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Old 06-09-2014, 09:34 PM
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I Just Can't Figure It Out?

I have a new rebuilt 357 SBC (350 bore .040 over) I have some issues that I just can't seem to figure out. Here is the run down on the engine

Camshaft- 275/285 .470/470 112 LSA (Flat Tappet Hydraulic lifter)
Heads - Aluminum 64cc Camber 190cc intake runner 2.02/1.60
Intake- Edelbrock Performer RPM
Pistons- Compression Distance 1.560 in Piston Head Volume (cc):+6.00cc
Roller Tip Rocker Arms
GM HEI Distributor, Changed springs, After market coil
Spark Plugs-NGK 7373 gap .038
Timing 34deg at 3000 RPM
Carb- Holley 4150 .028 Primary .031 Secondary 650CFM, float set correctly.

I am having several issues one being it keeps fouling the spark plugs, the plugs are black and fuel fouled with less that 10 min run time. The engine surges at idle (800 RPM) but timing adjustment and idle screw adjustment does not level it out. I started at 1 1/2 turns on the idle screws, opening them up from there, and adjust the timing anywhere from 28deg all the way to 38deg but never could get the surging to level out. If I set the RPM above 1000 the surging is less. With a vacuum gauge connected I get a reading that goes between 16-19 at idle, which the camshaft manufacture said is very good for this camshaft.

I took the carb to have it looked at thinking this had to be my problem, the builder check it out and found everything was fine.

It seems like I would be running lean if anything. I have checked and rechecked for Vacuum leaks, isolating everything I could with no change.

Does anyone have any ideas?

Thanks

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Old 06-09-2014, 10:08 PM
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As the carb is set right now, how much transition slot is showing below the primary throttle blades??

You'll have to remove carb to see this.

Can you screw one idle mixture needle in tight and kill the motor, or does it make very little difference??

Have you tried the idle mix needles in tighter than 1-1/2 turns out...no rule says they have to be 1-1/2 or more out, 1-1/2 out is just a starting point to get engine up and running,..... to go in OR out from there.
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Old 06-09-2014, 10:18 PM
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Fuel pressure? Blown power valve? Bad needle and seat? Are you sure the float is right?
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Old 06-10-2014, 06:50 AM
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The idle mixture screws if turned in will kill the motor, that seems to be good.

Fuel Pressure is 6 psig, I have a regulator inline.

Float level is set to where the fuel is just under the screw on the side.

When I took the carb to the builder yesterday, he pulled and check the power valve, needle and seat, and everything checked out fine.

I wish I had another carb to check against this one, something just does not seem to be agreeing.
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Old 06-10-2014, 06:56 AM
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You have 34 degrees total timing. What initial timing do you have and what sort of advance mechanism are you running. ie vacuum or mechanical?
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Old 06-10-2014, 08:47 AM
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have you looked into the carb at idle to see if there is any fuel dripping?
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Old 06-10-2014, 10:12 AM
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Quote:
Originally Posted by TGBTG7701 View Post
I have a new rebuilt 357 SBC (350 bore .040 over) I have some issues that I just can't seem to figure out. Here is the run down on the engine

Camshaft- 275/285 .470/470 112 LSA (Flat Tappet Hydraulic lifter)
Heads - Aluminum 64cc Camber 190cc intake runner 2.02/1.60
Intake- Edelbrock Performer RPM
Pistons- Compression Distance 1.560 in Piston Head Volume (cc):+6.00cc
Roller Tip Rocker Arms
GM HEI Distributor, Changed springs, After market coil
Spark Plugs-NGK 7373 gap .038
Timing 34deg at 3000 RPM
Carb- Holley 4150 .028 Primary .031 Secondary 650CFM, float set correctly.

I am having several issues one being it keeps fouling the spark plugs, the plugs are black and fuel fouled with less that 10 min run time. The engine surges at idle (800 RPM) but timing adjustment and idle screw adjustment does not level it out. I started at 1 1/2 turns on the idle screws, opening them up from there, and adjust the timing anywhere from 28deg all the way to 38deg but never could get the surging to level out. If I set the RPM above 1000 the surging is less. With a vacuum gauge connected I get a reading that goes between 16-19 at idle, which the camshaft manufacture said is very good for this camshaft.

I took the carb to have it looked at thinking this had to be my problem, the builder check it out and found everything was fine.

It seems like I would be running lean if anything. I have checked and rechecked for Vacuum leaks, isolating everything I could with no change.

Does anyone have any ideas?

Thanks
X2 on ericnova72's comment to transition slot. With as much cam as you have this is a common problem that is to get enough air to idle the engine the transiiton slot becomes exposed by the open throttle blades. Check this carefully (carb off the engine) as the bottom of the throttle blade needs expose no more than .020 inch of the transition slot per Holley. If more than that much of the transition slot is open both it and the idle port will feed fuel and there will be a loss of control of the idle screws. In bad cases the mains may start to feed as well.

If you need more idle air it needs to come from a source other than the primary throttle position. There is a set screw that stops the secondary closed position. This can be adjusted to crack open the secondaries to allow a little more idle air. Other techniques offer drilling a small hole in the primary throttle blades to allow air entry that does not alter the transition slot exposure. Another method is to put a length of hose on the full time vacuum port so as to allow a metering valve connection to an acquarium flow control valve. Then use the adjustment on that valve to control enough air to dial in the idle speed without changing the throttle position. Thread Stainless Steel Aquarium Air Distributor Lever Valve - Tmart.com

Bogie
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