Holley problems

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dubldart340

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I have a holley 780 vacuum secondary carb that causes the engine to break up at high RPM (I assume from loss of fuel). I have gone throught the carb and cleaned everything, replaced the power valve, needle valves, floats, gaskets, secondary diaphram, etc. and it still does it. The car runs great for general driving, just breaks up when I get on it.

I know it is in the carb, because if I replace it with another carb of the same model, the engine runs fantastic at RPM. I don't want to just use the carb that RPM's well because it is very old with a worn out throttle plate, thus causing the car to not run well in traffic, where I tend to use it the most.

Any ideas what could be wrong?
 
well, not knowing the history, it may be a tuning issue. What carb does it run well with? when does it stumble? right off idle, when you acccelerate from cruise, at cruise?
 
The carb I am questioning runs well right off idle until about 4000 RPM and then begins to expire (when making a full throttle launch). It will also do it if accelerating from a slow speed (rolling start). It runs well for general driving, it is only a problem when doing a full throttle run (like at the strip).
 
try the sec, spings they come in diff.waights lite to hevey if to lite it will come on to strong bog or backfireing sometimes,play whith dif.spings till you get it. lon
 
I would do a couple of things. First, make sure the float levels are correct in both bowls. If they are fine, find a cheap fuel pressure gage, and plumb it inline so you can watch it while you drive (like run a long piece of fuel hose out the back of the hood, and tape the gage to the windsheild, away from heat sources..lol). What is the rest of the fuel system? What pump? line size, etc. I'm thinking either you're uncoverring the jets under accel, or draining the bowls from not enough fuel delivery.
 
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