Old School Techniques: Choking and Line Pinching


(Craig Stubbs) #1

I’ve noticed in some videos that it appears the driver is choking the engine at times while racing. When would this be beneficial? Why does one need to do this in the first place? What are the advantages and disadvantages?
We race IAME air cooled 125cc engines (the old Easy Kart class) at our local track in Monticello, FL. It is plenty hot and humid in the summer and really nice in the winter. Would this technique be useful under these conditions?
Sorry if this seems like a stupid question, but I gotta know!
Thanks for your input.

How bout some links to cure the boredom?
(TJ Koyen) #2

In the old days of 100cc direct-drive (ICA, FA) the karts ran the engines so lean that if you didn’t choke them at the end of the straight to give them a shot of fuel, they’d seize.

I used to do it on Leopards when breaking them in sometimes but that was mostly just to look cool. The engines are more durable now so there isn’t really a need to choke it during a race.

(Eric Gunderson) #3

I never knew it was to keep the engines seizing, that makes sense. I was always told it was to give the engine an extra shot of fuel to burn out of slower corners.

In the JICA days and ICA you saw people do it. Now, as TJ says, most people only do it to look cool. And, admittedly, it does look pretty steezy.

(Aaron Hachmeister) #4

I tried that out this past weekend when we put on a new clutch and had it set at 4,700 accidentally. Figured maybe choking the engine on the start would help (it didn’t, still got mugged)

As far as ICA stuff, there’s a video of Jordan Lennox running a classic CRG. He always choked the kart going into the slower corners, it looked super cool

(James McMahon) #5

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…

  1. To cool the motor.
  2. Shut it down and stop it coming apart.
  3. 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.

(James McMahon) #6

This video might give a good example of using choke to manage the motor. Skip to 3min in for the start of the race.

Clay is one of those tracks where you’re at sustained high RPM coming into the first turn, some laps you see Glenn let it run without choke (cringe), others he’ll give it varying amounts of choke to conserve the motor when he can by shutting it down.

It’s very much a feel thing. Not scientific.

(Bill Holt) #7

I love choke braking, as mentioned looks super cool and I guess, somewhat more importantly does give the engine a hit of cooling and lubrication. When the KA first came out the question did come up about choke braking. Obviously as mentioned its not really needed, but because a “name” driver was doing it, everyone else started too as well. "Insert a cheeky giggle."
This is a link to a video of KZ world final at Genk in Europe, Max Verstappen decides to start choke braking at the end of the straight. at about the lap 13 mark (12.00 minutes roughly). How you fit that in with changing gears, braking and turning, just goes to show what sort of coordination some of the top guys have.

(TJ Koyen) #8

I only did it on the Leopard specifically at New Castle on that straight when breaking it in. My balls weren’t large enough to go full chat into that last corner on the first hot laps of break-in without choking it. I was always at full pucker mode then.

(Terence Dove) #9

This looks like a great excuse for me to add my favourite bit of video!

Gives me goosebumps…

Like James says, you used the choke like a manual rev limiter - it allows you to stop the engine accelerating at the end of the straight without lifting early and starving the engine of fuel (lifting without braking at high rpm is asking for a seize with the freely revving 100cc motors)

(Andre Molina) #10

How do 3 jet carbs play into this?

I’ve heard a few times that the 3rd jet is supposed to substitute choking the carb, injecting a bit of fuel in the engine at the very top of the rev range.

(James McMahon) #11

Because it’s so far away from the Venturi, my understanding is that the power jet affects most where fuel demand is strongest, usually peak torque.

(James McMahon) #12

Fantastic Pantanooooooooooooooo

(Nik Goodfellow) #13


Well, like James said choking is mainly for reliability but I had always used it for more fuel in the bottom end as well.

In fact at one track we would switch the jet configuration. Normally we would 1.5 turns on the low jet and 1 turn on the high jet. At this track we would run 1 turn on the low and 1.5 turns on the high and (get ready for this) open the throttle and choke mid corner and release the choke when I actually wanted to accelerate. I actually read about Senna doing it and there was only one track in the UK I could make it work.

Although I miss those days (and if anyone lets me near an X30 I will be choking it) i don’t miss half the field being wiped our by a high pressure region passing over the track.

(TJ Koyen) #14

I showed this clip to the young bucks at the track a couple weeks ago and they were like “who is Pantano?”

They thought it was awesome though. “That’s so euro!” as they say.

(Craig Stubbs) #15

Thanks for all of the info!
I appreciate your time and comments.
I’m getting that the 125s don’t really need to be choked like that but it may be something to play around with just to see where it might come in handy.

(James McMahon) #16

It’s not a bad thing for aircooled. Are they piston port motors, or SUDAMs or?

(Craig Stubbs) #17

These motors are the old Birel Easy Kart series engines. They have a Leopard bottom end and an air cooled jug and head, 125cc. It is a reed valve motor made by IAME. They run a Tillotson 384 carb.

(James McMahon) #18

Sounds like a cool motor to be honest. Got any videos?
Again, just be aware to not choke it too heavily or you can break the reeds.

(Craig Stubbs) #19

I am out of town at the present time. When I get home I’ll post a few pictures to the post.
It is a good motor. They have great power from about 13k to 15k, will rev right on up to 16k with no problem. I had a few pistols stick when I was first trying to figure out the whole lean-rich thing with them. Only had one bottom end failure. That was the result of a crash that tore off my air filter, sand flying all around and I kept driving. I should have shut the engine off.

(Aaron Hachmeister) #20

Would this still be effective on an engine today? Would choking the engine help get out of some turns if it’s done correctly?