TeemuM wrote: ↑Fri Dec 04, 2020 1:59 am
I have now all electrics working and have been trying to get engine running. I washed SU´s and changed gaskets and needles. I have never had car with carburetors so I have zero experience working with them but after some youtube videos and online instructions I got engine running somehow. It is still running very rough and rich but problem with tuning them is that when engine stops it doesn´t start again. Even if it only runs few minutes and stops then it won´t start again before engine is become fully cold.
What could cause that? It tries to start but only coughs. It gets spark so I quess it is fuel related then.
SU carbs are a very simple design with few parts, working on vacuum only... no accelerator pumps, jets etc... just a needle and seat that rely on vacuum to pull fuel into the engine. The amount of vacuum is determined by the throttle plate, and the balance of the two carbs determined by the linkage. Problems occur when something stops those pieces from working together or properly. The seats need to be nice and round, the needles equally graduated and mounted, the pistons need to raise up without binding and in a controlled fashion (the reason for the oil in the dash pot... it controls how fast the piston rises which controls the amount of fuel and air delivered), and the throttle plates need to be equally open throughout their movement (balance), the amount open to determine rpm (other than an intake leak which is bypassing the throttle plates).
Assuming spark and ignition advance are all working properly, start off by making sure everything is set the same in both carbs, making sure the pistons raise and lower without a hitch. Disconnect your linkage, look it over and understand how it works both individually to set the throttle plates, and then together to set the balance, and note what happens to the throttle plates when you reattach your dog bones (do they change the position of the throttle plates when reconnected? All they should do it connect things, not change them). The bottom dial on the carb sets the position of the needle in the seat by raising or lowering it on the needle. This is used to set idle mix. As the engine vacuum pulls on the piston, it raises the needle up higher and thus increases the amount of fuel available to mix with the incoming air.
I'm not sure why it won't start when warm, but it's either too rich or too lean. I would have a second set of clean spark plugs at hand (you'll need them someday anyway!), and read your plugs when it won't start... black is rich, white is lean, either will stop the engine from running. If the plugs are white, turn the dial so it drops the seat down lower to richen the mix. Black, then replace the plugs, turn the dial to raise the seat and lean the carbs, and start it again. Some starting fluid can help an engine start in both conditions, but go lightly if you suspect it's rich. You can individually set the throttle plates on each carb with the linkage disconnected, but you wont be able to rev the engine up. Once it's running without dying, you will be able to balance the carbs (please use a carb balancer, either a snail or a unisyn, snail is better/easier to use), then connect your linkage and make sure it doesnt change the throttle plates position, check balance and adjust the linkage at the balance screw until they are equal, then set the idle, adjusting the individual throttle plate screws to allow adjustment if necessary to make sure they are not stopping the linkage from creating the balance or allowing the engine to idle at 800rpms.
Confounding factors can be intake leaks via worn throttle shafts or intake tract gaskets (locate with carb cleaner by spraying gaskets and noting changes in rpm), screwy linkage, worn needles and/or seats, or pistons not raising and dropping normally or equally.
Hope this helps! Spend the time to fully understand how the linkage works and what does what... it will help a lot in trouble shooting!