Matt69olds
Full Access Member
- Joined
- Jan 8, 2018
- Posts
- 2,451
- Reaction score
- 3,819
- Location
- Central Indiana
- First Name
- Matt
- Truck Year
- 81
- Truck Model
- GMC 1/2 ton
- Engine Size
- 455 Olds
**** can the Holley, Q-Jet swap it!!!
Assuming the float is set correctly, you don’t have too much fuel pressure, the timing is correct, and no vacuum leaks, I would think a engine with a camshaft mild enough to idle at 15inches of vacuum shouldn’t need the secondaries opened up. And since it’s a brand new carb, I certainly wouldn’t go drilling holes in anything before asking some serious questions.
What air cleaner are you using? Hood clearance on a truck probably isn’t a concern, but if the air cleaner is too close to the top of the carb the float bowl and air bleeds can’t vent correctly, causing all kinds of calibration issues.
Main jetting will have almost zero influence on idle mixture. Holley calibrated the carbs a little on the rich side as a safe alternative. It’s easier to set the carb a little rich for a customer using the carb on a big block. If they set the carb to be ideal for a small engine, they will have plenty of complaints from people using it on a big engine. My point? You shouldn’t need to change keys more than a couple sizes either way from the factory setting. If you need a bigger change, either something is screwed up it’s the carb, or you have the wrong carb for the application.
This is my suggestions: make absolutely sure the timing is correct, the vacuum advance is on manifold vacuum and functional. Make sure the float is set correctly, make sure the fuel pressure isn’t too high. Does your fuel pump have a return line? Is it restricted? Engines equipped with A/C usually use a fuel pump with a small return line. That return line had a small orifice inside the pump. The idea is the small orifice provides a small calibrated leak, fuel is constantly circulating thru the pump, keeping the fuel and pump cooler. If the return line is capped off or restricted, the pressure will be too high. Not saying that’s your issue, but I have experienced it enough times that it’s a suspect problem.
If you have messed around with the idle screw on the secondary side, put it back they way it was. Ensure the choke opens completely. Spray a bunch of carb cleaner into the air bleeds. Lightly seat each idle mixture screw, then back then out evenly 3 turns (unless Holley has a different spec). That should be close enough to drive. Take the truck out for a few full throttle runs. Make sure the truck is fully warmed up, drive it long and hard enough that it burns off all the carbon from the plugs snd combustion chamber. Once that’s done, set the idle to around 650-700rpm. Connect a vacuum gauge to manifold vacuum, pick a idle mixture screw, and slowly back out the screw a 1/2 turn. Does the idle or vacuum go up? If so, go to the other screw and turn it out a 1/2 turn. Keep going back and forth until you get the highest idle speed/vacuum reading. Reset the idle speed.
If you have done all that and it still runs pig rich, I’d suggest calling Holley and explaining the issue. Did the engine run like crap with the other carb? Who knows, just because it’s brand new doesn’t mean it was machined correctly.
Assuming the float is set correctly, you don’t have too much fuel pressure, the timing is correct, and no vacuum leaks, I would think a engine with a camshaft mild enough to idle at 15inches of vacuum shouldn’t need the secondaries opened up. And since it’s a brand new carb, I certainly wouldn’t go drilling holes in anything before asking some serious questions.
What air cleaner are you using? Hood clearance on a truck probably isn’t a concern, but if the air cleaner is too close to the top of the carb the float bowl and air bleeds can’t vent correctly, causing all kinds of calibration issues.
Main jetting will have almost zero influence on idle mixture. Holley calibrated the carbs a little on the rich side as a safe alternative. It’s easier to set the carb a little rich for a customer using the carb on a big block. If they set the carb to be ideal for a small engine, they will have plenty of complaints from people using it on a big engine. My point? You shouldn’t need to change keys more than a couple sizes either way from the factory setting. If you need a bigger change, either something is screwed up it’s the carb, or you have the wrong carb for the application.
This is my suggestions: make absolutely sure the timing is correct, the vacuum advance is on manifold vacuum and functional. Make sure the float is set correctly, make sure the fuel pressure isn’t too high. Does your fuel pump have a return line? Is it restricted? Engines equipped with A/C usually use a fuel pump with a small return line. That return line had a small orifice inside the pump. The idea is the small orifice provides a small calibrated leak, fuel is constantly circulating thru the pump, keeping the fuel and pump cooler. If the return line is capped off or restricted, the pressure will be too high. Not saying that’s your issue, but I have experienced it enough times that it’s a suspect problem.
If you have messed around with the idle screw on the secondary side, put it back they way it was. Ensure the choke opens completely. Spray a bunch of carb cleaner into the air bleeds. Lightly seat each idle mixture screw, then back then out evenly 3 turns (unless Holley has a different spec). That should be close enough to drive. Take the truck out for a few full throttle runs. Make sure the truck is fully warmed up, drive it long and hard enough that it burns off all the carbon from the plugs snd combustion chamber. Once that’s done, set the idle to around 650-700rpm. Connect a vacuum gauge to manifold vacuum, pick a idle mixture screw, and slowly back out the screw a 1/2 turn. Does the idle or vacuum go up? If so, go to the other screw and turn it out a 1/2 turn. Keep going back and forth until you get the highest idle speed/vacuum reading. Reset the idle speed.
If you have done all that and it still runs pig rich, I’d suggest calling Holley and explaining the issue. Did the engine run like crap with the other carb? Who knows, just because it’s brand new doesn’t mean it was machined correctly.
