440 Vacuum Leak, where to start?

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BigBen

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I have finally got my 1970 Duster with 440 on the road. One niggling problem that I am having seems to be coming from a vacuum leak. The motor was running an AFB Competition 750cfm carby, when idling it would run a little rough with a big flat spot off the mark, then under load a loud whistling noise would come in and remain until the motor is turned off. When restarting the whistling would be gone until under load again. I replaced the carby with a brand new Holley 750, motor now starts a lot easier, idles fantastic, no flat spot...but...it still whistles. Not quite as bad as before but enough to be annoying. I suspect the leak may be coming from the 1" Phenolic riser that is under the carby as I did not remove this when I installed the new carby. This will be quick and easy to check, but I'm not sure where to check next if this is not the source. I guess what is throwing me is that the noise doesn't come in until your in gear driving. Could the leak be associated with the 727 ?
 
Carby? That makes it sound all warm and fuzzy. Performance carburetors are supposed to be mean and nasty and suck your HEAD in if you get too close. "Carby" just doesn't sound right. On to your problem. Whistling sounds are always always ALWAYS at the joint where the carby (or spacer in this case) joins the intake manifold. Simple to locate. Get you a spray bottle full of water and while idling, spray all about the carby base and around the spacer plate where they bolt together and where the spacer plate bolts to the intakey. When you hear a difference in idle speed, you found your vacuum leaky. Since the torqueflites have no vacuum assist, I fail to see how the transmission could be involved. Keep us posted.
 
Thanks Scampy, we like to throw a "y" on most things down here, carby, dizzy (distributor), etc..thanks for your help though I will try it today.
 
Problem fixed. First suspicion proved correct, the leak was coming from under the 1" riser. Sprayed some engine start around the base with the motor running and sure enough the revs picked up slightly and the whistling stopped momentarily. Two new gaskets and all is now good.
 
Coolness. But I recommended water for a reason. SAFETY!!! I've seen spary solvents cause fire when checking for leaks. Fire sucks. Glad you got it fixed!!! You got some pics? I wanna see that carby. LOL
 
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