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JaimeReyes
Samba Member
Joined: May 30, 2018
Posts: 165
Location: San diego CA
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 Posted: Sun Jul 15, 2018 5:42 pm Post subject: fuel pressure |
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| so i installed a new brosol fuel pump on my 71 super and hooked up the fuel pressure gauge to see what it’s putting out. was super high! like around 8psi. did what i could and i got it down to about 4.5 psi. is 4.5psi still too high? does it need to be around the 3psi mark or am i okay to leave it where it is? |
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Cusser
Samba Member
Joined: October 02, 2006
Posts: 31358
Location: Hot Arizona
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| Posted: Sun Jul 15, 2018 7:01 pm Post subject: Re: fuel pressure |
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| JaimeReyes wrote: |
| so i installed a new brosol fuel pump on my 71 super and hooked up the fuel pressure gauge to see what it’s putting out. was super high! like around 8psi. did what i could and i got it down to about 4.5 psi. is 4.5psi still too high? does it need to be around the 3psi mark or am i okay to leave it where it is? |
That all depends whether the needle valve in the carb works properly with that psi.
_________________
1970 VW (owned since 1972) and 1971 VW Convertible (owned since 1976), second owner of each. The '71 now has the 1835 engine, swapped from the '70. Second owner of each. 1988 Mazda B2200 truck, 1998 Frontier, 2014 Yukon, 2004 Frontier King Cab. All manual transmission except for the Yukon. http://www.thesamba.com/vw/forum/album_page.php?pic_id=335294 http://www.thesamba.com/vw/forum/album_page.php?pic_id=335297 |
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JaimeReyes
Samba Member
Joined: May 30, 2018
Posts: 165
Location: San diego CA
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| Posted: Sun Jul 15, 2018 7:03 pm Post subject: Re: fuel pressure |
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| Cusser wrote: |
| JaimeReyes wrote: |
| so i installed a new brosol fuel pump on my 71 super and hooked up the fuel pressure gauge to see what it’s putting out. was super high! like around 8psi. did what i could and i got it down to about 4.5 psi. is 4.5psi still too high? does it need to be around the 3psi mark or am i okay to leave it where it is? |
That all depends whether the needle valve in the carb works properly with that psi. |
hey thanks for taking the time and helping me out i really appreciate it! my really sure how to determine that either. sorry i’m a compete newbie and don’t know much at all :/ |
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AlmostHeavenWV_VW 
Samba Member
Joined: October 12, 2017
Posts: 1965
Location: WV
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| Posted: Sun Jul 15, 2018 7:24 pm Post subject: Re: fuel pressure |
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| JaimeReyes wrote: |
hey thanks for taking the time and helping me out i really appreciate it! my really sure how to determine that either. sorry i’m a compete newbie and don’t know much at all :/ |
Basically, Cusser is saying:
"Does the carb/engine become flooded with fuel with the PSI that pump is putting out?"
The needle valve (or float valve) is the valve which allows fuel from the pump into the carb fuel bowl. Once the float inside the bowl (and the fuel level) get high enough-"Full" the needle valve closes and no more fuel goes into the float bowl until some fuel is used and the float drops enough to re-open the needle valve.
Cycle repeats so long as the engine is running.
If the pump puts out too much PSI, it can 'overwhelm' the needle valve (aka can 'force' fuel into the float bowl when the needle valve should be closed) this causes the fuel level in the bowl to be 'overfilled' and can then simple drip/run into the carb and intake. Essentially then 'dumping' extra fuel into the engine.
Long story short:
If your car (and carb needle valve) seem to do fine with that PSI, it's pretty close to specs.
Your choice if you want to find a solution to further reduce PSI output.
Trouble is: once you begin stacking lots of gaskets to reduce pressure, there's a chance you begin decreasing both the PSI and volume of fuel being delivered to the carb. This reduction in volume can cause fuel 'starvation' at high RPMs (highway driving). This can cause a cycle of loss of power (while the engine isn't getting enough fuel) until the speed/RPMs drop for a long enough time to get the bowl fuel level 'caught up' and then you will have good power until the fuel level drops again (because the pump isn't putting out enough volume to keep up with the high RPM fuel demands).
