Carb jet sizing, input needed Options

[dfelz]

I have dual webber 40 IDFs I am in the process of rebuilding for my 1911cc engine. One of my idle jets needs replacing and since i am order one i wanted input of sizing, to see if i should change the other three as well. I looked at a sizing chart and factory is a 28 venturi with 1.15 main and .5 idle. I have 30 venturi installed, so that means more air flow, which should also be accompanied by more fuel flow to stabilize the ratio. I was thinking a 1.30 main and a .55 idle...... want your input.

ALSO, i am either ordering from cb performance or aircooled.net, unless someone else has other sources.... but when i was comparing the prices, aircooled is over double per jet... but they do state that their product is NOT from china, and cb doesnt list a supplier..... anyone know if cb's are china products???

Thanks!! (IMG:style_emoticons/default/beerchug.gif)

[michael7810]

My 1911 has 50/135s and runs good. I've tried bigger idles and it runs ok but the fuel mileage dropped from 25 to19. I've found it will run good on smaller idles as long as sync timing idle speed are right on. My AF is 12.7 (idle) 15 (highway no hill) and 13.5 WOT. I have a local tuner shop that sells jets for $7.50 and let's you exchange for different sizes for free

[ThePaintedMan]

David,
Those venturis are on the border of too big. Even though it's a 1911, that doesn't necessarily mean that bigger venturis are in order. That is highly dependent on head flow and other variables. Not saying they don't work, but what you're looking for is good velocity, which smaller venturis usually provide. Remember, putting more air through the carburetor does not necessarily mean the engine is sucking more. Feed it what it needs, no more, no less.

Michael's suggestions seem pretty close, but hard to tell if we don't know what vents he is using. I don't have my Weber book in front of me, but I'll check Tomlinson's recommendations when I get home - he has a whole chart in there.

115 mains are definitely really small for even a 1.7. You're looking for something probably in the 135-40 range. The 50 idles should be fine, as they are spec for the 1.7 and usually a little bit big for that engine.

[michael7810]

I'm running 32 vents. The engine died at 5k rpm with 28s and pulls good to RL with 32s. However I have made other changes since increasing the vents and have thought about trying the 28s again. Just not ready to start over at this point.