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  #1  
Old 06-15-2008, 12:21 AM
ramarado ramarado is offline
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Power valve in carb purpose symptoms of bad one

What exactly does the power valve do in a carb and what would be symptoms of a bad one?

I have a 2100 or 2150 2bbl on a 360. It was rebuilt about 2000 miles ago. I just replaced the rings, and rod bearings and had the heads done, however it seems to have less get up and go now than before I worked on it. I'm also using a stock 4bbl manifold with Edelbrock adaptor plate, and put on a Pertronix ignition. The engine backfired whenever I was trying to make the initial start after the rebuild, (timing way off), so it could have screwed up. The engine runs smoother than before and can't find any vacuum leaks.
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Old 06-15-2008, 01:01 AM
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The power valve is held closed by engine vacuum, and opens at a pre-set vacuum level to richen the fuel mixture in partial-throttle operation before the main jets become fully involved.
A bad power valve will cause extremely rich running if it's leaking, or a mid throttle stumble if it's not opening.
That adapter isn't doing you any favors, since you're dumping all the air and fuel against the flat surface between the primary and secondary openings on the 4BBL intake.
What problems are you having?
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Old 06-15-2008, 01:41 AM
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Actually the power valve is to enrichen the mixture when the engine is under load or accelerating hard. The mains come on line, normally before the power valve opens, it's the accelerator pump's job to cover up the lean condition before the mains start flowing, not the power valve. A bad power valve almost alway blows the diaphragm, usually caused by a back fire, and leaks full into the manifold cause a very rich condition at idle and low speed. There are kits available for older Holley carbs for a valve in the vacuum signal to the valve that stops the pressure from the back fire from reaching the power valve. Newer Holleys come with the protector installed. I've never seen a stuck power valve, mainly because when the engine is off the valve is in the open position
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Old 06-15-2008, 06:49 AM
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Quote:
Originally Posted by Ford_Six View Post
That adapter isn't doing you any favors, since you're dumping all the air and fuel against the flat surface between the primary and secondary openings on the 4BBL intake.
What problems are you having?

First, it seems to take longer to start than it did on the first start of the day or if it has sat for a while. I've toyed with the timing between 8 and 15 and it doesn't seem to make much difference. The reason for the question about the PV is that it seems to be more sluggish when climbing, accelerating, etc., I also have a very slight pause or hesitation if I just "get on it" from a light. If I ease into it, for just a second then get on it it'll go ahead and go. My main thing is the somewhat sluggish acceleration though.

As far as the adaptor, the adaptor simply covers the secondaries holes on the manifold. There is no "dead spot" between the carb and the manifold. At least in my mind it should be just like running a 4 bbl without using the secondaries.
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Old 06-15-2008, 12:39 PM
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Sounds more like the accelerator pump...
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Old 06-15-2008, 02:08 PM
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OK, so where is that? Do both come in a kit or is one separate?
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Old 06-15-2008, 04:04 PM
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Depending on the exact model of the carb, you can sometimes purchase just the parts for the pump, or it will come with a complete rebuild kit.

Putting it bluntly? If the accel. pump has gone bad? (easy to verify, look down the throat of the carb with the engine off and move the throttle linkage to wide open--you should see and smell raw fuel being sprayed right into the throat.) At any rate--If the pump has gone bad? Might just as well rebuild the carb with all new parts. Most of these items come complete in the "Kit", but be aware--the float can be either brass or a black plastic type material. I normally replace the float at the same time a rebuild is done.