More gaskets doesn't always mean better/correct.
A good intro into precisely how a carburetor works:
https://en.wikipedia.org/wiki/Carburetor
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1973 Standard Beetle
1600DP AK case
Solex 34PICT3 Carb
Bosch DVDA 205AJ Distributor |
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JaimeReyes
Samba Member
Joined: May 30, 2018
Posts: 165
Location: San diego CA
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| Posted: Sun Jul 15, 2018 7:38 pm Post subject: Re: fuel pressure |
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| AlmostHeavenWV_VW wrote: |
| JaimeReyes wrote: |
hey thanks for taking the time and helping me out i really appreciate it! my really sure how to determine that either. sorry i’m a compete newbie and don’t know much at all :/ |
Basically, Cusser is saying:
"Does the carb/engine become flooded with fuel with the PSI that pump is putting out?"
The needle valve (or float valve) is the valve which allows fuel from the pump into the carb fuel bowl. Once the float inside the bowl (and the fuel level) get high enough-"Full" the needle valve closes and no more fuel goes into the float bowl until some fuel is used and the float drops enough to re-open the needle valve.
Cycle repeats so long as the engine is running.
If the pump puts out too much PSI, it can 'overwhelm' the needle valve (aka can 'force' fuel into the float bowl when the needle valve should be closed) this causes the fuel level in the bowl to be 'overfilled' and can then simple drip/run into the carb and intake. Essentially then 'dumping' extra fuel into the engine.
Long story short:
If your car (and carb needle valve) seem to do fine with that PSI, it's pretty close to specs.
Your choice if you want to find a solution to further reduce PSI output.
Trouble is: once you begin stacking lots of gaskets to reduce pressure, there's a chance you begin decreasing both the PSI and volume of fuel being delivered to the carb. This reduction in volume can cause fuel 'starvation' at high RPMs (highway driving). This can cause a cycle of loss of power (while the engine isn't getting enough fuel) until the speed/RPMs drop for a long enough time to get the bowl fuel level 'caught up' and then you will have good power until the fuel level drops again (because the pump isn't putting out enough volume to keep up with the high RPM fuel demands).
More gaskets doesn't always mean better/correct.
A good intro into precisely how a carburetor works:
https://en.wikipedia.org/wiki/Carburetor |
hey thanks for the long and imformative reply! i get the concept of it but i’m having trouble figuring out how to check if it’s flooding/overfilling |
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bluebus86
Banned
Joined: September 02, 2010
Posts: 11075
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| Posted: Sun Jul 15, 2018 9:02 pm Post subject: Re: fuel pressure |
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Set the pressure to specification. Per the orange Bentley Official VW Service Manual section 4.2....... 3.0 to 5.0 psi.
Once this is done, you may wish to check and adjust float bowl level, procedure in the afore mentioned Bentley Manual.
Bug On!
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Help Prevent VW Engine Fires, see this link.....Engine safety wire information
Stop introducing dirt into your oil when adjusting valves ... https://www.thesamba.com/vw/forum/viewtopic.php?t=683022 |
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Eric&Barb
Samba Member
Joined: September 19, 2004
Posts: 24732
Location: Olympia Wash Rinse & Repeat
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| Posted: Mon Jul 16, 2018 12:58 pm Post subject: Re: fuel pressure |
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Also you can remove the air cleaner with engine running and see if fuel is pouring out of the vent tube at top of the carb throat. Do wear eye protection just in case of back fire in your face....
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JaimeReyes
Samba Member
Joined: May 30, 2018
Posts: 165
Location: San diego CA
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| Posted: Mon Jul 16, 2018 5:48 pm Post subject: Re: fuel pressure |
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| Eric&Barb wrote: |
| Also you can remove the air cleaner with engine running and see if fuel is pouring out of the vent tube at top of the carb throat. Do wear eye protection just in case of back fire in your face.... |
not sure what the vent tube is?? is that the same thing as the little accelerator tube that looks like a faucet? |
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busdaddy
Samba Member
Joined: February 12, 2004
Posts: 51117
Location: Surrey B.C. Canada, but thinking of Ukraine
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| Posted: Mon Jul 16, 2018 5:50 pm Post subject: Re: fuel pressure |
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| JaimeReyes wrote: |
| Eric&Barb wrote: |
| Also you can remove the air cleaner with engine running and see if fuel is pouring out of the vent tube at top of the carb throat. Do wear eye protection just in case of back fire in your face.... |
not sure what the vent tube is?? is that the same thing as the little accelerator tube that looks like a faucet? |
No, but nearby, it'll be the one puking out gas 
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Rust NEVER sleeps and stock never goes out of style.
