Need some help with a 750

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1973dusterkid

1973dusterkid

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Ok so me and my dad have been trying to set my carb and we cant get it right.
Its a 750 holley DP we set the idle with the 508 purple shaft at about 850-900 rpm you go and drive it and stop at a red light the idle is between 1100 to 1400 rpm why is it doing this we have tried every witch way to fix it and still nothing.
 
Once you set it are you revving it and it returns to what you've set it at? The floats set so fuel barely trickle out when you remove the bowl screw or in the site glass? All the vacuum ports plugged? I forgot the large one in the rear once... whats the fuel pressure set to?
 
Captain Chaos said:
Once you set it are you revving it and it returns to what you've set it at? The floats set so fuel barely trickle out when you remove the bowl screw or in the site glass? All the vacuum ports plugged? I forgot the large one in the rear once... whats the fuel pressure set to?
Click to expand...


All vacuum ports pluged

When you rev it at idle jumps from the 850 to about 1100

Floats are good and as far as fuel psi I don't know
 
Try to close the secondary blades a little more and see if that helps. Don't forget to reset your idle mixture after the adjustment
 
Does the carburetor have some kind of choke on it?
 
Check distributor advance. Had the same problem on a new build. Found out the advance was not in quick enough.
 
1973dusterkid said:
Electric choke
Click to expand...

Ok. Have you double checked to see that the choke adjustment screw is not holding the throttle open? More times than I can count, that's the problem. Either that, or the choke is adjusted just enough that the flap opens, but the choke cam catches the screw in fast idle.
 
Same as most threads like this.

What is the initial timing?
 
Ok I have been talking with holley and going to down jet and get a new power valve.

62 front jets
64 rear jets

5.5 power valve
I have 10 vacuum at idle
 
Ok guys

I changed to a
5 power valve
and went from 72 to 70 jets

The idle problem is still there but not as bad

I set the idle at 850 and it runs good go down the road and the idle hangs up at about 1000 at idle
 
I'm guessing that what is happening is that the mechanical advance is coming in and staying in after it's been reved up. It could be the vacuum advance too, if that's connected. You could try unhooking and plugging the vacuum advance to test this.

You'll need to do some work on the distributor to get the advance curve set up or get an aftermarket distributor that has adjustments.
 
73Scamp318 said:
I'm guessing that what is happening is that the mechanical advance is coming in and staying in after it's been reved up. It could be the vacuum advance too, if that's connected. You could try unhooking and plugging the vacuum advance to test this.

You'll need to do some work on the distributor to get the advance curve set up or get an aftermarket distributor that has adjustments.
Click to expand...

Well said. My 340 idle was all over the place and it was the vacume advance sticking. New distributor with timing curve adjusted and problem solved.
 
Ask for help and ignore pertinent questions seems to be the norm anymore.

Holley likely didn't give you anything that will solve the root problem...

If at idle it doesn't have at least 18* of timing, you aren't in the ballpark!
 
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