As others pointed out, it goes back to the days when high output aircooled motors were the standard. They were run on (and beyond) the ragged edge of mechanical limits with mean piston speeds matching F1 motors.
It can be used for the following…
- To cool the motor.
- Shut it down and stop it coming apart.
- Provide little extra lube before you shut the throttle at 19,000+
The less choke you use, the faster you (might) be. But the risk of grenading the motor increases too. These motors will rev until they come apart, so you had to protect them. While also trying to out race your opponents. In a lot of cases you’re geared for punch off the turns and basically playing Russian roulette, letting the motor rev as high as you dare with varying amounts of choke
It made for pretty interesting racing. Carburation came into play too, especially with aircooled. Often you could really only run right on the edge for a short period of time. Choking might let you get away with it for longer. So you’ll find racers alternating between running the motor hard with choke, without choke, fall into line and run a little rich and anything in between as a tactic.
Put all that together, with direct drive that locked the wheels solid when the motor gave up (frequently) made for some very interesting racing that was part driving, part chess and part roulette.
I’ve heard it said it’s used to help drive off the turns. To me, if that’s the case I think your jetting is off. @NikG and @Mynameismcgyver might have some thoughts on that.
@Terence_Dove and @DavinRS talked about these motors (Classes ICA, Formula A & Formula Super A) in part of the three part podcast they did on Terence’s book. I’m not sure which one of the three it was though.
For watercooled it was more 2 & 3. One thing to be mindful of is that excessive choke can cause a reed motor to ingest its reeds. That’s bad.
For TaG and so on, probably not much need for choke, but I can understand someone using it as piece of mind or general mechanical sympathy. For a turn where you have a quick lift then right back on the power, you could go for a choke there.