Weber IDF Main jet help Options

[tornik550]

I am in the process of tuning my car after a rebuild. I have dual weber 40's. My engine is a 2.27L. Everything is going pretty well. I am slowly getting the jetting closer and closer. I was driving around at about 60mph yesterday. The head were hotter than I like (325 degrees). I checked the spark plugs and they were bone white. The main jets were at 160. I increased them to 170 (above 160 I only have increments of 10). With the increase to 170, the car is running very well and the temp at around 60mph cruising is around 275. I sped home, tuned off the car and took out the plugs- bone white. I was quite surprised.

The main jet size sounds high too me. Does it sound high to everyone else? Is it possible that I am needing a high main jet size because of the float level? I have my floats set at 10.5 with the gasket.

Any opinions would be greatly appreciated

[john rogers]

From several of the posts here I seem to read that many think that the float raises up and at a certain level shuts off the fuel flow and acts like the float in a toilet tank. The guys at Pierce Manifolds (Weber repair masters) explained it to me this way as did several old German Porsche mechanics: The float and it's attached needle valve is bouncing like mad inside that carb body and on a race/street car with stiff springs and shocks it is worse than the carb was ever designed for. That is one of the items that PMO rectified with their design but unfortunately they do not make a 4 cylinder version. Anyways, since the needle does not actually shut tight and stay that way when you raise fuel pressure at the bowl it causes some spill over due to the increased force against the needle valve. I was not sure of this but when notable racers such as Wayne Baker and others said to bump up pressure by 1/2 to a full pound on a track such as CA Speedway or Willow Springs Big Track where you are at full throttle a very long time and this will provide some insurance against running out of pump capacity and going too lean. Sure enough, when at full throttle the O2 mixture would go rich by 1 full number, usually from 13.51 mixture to 12.5:1 mixture. On short tracks I would lower fuel pressure to about 3.5# and the car was very happy.

Of course this bouncing also affects the set float level and is one of the reasons why it changes over time. As a matter of fact the guys at Pierce Manifolds strongly suggest that float levels NOT be set until the car has been towed to the track as the vibration of the trip can affect the float settings!