holley 750 help
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Senior User
Joined: Jun 2005
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From: Bethlehem, PA
holley 750 help
ok so i just rebuilt a 400 punch 30 over...upgraded the cam, the springs and all that jazz....so i got an edelbrock performer intake for it and a good deal on a holley 750 double pumper...got it all setup and dropped in the truck, using a 1 inch spacer and im at the point where im trying to set the timing/tune the carb in. just when i think i get the timing pretty much on...i set tweak the carb a lil bit it idles fine...revs pretty decent, but the carb backfires...sometimes just a bang...sometimes a nice flame...im thinking too much fuel?...i dont have a lot of experience with tuning a fresh carb...any help would be majorly appriciated
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Joined: Nov 2005
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will it backfire if you accelerate slow? with the engine off look down the carb and see how far you open the throttle before the pumps start squirting gas? if you have no intke leaks this carb should run without backfiring. do you know what size pump is on the secondaries. the linkage to the pumps probably need adjusting or you might have to put differnt squirters in it.
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From: Bethlehem, PA
i used a four hole spacer for the carb....i doubt the intake is leaking but if it was sucking air would that cause a large backfire like that?....when i accelerate slow it doesnt backfire...just when i do a quick acceleration
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Joined: Dec 2001
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From: Louisville,Ky.
You need to look down the carb with the engine off, push the throttle open and make sure that you have 2 streams of fuel that shoot out of the squirter nozzle. Also make sure that the vacuum advance is working properly. It should be hooked to the passengerside port on the front metering block just ahead of the choke.
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From: Bethlehem, PA
the two squirter nozzles are working properly as far as i can see i took off all of the vacum lines on this motor except for the few essentials...its my first rebuilt so i may be slightly off...but this is how its all hooked up...the pcv is "T" ed into a line with the booster going to the back of the carb, the distributer is hooked to the fron of the carb at the bottom, and the tranny is hooked to the top of the carb just infront of the choke..also i dont think it makes a diff. but this carb did not come with a choke on it, i figured for right now because of the warm weather i could get away with not having a choke on it for now...
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Joined: May 2004
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From: Phoenix, Az
Modulator line needs to go to intake manifold. PCV can go to fitting bottom rear of carb. You may want to add a vacuum tank/reservoir for your power brake booster. I believe on your carb, there should be a vacuum fitting in the piece between the body of the carb, and the float bowl. I can't remember what it is called right offhand. A senior moment 
!
As previously noted, the front squirter should squirt as soon as you move the throttle linkage. The secondary squirter as soon as the secondary throttle plate moves. Check that the fuel level is at the bottom of the hole in the sides of the bowls at idle.
Since you have had several backfires, you should check the power valve(s) as they are prone to rupture.
The carb should work fine on that motor. I ran a 750 double pumper on a 1971 302 Maverick with no problems, years ago, except keeping the tires hooked up!
!As previously noted, the front squirter should squirt as soon as you move the throttle linkage. The secondary squirter as soon as the secondary throttle plate moves. Check that the fuel level is at the bottom of the hole in the sides of the bowls at idle.
Since you have had several backfires, you should check the power valve(s) as they are prone to rupture.
The carb should work fine on that motor. I ran a 750 double pumper on a 1971 302 Maverick with no problems, years ago, except keeping the tires hooked up!
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From: Bethlehem, PA
ok so i figured it out...and man did i kick myself in the a@@ ....i had the firing pattern off at the distributer by one all the way around...so after fixing that i set the timing and advanced it about 8 degrees and adjusted the carb...no more backfire but it kind of lags on the get go...right off the line its laggy, but once it hits higher rpms it really picks up nicely, im thinking the carb is dumping too much gas maybe? would i be able to advance the timing more to get a better take off or do i have to rejet the carb?
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Joined: May 2004
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From: Phoenix, Az
By "laggy", should we assume you mean it has a flat spot? Kinda bogs, or doesn't want to take off as good as at less throttle?
If so, then you may not be getting enough of a pump shot. Holley makes kits and has available, different size "shooters", accelerator pumps, and cams to "fine tune" your carb. Holley also publishes a very good book for tuning your carb. It provides a lot of insight as to what will actually accomplish what.
If so, then you may not be getting enough of a pump shot. Holley makes kits and has available, different size "shooters", accelerator pumps, and cams to "fine tune" your carb. Holley also publishes a very good book for tuning your carb. It provides a lot of insight as to what will actually accomplish what.
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From: Colville, Washington
The distributor line and the tranny lines are not hooked to the proper ports. The distributor vacuum advance should be hooked to the small port on the passenger side about mid way up the carb in front of the choke. It is the only vacuum fitting in the metering block. It will have no vacuum at idle and then increase as the throttle is opened. The tranny line should hook to manifold vacuum, it is usually hooked to a fitting at the back of the intake manifold.
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From: Bethlehem, PA
ok so its been a while but i finally got everything pretty straightened out, but when i got my new lifters one of them was a defualt, there was no hole drilled in the top for the oil to cycle and that led to problems quick, still trying to get a hold of comp cams...i guess they dont like to be bothered about that stuff but after a whole day rippin down the top end to fix it and new gaskets...yea i was a lil steamed...So anyway one last thing im workin on here i have the timing adjusted good and in high rpms the truck really flies, but right off the get go its really kind of slow and stutters a lil any ideas?...possibly needs a rejeting?
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