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Old 03-10-2015, 05:28 PM   #1
Rand
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Default fuel pressure?

I know it's here somewheres..but I'm feeling lazy.. what is the ideal stock fuel pressure for a 50Merc
Tks, Randy
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Old 03-10-2015, 06:56 PM   #2
rotorwrench
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Default Re: fuel pressure?

Pump pressures should be not less than 3 1/2 PSI @ 1800 rpm according to the manual.
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Old 03-10-2015, 09:23 PM   #3
Drbrown
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Default Re: fuel pressure?

Some Service Bulletins recommend first testing pressure at engine start-up, leave running at idle without increasing rpm, note pressure. Next increase rpm and note pressure. You may wish to check the vacuum reading of your pump (most around 10 psi) but check spec's on that. Latter may indicate faulty check valve.
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Old 03-10-2015, 09:41 PM   #4
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Default Re: fuel pressure?

Thanks a bunch!
Randy
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Old 03-10-2015, 10:38 PM   #5
WestCoast
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Default Re: fuel pressure?

2 1/2 to no more than 3 1/2#
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Old 03-11-2015, 07:57 AM   #6
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Default Re: fuel pressure?

Dumb ? For most...
Is it possible to have to much fuel pressure on a stock system?
Meaning fuel is leaking out the heat riser thingie going into the carb and gas line fitting going into the carb. on a 50merc...
carb was rebuilt and did the exact thing before being rebuilt
Is a regulator needed on a stock system? Wasent 65 yrs ago..why now? I do have a in line electric pump that I only use on start ups after sitting awhile.
Thanks Randy
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Old 03-11-2015, 09:07 AM   #7
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Default Re: fuel pressure?

I have 53 Victoria with dual 2 barrels, I run around 2 1/2 lbs, no problem.
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Old 03-11-2015, 09:29 AM   #8
JSeery
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Default Re: fuel pressure?

Need to put a pressure gauge on it and see what the pressure is, otherwise you are just guessing.
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Old 03-11-2015, 09:43 AM   #9
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Default Re: fuel pressure?

Sounds like a leak inside, most likely where the brass chap that screws in for the needle and seat. Your gonna have to pull the lid off the carb and have a look.
Go gentle, they can be a bear to get lid and power valve rod and the gasket and plate all lined up for refit. A couple of lengths of all thread bout 6" long screwed into a couple of lid screw threads in the base are a great help here.
Sorry can't remember what the thread size is, 1/4 fine or 10 32 I think.
Martin.
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Old 03-11-2015, 09:46 AM   #10
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Default Re: fuel pressure?

The specs are for a stock Holley 885 carburetor. If you are running a Strombeg 97, you may need a regulator. The manual for the 49 to 51 Merc lists 3.5 PSI with 7 inches HG @ 1800rpm minimum and 4.5 psi with 9 inches HG @ 1800rpm maximum but it doesn't specify any difference between the vacuum assist type pump and the plain 8BA type pump. The Lincoln pump is also listed but they don't identify it as being any different for test purposes. The fuel pump section was not updated after 1949 so there were likely no changes for these years that they felt worth revision.
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Old 03-11-2015, 10:14 AM   #11
Rand
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Default Re: fuel pressure?

Yes..stock back draft Holly carb.. with 8ba pump.as stated above was rebuilt by a reputable builder here on the Barn. And he has gone the extra mile for me for sure. What I don't get is its leaking just like before the rebuild?? Which leaves be to believe I have to much pressure? But weird to me with a bone stock carb and pump..looks like I need to get a pressure gauge to see?
Thanks a bunch for all your help...
Randy
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Old 03-11-2015, 05:26 PM   #12
scooder
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Default Re: fuel pressure?

Randy,
If the fuel pump has been rebuilt before, it's quite possible that the wrong spring has been fitted under the diaphragm. Some of the rebuild kits have and still do occasionally have a spring that's to stiff. This would raise the pump pressure.
I think the gauge is a good idea for testing.
Another thing to be sure of is clean gas, tiny bits of stuff and things will prevent the needle seating, leading to flooding.
Got a filter?
Good luck,
Martin.
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Old 03-11-2015, 05:42 PM   #13
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Default Re: fuel pressure?

