GMCforum
For enthusiast of the Classic GMC Motorhome built from 1973 to 1978. A web-based mirror of the GMCnet mailing list.

Home » Public Forums » GMCnet » Onan (4K) Priming / Fuel Pump Questions...
Onan (4K) Priming / Fuel Pump Questions... [message #328478] Sun, 21 January 2018 21:25 Go to next message
SteveW is currently offline  SteveW   United States
Messages: 538
Registered: June 2005
Location: Southern California - Ora...
Karma: 1
Senior Member
Greetings from Southern California and THANK YOU in advance for your suggestions.

My generator (4K) has been difficult to start so I finally jumped into it...

It seems that the Onan starts quicker when I keep the "prime" circuit energized while cranking. Does this sound reasonable ?? From my reading of the various electrical diagrams - it sure looks like the fuel pump only runs with the prime circuit energized or from the positive side of the coil while the engine is running. Should the fuel pump be pumping while cranking ??

And... With the priming circuit - shouldn't the pump slow down after the carburetor bowl gets filled ?? Is seems that mine continues to pump - no changes in the sound it makes... (it's the "direct replacement pump" from Applied.)

Everything seems to work once it's running. No hunting, no stalling, 120VAC... Just a little difficult to start.

And THANK YOU Gene Fisher. From the 49er blog post on your website - I got my Onan electric choke properly wired and working.

Steve W
19873 23'
Southern California



Steve W 1973 : 23' Southern California

[Updated on: Sun, 21 January 2018 21:27]

Report message to a moderator

Re: Onan (4K) Priming / Fuel Pump Questions... [message #328480 is a reply to message #328478] Sun, 21 January 2018 22:08 Go to previous messageGo to next message
Rick Staples is currently offline  Rick Staples   United States
Messages: 126
Registered: May 2014
Location: Johnstown, Colorado, USA
Karma: -1
Senior Member
SteveW wrote:

shouldn't the pump slow down after the carburetor bowl gets filled ?? Is seems that mine continues to pump - no changes in the sound it makes...

This is just a SWAG, but it sounds like it might be sucking air. I'd double check all the fuel lines back to the tank.
Other possibilities that come to mind include a defective fuel shut off valve or wiring thereto. (If you're still using the cut-off valve.) Or possibly leaking check valves (dirt/rust) in the pump. Just some places to start....
HTH
Rick Staples


Rick Staples, '75 Eleganza, Johnstown, CO "Advice is a dangerous gift, even from the Wise to the Wise, and all paths may run ill." -Tolkien
Re: [GMCnet] Onan (4K) Priming / Fuel Pump Questions... [message #328484 is a reply to message #328480] Mon, 22 January 2018 00:33 Go to previous messageGo to next message
jimk is currently offline  jimk   United States
Messages: 6734
Registered: July 2006
Location: Belmont, CA
Karma: 9
Senior Member
We have a great primer swith for Onan , go to our site.

On Sun, Jan 21, 2018 at 8:08 PM, Richard H Staples
wrote:

> SteveW wrote:
>
> shouldn't the pump slow down after the carburetor bowl gets filled ?? Is
> seems that mine continues to pump - no changes in the sound it makes...
>
> This is just a SWAG, but it sounds like it might be sucking air. I'd
> double check all the fuel lines back to the tank.
> Other possibilities that come to mind include a defective fuel shut off
> valve or wiring thereto. (If you're still using the cut-off valve.) Or
> possibly leaking check valves (dirt/rust) in the pump. Just some places
> to start....
> HTH
> Rick Staples
> --
> Rick Staples, '75 Eleganza, Johnstown, CO
>
> "Advice is a dangerous gift, even from the Wise to the Wise, and all paths
> may run ill." -Tolkien
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>



--
Jim Kanomata
Applied/GMC, Newark,CA
jimk@appliedairfilters.com
http://www.appliedgmc.com
1-800-752-7502
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org



Jim Kanomata
Applied/GMC
jimk@appliedairfilters.com
www.appliedgmc.com
1-800-752-7502
Re: Onan (4K) Priming / Fuel Pump Questions... [message #328485 is a reply to message #328478] Mon, 22 January 2018 00:50 Go to previous messageGo to next message
SteveW is currently offline  SteveW   United States
Messages: 538
Registered: June 2005
Location: Southern California - Ora...
Karma: 1
Senior Member
Thanks Rick - I'll check all of the things that you mentioned.

Thanks Jim - I've got a good momentary switch in the circuit... Have you had any issues with the "direct replacement" onan fuel pumps not developing enough pressure ??

Steve W
1973 23'
Southern California


Steve W 1973 : 23' Southern California
Re: Onan (4K) Priming / Fuel Pump Questions... [message #328486 is a reply to message #328478] Mon, 22 January 2018 06:22 Go to previous messageGo to next message
Ken Burton is currently offline  Ken Burton   United States
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
Senior Member
SteveW wrote on Sun, 21 January 2018 21:25
Greetings from Southern California and THANK YOU in advance for your suggestions.

