee head, I'd think one need not dink with SPI in order to have a decent R/C engine.