Re: 67.5. 1600. Carb issue
Posted: Wed Mar 31, 2021 9:42 am
Bajaroadster, my suggestion as always is to be systematic in your approach. You have done well to read up on the carbs and how they work. What you have is a graduated or tapered needle that sits into a nozzle (basically a tube with a hole in it that delivers fuel to the carb) that increases in flow when the vacuum from the engine causes the carbs piston to raise, thus pulling up the needle which progressively feeds more fuel to the engine since the needle is tapered. The taper of the needle determines the overall air to fuel ratio. At idle the needle is sitting in the nozzle which allows little fuel to get past it. By adjusting the knob or dial on the nozzle you are changing the position of the nozzle on the needle by raising or lowering it, thus changing the amount of fuel, both at idle and then as the vacuum causes the carbs piston to raise and expose more space around the needle while you drive. Tighten it and you lean, loosen and you richen.
Your back carb is rich. That can be due to a sunk float (pushes fuel past the needle due to increased pressure that is not being regulated by the needle and seat via the float) so check that. Carbs airflow must be equal, so a unisyn or similar carb flow meter is required (get a "Snail" version if you don't already have a tool) and make sure they are the same (if you struggle here we'll discuss carb linkage so you can adjust). Then the nozzle adjustment needs to be equal for both carbs. I won't discuss adjustment at this moment because you have probably already read about it, but it's important to understand that when you're disabling one carb at idle (pushing up the plunger underneath the carb) you're checking the air/fuel ratio on the OTHER carb. When you push up the plunger on one carb to test the other carb, idle should raise up for a second then drop, with the engine acting like it's running on two cylinders but not dying. I might try leaning it until it dies while testing, then adjust it a 1/4 turn richer until it continues to run while testing, then do the same to the other carb and see how it runs. Backfiring through the carbs while driving is too lean (white plugs), black of course is too rich and the car is boggy. Continue to adjust as needed. If you're unable to adjust it or lean it out then there may be worn parts that need attention.
Hope this helps!
Your back carb is rich. That can be due to a sunk float (pushes fuel past the needle due to increased pressure that is not being regulated by the needle and seat via the float) so check that. Carbs airflow must be equal, so a unisyn or similar carb flow meter is required (get a "Snail" version if you don't already have a tool) and make sure they are the same (if you struggle here we'll discuss carb linkage so you can adjust). Then the nozzle adjustment needs to be equal for both carbs. I won't discuss adjustment at this moment because you have probably already read about it, but it's important to understand that when you're disabling one carb at idle (pushing up the plunger underneath the carb) you're checking the air/fuel ratio on the OTHER carb. When you push up the plunger on one carb to test the other carb, idle should raise up for a second then drop, with the engine acting like it's running on two cylinders but not dying. I might try leaning it until it dies while testing, then adjust it a 1/4 turn richer until it continues to run while testing, then do the same to the other carb and see how it runs. Backfiring through the carbs while driving is too lean (white plugs), black of course is too rich and the car is boggy. Continue to adjust as needed. If you're unable to adjust it or lean it out then there may be worn parts that need attention.
Hope this helps!