My generator (4K) has been difficult to start so I finally jumped into it...

It seems that the Onan starts quicker when I keep the "prime" circuit energized while cranking. Does this sound reasonable ??

Holding the prime switch while cranking to keep the pump and ignition going during the change over from cranking mode to run mode. This change over is normally caused by the internal alternator starting to generate approximately 28-30VAC which tells the board that the engine is running and to keep the pump and ignition going. Mine is slow to come up to voltage and holding the prime switch overcomes that delay.

On that generator from hell that I worked on last August it had the same problem. I had the flywheel off for other reasons so I cleaned all of the alternator pole pieces and and also polished the inside of the flywheel where the magnets are. That eliminated this problem. I need to do the same to mine but I never seen to get around to doing it. It is much easier to just hold the button a few seconds longer when starting. Maybe some day I'll fix mine too.


From my reading of the various electrical diagrams - it sure looks like the fuel pump only runs with the prime circuit energized or from the positive side of the coil while the engine is running. Should the fuel pump be pumping while cranking ??

No, the pump runs while cranking and while running. The ignition coil, fuel pump, and fuel solenoid (if equipped) are all wired together.

And... With the priming circuit - shouldn't the pump slow down after the carburetor bowl gets filled ?? Is seems that mine continues to pump - no changes in the sound it makes... (it's the "direct replacement pump" from Applied.)

Yes it should slow down after reaching pressure (around 4 to 6 PSI). Several things can cause this. sucking air on the intake side (I doubt this or you would probably have problems running under load). More likely is the check valve inside the pump that is not sealing completely due to a piece of grit or something. It is very easy to take the pump apart and clean / inspect it. If you want a new base gasket and filter are available from NAPA because they sell that same pump under the NAPA brand name. I would screw in a tee and small fuel gauge on the output of the pump to see if this is what is really happening. A PO installed one permanently on mine.

Everything seems to work once it's running. No hunting, no stalling, 120VAC... Just a little difficult to start.

And THANK YOU Gene Fisher. From the 49er blog post on your website - I got my Onan electric choke properly wired and working.

Steve W
19873 23'
Southern California




Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
Re: [GMCnet] Onan (4K) Priming / Fuel Pump Questions... [message #328487 is a reply to message #328486] Mon, 22 January 2018 07:40 Go to previous messageGo to next message
jimk is currently offline  jimk   United States
Messages: 6734
Registered: July 2006
Location: Belmont, CA
Karma: 9
Senior Member
Steve,
It seem like the origional style pumps developemore once in a while.

On Mon, Jan 22, 2018 at 4:22 AM, Ken Burton wrote:

> SteveW wrote on Sun, 21 January 2018 21:25
>> Greetings from Southern California and THANK YOU in advance for your
> suggestions.
>>
>> My generator (4K) has been difficult to start so I finally jumped into
> it...
>>
>> It seems that the Onan starts quicker when I keep the "prime" circuit
> energized while cranking. Does this sound reasonable ??
>>
>> Holding the prime switch while cranking to keep the pump and ignition
> going during the change over from cranking mode to run mode. This change
>> over is normally caused by the internal alternator starting to generate
> approximately 28-30VAC which tells the board that the engine is running and
>> to keep the pump and ignition going. Mine is slow to come up to voltage
> and holding the prime switch overcomes that delay.
>>
>> On that generator from hell that I worked on last August it had the same
> problem. I had the flywheel off for other reasons so I cleaned all of
>> the alternator pole pieces and and also polished the inside of the
> flywheel where the magnets are. That eliminated this problem. I need to
> do the
>> same to mine but I never seen to get around to doing it. It is much
> easier to just hold the button a few seconds longer when starting. Maybe
> some
>> day I'll fix mine too.
>>
>> From my reading of the various electrical diagrams - it sure looks like
> the fuel pump only runs with the prime circuit energized or from the
>> positive side of the coil while the engine is running. Should the fuel
> pump be pumping while cranking ??
>>
>> No, the pump runs while cranking and while running. The ignition
> coil, fuel pump, and fuel solenoid (if equipped) are all wired together.
>>
>> And... With the priming circuit - shouldn't the pump slow down after the
> carburetor bowl gets filled ?? Is seems that mine continues to pump - no
>> changes in the sound it makes... (it's the "direct replacement pump"
> from Applied.)
>>
>> Yes it should slow down after reaching pressure (around 4 to 6 PSI).
> Several things can cause this. sucking air on the intake side (I doubt
>> this or you would probably have problems running under load). More
> likely is the check valve inside the pump that is not sealing completely
> due to
>> a piece of grit or something. It is very easy to take the pump apart
> and clean / inspect it. If you want a new base gasket and filter are
>> available from NAPA because they sell that same pump under the NAPA
> brand name. I would screw in a tee and small fuel gauge on the output of
> the
>> pump to see if this is what is really happening. A PO installed one
> permanently on mine.
>>
>> Everything seems to work once it's running. No hunting, no stalling,
> 120VAC... Just a little difficult to start.
>>
>> And THANK YOU Gene Fisher. From the 49er blog post on your website - I
> got my Onan electric choke properly wired and working.
>>
>> Steve W
>> 19873 23'
>> Southern California
>
>
> --
> Ken Burton - N9KB
> 76 Palm Beach
> Hebron, Indiana
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>



