Effect of larger carburetor on jetting?
  • mustangnut
    Posts: 22
    Joined: Sun Jan 28, 2018 11:04 am

    Effect of larger carburetor on jetting?

    by mustangnut » Fri Apr 27, 2018 8:06 am

    Trying to understand the effect of a larger carburetor on jetting. If you have a carburetor which is operating correctly but want to install a larger one for better performance, how will the jetting & needle setting change?

    For example, a 125cc engine with a Keihin 20mm carburetor, 40 pilot jet and 118 main jet. If I went to a 24mm Keihin, would my jets be the same size or larger? I understand the velocity thru the carburetor will be lower...would that require larger jets?
  • Back2-2
    Posts: 1148
    Joined: Tue Jan 04, 2011 3:31 pm

    Re: Effect of larger carburetor on jetting?

    by Back2-2 » Fri Apr 27, 2018 1:21 pm

    More air requires more fuel. You will be having a higher amount of air flowing through the throat so more fuel will need to be supplied. As long as you have an engine that will use that extra amount of air flow you will have larger jets in the larger carb. Basically you will be starting from scratch to find the optimum jetting required for your engine and the new carburetor. Many factors come in to play, engine type and mod's, air box mod's, intake mod's ect...
    Neil
    Black Hills of SD
    Life without Motorcycles would just be boring, really boring
    Honda 450X. Yamaha Tracer GT900. HD Fat Boy. Triumph Bonneville. Yamaha Majesty 400. Yamaha Grizzly. Yamaha Wolverine. Yamaha TW200
  • Aussiecrf230
    Posts: 1962
    Joined: Fri Jan 06, 2006 11:11 pm

    Re: Effect of larger carburetor on jetting?

    by Aussiecrf230 » Fri Apr 27, 2018 3:53 pm

    Depending on the engine you may lose some low end tractability with the larger carby. If it was very under carbureted to start with there may be no real difference. Have you changed anything within the engine like the cam? Changing from a 20mm to a 24mm is an increase of throat area of 44% and potential flow. It can also be a loss of airspeed through the carby if your engine can't utilise the potential of the larger carby. Let us know how you go if you decide to give it a go as others might be thinking of doing it as well.
    Ray
    Australia

    CRF230F 2004
    C30F Power Up needle
    Mains 132
    Idle 45
    2 turns out
    Baffle out, Screens In

    It starts,it runs,it gets to where all CRFs can get to without the valve or valve plate dramas
  • mustangnut
    Posts: 22
    Joined: Sun Jan 28, 2018 11:04 am

    Re: Effect of larger carburetor on jetting?

    by mustangnut » Sat Apr 28, 2018 2:22 pm

    Thanks for the responses. The bike is a CRF125FB which is getting a Yoshimura exhaust, high flow air filter & the aforementioned jets this weekend in the stock 20mm carb. Otherwise its bone stock except the air snorkel is removed.
    No info around for this bike, but I'd read articles for other 125s (like the TTR-125) where they recommend 24mm carbs. Thought ours would benefit from the same after opening it up, but no idea on setting it up.
  • Aussiecrf230
    Posts: 1962
    Joined: Fri Jan 06, 2006 11:11 pm

    Re: Effect of larger carburetor on jetting?

    by Aussiecrf230 » Sat Apr 28, 2018 8:32 pm

    A stock carb of a larger engine model of the same family would be a good starting point. The jets are sized to work with the bore of the carby. There is also a good chance the needle would do the job or perhaps the original depending on it's sizing. I would look at a 150F carby as a possibility if you really want to go bigger. Play with what you have to start with so you know how far that takes you. Without a camshaft change you may not get a bigger top end anyway.
    Ray
    Australia

    CRF230F 2004
    C30F Power Up needle
    Mains 132
    Idle 45
    2 turns out
    Baffle out, Screens In

    It starts,it runs,it gets to where all CRFs can get to without the valve or valve plate dramas
  • mustangnut
    Posts: 22
    Joined: Sun Jan 28, 2018 11:04 am

    Re: Effect of larger carburetor on jetting?

