What are the results of having too small of a power valve? I'm having some issues, and I'm going to try a smaller power valve, but want to know what to look for if I go too small. I'm only pulling around 8-9 vac while idling with a few lower dips. I currently have a 4 power valve, but am going to try a lower one. Just wondering what results of too low a power valve are.
Rule of thumb is 2.0 less then vacuum reading @ idle for racing purposes
June 23 2011, 7:30 AM
but that may be too high of a tip-in point verses power/economy for a street piece. I'd install a vacuum gauge and road test at various rpm's because it's not uncommon for an engine to pull vacuum under acceleration, especially in high gear and effectively shut down the P.V.(s) Test and tune.
445 stroker running on E85. Proform main body & Quick Fuel metering blocks. No PCV, as I'm running an Evac system. Jetting was set on the dyno. Idle is set to 1100 rpms. 3500 stall. 5 lbs of fuel pres. Engine ran smooth but lumpy on the dyno. Now that it's back in the car, it is missing at lower rpms and running "rough". Once I'm into it hard, it runs smooth at around 3500prms and up. Holding it at 2000 rpms, it is coughing and missing almost like a dead cylinder. It's appearing that once I'm into the secondaries is when it smooths out. I've installed new rotor, cap, and wires. Running MSD 6A, and MSD coil. All plugs look fine. I don't believe that it's an ignition problem, because it gets better at high rpm's. I pulled the carb apart and did a complete wash down because E85 can gum up easily. For right now, I'm leaning towards the power valve being too big and dumping in a lot of fuel.
I'm wondering what symtoms will appear if I go too small on the power valve? The good news is that once the pedal is on the floor, I'm tring to hold back over 500hp, and the MT steet radials just go up in smoke!
This message has been edited by 66Bird on Jun 23, 2011 8:27 AM
A power valve that's too low (low number) will cause a lean condition when the engine is under heavy load or at full throttle. The power valve does not operate when the engine is idling or at part throttle cruise. If you're experiencing rough idle or tip in problems, you need to look at the primary jets, air bleeds, idle mixture, throttle plate positions at idle and other things. How much of the transfer slots are showing at idle? Do you have all the plates adjusted properly at idle for your cam?
a 1,100 rpm idle, bet the throttle blades are open too far...past the maximum .030 Holley recommends. Btw, less, like .025, is better here! Pop the carb off, mike some pieces of narrow newspaper strips (metal feeler gauges won't cut it here unless one hacks a few up) and check the bore-to-blade drag on both throttles.
The Tslot should be exposed slightly as shown. Not having any showing will cause a stumble as you apply the gas from it going lean. Having too much exposed will feed enough fuel to the engine at idle, to make adjusting the idle air fuel screws not have any effect, making it idle too rich. It is best to set the Tslot so the opening that is exposed is as long as it is wide, square. After doing this, make your idle speed adjustment using the secondary idle adjustment screw. If you are running a real radical cam, this may not be enough to control the idle. If that is the case, drilling small holes in the throttle blades may be required to provide enough air to the engine, while the blades are nearly closed. On some older carbs you may have to remove the secondary adjuster screw from the bottom of the base plate and put it in from the top to allow easy access. The Tslot controls the fuel delivery until between 2200 and 2800 rpms in most engines when the fuel starts feeding from the boosters. If you are rich or lean before that, it is most likely from a idle air adjustment, Tslot, or emulsion circuit that feeds the Tslot causing the problem.
This message has been edited by machoneman on Jun 23, 2011 10:01 AM This message has been edited by machoneman on Jun 23, 2011 9:57 AM This message has been edited by machoneman on Jun 23, 2011 9:52 AM This message has been edited by machoneman on Jun 23, 2011 9:52 AM
Holley does give that broad recommendation but....
For fine tuning, one should experiment with varying PV ratings as another poster noted above with more race-type engines. I have also found that dropping about 2 points off the nominal idle vacuum is a much better starting point for a 'hot' engine. Using Holley's 1/2 recommendation is better suited for pure street driven cars, close to stock engines and or those who strive for maximum gas mileage. Tuning with the PV for maximum responsiveness in your specific combo is no different than experimenting with metering block jetting, shooter sizes, pump shot cams, etc.
This message has been edited by machoneman on Jun 23, 2011 10:46 AM This message has been edited by machoneman on Jun 23, 2011 10:39 AM
more likely a bleed/booster/jet combination issue with the E85. At a steady state 2000 the power valve is closed in most cases. Sounds to me like you're feeling the main circuit popping and spitting as it tries to come in. Idle also sounds pretty high unless it's a real "rock & roll" cam in there.
You might even try setting it up on a borrowed gasoline carb to get a feel for how it should behave before going back to the E85 deal.