Dimension inside the screen of gray backplate

happydad Mon Aug 31, 2020 12:17 am

While tinkering tonight, i was removing the screen from the back of a gray backplate with a cracked side around the screen. I was thinking that the hole inside was round, but surprise it was not. It was sort of rectangular with rounded edges on the longest sides. see photo with measurements.

Dimension inside the screen of gray backplate Img_1811

Behind the screen. (Blue looked better when I was editing the pic).

Someone had asked for a measurement, but I couldn't find the thread.

happydad. RC Plane

Oldenginerod Mon Aug 31, 2020 3:20 am

I recall Warren Leadbeater did an "eBay Guide" on hopping up a SureStart but my bookmark will no longer open the page. He touched on the optimal venturi size as well as re-orientating the spray bar hole. It's a long time since I read it, and don't know how to get back to it.

Rod.

balogh Mon Aug 31, 2020 6:06 am

happydad wrote:While tinkering tonight, i was removing the screen from the back of a gray backplate with a cracked side around the screen. I was thinking that the hole inside was round, but surprise it was not. It was sort of rectangular with rounded edges on the longest sides. see photo with measurements.

Behind the screen. (Blue looked better when I was editing the pic).

Someone had asked for a measurement, but I couldn't find the thread.

happydad.

Happydad, is it just the entry cross section, and the rest of the venturi cross section up to the reed valve is circular? I could check it but I am not at home. I assume this oblong entry cross section is only to retain/support the screen?

sosam117 Mon Aug 31, 2020 7:37 am

happydad wrote:While tinkering tonight, i was removing the screen from the back of a gray backplate with a cracked side around the screen. I was thinking that the hole inside was round, but surprise it was not. It was sort of rectangular with rounded edges on the longest sides. see photo with measurements.

Behind the screen. (Blue looked better when I was editing the pic).

Someone had asked for a measurement, but I couldn't find the thread.

happydad.

Happydad,
What you are measuring is the venturi size (opening) for the air to mix with the fuel.
The straight sides are for the support of the spray bar.
If it was round, there would be little support for the spray bar when it was pressed in and probably collapse the "round hole".

As for the spray bar hole. It should be pointing inside. (not facing out or 90 degrees.)
Carefully look how the current spray bar hole is facing.

As for your screen. It should be round and about .20" in diameter.
As someone else posted. The oblong entry cross section is only to retain/support the screen?

balogh Mon Aug 31, 2020 8:51 am

People seem to be split over which direction the spraybar hole is to point? I e.g. would expect best suction and vaporization of the fuel is achieved with the hole pointing perpendicular i.e. 90 degree to the airflow where, partly because of air acceleration around the spraybar and partly because of the narrowest cross section in the venturi the air velocity is highest..this is fairly theoretical but 180 degrees I.e. parallel with the airflow seems sub-optimum??

Practical experience may overrule theory, of course

happydad Mon Aug 31, 2020 3:59 pm

well, i took some more data, digging down a little further and measuring the oblong area behind the needle valve assy. from both ends

Dimension inside the screen of gray backplate Img_1814

picture looking inside backplate after needle valve assembly was removed.

Dimension inside the screen of gray backplate Img_1813

drawing depicting side and top views, sorry can't spell needle.

My conclusion is that the area behind the valve assembly is oblong, flat on the long side and round on the short side, and approximately 0.039inch when measured from both ends. Two Cents

happydad.

**Cribbs74** Tue Sep 01, 2020 9:44 am

I would imagine that the airflow around the spray bar and through the venturii Is somewhat negated by the function of the reed. What I am saying is I think the reed itself and venturi size are the bigger players. Flow before the reed is altered as it goes around the reed.

Just my thoughts.

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