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Old 12-18-2006, 01:18 PM
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Problem: Temporary lean out - 406 with a demon

Guys,

have a strange problem - when I hammer on my car, 1st , 2nd, everything is fine. Then I hit third and fourth,and sometimes after I put my foot back all the way down (I lift a little to shift) the car lays down on me. All I have to do is let up just a split second - about as fast as you can get out and back in - and everything is fine. It will do it in third and sometimes fourth, and not every time. It's not the accell pump, the car starts to pull for maybe 1 sec., then the problem happens.

My LM-1 logs show it being a lean out when this happens, but of course an ignition miss will show up as a lean condition as well. I can't imagine it gets so much fuel that is misfires, so I think its going lean.

Running a demon 750 annualar boosters, changed MAB to .026" (holley size) to get rid of high speed leanout, which cured that problem. Fuel pressure is about 7-8psi, fuel level mid to mid top of sight. using a Mallory Comp 140 pump, so fuel is not an issue. I log fuel pressure, never drops below 7 psi.

car is a 68 camaro with 406, 6 speed, 4.86:1 9.9:1 SCR, Mallory HyFire CDI ign., 18 inital, 36 total adv in MSD billet dist.

anybody have any ideas??

my next step is to restrict the emulsion holes to the proper size with brass set screws drilled according to holley = .026". Demon drills them about .031", which is said to cause problems with the two phase flow of fuel and air. Imagine a percolating coffee pot, this is what the big e-holes are said to do. Perhaps that can cause my instantaneous lean out condition????

Sorry about all the words - felt I had to explain . ...
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Old 12-18-2006, 01:56 PM
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You better look closer at the pump. 7 pounds before the needle and seat don't mean it's gettin past the needle. look through the flaot bowl too. Sometimes casting flash right at the slot into the seat restrict flow in the bowl.
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Old 12-18-2006, 02:54 PM
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I can see that being a possibility, but why would it be fine if I pedal it slightly - I dont' even have to get all the way out - just a blip and she's back on track. You're suggesting that when I lift, the bowls fill slightly, enough to get fuel back up to the jets?

What do you mean look closer at the pump? It will flow 120GPH at 8 PSI, should be more than enough, especially once the g's level out in third and fourth. 1/2" fuel lines, big canister style fuel filter before the carb., screen type Earl's filter before the pump, I think like 100 micron for the big stuff.

I know my fuel system is far from overkill for a drag car, but it should be able to keep up with my little mouse motor I would think.

Haven't tried the 1 gallon time-to-fill test yet. Guess I could try that for more data. Anybody have any luck doing that test?
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Old 12-18-2006, 03:14 PM
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I just meant to get you looking at a volume restriction. You may have a failing pump or kinked hose, perhaps a air vent problem in the cell, cavition problem, filter blocked, ect...

Why would a ignition miss show as lean? The extra fuel loaded into the exhaust would show rich.
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Old 12-19-2006, 11:04 AM
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The O2 sensor only measures oxygen content - when the fuel doesn't get burned the oxygen content stays the same.

This is why an engine with a big cam will look like it is idling (irradically) at around 15:1 - 18:1, with jumps off the chart - basically it's near misfire on some cylinders and shows up as unused oxygen.

Here's an interesting tidbit. When I turn the pump on initially I hear funny sounds - don't know how to describe. Like a sqwuak or something as it builds up pressure, once the pressure builds, everything is fine.

I have been wondering if that could be something wrong in the pump or regulator causing the funny noises. Any ideas?? I got the pump used from my neighbor and I have not rebuilt it or anything (same with the regulator).
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Old 12-19-2006, 11:11 AM
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Hotrod... are you running a Holley pump by chance? The reds, blues and blacks will make noise on start up if on a dead headed system.
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Old 12-19-2006, 12:11 PM
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I'm running a Mallory Comp 140 pump, with the regulator that they sell with it. (dead-head style).

I'm wondering if a return style would be better.
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Old 12-20-2006, 03:07 PM
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Hotrod... I have run a dead headed system on my car for both the engine and nitrous for years with no issues(2 1/2" systems). I don't see it as a problem.
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