--
Jim Kanomata
Applied/GMC, Newark,CA
jimk@appliedairfilters.com
http://www.appliedgmc.com
1-800-752-7502
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org



Jim Kanomata
Applied/GMC
jimk@appliedairfilters.com
www.appliedgmc.com
1-800-752-7502
Re: Onan (4K) Priming / Fuel Pump Questions... [message #328492 is a reply to message #328478] Mon, 22 January 2018 09:39 Go to previous messageGo to next message
JohnL455 is currently offline  JohnL455   United States
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
Senior Member
Does your choke work?? My 6khas NO add on primer button and starts in like 6 rotations after sitting for months. When run recently it starts as soon as you touch the start switch. My choker works as designed. Also check for pinched fuel hose when the drawer is closed. That one will drive you crazy. Fuel lines can have nearly invisible splits where they go over the brass barbs and suck air.

John Lebetski
Woodstock, IL
77 Eleganza II
Re: [GMCnet] Onan (4K) Priming / Fuel Pump Questions... [message #328495 is a reply to message #328492] Mon, 22 January 2018 10:12 Go to previous messageGo to next message
jimk is currently offline  jimk   United States
Messages: 6734
Registered: July 2006
Location: Belmont, CA
Karma: 9
Senior Member
John,
Your lucky that it does that, majority needs to crank for too long, so the
optional primer switch comes in handy

On Mon, Jan 22, 2018 at 9:40 AM John R. Lebetski
wrote:

> Does your choke work?? My 6khas NO add on primer button and starts in like
> 6 rotations after sitting for months. When run recently it starts as soon
> as you touch the start switch. My choker works as designed. Also check
> for pinched fuel hose when the drawer is closed. That one will drive you
> crazy. Fuel lines can have nearly invisible splits where they go over the
> brass barbs and suck air.
> --
> John Lebetski
> Woodstock, IL
> 77 Eleganza II
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
--
Jim Kanomata
Applied/GMC, Newark,CA
jimk@appliedairfilters.com
http://www.appliedgmc.com
1-800-752-7502
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org



Jim Kanomata
Applied/GMC
jimk@appliedairfilters.com
www.appliedgmc.com
1-800-752-7502
Re: Onan (4K) Priming / Fuel Pump Questions... [message #328496 is a reply to message #328478] Mon, 22 January 2018 10:26 Go to previous messageGo to next message
JohnL455 is currently offline  JohnL455   United States
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
Senior Member
That Walbro fuel bowl is pretty small and once there is some fuel, with the choke working it gets pulled over and starts right up. Then the elec choke gets realeased (no 12v) as soon as it starts so won't run rich and foul. Pretty well designed system going back to 60's tech.

John Lebetski
Woodstock, IL
77 Eleganza II
Re: Onan (4K) Priming / Fuel Pump Questions... [message #328497 is a reply to message #328478] Mon, 22 January 2018 11:03 Go to previous messageGo to next message
jhbridges is currently offline  jhbridges   United States
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
Senior Member
I took out a yomamamaha set and installed an Onan in our 'giant boom box' some decades ago. It always started on about the second spin. I bought several others over the years, ending up with the belt drivwe replacement which used a V-twin OHV engine to make the EPA happy.
Only hard statrts I've even seen are RV sets. This leads to the conclusion that they're getting too hot on shutdown and boiling off their gas, or there's some other compromise in the fuel feed. The primer switch usually cures the symptom, but doesn't address the problem. A working choke, tight lines, and a working fuel pump and they should spi8n right off after a few revolutions on the starter.

--johnny


Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons. Braselton, Ga. I forgive them all, save those who hurt the dogs. They must answer to me in hell
Re: Onan (4K) Priming / Fuel Pump Questions... [message #328500 is a reply to message #328497] Mon, 22 January 2018 12:32 Go to previous message
Matt Colie is currently offline  Matt Colie   United States
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
Karma: 7
Senior Member
Having a 23 with a separate garden tractor battery for the Onan, limits how long I want to crank. If we come in off the road and I hit the prime switch, the pump will do the fast click for about 10 seconds. If it is cranked then, the start is about instant. I have tried to figure out where the missing fuel went with zero success. As it is, I love that old beast.

Matt


Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
Previous Topic: How Do I..??
Next Topic: [GMCnet] Ken. Came we have new windshields installed in Amana Iowa in September?
Goto Forum:
  


Current Time: Thu Mar 28 08:59:18 CDT 2024

Total time taken to generate the page: 0.01247 seconds