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Carburettor tuning help

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  • Carburettor tuning help

    Hey everyone so I've got a Mazda 121 1989 and the previous owner had messed with the carburettor. Was idling too high so I got under the bonnet and started to mess with all the adjustable screws, not she sounds worse than ever . Anyone dealt with one of these carburettors before? What does each screw control? There's some sort of button mechanism on the far left side connected to a wire with a screw to adjust how far it is pushed down, with no acceleration do I want it as far pushed down as possible? half way? So confused and stressed about this as the car is now running really rough . Any help would be fantastic guys!!

  • #2
    The second picture appears to be an idle speed control screw, with the wire connected. On most carburettors this should be as far out as possible, usually just touching and a little bit in. If the car is running rough like this it probably is too rich or too lean. If it isn't backfiring when you floor it than probably too rich. Might be helpful to buy a cheap air/fuel mixture gauge that can hook on to your oxygen sensor (if it has one) and definitely a cheap vacuum gauge.


    • #3
      Only gauges in my area cost around $200, otherwise I'd have them, just don't have the money lying around at the moment. So it is supposed to backfire at really high revs?


      • #4
        From your use of the word "bonnet" you would seem to be outside North America. Still, the price of a tachometer or vaucum guage should not be more than $50, probably more like $30. I'm in Canada and our dollar is about the same as the Austrailian dollar. Try a search of the Internet, especialy the for-sale-by-owner sites for used equipment. First I'd give the carburettor a cleaning by spraying carburettor cleaner inside and on the outside linkages. Your first photo shows the white plastic thumb screw which adjusts the idle speed. If you have the haynes manual look on page 5-6 under "Ignition timing - check and adjustment" to see how to check the idle speed. You don't need to go all the way and check the timing, just the first part about checking the dile speed. You have to pull the top hose off the distributor and plug it, then start the egnine and let it warm up, then adjust the screw until the tachometer reads 750 rpm.

        Your second photo shows the accelerator cable linkage. The Haynes manual says to adjust the cable using the lock nut on the support. Fuilly depress the accelerator and check the throttle inside the carburettor is fully open.Make sure the throttle closes fully when the accelerator isn't depressed.

        The Haynes manual has a long list of things to check outside the carburettor from vacuum hoses to "electronc engine and carburettor controls" Your performance problems may not be in the carburettor itself.

        Good luck.
        Original owner of silver grey carburetted 1989 Festiva. 105k km as of June 2006.


        • #5
          Here's what you need. At this price I may buy one. You gotta wonder how accurate it can be but then again your most expensive units are probably assembled with a few dollars worth of components..

          '88 Festiva LX 5 speed, A/C, Carb, restored $$$ body paint, badly wrecked @ 200k.
          '93 Festiva L, 5 speed, Aqua, bought from the original owner,.Zero rust but very nasty otherwise. Awaits the B6T.
          '91 Festiva L, 5 speed, bought to drive while putting the B6T in the '93. now B6ME powered.


          • #6
            You guys are fantastic, thanks for all the help, been meaning to grab a Haynes manual just haven't got around to it yet, survived so far without one, but I think now's about time I got one. Haven't looked online for a gauge yet but they're awfully expensive in store here. yes I do live in the outback Australia.


            • #7
              No need to buy gauges, just tune it by ear. Set the idle speed as low as possible, but high enough that the engine is running fast enough to run smoothly and it doesn't die easily while letting the clutch out slowly.

              The thing with the wire attached is probably a solenoid that bumps the idle speed up when you turn the lights or hvac fan on settings 2/3 to compensate for increased electrical load on the alternator, or AC compressor load. Adjust that with the fan on high and headlights on, or AC on.

              The fast idle needs to be adjusted as soon as the engine is first started, and fully cold so the choke is engaged. That's the only time the fast idle screw does anything.

              The idle mixture screw may be blocked by a brass cover, many carbs had it blocked to keep the owners from adjusting it. Find it and adjust it using the "lean best idle" technique found here:

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              • #8
                Does anyone know if a Weber carburetor or another aftermarket one would be a direct fit, if so which one? Is it worth it? Would it be easier to tune or harder? Thanks guys


                • #9
                  Did you get this problem solved yet? The "button thing" is an idle switch. Its only function is to signal the computer that the throttle is in idle position. In the second picture, the screw that is under the fast idle screw is the idle-up adjustment screw (not showing) used for adjusting accessory load (blower, lights, AC). There has been information posted before about Weber conversion. You may find it using the search. How hard it is depends on your ability to make modifications and adjust carburetors. Your carb may not be bad. The carb system has many points where there could be an air leak, especially the vacuum hoses. The engine may have been idled up to compensate for some other problem such as a bad oxygen sensor.
                  Last edited by tooldude; 04-24-2020, 02:19 AM.
                  When I'm good I'm very, very good and when I'm bad I'm HORRID.


                  • #10
                    If you go to the, he still has rocketman carb adapters for webers and escort carbs for $85.00. I've had an Escort carb with that adapter for years and I love it! Just my opinion but I think the Escort carb is far superior to the Weber. I have had both.


                    • #11
                      Originally posted by zoe60 View Post
                      If you go to the, he still has rocketman carb adapters for webers and escort carbs for $85.00. I've had an Escort carb with that adapter for years and I love it! Just my opinion but I think the Escort carb is far superior to the Weber. I have had both.
                      The escort carb is a single barrel, won't I be loosing power?


                      • #12
                        No, the Escort carb is a two barrel and will have more power than the original Festy carb. Just look around on Ebay for an 82 - 86 Escort carb and check it out. Very similar to a Weber 32 36 but IMHO constructed way better. Not to mention way cheaper.


                        • #13
                          Here's one. I'd shop around more. See that vacuum port on the upper left? That makes the idle go upu for the air conditioner. Can't do that with a Weber, and it works perfectly and better than my original. I do have air though, so that is important to me. may not be for you. Good luck!