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EFV-8 Club Forum / General Mercury Discussion / carb question

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Discussion Topic: carb question -- page: 1 2

40merc

01-25-2010 @ 8:29 PM

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does anyone have any links to help me better understand powervalve's? im wondering if im using the wrong one, the engine is a 47 merc. pickup

TomO

01-26-2010 @ 8:54 AM

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There is a lot of theories on power valves posted on the internet, but the only place that I have found to accurately describe power valve operation in stock applications is the Ford Service letters or the Canadian Ford Shop Manuals. Both are available in reprints.

The power valve opens to let more fuel into the manifold when vacuum drops below its set point. Example: power valve has 6.5 inches for its opening value, when the vacuum drops below 6.5 inches it will open th allow an extra shot of gasoline.

The 6.5 value would be normal for all 32-53 engines with a single carburetor. Higher vacuum ratings may give you a mixture that is too rich for normal driving and lower ratings would be for higher altitudes.

What are the symptoms that lead you to believe that you are using the incorrect valve for your engine?

Tom

supereal

01-26-2010 @ 11:26 AM

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There is an excellent article in the latest V8 TIMES regarding the need to machine power valves to provide a proper gasket seat in older vehicles. Given the ongoing problems with leaking power valve gaskets, it should be an welcome "fix". In most cases, a leaking valve gasket is the problem, rather that the correct rating of the valve. We have a box full of various rated valves, and usually a mismatch doesn't have a marked effect on drive ability unless something else is going on in the carb.

40merc

01-26-2010 @ 6:03 PM

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well if its based on the vacuum then i immagine its not the problem.

its a ford 2bbl model 94 carb. freshly rebuilt.
when its warm it runs great, but its often times hard starting, granted its easer then it used to be.
when i replaced the carb i also replaced the hard vacuum line that runs from the intake manifold to the timing cover plate. throttle response is great and all is well. but it just has truble starting.

when i start it it will run for two or so seconds then die out... ive tried messing with the air/fuel mix screws for a week or so now and it had no affect when starting it cold.

im just kind of puzzled, timing is set correctly, replaced the brass screen on the fuel pump, good vacuum pressure when warm, vacuum break system is working correctly, and a fresh carb.... what else am i missing? i know these old flat heads should just start right up...

the only thing that comes to mind is an odd fuel pressure, but that would affect how it runs in general, the engine is only problematic when cold

This message was edited by 40merc on 1-26-10 @ 6:04 PM

TomO

01-27-2010 @ 9:05 AM

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Hard starting can be caused by many different things. Is it hard starting when cold, when it has been run for a while and then sits for 1 hour or less, or when it is warm?

Tom

40merc

01-27-2010 @ 12:11 PM

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only hard starting after sitting over night, once it warms up and i start it a few hours later it has no problem.

supereal

01-27-2010 @ 1:16 PM

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Hard starting after sitting overnight often is due to leakdown of fuel from the carb, or eveaporation of the fuel from engine heat. This does two things: requires lots of cranking to refill the carb bowl, and cranking to lessen the accumulation of fuel in the manifold, which will cause flooding, if there is a leak. If the power valve is leaking, it will cause this problem. Have a can of starting fluid handy, and if the engine doesn't start after cranking, give it a couple of squirts of the fluid and try again. It is likely the engine will then start. My '47 used to give me fits, until I learned it had to have several pumps of the accelerator before hitting the starter button in order to provide sufficient fuel for starting. I knew my power valve was OK, but the evaporation of the fuel in the carb from engine heat didn't leave enough to provide an easy start. I run the car infrequently, and installed an electric booster pump to reduce strain on the battery. Now, it pops off every time, hot or cold.

40merc

02-09-2010 @ 11:39 AM

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i rebuilt the carb and fuel pump and had the tank stripped and a new gas line, seems to be okay now. little rough when cold but it doesn't die like it used to. also have a little backfiring when warm, probably a timing issue though. not sure what else it could be. i suspect the problem before was the dirty gas tank and the four piece fuel line, as well as a bad diaphragm on the fuel pump.

This message was edited by 40merc on 2-9-10 @ 11:40 AM

supereal

02-09-2010 @ 3:21 PM

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Backfiring can be caused by a too lean mixture or, often, crossfiring between spark plug wires in the loom, or a carbon path in the distributor. As you report that the engine dies soon after starting, I'd suspect either a float level set too low, or a leak down caused by the power valve gasket. The vacuum control of the distributor is actually a "retard" device, rather than an advance control. When manifold vacuum falls, a spring forces a snubber against the disk in the distributor, causing the timing advance of the weights to be reduced. That said, since you say you replaced the vacuum line to the distributor, double check for a leak there. Such a leak will also cause backfire.

40merc

02-10-2010 @ 12:30 PM

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tinkerd with the airfuel mix and its doing better. i am curious about the carbon footprint your talking about, is this a common problem in flatheads? ive not really messed with the distributor other then setting the timing and points, general maintenance stuff...

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