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cletus_zuber
Samba Member
Joined: January 08, 2010
Posts: 2406
Location: Gladstone, Ore
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| Posted: Mon Jul 16, 2018 5:56 pm Post subject: Re: fuel pressure |
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How did you measure the fuel pressure and how accurate is your gauge?
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1972 1302
HPMX 40's & 019 |
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Dark Earth
Samba Member
Joined: December 22, 2015
Posts: 1054
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| Posted: Mon Jul 16, 2018 5:59 pm Post subject: Re: fuel pressure |
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| JaimeReyes wrote: |
| Eric&Barb wrote: |
| Also you can remove the air cleaner with engine running and see if fuel is pouring out of the vent tube at top of the carb throat. Do wear eye protection just in case of back fire in your face.... |
not sure what the vent tube is?? is that the same thing as the little accelerator tube that looks like a faucet? |
It's the big one in the center cut at an angle. It vents the float bowl where all the fuel comes into the carburetor.
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My Build: '69 Baja - Dark Earth Version
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JaimeReyes
Samba Member
Joined: May 30, 2018
Posts: 165
Location: San diego CA
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| Posted: Mon Jul 16, 2018 8:40 pm Post subject: Re: fuel pressure |
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| busdaddy wrote: |
| JaimeReyes wrote: |
| Eric&Barb wrote: |
| Also you can remove the air cleaner with engine running and see if fuel is pouring out of the vent tube at top of the carb throat. Do wear eye protection just in case of back fire in your face.... |
not sure what the vent tube is?? is that the same thing as the little accelerator tube that looks like a faucet? |
No, but nearby, it'll be the one puking out gas |
ooh okay gotcha!! is it suppose to be puking out gas? |
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JaimeReyes
Samba Member
Joined: May 30, 2018
Posts: 165
Location: San diego CA
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| Posted: Mon Jul 16, 2018 8:42 pm Post subject: Re: fuel pressure |
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| Dark Earth wrote: |
| JaimeReyes wrote: |
| Eric&Barb wrote: |
| Also you can remove the air cleaner with engine running and see if fuel is pouring out of the vent tube at top of the carb throat. Do wear eye protection just in case of back fire in your face.... |
not sure what the vent tube is?? is that the same thing as the little accelerator tube that looks like a faucet? |
It's the big one in the center cut at an angle. It vents the float bowl where all the fuel comes into the carburetor.
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awesome! thanks for the picture. |
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busdaddy
Samba Member
Joined: February 12, 2004
Posts: 51117
Location: Surrey B.C. Canada, but thinking of Ukraine
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| Posted: Mon Jul 16, 2018 8:58 pm Post subject: Re: fuel pressure |
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| JaimeReyes wrote: |
| busdaddy wrote: |
| JaimeReyes wrote: |
| Eric&Barb wrote: |
| Also you can remove the air cleaner with engine running and see if fuel is pouring out of the vent tube at top of the carb throat. Do wear eye protection just in case of back fire in your face.... |
not sure what the vent tube is?? is that the same thing as the little accelerator tube that looks like a faucet? |
No, but nearby, it'll be the one puking out gas |
ooh okay gotcha!! is it suppose to be puking out gas? |
No, that only happens with excessive fuel pressure and/or a faulty needle and seat or sunk float.
_________________
Rust NEVER sleeps and stock never goes out of style.
Please don't PM technical questions, ask your problem in public so everyone can play along. If you think it's too stupid post it here
Stop dead photo links! Post your photos to The Samba Gallery!
