David
I am familiar with Webers having a few but, one of my TC’s has Delorto’s (S3SS), late last year I had a petrol leak below the front carb, found out they have diaphragms! Got 2 new ones form PM, replaced them thinking all was well.
All was not well, I forgot that petrol eats most sealants, I had put a VERY thin layer of Silicone on the face of the diaphragms, put the carbs back on the engine a day later so it would dry. Engine started fine, got to tune it and thought I was ready for spring! Well, the next time I tried to start it, no such luck, really had me baffled!!! Tried all the normal stuff, slept on it a few times. For some reason, no idea why, I took the K&N filters of and when trying to start the engine, I covered two of the trumpets, the engine came to life, ran rough but on all 4 and would rev but slowly?
So, decided to take the front carb off and check to see if I messed up putting in the new diaphragm, well I did, little bits of silicone evident and, I assumed blocking all the accelerator jets, so out they came and got a good cleaning, all back together without ANY kind of sealant, started and ran fine, back to the same outstanding performance from a rebuild done by Vegantune 28 years ago then parked for 27 years or so! Remember to check and clean the ball seat valve in the plate where the diaphragm screws to!
No I am not saying you have silicone blocking your jets, your problem sounds so much like mine, all seems well, spark, battery, was running etc., you very well just may have some dirt in the accelerator jets? I noticed that the Delortos do not squirt fuel into the head, it is more of a dribble?
I have done stupid things in the past, will do stupid things again, it is great the I am still capable of figuring my stupidity out and fixing it!
Good luck and keep us posted!