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I have an '85 Mustang with a 306c.i. / 10.5:1 comp. / Custom Hydraulic Roller Cam @50-230dur,533lift / Victor Jr. / Demon 650 Race / MSD billet distributor with MSD 6AL ignition & blaster 2 coil.

The carb came with 71 primary/71 secondary jets and upon driving the car, it hesitated beyond belief and ran extremely lean. After going up in various jet sizes for the best fuel delivery, I ended going up 20 sizes to 90/91. With these jets, the secondaries run fine without hesitation, however, at off idle speed and low rpm's up to approx 3,500rpm, the car still hesitates and blubbers. After driving it for 60plus miles and checking the plugs, they're still white as can be.

I contacted the Demon tech's, and they told me that the 650 is plenty of carb for my setup and said that I may have a fuel delivery/volume problem. They recommended testing my fuel pump delivery by disconnecting my fuel line and placing it into a 1gal. can and timing how long it took the pump to fill the container. They said that it should take 24 seconds or less.

When I did this with my Holley Blue (110gph), it took close to a minute. When I told them that it took well over 24 seconds, they told me I wasn't delivering enough volume to the carb and they recommended their 220 HR which delivers 220gph. The tech stated that with this pump, my carb will get enough fuel, and that I will more than likely have to down size my jets.

After thinking about this, I though about the stock alternator (74amp) and wondered wether I was delivering enough amps to the pump. Regardless, with all described above, is it possible that my carb may be starved for fuel even when the bowls appear to be constantly full while running 7lbs of pressure through the regulator and with the larger jets? Also, as I mentioned earlier, the car doesn't seem to be starved when I throttle it and get up in the rpm's, only off idle and at a constant rpm below 3500.

Any input/suggestions extremely welcomed.

Thanks
 

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Well, in my experiences, if the fuel pump can't keep up with the carburator it runs out of gas and quits. Even if you plugged your power valve you would not need to go up that high on your jets unless you are running alky maybe? It sounds to me like you have a vacuum leak of some kind or maybe one of your fuel floats is just stuck so you are only running off 2 barrels or the float level is set too low and its running out of gas.



<font size=-1>[ This Message was edited by: Hottarod on 2/13/02 2:55pm ]</font>
 

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Discussion Starter #3
Float bowl levels are fine and the car ain't running out of gas even under extreme throttle. The car hesitates at a constant RPM and the plugs are still white even after 60+miles. I'm at a loss.
 

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I run a 650 doublepumper on a 289. It came with 68/73 jets and it finally settled down at 74/78 with a front power valve. Without a power valve my jets would be in the low 80's. The car runs mid 12's right now and is making around 400 flywheel but its not quite optimized so my times are suffering a bit.

I would say its air getting in the motor somehow and bypassing your carb like in a small vacuum leak, pcv open under load, intake to carb gasket leaking, something like that. I had a carb spacer gasket leak on mine that would only show up beyond idle when I rev'd the motor. I could jet it to the moon and it wouldn't fatten up until I fixed that.
 

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I had similar problems with my 650 speed demon and found the idle air bleeds to be too large.

They sent me press-in brass idle bleeds, but your race demon should come with screw-in air bleeds. Try out some different sizes.

I heard a lot of grumblings about the earlier production Demons but the problems were all supposed to be fixed by now.

The whole thing about the gallon of gas in 24 seconds is blown a little out of proportion. My car runs a Holley blue and like you it took me about a min (55.19 seconds) to fill a gallon of gas. Despite my car having "inadequate fuel delivery" it still spits out some decent times. Would more fuel show some improvement? Probably, but is lack of fuel hurting my motor? Not if 1 &1/2 years of driving & spraying mean anything.

Have you whacked an LS1 today?
 
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