I rebuilt my Rochester quad a few months ago. It runs good and starts good. I went to fine tune the idle adjust, and when I screw both jets all the way in nothing changes. RPM's stay the same, never studders or misfires. Any idea what might be the problem? I had it out on the water today with both screws at 2 1/2 turns out, and it ran fine.
@@ashtielgreymark5465 I wasn't trying to change the rpm's at which the engine idles. I was making the initial adjustments according to the instructions.
Either the fast idle screw is set wrong or a vac leak, most likely the secondary throttle shafts. Either condition can put the carb into 'off idle' or 'part throttle' condition which bypasses the idle circuit, rendering the idle screws useless. I'll bet idle with the idle screw all out will still maintain @ 1000rpm instead of bogging down. A carb is an air-pump, and will pull fuel with high vac from the secondaries, feeding the motor. If the fast idle is wrong, it opens the primary throttle plates high enough to allow vac to pull fuel from the transfer slot, bypassing the idle screw slot
@@gypsy6211 Thanks ! I'll troubleshoot it when I break it out of winterization this Spring. It ran fine and accelerated just like it did before I rebuilt the carb.
@@daisy8luke left side, under the choke is the fast idle cam. Back that all the way out. Set idle mixture screws to 3½ turns out from lightly seated. Unload the choke so that the primary plates are fully closed. Set the right side idle screw @ 1½ turns in from touching the throttle. Choke should be set to where it snaps the choke plate fully closed. When you pull on the left side vac pot, it should crack the choke plate open @ ¼" at the back, adjust that as needed for smooth running after start. Reset the choke so the choke plate is fully closed, hold the choke linkage in fully up position, then adjust the left side fast idle screw until the cam is barely on the last notch, about to drop into the middle. That's essentially the default exterior settings, minor adjustments using vac guage, tach and ears should be made. On start, the motor should sit @ 1200-1800rpm depending on you and should idle smooth. Once the choke comes off and engine is warm, it should idle @ 700-800rpm in park and drop to @ 500rpm in drive. Fast idle screw is only for initial warmup, it should drop off entirely soon as to tap the gas pedal on warm, but during warming will only step down a bit. Once off, that's when your idle screw, mixture screws take over. Of course if there is a vacuum leak, from anywhere, it's going to prioritize everything and nothing will set correctly. There's no screw or bolt in a carb or mount that requires high torque, everything should be hand tight snug, not cranked down, or that warps the carb, especially the airhorn, and makes things worse.
Why not just use a vacuum gauge and tune to maximum vacuum instead of this guessing way I’m not saying your wrong but I don’t think your right just saying
@joshbohner1712 You may not want to say he’s wrong but I’ll say he is definitely wrong, There is nothing in this video that is helpful to anyone whatsoever except for the person who wants to have trouble docking! These carburetors were never meant to run on modern day fuels, This carburetor should definitely have been rebuilt and modified to work with modern fuel! Not only that you must add a fuel pressure regulator and drop the fuel pressure down!
I rebuilt my Rochester quad a few months ago. It runs good and starts good. I went to fine tune the idle adjust, and when I screw both jets all the way in nothing changes. RPM's stay the same, never studders or misfires. Any idea what might be the problem? I had it out on the water today with both screws at 2 1/2 turns out, and it ran fine.
If you want to change the rpm you idle at you need to change the high and low idle screws not the mixture screws
@@ashtielgreymark5465 I wasn't trying to change the rpm's at which the engine idles. I was making the initial adjustments according to the instructions.
Either the fast idle screw is set wrong or a vac leak, most likely the secondary throttle shafts. Either condition can put the carb into 'off idle' or 'part throttle' condition which bypasses the idle circuit, rendering the idle screws useless. I'll bet idle with the idle screw all out will still maintain @ 1000rpm instead of bogging down. A carb is an air-pump, and will pull fuel with high vac from the secondaries, feeding the motor. If the fast idle is wrong, it opens the primary throttle plates high enough to allow vac to pull fuel from the transfer slot, bypassing the idle screw slot
@@gypsy6211 Thanks ! I'll troubleshoot it when I break it out of winterization this Spring. It ran fine and accelerated just like it did before I rebuilt the carb.
@@daisy8luke left side, under the choke is the fast idle cam. Back that all the way out. Set idle mixture screws to 3½ turns out from lightly seated. Unload the choke so that the primary plates are fully closed. Set the right side idle screw @ 1½ turns in from touching the throttle. Choke should be set to where it snaps the choke plate fully closed. When you pull on the left side vac pot, it should crack the choke plate open @ ¼" at the back, adjust that as needed for smooth running after start. Reset the choke so the choke plate is fully closed, hold the choke linkage in fully up position, then adjust the left side fast idle screw until the cam is barely on the last notch, about to drop into the middle. That's essentially the default exterior settings, minor adjustments using vac guage, tach and ears should be made. On start, the motor should sit @ 1200-1800rpm depending on you and should idle smooth. Once the choke comes off and engine is warm, it should idle @ 700-800rpm in park and drop to @ 500rpm in drive. Fast idle screw is only for initial warmup, it should drop off entirely soon as to tap the gas pedal on warm, but during warming will only step down a bit. Once off, that's when your idle screw, mixture screws take over. Of course if there is a vacuum leak, from anywhere, it's going to prioritize everything and nothing will set correctly. There's no screw or bolt in a carb or mount that requires high torque, everything should be hand tight snug, not cranked down, or that warps the carb, especially the airhorn, and makes things worse.
You can tune a piano, but you can't tuna fish or a Q jet.
Seriously, when the Q jet is tuned, they are a great carb!
What? I have some serious doubts about this video.
Why? That’s the “tip in procedure”. Check Cliff Ruggles book.
Why not just use a vacuum gauge and tune to maximum vacuum instead of this guessing way I’m not saying your wrong but I don’t think your right just saying
@joshbohner1712 You may not want to say he’s wrong but I’ll say he is definitely wrong, There is nothing in this video that is helpful to anyone whatsoever except for the person who wants to have trouble docking! These carburetors were never meant to run on modern day fuels, This carburetor should definitely have been rebuilt and modified to work with modern fuel! Not only that you must add a fuel pressure regulator and drop the fuel pressure down!
I have never TUNNED a carburetor. I have tuned many though......