    by mustangnut » Sun Apr 29, 2018 4:35 am

    The CRF150F has a 24mm; the CRF100F had a 22mm; but the CRF125F only has a 20mm. The 22mm and 24mm carbs appear to have the same dimensions and manifold side o-ring as the 20mm, so installation shouldn't be an issue.
    My thinking was to buy a CRF150F carb as a direct bolt-on for our little 125. Now wondering if it may be too big and go for the 22mm off a CRF100F(?) I can get an OEM 22mm carb for about $120; 24mm is $250.
  • Leardriver
    Posts: 462
    Joined: Wed May 05, 2010 10:33 am

    Re: Effect of larger carburetor on jetting?

    by Leardriver » Sun Apr 29, 2018 8:17 pm

    My guess is that it will never run like you want, and that you will hate it.
    Smaller carbs have higher velocity, better low and mid range.
    Last edited by Leardriver on Mon Apr 30, 2018 10:27 am, edited 1 time in total.
  • Aussiecrf230
    Posts: 1962
    Joined: Fri Jan 06, 2006 11:11 pm

    Re: Effect of larger carburetor on jetting?

    by Aussiecrf230 » Sun Apr 29, 2018 11:07 pm

    My guess is that it will never run like you want, and that you will hate it.
    smaller carbs have higher velocity, better low and mid range.
    Have to agree with Leardriver on this, unless you have a head and cam to use the extra airflow you could be just throwing money away. Ensure you have the right springs for your weight and sag set properly would be a better way to spend your dollar.
    Ray
    Australia

    CRF230F 2004
    C30F Power Up needle
    Mains 132
    Idle 45
    2 turns out
    Baffle out, Screens In

    It starts,it runs,it gets to where all CRFs can get to without the valve or valve plate dramas
  • mustangnut
    Posts: 22
    Joined: Sun Jan 28, 2018 11:04 am

    Re: Effect of larger carburetor on jetting?

    by mustangnut » Mon Apr 30, 2018 7:56 am

    Well, it was worth investigating. Not that familiar with these tiny carburetors on tiny motors and figured I would ask here before I invested money. What works on the Yamaha TTR125 may not work on the Honda. If I do ever give it a shot I'll post my results.

    I posted over in the CRF125 forum that I installed the exhaust, removed the air snorkel & re-jetted the factory carburetor this weekend. Difference was incredible. That little thing actually pulls good in every gear now. Dad may have to get his own trail bike soon \:D/
  • Aussiecrf230
    Posts: 1962
    Joined: Fri Jan 06, 2006 11:11 pm

    Re: Effect of larger carburetor on jetting?

    by Aussiecrf230 » Mon Apr 30, 2018 11:09 pm

    Glad to hear you have had success in rejetting. Is it more like you want now?
    Ray
    Australia

    CRF230F 2004
    C30F Power Up needle
    Mains 132
    Idle 45
    2 turns out
    Baffle out, Screens In

    It starts,it runs,it gets to where all CRFs can get to without the valve or valve plate dramas
  • mustangnut
    Posts: 22
    Joined: Sun Jan 28, 2018 11:04 am

    Re: Effect of larger carburetor on jetting?

    by mustangnut » Tue May 01, 2018 5:34 am

    For the time being, yes. My son had never ridden until I got him this bike a couple of months ago. Once he progressed past learning to ride, I wanted to creep up on the power for him. I understand there is a lot of power to be had with higher compression pistons, ported heads & cams... or just get a bigger bike. I love to build and work on things, so who knows which route I may end up going.

    Just to add to the conversation, I've read a couple of theories on sizing carbs while maintaining good "rideability". Love to hear some input from those with more experience:

    1. To keep up velocity, stay 1 to 2 millimeters less than the valve head diameter, but never larger than the valve diameter. The intake valve for the CRF125F is apparently 25.5mm (same valves as the CRF110).

    2. Take the carb size & divide by two. Square your result to determine the engine displacement in cc's for good performance with stable jetting:
    20 mm → 100 cc
    22 mm → 120 cc
    24 mm → 145 cc
    26 mm → 170 cc
    28 mm → 200 cc
    30 mm → 225 cc
    32 mm → 255 cc

    I'm still interested in the whole "airflow past the venturi requiring more fuel, even with same cc's" information. I may make a trip to the library and tinker with a 22mm carburetor just for fun. Still think I need a CRF230F for me though 8)

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