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bluebus86
Banned
Joined: September 02, 2010
Posts: 11075
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| Posted: Mon Jul 16, 2018 9:01 pm Post subject: Re: fuel pressure |
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Not that if the tube in question does not burp out fuel, that does NOT mean fuel pressure or float bowl height is correct. If fuel does burp fuel, some thing is wrong, but no burping is not an assurance that all is correct with fuel pressure or fuel level in bowl.
Set the pressure with use of a gage, then measure float bowl level as per the factory manual.
Good Luck, Bug On!
_________________
Help Prevent VW Engine Fires, see this link.....Engine safety wire information
Stop introducing dirt into your oil when adjusting valves ... https://www.thesamba.com/vw/forum/viewtopic.php?t=683022 |
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Eric&Barb
Samba Member
Joined: September 19, 2004
Posts: 24732
Location: Olympia Wash Rinse & Repeat
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| Posted: Mon Jul 16, 2018 9:10 pm Post subject: Re: fuel pressure |
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Vent tube allows normal air pressure to get into the float chamber to push down on the pool of fuel in there. With the drop in air pressure just below the venturi, the fuel then will get sucked thru all the passages starting in the bottom of the float chamber and thru jets to spray into the lower throat of the carb.
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Dark Earth
Samba Member
Joined: December 22, 2015
Posts: 1054
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| Posted: Mon Jul 16, 2018 9:18 pm Post subject: Re: fuel pressure |
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Fuel comes in at "A". Normally the float bowl will fill with fuel causing the float "F" to rise closing the needle and seat "P" (as pictured in the diagram). If the fuel pressure is too high, the pressure will force the needle and seat "P" open, forcing the float "F" down. This will overfill the float bowl causing fuel to spill out of the vent tube.
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Towel Rail
Horizontally Opposed
Joined: April 15, 2005
Posts: 4622
Location: SE CR IA US NA PE
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| Posted: Tue Jul 17, 2018 9:24 pm Post subject: Re: fuel pressure |
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| bluebus86 wrote: |
Set the pressure to specification. Per the orange Bentley Official VW Service Manual section 4.2....... 3.0 to 5.0 psi.
Once this is done, you may wish to check and adjust float bowl level, procedure in the afore mentioned Bentley Manual.
Bug On! |
3-5 psi is the *maximum* spec. Earlier in the text is a target of 2.8 psi at 3400 rpm.
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1974 Thing -- under the knife
1967 Beetle -- spring/summer/fall driver
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Meiang
Samba Member
Joined: September 27, 2016
Posts: 656
Location: Perth, Western Australia
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| Posted: Wed Jul 18, 2018 4:36 am Post subject: Re: fuel pressure |
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This whole fuel pressure measurement issue is so !!!
Trying to find a gauge that can measure 3psi accurately is very hard. 3, 4, 5, 6, 8, psi, most cheap gauges would not distinguish the difference accurately. Most are 15psi miniumum FSD
Unless a gauge has a calibration certificate it means nothing. This means you are going to pay US$150-200 plus for the gauge. Who pays that.
For those who dispute this. Have they had their gauge tested by a certified laboratory. Check the price - its probable a lot more.
Just because the cheap gauge indicates 3psi means nothing. |
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Dark Earth
Samba Member
Joined: December 22, 2015
Posts: 1054
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| Posted: Wed Jul 18, 2018 7:29 am Post subject: Re: fuel pressure |
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| Meiang wrote: |
This whole fuel pressure measurement issue is so !!!
Trying to find a gauge that can measure 3psi accurately is very hard. 3, 4, 5, 6, 8, psi, most cheap gauges would not distinguish the difference accurately. Most are 15psi miniumum FSD
Unless a gauge has a calibration certificate it means nothing. This means you are going to pay US$150-200 plus for the gauge. Who pays that.
For those who dispute this. Have they had their gauge tested by a certified laboratory. Check the price - its probable a lot more.
Just because the cheap gauge indicates 3psi means nothing. |
Here's the one I'm gonna use ( If I ever finish my build ) ...
https://www.harborfreight.com/fuel-pump-and-vacuum-tester-62637.html
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My Build: '69 Baja - Dark Earth Version
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