Home » Public Forums » GMCnet » Lack of power at speed (Carb problem?)
| Lack of power at speed [message #89337] |
Sun, 20 June 2010 17:32  |
docsdad
Messages: 32
Registered: May 2008
Karma: 0
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I fired up the Mother Ship a few days ago with the intention of going to my son's in Illinois. Temp was 100 degrees and just a few miles down the interstate, I experienced what I assumed was the dreaded "vapor lock" Struggled to the next exit and turned around for home. It missed, sputtered and actually died a couple times, I pulled over and it would start back up and I struggled home on a local 2 lane at 35 miles per hour. When I tried to step on gas to increase speed, It was like I turned off the key and it would slow down and sputter along. Pumping the foot feed seemed to help but for the last 3 miles just running at 35 to 45 it seemed to run ok if I did not push it.
When I got home, I decided to install new electronic module as the symptoms seemed to point me in this direction. Next day, cooler weather, well cloudy and in the 90's. Took the motor home out, It run fine on two lane road 45 to 55 if I did not push accelerator down very strongly. If I floor it it does not speed up just seems to slow down, like it is not getting any gas. But if not pushed hard, it seems to run ok.
I am thinking the secondaries are not functioning. Need you collective brain power. I have had the carb off three times now trying to find something wrong. The only thing that I find is as follows. The baffle in the secondary throttle bore, is missing, has always been this way since I have owned the rig. The upper plates on the secondary bores seem to have a bit of a sticking condition when they go to the fully closed position. I am not an expert on carbs, but I do not understand exactly how the secondaries are supposed to work. It seems on my carb the upper plates on the secondary control the secondary jets through cam action, but I do not understand what causes the secondaries to open other than the flow of air when the throttle plate opens as there seems to be not connection between the upper plates and the throttle plates. .
I am about ready to change over to Fuel injection, but I need to know that I am not overlooking something simple.
Your thoughts and advice would be appreciated.
Jim Thompson
Georgia
75 Avion that used to run like a top.
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| Re: Lack of power at speed [message #89339 is a reply to message #89337] |
Sun, 20 June 2010 17:42  |
midlf
Messages: 2212
Registered: July 2007
Location: SE Wisc. (Palmyra)
Karma: 1
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You are correct the air door over the secondaries opens from vacuum/air flow, There is no direct mechanical connection between the air door and the secondary throttle plates. There is an excellent, non-technical description of Q-Jet operation in the maintenance manual, section 6M ENGINE FUEL.
Steve Southworth
1974 Glacier TZE064V100150 (for workin on)
1975 Transmode TZE365V100394 (parts & spares)
Palmyra WI
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| Re: Lack of power at speed [message #89343 is a reply to message #89337] |
Sun, 20 June 2010 18:07 |
Craig Lechowicz
Messages: 541
Registered: October 2006
Location: Waterford, MI
Karma: 0
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Jim wrote,
>When I tried to step on gas to increase speed, It was like I turned off the key and >it would slow down and sputter along.
Jim, your secondaries may not be opening, but that won't cause the symptoms you describe, it will just have less power, but not miss or stall. On mine, the metal rod that runs between the vacuum break (can) at the right front of the carb, and the upper metering plates was torsionally twisted, and caused a bind in the linkage that kept the secondary air metering valves from opening (upper "throttle plates"). It was slow, but ran fine other than that. Your problem does sound like a plugged fuel filter or vapor lock. If you still have the small filter in the carb inlet nut, and it is plugged it will act as you described as at low speed the spring behind it lets enough fuel in to run slowly/sporadically. You can take it out and blow through it to see if it's plugged. Mine has had some, but not a lot of vapor lock issues, usually when running late fall or winter blend gas in the summer, and after coming to a stop where the engine heat soaks the fuel. Generally, I've been able to nurse it along for 1/2 mile or so, then the air flow seems to cool it down enough to let it run normally, but that's a tough issue to solve.
Craig Lechowicz
'77 Kingsley, Waterford, MI
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| Re: Lack of power at speed [message #89347 is a reply to message #89337] |
Sun, 20 June 2010 18:18 |
fred v
Messages: 999
Registered: April 2006
Location: pensacola, fl.
Karma: 0
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is it possible that the secondaries are opening too soon and bogging down the engine?
Jim, have you drive it with the hatch open and air filter top removed so you can see what is happening? what baffle are you talking about?
Fred V
'77 Royale RB 455
P'cola, Fl
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| Re: Lack of power at speed [message #89348 is a reply to message #89337] |
Sun, 20 June 2010 18:20 |
jknezek
Messages: 1057
Registered: December 2007
Karma: 5
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Senior Member |
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What does your vacuum look like when you step on the gas? Is your intake manifold blocked? I had similar symptoms that kept getting worse. Ended up being a crack in the intake manifold that expanded when it got hot. Wasn't so much an issue of lack of power at speed, more an issue of lack of vacuum at heat. I would also check your fuel filter. Could be fuel starvation from a plugged filter?
