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Discussion Starter #1
I have a Holley 650 double pumper (4150 part number 0-4777s) that is running WAY rich for my combination out of the Box. I get backfires and sluggishness on medium to hard acceleration and the plugs are pretty black showing me I am not burning all the fuel.
I am pretty sure the answer is in the Power Valve and the Jetting. I have purchased the full Holley Jet set, but have not installed any YET.

The car is a 66 Mustang with a 289 (freshly rebuilt with JE, Eagle, ARP) Eldebrock Performer RPM heads, Performer RPM Air-gap Intake Manifold, Comp Hyd Roller cam with a 224 duration (intake and exhaust) and a .533 Lobe lift, Crane golds 1.6 rockers, T-10 4 speed, 3:55’ gears, Hooker 1/58 headers, X pipe dual exhaust. I think the compression is 9.5 :1.

There is no vacuum hoses set up on this engine except a breather hose from the carb to the valve cover (I am getting 7-8 of vacuum at idle and about 17 at 2500 RPM on this hose).

I am looking for some guidance or any input from a Holley or tuning Guru’s before I start the experiment. Any comments from anyone running a similar combo will be helpful!


Rex
1966 GT 350 Clone
289 (rebuilt with JE, Eagle, Arp)
Edleborck RPM Heads
Comp Cams hyd roller, Crane 1.6 rockers,
Holley 650 DP, Hookers 1 5/8, T-10, 3:55 Gears, Custom Dual exhaust with X Pipe
 

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Good luck. I rebuilt my Holley 600 today. I put the next size jets 65's to 69's in mine. Try putting the smallest jet in & test it. Lean your choke out & check your idle mixture screws 1 & 1/2 turns. Do one thing at a time & test drive it.

66 Coupe, 302, Auto, 3.25 gears. [color:red]Candyapple Red</font color=red> with White Pearl Stripes. Cragars.
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Your vacuum level sounds low to me. I suspect that you need to increase your idle rpms up to around 1000 and also run more initial timing like around 16-18. It would help to find out what Comp Cams says your idle vacuum level should be. I run a Comp [email protected] with 560 lift and I get 11-12 inches vacuum in neutral but my idle rpm is up at 1200.

If you can't get enough idle from the idle speed screw without kicking in the primaries you will need to turn in the secondary throttle stop screw a little(1/8 or 1/4 turn at the most). The symptom you are seeing probably is the power valve but just dropping to a lower number may not be the correct fix by itself. Once you get your idle set take some more vacuum readings. You may still need to go to a 5 1/2 or 4 1/2 power valve.

The popping and spit back syndrome may be not enough timing or it could also be not getting enough pump shot.



You can see my 65 fastback at: http://hottarod.stangnet.com/
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"There is no vacuum hoses set up on this engine except a breather hose from the carb to the valve cover "

I hope you have a PCV valve in that line. If not, try plugging it off and see what happens before you go into the carb. The factory setup should be pretty close for your combination.


Dave J

My wife says if I buy another Mustang, she's gonna divorce me. I'm sure gonna miss her....
 
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Update: I put the timing at 16 btdc and leaned it out a little bit more. It is running much better, no backfires''!! :) Thanks to everyone who responded with information! I now have it idling at 1000 RPM and I am getting 10-11 in vacuum (normal with a big cam??). I think there is more power to be had. I talked to Holley Tech and they suggested trying a 4.5 or 5.5 Power valve.

Any opinions?

Thanks again on all of the help!


Rex
1966 GT 350 Clone
289 (rebuilt with JE, Eagle, Arp)
Edleborck RPM Heads
Comp Cams hyd roller, Crane 1.6 rockers,
Holley 650 DP, Hookers 1 5/8, T-10, 3:55 Gears, Custom Dual exhaust with X Pipe
 
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