Cool weather jetting

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    Wade Metzinger

    33mm carb on a ZR3 3×3 motor. Works great until the temps drop then it doesn’t want to do a burnout and stumbles just off the line.

    I think that means a pilot jet adjustment but bigger or smaller?


    (in my big car, you go bigger jets because it can burn more fuel with the cooler and dryer air but everything seems to be backwards on these Jrs so I figured I’d ask)

    2005 Halfscale ZR3 Shockwave 8.90

    Matt Parlett

    If your car is slowing down or stumbling off the line when the temps drop then your issue is likely either your not getting the engine warm enough, or your air / fuel mixture is off and your hitting your lean wall.

    So the more likely of the two is that you are having an issue where the temp drops and the engine isn’t getting enough heat in it. If this is the case, we can confirm it by looking at what the cylinder head temp was at the zero time point of both the run where it was fine and the run where there was a problem. If there is more than a 15 degree difference then a lack of heat in the engine is likely the culprit.

    If you need to get more heat in the engine, the easiest solution is to install a lean out valve. This will allow you to lean the car up on the starting line to help build temperature easily. Then you just close the valve when its at temp and your back to your normal idle rpm. The other option is to lean out your pilot, however I would do this before a race and not in between rounds if we can avoid it. Either go up on your air bleed or open your air bleed screw more and it should cause that starting line temps to be warmer.

    If the issue is truly a jetting or air / fuel mixture issue then you’ll likely be able to see it in your graph and the starting line temps will be similar. If the weather is cooling and the humidity grains aren’t jumping, then it means that we are adding more air to our air / fuel mixture. And if you stumble when that happens it means you don’t have enough fuel and you likely need to richen your entire mixture by increasing your main jet size. It would mean that you have hit your lean wall.

    With our RaceBase software we have a Mikuni jetting program that you would be able to enter your jetting information in, tell it you want to richen it by 2% and it will tell you what jet size to run. It will then allow you to maintain that same air / fuel mixture throughout the entire season. We used in in the @RowansRacing Top Jr Dragster this season and we didn’t have to move more than 15 pounds the entire season.

    Wade Metzinger

    Thanks for the feedback, I’ll try to implement some of the info this spring.

    Do you have a picture of your lean out valve? Did you use the port on the intake for the fuel pump and just added a valve?

    2005 Halfscale ZR3 Shockwave 8.90

    Jr Dragster Plus

    It’s a pretty simple part you can get from ace or true value. I do know if a company making a nicer one but it’s not out yet. 

    Nic Woods


    when its cold out side so is your engine.  best thing to do if its been running great all day and then the struggles come when the sun goes down and it cools off – is to just get a head start on trying to get some more heat in the motor by starting early and possibly doing a few burnouts.


    If that doesn’t fix it..  going richer on the pilot. and a tickle leaner on the main jet (due to the humidity/ mositure will get you out of the stumble.. But you still have to make sure you get some heat in the motor on a cool night.



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