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Old 06-16-2008, 09:26 PM
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would a bad power valve cause a rough idle to the point it acts like the butterflies in the carb are being completely closed off?? its hard to describe. this problem struck my 1972 302 the second day after a timing chain head swap. first two days it was fine then dieseling and now the idle has gone to hell
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Old 06-17-2008, 04:51 PM
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Quote:
Originally Posted by jack048307 View Post
would a bad power valve cause a rough idle to the point it acts like the butterflies in the carb are being completely closed off?? its hard to describe. this problem struck my 1972 302 the second day after a timing chain head swap. first two days it was fine then dieseling and now the idle has gone to hell
One tiny little back fire and you can blow the diaphragm in the power valve. Dieseling acts like a back fire lots of times. Run the idle screws in and see if the motor continues to run. If it does, chances ae you blew the power valve. When you replace the valve, get a power valve protector kit and install it so you don't blow another power valve.
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Old 06-27-2008, 08:30 AM
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i went ahead and changed the powervalve and nothing changed my problem is somewhere else
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Old 06-27-2008, 08:50 AM
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Bear: I've never seen a stuck power valve, mainly because when the engine is off the valve is in the open position

I think that a spring holds the power valve closed when the engine is off, and that vacuum in the venturis pulls it open.

Jack: check for leaks with a piece of hose and your ear, or smoke machine, or propane. When you did the heads, you may have gotten a hose loose, or gaskets... or the valves may have been adjusted too tight. The geometry of the valve train changes with wear and different parts, and you may be just on the edge of needing shorter pushrods or a tad looser lifter adjustment.
You can check by taking a turn back on all the rocker arm nuts and if it cleans up the idle, you have a valve train problem.

Ramarado: The accelerator pump squirts some gas into the carb above the butterfly to briefly enrichen the mix. Air goes zoom when you open the throttle, but the fuel, as liquid, is slower to get going, so to avoid a momentary lean out while waiting for the gas, they mechanically squirt some gas. It may just need adjustment. Have you broken in the rings yet? Is the engine still 'tight'? Did you get the timing adjusted? How much slop in the timing chain?
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Last edited by tomw; 06-27-2008 at 09:05 AM. Reason: more thoughts
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Old 06-27-2008, 11:49 AM
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Quote:
Originally Posted by tomw View Post
Bear: I've never seen a stuck power valve, mainly because when the engine is off the valve is in the open position

I think that a spring holds the power valve closed when the engine is off, and that vacuum in the venturis pulls it open.

tom
When vacuum drops the valve opens. High vacuum holds it closed, hence the spring holds it open. But with engine off, no vacuum means the engine does not suck and no fuel flow.
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Old 06-27-2008, 08:46 PM
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what size of power valve are you using? usually carbs have a 6.5hg which is too small for a stock engine, you should have 18-19hg of vac. from the manifold, so you should use a 9 or an 8.5 power valve.

also that carb. setup sounds sketchy, is it a four barrel or a two barrel?

since you just rebuilt the top end, the engine may be sluggish because the rings havent set yet.

i had the same problem with my 390, and i was told i had too much carb for the engine, and at low speed the engine didnt have the velocity to richen the mix when i had the carb open.
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Old 06-27-2008, 08:51 PM
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Quote:
Originally Posted by tomw View Post

Ramarado: The accelerator pump squirts some gas into the carb above the butterfly to briefly enrichen the mix. Air goes zoom when you open the throttle, but the fuel, as liquid, is slower to get going, so to avoid a momentary lean out while waiting for the gas, they mechanically squirt some gas. It may just need adjustment. Have you broken in the rings yet? Is the engine still 'tight'? Did you get the timing adjusted? How much slop in the timing chain?
tom

I put a new kit and pv in the carb a couple of weeks ago and it is a lot better! I'm still having some idle issues, just not smooth as I think it should be, but it doesn't die or anything at a stop. Whenever I have time I'm going to re adjust the idle screws. Timing chain is about 7-800 miles new. Rings seem to be adjusted fine. Engine doesn't seem to be too tight and runs good. Just not getting the mileage I used to before all the changes seems like but don't really know for sure.
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Old 06-28-2008, 06:31 AM
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Well they also make blow proof power valves now...altho I have never used one...and as mentioned above check the accelerator pump for adjustment... some call for a .020" clearence, but I have always run them just where the actuating arm is just touching..Not pushing on the pump but just touching...

RJ
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Old 06-28-2008, 06:31 AM
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