Mailbag: Tuning Your Carburetor for Mid and Upper RPM Power
You’ve got questions. We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. This week, we’re setting up a Holley carburetor for mid and high rpm power production.
Q: I have a small block Chevy 350, bored .030-inch over, in a 1970 Buick Skylark. It has TRW flat top pistons, stock 1.94-inch intake heads with Crane roller tip rocker arms, a COMP Cams 280-degree duration (advertised) cam, Edelbrock Performer RPM intake manifold, and a Holley 650 cfm double pumper carburetor. The ignition is stock HEI with a Moroso curve and vacuum advance kit and a Crane HI-6 ignition. Exhaust is Hooker headers with dual Flowmaster mufflers and an equalizer tube.
My question is, why don’t I have any power? My throttle response is good, but when I get on it, the engine falls flat on its face. It has no power anywhere in the mid to upper rpm ranges.
A: We suspect your power loss is due to carburetion problems. If you did nothing but bolt the Holley carburetor on, then you probably have some tuning to do. First, set the float levels and the air/fuel mixture screw. Check to make sure that your throttle cable or arm opens the throttle blades all the way; have someone sit in the car and push the gas pedal to the floor as you watch the blades open.
If the throttle blades are OK, then you need to work on the accelerator pump discharge nozzles, more commonly known as squirters. The pump cam (the plastic piece on the throttle linkage), controls the amount of fuel delivered, while the nozzle controls when the fuel is delivered. Larger nozzles quicken initial fuel delivery; smaller nozzles delay it.
Once you work out the bog with the accelerator pump circuit and have the fuel level set correctly (and providing the jetting is correct), you should have enough fuel to make the mid and high rpm power you’re now lacking.
You may want to add “with the motor turned OFF” to the part about checking the WOT position.—>have someone sit in the car and push the gas pedal to the floor as you watch the blades open. Just saying, Some people just don’t think.
Lol I thought the same thing when i read it. just picture someone getting a face full of fire as their kid punches it to the floor. Some people lack the common sense nowadays
If they are running a stock fuel pump they may need to upgrade to a high volume fuel pump.
Well that was two paragraphs of wasted time.
If u accually read the info that comes with your carb u wouldn’t have these problems 2 begin with…. No such thing as bolt it on n go people.. Read the Shit that comes with your stuff and u won’t be a DUMB ASS ANY MORE!!!!
To diagnose this problem the writer would need to supply more information. The problem is not referred to as a bog or flat spot, but “no power” at a certain part of the power band. The true problem needs to be clarified. It could be a carb tuning issue, but also may not be. Many other basic diagnostics, such as ignition timing, valve timing and adjustment, and compression testing should be performed to help determine the the true cause(s) of the problem.
I discovered that the fuel tank 3 port valve has tiny holes for fuel flow even though the fuel lines are 3/8″ diameter. My q-jet was emptying at about 4000 rpm under wot in a 72 1 ton 4×4. It’s a warm 400 sbc. I’m switching to 1/2″ stainless line with dash fittings to keep the fuel flowing.