Yes 3 filters in line one at tank with the electric pump one near the oem pump and one in the glass bowl.
The carb was rebuilt...not the pump..
Randy
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Old 03-11-2015, 06:59 PM   #14
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Default Re: fuel pressure?

Quote:
Originally Posted by JSeery View Post
Need to put a pressure gauge on it and see what the pressure is, otherwise you are just guessing.
Put a gauge in the fuel line between the pump and carb. Here's the problem. As the fuel pump heats up the fuel pressure drops and in hot weather with this new gas it forms vapor lock. I find in 90 degree plus outside temperature I set the pump to 4 lbs or a little more cold and as the pump gets hot it will drop down between 3 and 3.5. When it get real hot it will drop down in the 2 lb range and start to get a little jumpy in pressure. When it gets down in the 1.5 to 2 lb range the pressure will start jumping up and down as vapor lock starts and the engine will get a little rough. If the engine is shut down it is doubtful it will restart until it cools down. Pouring cold water or a bag of ice on the pump will allow you to restart. With an initial cool low pressure pump setting the problem increases. G.M.
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Old 03-11-2015, 09:29 PM   #15
Rand
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Default Re: fuel pressure?

Here is a picture of the cheep'o gauge that i got at Autozone showing 5+ psi of fuel pressure...would that cause my freshly rebuilt carb to leak?
and a picture of the leak itself in a mirror looking up
Thanks a bunch for your thoughts
Randy
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File Type: jpg gauge.jpg (58.6 KB, 75 views)
File Type: jpg leak.jpg (30.2 KB, 68 views)
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Old 03-11-2015, 11:11 PM   #16
Ralph Moore
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Default Re: fuel pressure?

5 psi is high, needs to be closer to 2-1/2 to 3#
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Old 03-12-2015, 05:19 AM   #17
scooder
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Default Re: fuel pressure?

Quote:
Originally Posted by Ralph Moore View Post
5 psi is high, needs to be closer to 2-1/2 to 3#
Not on the big Merc carb. This carb is designed to work with 3 1/2 - 4 1/2.
Martin.
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Old 03-12-2015, 05:41 AM   #18
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Default Re: fuel pressure?

5 is a bit to high,
I'd pull the lid to see where the leak is coming from.
The gas is coming down the gas line and the inlet pipe for the choke. For it to come down the choke inlet pipe, I'd say it's quite a leak.
Pull the lid and have a look, could be as easy as a fitting not snugged down tight enough.
I know you haven't had the pump rebuilt, but it would have been rebuilt or replaced in its life, they don't last forever. So could have the wrong spring put in it at this point. I mentioned this as you said it was leaking exactly like this before the carb was rebuilt. Which usually would eliminate the carb as being the issue. Usually, but this would be an assumption.
I'd still pull the lid, and guess I'd be looking at the brass fitting up the back of the needle and seat assembly.
Could be an easy fix, rather than guessing.
Martin.
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Old 03-12-2015, 10:06 AM   #19
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Default Re: fuel pressure?

All the top mount bowl Holley carbs are very sensitive to warpage from heavy handed maintenance over the years. If all the parts are in good condition with proper seals in place, they hold their mud pretty well. If not, they leak. I've gone through three carbs before just to find enough parts to put together one good one.

A person could just put a regulator on the fuel feed side to keep the pressure less than 4 PSI but you have to have a good gauge to check pressures that low. A manometer on the suction side would be more accurate to check a pumps draw but the output pressure is just as important.
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Old 03-28-2015, 10:53 AM   #20
Ken Bull
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Default Re: fuel pressure?

My 94 Holley Carb was flooding, so I got a restore Kit #601 from Daytona Parts Co. that includes their "Daytona Float Valve". The valve Plunger has a Fluorocarbon Elastomer Rubber Disc (its alcahol resistant). Best part is it solved my flooding problem.?s contact them @ 386 427 7108 in New Smyrna Beach Florida. Very helpful!
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