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Old 03-26-2002, 10:27 PM
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bbeckwith bbeckwith is offline
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Default fat mix but lean surge?

I have a problem with a lean surge condition on acceleration at around 3500 rpm. The motor is a 69 340 in a duster with manual trans, 650 dub pump holley 4150, Edelbrock Air Gap, dual point (I know Im a dinosaur) at around 12 btdc and 34 total.
I recently switched to AN fittings to the carb because the cheap chrome dual feed fuel line had the chrome flaking off where it attached to the carb, and I worried it would get in the carb. I also attached a new filter and marshall fuel pressure gauge at the same time. Some short while prior to this I had noticed the lean surge so this was not a factor.(I dont think!) The gauge showed 4.5 psi which Im told is a bit on the weak side, so I want to replace this pump. The plugs had around 800 (squirrely street miles on them) and they were ALL moderately carbon fouled, with numbers 7 and 8 slightly less fouled. The motor idles at 800 rpm with 10" vacuum and 1000 rpm fully warmed up at 13" vacuum. I ran a 10.5 power valve which does not show any visible sign of being blown (the motor will backfire on rare occasion when cold)(I have the newer design throttle body with checks), and 68 jets primary and 76 secondary in the 650. This combo has performed nearly perfect for the last year, and now I have a lean surge at 3500, yet the mix is fat enough to carbon foul. I tried replacing the 10.5 with a new 6.5 that I had on hand but there seems to be no difference after a test drive. What am I missing? I checked for vacuum leaks around the head/intake with wd40, no difference. Could the intake be leaking on the underside/cam valley side of head to intake seal? the carb to intake seal was good, so Im perplexed. To add to the fun, the fuel gauge now shows 0 psi, but the motor still runs. I removed the gauge and tested it and it seems ok. I put it back on, and it shows 3.5 - 4 psi, I go for a test ride, and it shows 0 again?
bad fuel pump probly. gonna put another gauge on pump to verify and go through the carb again, for the 100th time, and clean out again, just to make sure theres no foreign material somewhere. If it were just a couple plugs carbon fouled I would say theres sticky valves or something with the valvetrain, but all of them, and a lean surge? any ideas will be appreciated.
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Old 03-26-2002, 10:36 PM
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Default more info

forgot to add the cam is of unknown origin, as I didnt assemble this motor, bought it this way, and previous owner couldnt tell me anything about it. It is a big rumpety rump tho, lumpy idle with lots of power in upper rpms.
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Old 03-27-2002, 11:24 AM
Jims451 Jims451 is offline
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From your low fuel pressure it sounds like a fuel delivery problem?
I would check for air getting sucked into the fuel line from a loose connection or cracked fuel line, maybe back at the gas tank.
Also, you may have a fuel tank venting problem?
Try running the car with the gas cap removed to see if the tank is not properly venting.

Double check the carb passages, and air bleads and blow them out with carb cleaner.
My friends 360 has a lean searge with his 600 cfm holley, found some old bowl gasket material stuck into one of the jets.

How much vacuum do you have? I have never ran a power valve higher than the 6.5?
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Old 03-27-2002, 11:34 AM
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Twisted360 Twisted360 is offline
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The 10.5" power valve sounds very high. Holley has a formula for choosing a power valve. Take a vacuum reading at idle (13" in your case) divide it in half ( 6.5" ) and add a half. So you should be running a 7" power valve not a 10.5. After changing this you will have to rethink your jetting. After you make these changes post the results please.
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Old 03-27-2002, 12:25 PM
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I thought the power valve was supposed to be 2 smaller than vacuum reading at idle, so if I was at 13", I should run an 11, or in my case, a 10.5 changing to a 6.5 didnt make any difference, still had the lean surge when accelerating through 3500 rpm, not WOT. when I get home from work today, Im testing the pump, will update this thread later
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Old 03-27-2002, 07:07 PM
Jims451 Jims451 is offline
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Hook up a power valve so you can see it while driving.
You want the power valve to be a bit below the vacuum reading when driving (crusing) down the highway with only a mild load on the engine.
When the load increases, like going up a steep hill, you want to power valve to be above the vacuum reading. The lower the power valve you can use (without causing drivability problems) the better gas mialage you will get.

If the fuel pump / pressure checks out, then it may be possable that by using too high a power valve, you jetter the carb primary side down too much?
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Old 03-27-2002, 08:40 PM
raymond raymond is offline
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You could have too much timing in it. If you are running vacuam advance as well as centifugal , try disconnecting the vacuam line. If not then twist the distributor back a few degrees and give that a try. Too much advance will often show up as a surge problem .
Hope this helps
Raymond
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Old 03-27-2002, 10:55 PM
HOLESHOT HOLESHOT is offline
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for sure the lowest vacum you have is 10,therefore you should be at 7.5/8.5 PV.This carb came with the 6.5.The enrichichment just occurs faster with the 10.5 which will load plugs and hurt mileage.The lean surge is likely too much timing.
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Old 03-28-2002, 12:19 AM
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Default tonights update

Seems it was a fuel delivery problem, I guess I put that fuel gauge on at the right time. I swapped fuel pumps today after work and as soon as it fired up, the gauge jumped to life registering 7 psi. I didnt take the car out as it was getting late and I dont like driving at night anymore, my sight is pathetic at night, so I will test it out tomorrow after work. I still suspect the new anodized fuel filter I installed between the pump and carb may be restricting flow. I will try the fram I had before, it is known to be good. I will update tomorrow eve, ps. as far as the timing goes, I have an Accel dual point distributor, with no vacuum advance, and Im running the same timing I always have, with no problems. I think this was the fuel pump primarily.

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