Mystery Vacuum leak

[powerwaggin]

My 1975 318 powered dump truck starts and idles fine, revs up nice but lacks power and backfires thru the carb at mid throttle even in neutral with no load.. Definitely a lean burn problem. Carb works fine on my other truck...have tried with two other complete ignition systems with exact same results. Has new plugs gapped at .035. The weird thing is that every now and again she'll run like a scalded dog...then later start acting up again...sometimes when hot, sometimes when cold. Today I sprayed starter fluid around the carb flange on both sides and it picked up noticably....gasket is new and carb is tight...could this be the famous intake crack I've read about? I'm going nuts as I generally know my way around this engine's various symptoms having plowed snow for 25 years with a 1973 W200.

 

[Demonracer]

It might be the throttle shaft bushing, I've had that problem from time to time with older carbs. You can either change the carb, or find some one who has the ability to rebush the throttle shafts.

Good luck,

Bill S.