Questions about Jetting Changes on a Comer c50/c51

So we finally had a chance to Fire-Up my Daughter’s kart the other day. Not a true test as she just putting around an empty parking lot, but it got me thinking about jetting. I had the sense to pick up a set of pre-pin gauge measured jets when I first started restoring the kart. My Dad sent me his Air Density gauge to use for deciding when to change the jetting up or down and I understand that when the gauge drops, so should the jet size.

My question is what is the percent of change in Air Density that would warrant a change in Jet size? It seems every jet manufacturer has a different flow rate for a given jet size and I have not been able to find any information on the Dellorto Jets. The set I bought for it came Numbered/Pinned Gauge measured 57/0.024, 59/0.0245, 61/.0250 and 62/.0255. My understanding is a 0.026 is a No Go. I also understand that any Jet change will be Relative to whatever Base Line is established for this Engine and Carb combo and one could assume that each step in jet size would yield a similar change in fuel flow percentage (example going from a 56 jet to a 57 would be a 3% increase in fuel flow rate).

Is this even possible to know without a Dyno and Measuring equipment?

Is there a ballpark number that can even be assumed from one jet measure to another?

Thank you in advance for any tips or advise.

Are you calculating the 3% by area or diameter?

For now, I’d say run the biggest jet it will take and try to resist the temptation to tune it until your driver is able to utilize it.

I did not actually calculate it for that example, it was just a way to represent the correlation I was looking for.

I did find this on a site talking about Jet size and Percent of change:
“New jet diameter = √ (Old jet diameter² x required flow factor). Flow factor is e.g. 4%=1.04, -7%=0.93 etc.”

If I plug in the numbers for a change from a 57/.0240 to a 59/.0245 jet it came out to be around 4.21% change in flow factor. However diameter alone may not be enough to calculate change of flow. I guess I am just looking for some confirmation that I am on the right track or if anyone has seen a variation in this relationship with the Comer engine?

1 Like

You can swirl in the jetting math and get lost like I did for a while. I was also using air density calculations for a bit, but ultimately using a mychron to test different jets and see which gives you the most RPM on the straights and which turns the fastest laps is just as effective in my experience.

I also found that once you have the jet size for your comer and carb that work best you’ll rarely change unless there are extreme temperature and humidity changes. I would change jets for a weekend with anticipated high temps or humidity and ultimately end up back where I started.

You won’t really be able to rely on the data to know which is a better jet until your daughter is running the kart in a consistent manner, so for now choose a slightly larger jet that runs well and consistent and just let her go. For a blueprinted Comer the ideal jets will almost always be max or very close to max size in my experience.


I agree completely with you and @KartingIsLife. I am not trying to make the kart a rocket, but just don’t want to limit it either because I did not foresee the change. Based on the Math (I am a math and measures kind of guy), it does not look like jet changes will be frequent (4% change in Air Density to Qualify a jet change is significant). In my research a slight tilt of the carb one way or another can have subtle changes in fuel rate between jets.

Today I did a Carb rebuild and threw in one of the pre-pinned jets I picked up. Surprisingly to me, the engine did not like full throttle application from low RPM. It bogged really bad. I went up one Jet (the #62/.0255") and it helped, but not to the extent I thought it would. What worked best was progressive application of the throttle to allow the engine revs to match the fuel delivery. I have heard of the Rotax engines having similar characteristics (another Dellorto Carb). I will instruct her that she cannot just burry the throttle, but rather slowly increase it in harmony with Revs to maximize acceleration. I am so used to the Tillottson Carbs, this is a new way of thinking for me.

1 Like

Longacre, on their website, has information about their air density gauge that may help you.
Briefly, a 1% change in air density requires a 1% change in flow. .038" – .039" equals a 5%+ change in flow.

I’m a little confused by your jet size numbers. Are we talking decimal or metric?

1 Like

From what I read, you could have multiple jets with the same Number and actually have multiple values for their actual diameters. I purchased a set of jets that were measured with a pin gauge and marked before sold. As far as I can tell, the Number and the Diameter have little reference to each other that a larger Number is supposed to a larger Diameter, which is why I spent the extra $10 or so to have a set that I at least knew how much each change was. Diameter is measured in Inches.

They also sold "Outlaw jets that went larger than the .0260 No-Go, but without an “Outlaw Carb” there is no benefit.

Here is the info you are looking for that I wrote. And like James said, resist doing any changes until your kid can actually benefit from it. Run the biggest jet you can, and if the engine burbles on the end of the straight then lean it a jet. Here is the article

1 Like

Thanks Jim, I already read that. It was my reason for going with the pre-Pinned jet set. I agree, going to throw in the fattest jet and see what does on the track. Moreover I was curious what the qualifier was to change jets. I think I answered my own question when I ran the math. A 4.3% change in RAD qualifies a Jet change. Now I just need to establish a base Jet # for which to evaluate a change. If you have a window based on jet diameter to gauge this, that would be helpful.

