Thread: New 496 wont run??

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Six main functions of the carb: #1 would be to deliver the improper fuel to air ratio under all conditions. Hence EFI.
(Sarcasm alert)

Also, a single backfire can destroy the power valve (I seem to recall this is especially true for Holley, and anti backfire kits are available)

How do I know if a power valve is bad, what can I see if anything or what are the symptoms?

Gently seat/shut the idle mixture screw (close, turn in, righty tightly). If the motor runs with the idle mixture screws shut it is getting fuel from somewhere else other than the idle circuit. This would lead to a possible power valve bad or mismatched to motor vacuum. It is also a sign of a possible vacuum leak.

However, vacuum leaks usually run lean. With your motor running rich, I would suspect high fuel pressure, high float setting or a mismatched power valve.

Fuel pressure needs to be confirmed first (or at least do what another suggested and disconnect the fuel and see if it runs on the float bolt's fuel only OK), otherwise you're chasing your tail.
Second would be to check the power valve to see what rating is marked on it, and change it to a higher vacuum rating..

Originally Posted by 36 sedan

With only 6hg vacuum, I would be suspicious of the power valve opening and flooding, another reason I suggested closing the the idle mixture screws.

Din Ding Ding..... we have a winner! Run the carb dry and pull the metering plate(s) to see what number is on the power valve. It sounds like fuel is dripping past with that low vacuum signal. Perhaps the paperwork can give you the value too??? Or ask the OEM what they put in'em from the factory.

Originally Posted by Camaro350

How do I know if a power valve is bad, what can I see if anything or what are the symptoms?

It'll run but power sucks.

The valve uses vacuum to open and close. When you stomp down on the gas pedal the vacuum in the manifold drops to a very low number, this makes the power valve open. So when you start with a low vacuum number of 6 inches.. the valve may already be opening and now you've got to much gas in the manifold - once you turn up the idle screw a little, the idle jumps way up because you've already got plenty of fuel in the manifold that you don't/didn't plan on being there. I'm not certain or familiar with that carb, if it is a Holley copy you can get a power valve eliminator and run without it. Or, see if you can get a different power valve with a lower setting?? But I think your vacuum signal is to low already.

Again, talk with the OEM, the folks that built that carb can help you tune it best. HTH.

34-40 has summed it up.
One caveat though, if you eliminate the power valve (which I don't recommend for the street), you will need to drastically richen WOT (wide open throttle) as the power valve adds fuel for that circuit.

And, continuing to run the motor rich can (will) do harm to the motor. May I suggest you find a local tuner familiar with carburetors (check with your local car clubs for a recommendation) and get some hands on help before it costs you dearly.

Okay sorry for the late reply, thanks for all the good info. I have it running but like I stated I have to have the idle really high, over 1200rpms to keep it running. Fuel pressure is about 7psi at the carb. I have shut the idle screws and the vehicle does die after a min, it will run with the screws about 1/2 turn out but beyond that it struggles and smells really rich. I reset the choke and fast idle and still no change. When I read vacuum from the carb full manifold vacuum port it’s around 13-15 psi at 1400rpms. But when I close the throttle to bring the idle back down the vacuum drops and the vehicle will eventually die. I sprayed down the whole intake and carb with carb spray and no leaks were detected. Not sure how it’s running so rich with only 1/2 turn out on the mixture screws u less I have a power valve issue. I even checked the breathers on the valve covers to see if they were pulling vacuum under load and they were not so I don’t think the intake manifold is leaking either.

have you tried another carb ? first step i take .

I have not tried a new carb, I don’t have another one to swap out and try. If I could for sure know that it is a carb issue I could then possibly get another one or take this one apart

sounds like a power valve problem. loads up at idle . but all i can do is guess . denny is the big block guy i check to . our old bracket car was a bitch to to tune . had to drill the butterfly's to get it to idle right ..

You are running way too rich at normal idle speed, your vacuum is way low at normal idle speed. Logic tells me your power valve is opening at normal idle speed because the vacuum is less than the power valves opening threshold. You probably have a 6.5 power valve, try an 8 or 10 see if it gets you an idle.

I’ll reiterate my previous post and highly recommending you take it to a experienced tuner before damage occurs...

why are you running 7 psi on fuel pressure ? is it street driven or an 1/8 mile car ?

divide your vacuum at idle x2 . this will give you the size power valve you need. if you have 13-14 then the 6.5 will work . most likely the valve is blown .

Originally Posted by shine

why are you running 7 psi on fuel pressure ? is it street driven or an 1/8 mile car ?

divide your vacuum at idle x2 . this will give you the size power valve you need. if you have 13-14 then the 6.5 will work . most likely the valve is blown .

Agreed, 7 psi is a little high, while the manufacturer may claim higher psi is OK, I, like you would not.

Also agree under normal circumstances this is the formula for power valve tuning, normally with a tuned carb this will allow the power valve to open under acceleration, and good a starting point to tuning the circuit.

However, and JMHO,
Not getting enough fuel at WOT is not the OP's case, and a lower value power valve may make matters worse until the cause of the way overly rich condition is sorted out. The power valve may be bad (blown), and why I recommended a higher value power valve to help source the cause for the problem.

Again, JMHO, I believe there is more going on here than the OP has the skills and knowledge for, and his continued hit and miss approach may cause damage to his motor before he sorts it out.

I reiterate my recommendation for the OP to seek the skills and experience of a motor tuner knowledgable in carbs to help sort this out. A few dollars spent on a skilled tuner now may save thousands of dollars later.