Thanks,
Jeremy Knezek
1976 Glenbrook
Birmingham, AL
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| Re: Lack of power at speed [message #89356 is a reply to message #89348] |
Sun, 20 June 2010 20:15 |
docsdad
Messages: 32
Registered: May 2008
Karma: 0
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Changed out the filter. Can run at 65 on interstate but still if I use full throttle, it will bog when going up a slight hill. If I nurse it and just let the speed drop, no problem, but if I add throttle, it tends to bog down. My concern is if I use the cruise, it will when going up hill, continue to ask for more throttle, until it bogs out. This did not happen last year. When I talk hill, I am in Middle Georgia, and it is pretty flat here. I may have other problems but I sure do not have any power at cruise under load. At least now since I changed out the ignition module, It does not die on me.
Jim T
Georgia
75 Avion
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| Re: Lack of power at speed [message #89361 is a reply to message #89356] |
Sun, 20 June 2010 20:55 |
C Boyd
Messages: 2629
Registered: April 2006
Karma: 18
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| docsdad wrote on Sun, 20 June 2010 21:15 |
Changed out the filter. Can run at 65 on interstate but still if I use full throttle, it will bog when going up a slight hill. If I nurse it and just let the speed drop, no problem, but if I add throttle, it tends to bog down. My concern is if I use the cruise, it will when going up hill, continue to ask for more throttle, until it bogs out. This did not happen last year. When I talk hill, I am in Middle Georgia, and it is pretty flat here. I may have other problems but I sure do not have any power at cruise under load. At least now since I changed out the ignition module, It does not die on me.
Jim T
Georgia
75 Avion
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Sir: maybe it has another added filter in the fuel line somewhere that might be clogged. Usually in front of the switching valve on drivers side inside the frame rail or/and inbetween the lf fender and the fuel pump. Also when stopped and engine off, remove the aircleaner top and mash the gas pedal to see if a full squirt of gas is coming from the accelerator pump. Seems since what you have done has helped, you should be on the right track. On my coach I added a filter after the switching valve but before I hooked it up I blew shop air back thru the valve into the gas tank then switched the valve and blew back in the other tank to clear the pick up socks. I removed the gas cap before doing this. Since then I added a quick disconect on my air tank and I carry a air hose with blower and chuck. It really helped my issues with fuel.
Good Luck..
C. Boyd
76 Crestmont
East Tennessee
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| Re: [GMCnet] Lack of power at speed [message #89368 is a reply to message #89356] |
Sun, 20 June 2010 21:23 |
k2gkk
Messages: 4452
Registered: November 2009
Karma: -8
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Be glad yours just "bogs" when going up a hill!
The motor in mine simply dies!
* Mac Macdonald *
* USAF, Retired *
* Oklahoma City *
** "Money Pit" **
* '76 ex - P.B. *
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> To: gmclist@temp.gmcnet.org
> From: tjamessherry@bellsouth.net
> Date: Sun, 20 Jun 2010 20:15:46 -0500
> Subject: Re: [GMCnet] Lack of power at speed
>
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> Changed out the filter. Can run at 65 on interstate but still if I use full throttle, it will bog when going up a slight hill. If I nurse it and just let the speed drop, no problem, but if I add throttle, it tends to bog down. My concern is if I use the cruise, it will when going up hill, continue to ask for more throttle, until it bogs out. This did not happen last year. When I talk hill, I am in Middle Georgia, and it is pretty flat here. I may have other problems but I sure do not have any power at cruise under load. At least now since I changed out the ignition module, It does not die on me.
>
> Jim T
> Georgia
> 75 Avion
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| Re: Lack of power at speed [message #89371 is a reply to message #89337] |
Sun, 20 June 2010 21:30 |
fred v
Messages: 999
Registered: April 2006
Location: pensacola, fl.
Karma: 0
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Senior Member |
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you definitely need that little spring and your problems will be over. you can prove this by taking a piece of wire and hold the air valve shut and go for a drive. what is happening is the secondaries are opening way too soon and your mixture is getting so lean that you loose all power. this is not good for the engine and needs to be fixed.
turn the little screw 3/4 of a turn past the shut position and lock it down. you should be able to get a spring from either of the Jim's or Sirum or maybe someone here can send you one.
also the vacuum diaphragm on the right front is supposed to lock the air valve closed at high vacuum.
good luck,
Fred V
'77 Royale RB 455
P'cola, Fl
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