The Sopwith Camel was equipped with a few different engines. The early models using the LeRhone 130 and Clerget 130 had the ability to throttle down to about 2/3rds to 1/2 throttle. Also dependent upon whether they were descending, you could pull the fuel mixture quadrant out of synch with the air volume lever and lean the engine and slow it even more for a short period of time. The low compression ratios on these engine actually withstood leaning of the mixture somewhat as long as they weren't being loaded much.
The 160 hp Gnome Monosuapape was entirely different as it was not much different from the earlier Gnomes that could not be throttled. It used an elaborate clockwork ignition distributor that turned off rotating sets of cylinders in alternation, so you didn't foul the plugs on what would have normally been the ones always shut off.
Hard to explain, but it worked as they shut of 3 or 5 or 7 sets of cylinders depending upon where the mag switch was set. I also believe the Gnome Mono did have a fine mixture setting lever for the engine when running at "partial ignition throttle"
The Bentley Rotary used on the later F.1 Camels, was a British manufactured, upgraded Clerget 130 that also had a half throttle carb "idle" position. Because most rotaries were in a pretty low state of tune, the partial throttle was more close to having a reasonable amount of throttling, mainly because of the low state of engine power per cubic inch. These things only spun at about 1200~1300 RPM max.