On my article I refer to different Density altitudes and the jet to use. The work has already been done. I say on there that you may need to make your own chart because your jets may vary slightly, or your driver may handle the gas pedal differently. Start with what I have there, and adjust as needed.

[quote=“Jim_Maier, post:10, topic:6902, full:true”]
On my article I refer to different Density altitudes and the jet to use.

I didn’t see your article, do you have a link?
“Density altitude” what exactly does that mean? I know at any altitude air ddensity can vary, as much as 14 points, maybe more, Over one years period of time.

Thanks for your input Jim,

I presume you are referring to this article:
…and more specifically this chart:

  • Altitude = Negative 1500’ = 60 Jet
  • Altitude = Negative 500’ = 59 Jet
  • Altitude = 0’ = 58 Jet
  • Altitude = 500’ = 57 Jet
  • Altitude = 1500’ = 56 Jet
  • Altitude = 2500’ = 55 Jet
  • Altitude = 3500’ = 54 Jet

However I am not using a gauge that gives Density Altitudes, I am using a Relative Air Density Gauge. According to the change in air density for change in elevation found here: , for every 500 ft of elevation change (at standard atmospheric pressure) the percent change in air density is only 1.38%. (***The linked chart shows the change i air density over a larger range and is parabolic, but at -1500 to +3500 ft one can assume a near linear change.) The percent change in fuel delivery per jet change is around 4.21%. That is nearly double the change your altitude chart is suggesting.

Am I missing something here?

“Relative air density gauge” can you explain relative air density to me?

Referenced from:

It simply put is:
" Relative Air Density (RAD):

To allow us to get easy tuning information from air density data it is better to compare our value to a standard set of conditions, this is based on the international standard of 0 meters altitude, 15ºC, 1013.25mb pressure and 0% relative humidity.

So what we end up with is a simple comparison relative to a standard. If RAD goes up 3% it means 3% more density, if it goes down 3% it’s 3% less dense (and therefore 3% less fuel required).

When you have our engine tuned correctly you simply note down the RAD reading (or save it with the press of a button with ‘Weather Watch’). When racing under different conditions you note the new RAD reading, the difference in percentage is the difference in fuelling required i.e. if RAD is now 5% higher we need to set a 5% increase in fuel. Weather Watch displays this difference for you! "

It is just a way saying the reading on the Air Density Percent gauge is “Relative” to Standard Atmospheric Pressure, Temperature and 0% Humidity at Sea Level. For example, my Gauge is reading just below 95% with the current conditions of 30.27 inches of Mercury, 54.5 Degrees F, 49% Humidity at approximately 500ft above Sea Level, so it is currently 5% less dense than that of the Standard.

|Date↓ | Baro|Temp|Humd|Dew|CF-%|
|11/2/2019 3:35 PM|30.24|54.5| 0|34|93.7%|

Excuse formatting ( it looks better in Excel) the bottom line is my formula for air density. I got this from Shellquest web site. It calculates air density using only temperature and barometric pressure. Zero humidity. Can you explain the discrepancy in air density percenntage? It’s only 1.21 different from yours.

I am not sure what relative air density is. The description confused me even more. “Density Altitude” sums up everything air related and is the measure you should be using.

Slightly easier to read.

First off I am not trying to make this an argument. I am thankful for the input.

It is the Difference (in percentages) of Current Air Density Measured compared to what Air Density is Measured at the Standard Atmospheric Conditions. Its RELATIVE to the STANDARD.

What is confusing me is your chart correlating Altitude with Jet Diameter. Your chart shows Altitudes at -1500 ft, -500 ft, 0 ft, *500 ft, *1500 ft, *2500 ft and *3500 ft. Your Jet sizing progresses at a .01mm change for each progressing in elevation. For every .01mm in Jet size change you get a 3.45% change in fuel delivery, but for every 500ft change in altitude you only gat a 1.54% change in Air Density.

Why does the Jet size change up or down only .01mm from 0 ft to +/- 500 ft and +/- .01mm every 1000 ft after?

Found an article that talks about the relationship of Relative Air Density versus Altitude Density:

It included a link to an Engine Tuner’s Calculator:

I like that is shows the relative change in power output for an engine with change in Air Density.

I’m totally familiar with air density, there was a gauge in my pits all the time in the 90s. I totally understand what the air density gauge is measured. What I don’t understand is, what is it relative too?

I understand relative humidity, but I don’t understand the relationship between relative humidity and relative air density.