I dont believe it was Smurthwait. It was Bill Johnson, I am sure. What say you Heywood or Sawicki?
Wayne

I remember it being Bill Johnson.

The primary problem was the relatively close tolerance fit required on the exhaust slider to control the gas flow through the engine.

Some were used in conjunction with a fuel metering block to control the mixture with a fixed venturi.  These were usually crankcase pressure setups.

Others were used with carburetor.

In both cases the motion of the exhaust valve was connected to the other control device.

Harry Higley published a system that added a crankcase pressure bleed to the metering block and exhaust slide system.

Exhaust baffles can work wonders helping some engines idle.  A K&B 5.8 responds wonderfully to an exhaust baffle.  The old ST C35 would idle in the 2500 - 3000 RPM range on 50% nitro fuel with a baffle for 30 - 40 seconds at a time and would come right up (Mag-3 or Mag-4 carb) when the throttle was advanced. 

Without a baffle you may need to run the engine rich to get the desired idle speed.

The idle speed really only needs to be low enough that the model won't stay in the air at idle.  It doesn't need to be super slow unless you have an airplane that just floats.

Dave

I didn't remember any fuel metering device associated with this particular slide valve; but it's been a long time.  I do remember Bill's metering device, and pics of one made of brass tubing. 

The gadget I was asking about was just a slot cut through the exhaust stack and a flat metal piece with a somewhat tear-drop shaped opening that was moved by an arm on the J. Roberts bellcrank. The position of the flat slide could be changed, fore and aft wise; the hole in the slide was sized and shaped to progressively close off the exhaust stack as the trigger at the J. Roberts handle was moved.

Clear as mud?? 

Actually, everybody has a bit of the truth on this one.

Bob Smurthwaite did sell an exhaust blade as the J. Roberts "Vari-Speed" Throttle Control. It is shown on the plans for Vern Clements' Brewster Buffalo published in the July, 1960 issue of Model Airplane News. It was as described, a thin blade with an elongated teardrop opening. There was no fuel control. It is also shown on the plans for the Consolidated Dark Shark and the J. Roberts Mauler (Copyright 1960).

Bill Johnson, out of St. Louis, did manufacture the more involved system that included the fuel meter. It was very popular in the late 1960's. The St. Louis gang had things working pretty good in those days. I have a never used ST G40 with the complete Johnson set-up. (I'm at work right now ~ 7:37 AM EST ~ but will post a pic this evening).

There were a lot of home made knock-offs. I did one for my McCoy -60- Sterling Guardian. As Dave indicated, the fit was critical and some form of fuel control was also necessary. When the big bore Kavan carb arrived things got a lot better.

I asked about his because, if it works, it seems like a light, simple way to control speed.  Guess if it was so good, we wouldn't have R/C throttles...... 

Super Tigre G40 modified by Bill Johnson.

Wayne,
Did you get my PM?

The zoominternet.net addy didn't work for me........

Jim

Vern Clements was a good friend of AL Culver of Pocatello, Idaho, at the time I lived there.

Ty, You weren't by chance at one of the Prototype plants during that time were you...   I went through there in the late 80's but alas was not actively flying at the time... I think I spent too much time either at the site  or in the bars