I replaced my engine with a Blueprint engine, model # BP38318CTC1.

The engine starts, idles, and runs fine as long as you only run about 1/4 throttle. Anything more than that and my AFR gauge bottoms all the way out at 8:1 and the engine had no power. Once I let off the gas pedal the AFR levels back out to about 13:1 and it runs fine again.

My timing is set to 14 degrees initial at 1000 rpm, 34 degrees total at 3000 rpm. No vacuum advance, as suggested by the manufacturer. Pulling 12~13 inches of vacuum at idle. Fuel pressure is 6psi at the carb. The float bowl levels are adjusted to the middle of the sight glass. I'm using brand new NGK BKR5E plugs, new 10.5mm wires. Factory style in tank TBI fuel pump going to Mallory regulator with a return. 700r4 trans from an 87 Iroc, 3.23 Borg Warner rear end from an 87 Iroc.

I have attempted to swap the 750 cfm Holley supplied with a known good 670 cfm Holley. I have replaced the distributor cap, coil, rotor and ignition module from a known good unit. The are only two vacuum hoses attached to the carburetor, neither are cracked or leaking. I have double checked the reference mark made on the intake manifold and distributor for timing with a timing light and double checked my plug wiring.

I contacted Blueprint and they suggested my fuel level in the floats were too high. I disagree but I lowered it anyway. That didn't help.

The jets in the 750 carb are 72 in the primary side with a 6.5 power valve. Secondary side has a metering plate that I haven't checked the numbers on. I swapped the power valve to a 5.5 . That just made it run lean at part throttle.

The 670 carb has 70 jets in the primary with a 6.5 power valve, 74 in the secondary.

No matter what changes have been made, the issue remains. Anything above 1/4 throttle makes my gauge read full rich and the engine has no power. Can you please give me suggestions for what to do next?

 

O2 sensors work on the basis of sensing an absence of oxygen in the exhaust gasses. If the AFR readings are indicating "rich", then there is a great absence of oxygen in the exhaust gasses; meaning your engine is having no trouble burning fuel (consuming oxygen), it's just getting more fuel than it wants to optimize torque output. Engine will make more torque when you give it less fuel to get the AFR ratio back in the range where engine runs best.

 

Disclaimer: I am not a carburetor person.

But, it sounds like your secondaries are opening early and dumping fuel too soon. I believe you can put in a heavier spring to delay their opening? What carb exactly do you have?

 

The 750 that shipped with the engine is Holley Part # 0-3310S. The other 670 that I swapped on and tried was Holley Part # BR-67256. Both produced the same results. I'm at the point where I'll try anything, so thanks for the tip.

 

I'd put the 3310 back on. Take off the E-ring for the secondary diaphragm and see if it does it on primaries only. If it does, that's a sign the secondaries are not opening. Hard to say why that might be, but make the test first.

When the secondaries open, they primarily dump in AIR. Fuel comes along later, being pulled in by the air.

Regardless, 2 carbs that do the exact same VERY UNUSUAL thing, is a VERY STRONG hint that it's not the carb itself. Could be something like, some linkage part getting caught on some gasket or casting feature or the air cleaner or something.

 

I'm inclined to agree with Sofa on this. Secondaries causing a problem can be ruled out fairly easily. 2 fairly different carbs causing the exact same issue leads me to believe it has nothing to do with the carb.

 

Is it possible that the fuel pressure is uncontrolled at higher RPM and the needle and seat are being over powered?

 

I questioned that but it's an electric pump with a dedicated regulator. If it's controlled well at idle, it would be controlled at higher rpm. If anything it would go lean when revved up if the pump couldn't keep up.

 

Would you be using a dropped-base air cleaner with a 2" air filter element causing the lid to sit right on top of the carb, blocking the airflow?

If so, you might find some good info here: https://www.thirdgen.org/forums/carb...-base-air.html