raid, check fuel pressure and the primary float in that particular carb.
Tje red circled tube is our bowl vent, The other is a idle circuit airbleed. The other set of holes are for your power valves....
:hi: No gold star for you on this one.
The tube circled in red is the bowl vent. Remember, carburetors work based on the pressure differential between the top of the carb and the bottom of the carb and fuel in the float bowl must not be pressurized. Hence, the bowl vent which, theoretically, keeps the float bowl at atmospheric pressure.
The air bleed circled in green is a high speed air bleed.
The air bleed located outboard from the high speed is the idle air bleed.
Those particular carbs use only a primary power valve, one in the primary metering block. And since the power valve in a Holley relies on vacuum to hold it closed (the spring opens it), the only passage leading to the power valve is in the base plate of the carb.
Click on the above image to see a base plate from a double pumper, which has two power valve ports in it. The small openings next to the base plate screw holes lead to a passage in the main body that opens up in the recess the diaphragm side of the power valve occupies.
A quick lesson on what its doubtless the most misunderstood circuit in a Holley carb, the power enrichment circuit.
If you have a Holley power valve sitting in the palm of your hand, the valve is in an 'open' state. The valve is open so fuel can flow through it. The spring you see on the valve is holding it open. If you look at the diaphragm side of the valve, the plate will have a series of numbers etched into it, around the outer edge. You will see one pair of numbers that will appear as 6 and then a 5 just clockwise beyond it. The spring on that power valve is rated to be overcome by 6.5 inches of vacuum on the diaphragm. If the number is an 8 followed by a 5, that power valve will open sooner, as it takes 8.5 inches of vacuum to hold it closed. In other words, if your motor idles with 14 inches of vacuum, that vacuum level holds the power valve in a closed state. When you open the throttle, vacuum drops and when vacuum gets to 8.4 inches of vacuum (in a perfect world, mind), the power valve spring pulls the valve open and you have the additional enrichment to keep the motor alive in the low vacuum state.
People cry and moan about 'junk Holley power valves' and I just try not to laugh at them. If you have your choke set properly so the motor doesn't experience a lean backfire through the carb, if you have your valve lash set correctly so an intake valve is not open when the spark plug fires and if you have your timing set properly so a cylinder is not firing whilst an intake valve is still open and if your power valve is correct for your particular combination so the motor does not go lean under full acceleration, you will never a lick of problem with a Holley power valve. If the motor backfires through the carb, flame shoots right up the power valve vacuum port and 99.999% of time will burn the diaphragm, allowing fuel to run through the power valve all the time.
For years, I would drill down from the top of the base plate a wee bit, just large and deep enough to accept a small check ball. I would take an old Philips screwdriver, drop it into the hole and give it a sharp rap with a small hammer, to flute the bottom of the enlarged hole. Drop the check ball in, put on the gasket and install the main body to the base plate. With the fluting in the passage, the power valve was always able to 'see' manifold vacuum. And if the guy with the 'junk power valve' didn't have things set properly and the motor would sneeze into the intake, the valve would get blown up against the bottom of the main body, sealing off the vacuum passage to the power valve diaphragm. Holley finally got smart and started installing a check valve assembly of their own.
If your motor will not maintain a constant idle or if you notice a very heavy gasoline smell at the exhaust, it's time to check your power valve. It is either opening too soon for your idle vacuum levels, or it is blown.
Holley puts a 6.5 power valve in nearly everything they assemble, so keep a spare on hand. If your motor backfires and starts hunting and seeking, your first step is to
cure the problem causing the backfire. Don't just jam another 'junk power valve' in there, because whatever caused the first one to blow is going to ruin the next one too. Once the ignition timing, valve timing, choke operation issue/s are all handled, now it is time to install that 6.5 power valve. Hook up your vacuum gauge, preferably with a long enough hose you can get the gauge inside the car with you whilst you are driving. Start the motor and let it settle down to idle. Make note of the idle vacuum level. For purposes of the tutorial, let's say it is 10 inches of vacuum. Now, it's time to take the car for a drive. Do not drive the car hard, just squeeze the throttle. We don't want the motor to go lean and backfire again. Find yourself a long, level stretch of road and ease the car up to cruising speed. Once you have established a steady cruise speed (65-70 MPH is fine), hold it steady and make note of your vacuum level. Depending on a very long list of variables I'll not go into here, the cruise vacuum level may be different. Whatever it is, make note of the number and head back to the ranch.
Let's say your cruise vacuum level was 9 inches of vacuum. It was 10 at idle and 9 at cruise. Since we don't want the power valve opening up in either of these situations, we need to select a power valve that will open later. I always recommend a power valve opening point that is 1 to 1.5 inches below your lowest recorded vacuum level. Run down to you friendly neighborhood speed shop and purchase yourself a 7.5 inch power valve. This opening rate will tip in a bit sooner than the 6.5 did, so you will quite likely eliminate that lazy spot you've been experiencing when you stand on it. It will always be right there, ready to tip in, but it won't be sneaking open on you at idle or as you cruise down the road. Put your good 6.5 power valve back in your toolbox and it will be ready to help you tune out power valve problems in the future.
Here are some rules of thumb you need to tattoo on your forehead. Remember to tattoo them on backwards, so you will be able to read them in the mirror. :winkn:
1. Do not remove the power valve from a Holley carb, unless the carb is to be used in a WOT drag race application using a clutch. If you do so, you need to jet up at least 8 numbers to cover up the loss of the power valve. And your motor does not always require that much more fuel, so leave the power valve in and let it do what it was designed to do. There is only one exception to this rule. If you are running a carb with a secondary power valve and the carb is mounted in a fore and aft location, I recommend plugging the secondary power valve passage only. When you do so, stick 2-4 numbers more jet in the secondary side and tune the secondary accelerator pump to cover up the missing valve. Under hard acceleration, fuel can rush away from the power valve, causing a lean backfire.
2. Carburetors do not cause backfires. Engine problems and ignorant mechanics cause backfires. Is your choke working as it should? When was the last time you checked valve lash? Did you actually use a timing light to set ignition timing, or do you have one of those blinking ears that lets you set timing without a light? Have you chacked for vacuum leaks?
3. Go to rule 1 and repeat.
In all my days of working with carbs (and I hate to think of how many hundreds I have done), I have never seen a single one of them with a brain. They are just as dumb as a handful of rocks. So if you can just be smarter than the carb, you've won the battle.
