Tillotson HW38 Carb settings?

Hi everyone,
I just got into a TAG 100 class here at my local track. I am running the Vortex ROK VLR 100cc air cooled motor. In the heat and main the motor was acting like it was choking up on the start and everyone had 3-4 karts on me before I even got to the first corner and it felt sluggish coming out of the corners also. After removing the spark-plug we noticed we are running rich. What are the best High and Low settings to run for this motor and carb combination? thanks for any help!.
Been used to Fuel Injected Motors as I used to race Dirt Track before Karting so kind of at Sea on settings.


1 Like

The carb (optimal) settings can vary from day to day, sometimes even during the day as air density changes. The good news is that the late model 100cc TaG engines seem pretty user friendly when it comes to carb adjustments.

Have you been given settings as a starting point? If not we should be able to get some from vortex.

One thing that is critical, but sometimes overlooked is setting the “Popoff”. Any given screw positions are based around a popoff setting. Checking popoff also ensures that your carb does not have any vacuum leaks.

Here’s a guide on popoff:


If I can get baseline settings I can go from there, willing to learn from others. is there a link to VORTEX?.

Paging @Garett_Potter, do you have, or know if a baseline settings guide for the VLR?


I haven’t had a chance to run it personally but the info I have 1 & 1 on the needles is a good starting point, some are at 7/8 & 1. Looking for 1100 EGT I’m working on a tips and tricks manual for all the engines hope to have it up soon. Where are you located?

1 Like

Carb settings vary by day, so instead of a fixed needle setting like many 4 cycle guys do, what I did was a pretty simple process.

1.) set needles at factory recommended settings.
2.) take an easy lap to get things up to temp.
3.) at temp, turn LOW needle out until engine bogs on accelleration from low to mid RPM. This is called “4 cycling” because that’s what it sounds like…
4.) Turn LOW needle in very slowly, stopping once the accelleration “cleans up” and is a crisp accelleration from low to mid. I do this slowly to not overshoot this zone and accidentally go too lean.
5.) Repeat the process with the HIGH needle, turn out until it “4 cycles” from mid transition to high rpm. Turn in until accelleration cleans up again.

This will get you into a safe zone, close to the optimum window. From there, you can slowly turn the high needle in to fine tune accelleration. I tune by ear, typically, and found that in the ideal spot, the engine exhaust note is “crisper” than other settings. IT’s kinda’ hard to describle, but there’s almost a “Crack” to the sound but no detonation pinging or other anomalies… Tuning by ear is a lost art now that mychrons are around, I suppose. But it’s a good skill to have since you can do it while racing and spotting your apexes…

Perhaps TJ or someone else more experienced can chime in, but this was (is) my method.

Also - as things warm up (and longer races, particularly) the engine “heat soaks” and acts effectively leaner, so you’ll perhaps need to richen it to keep things cool. Given the large fin area of these engines relative to the ICA’s, etc., perhaps it’s not necessary. I never messed with my KA once on track…but then again, I was bringing up the rear due to chassis woes, so…

Arizona, any help appreciated.


Race at PKRA Phoenix live in Prescott Valley AZ 90 miles N of Phoenix.

My KA runs pretty good with just a crack over 1 on the low and about 1 1/8 on the high. I was just over 1 1/4 last time at track (test day) but EGT’s were low 1100, kept leaning out by tiny amounts each session till I got to 1250 ish and ran really good.

1 Like

Thanks Brian. For comparison, do you know what your pop off setting is, also the oil ratio you’re running.

Negative on the pop off, I let Doug Fleming set all that up for me. I run motul 8oz per gallon of c12

1 Like

I don’t have a tester to verify but I was told my ka pop off was set to 9