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Hey gang, I've looked around and didn't see anyone mention this issue about four bolt dome top SU carbs that run horrible after everything else on the engine is correct. Recently bought an early 1970 Series 1 and it's been having really bad stumbling issues when you open the throttle from idle. Timing is dead on, valves are within spec, compression is good on all cylinders, cleaned up the carbs and even swapped out a few bits and pieces from a donor set of carbs. Bushings are in really good shape and there is almost no vacuum loss from the throttle shafts. After tuning the carbs way out of spec I got it running half way decent. Nearly impossible to sync the front and rear carbs without the front being overly rich. Most of the hesitation upon accel was tuned out but it was smelling super rich and power was lacking. Luckily this is our project car so I wasn't driving it anywhere serious. I know I am doing everything right... why is it running like crap. Fast forward several weeks and we are replacing those aftermarket K&N filter cans off the SUs for a stock air cleaner assembly. In the process I start hooking all the hoses back up to stock and discover that the previous owner have rerouted all of the vapor hoses coming off the SU's into a T fitting that basically went nowhere. Hmm, that's weird. Then I start investigating the other vacuum hoses and discover the vacuum port on the front SU isn't connected to anything and the vacuum canister on the dist is connected to the fuel vapor fitting from the gas tank! WTF!!! *Light bulb goes off in head*!! Are you freaking kidding me!? I totally see how I missed this one glaring issue. The hoses went down out of sight and I assumed they were the same hose. DOH!! Needless to say, once the vac port and the canister were correctly joined - I did a 5 min basic tune on the SU's - the 240 purrs like a kitten and has full power through the entire acceleration curve. What do we take away from this experience... If you have a poorly running 240 it might not be the carbs. Make sure to check the vacuum canister is working properly and is receiving vacuum from the vac port on the back of the carb body. I would have saved myself a few weeks of aggravation if I had noticed this from the start.
