Home » Public Forums » GMCnet » Loss of Power - Need your thoughts
Loss of Power - Need your thoughts [message #308048] |
Fri, 30 September 2016 12:05 |
Steve
Messages: 506 Registered: September 2013 Location: East Greenville, Pa
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I experienced loss of power. Suspected fuel delivery and tested fuel pump. Test failed so replaced fuel pump. Still loss of power but noticed it only happens after warm up to operating temp, also noticed minor backfire through the exhaust so I suspected ignition. Replaced distributor with new Dick Paterson unit. Started strong but experienced power loss after warm up. Replaced fuel filter. Exhaust is new and not plugged.
What to check next?
Vacuum and vacuum lines, leaks?
12 volts to distributor?
Confirm that choke is opening?
Use manual pump to pump fuel from tank to confirm no kinks or blockage in fuel lines?
What else would cause loss of power that is operating temp dependant?
1978 GMC Royal
Eastern Pennslyvania
1968 Chevrolet C20 396 Camper Special
1969 Chevrolet C20 Camper Special
1985 Buick Electra Park Avenue
1992 Camaro 25th Anniversary Heretage Edition Black
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Re: [GMCnet] Loss of Power - Need your thoughts [message #308055 is a reply to message #308052] |
Fri, 30 September 2016 13:13 |
Steve
Messages: 506 Registered: September 2013 Location: East Greenville, Pa
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Thanks Jim,
I hope not. Is there a common location? How best to test. I also thought about other catastrophic items like worn out cam lobe for fuel pump?
1978 GMC Royal
Eastern Pennslyvania
1968 Chevrolet C20 396 Camper Special
1969 Chevrolet C20 Camper Special
1985 Buick Electra Park Avenue
1992 Camaro 25th Anniversary Heretage Edition Black
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Re: [GMCnet] Loss of Power - Need your thoughts [message #308056 is a reply to message #308055] |
Fri, 30 September 2016 13:19 |
lqqkatjon
Messages: 2324 Registered: October 2010 Location: St. Cloud, MN
Karma: 5
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I don't think it is a worn out cam lobe for fuel pump. does not really go on cam lobe as much as it rides on front of a not centered pully bolted onto the front of the cam.
I would lean back towards fuel or spark. However by pulling off the carb, you maybe able to see the crack in the intake. The backfiring though, I wonder if that would be able to be caused by the crack. I would think a loss of power, degraded performance, not a random chugging, or misfire.
just my thoughts.
Jon Roche
75 palm beach
EBL EFI, manny headers, Micro Level, rebuilt most of coach now.
St. Cloud, MN
http://lqqkatjon.blogspot.com/
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Re: [GMCnet] Loss of Power - Need your thoughts [message #308058 is a reply to message #308056] |
Fri, 30 September 2016 13:36 |
James Hupy
Messages: 6806 Registered: May 2010
Karma: -62
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Backfire is common with cracked manifolds. Could be something as simple as
bad plug wires, too large plug gap, crossed plug wires, vacuum leaks, or as
complex as burned valves or seats, stretched timing chain, and on and on.
Verify that the choke is fully open when warm, and that you have around 38°
total timing when driving at highway speeds. If you have a Paterson
distributor, odds are very good that your total advance (combination of
centrifugal and vacuum) is correct. If your base static timing is 8° Btdc.
you should be very close to correct. If it does not ping, you might get
away with 10° base, but I would not go much further than that. J.W.I.W.D.
JIM HUPY
SALEM, OR
78 GMC ROYALE 403
On Sep 30, 2016 11:24 AM, "Jon Roche" wrote:
> I don't think it is a worn out cam lobe for fuel pump. does not really
> go on cam lobe as much as it rides on front of a not centered pully bolted
> onto the front of the cam.
>
>
> I would lean back towards fuel or spark. However by pulling off the carb,
> you maybe able to see the crack in the intake. The backfiring though, I
> wonder if that would be able to be caused by the crack. I would think a
> loss of power, degraded performance, not a random chugging, or misfire.
>
> just my thoughts.
> --
> Jon Roche
> 75 palm beach
> St. Cloud, MN
> http://lqqkatjon.blogspot.com/
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
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Re: [GMCnet] Loss of Power - Need your thoughts [message #308060 is a reply to message #308055] |
Fri, 30 September 2016 14:29 |
James Hupy
Messages: 6806 Registered: May 2010
Karma: -62
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After the carb is removed, shine a bright led light down the throat of both
the primary and secondary passages. If you see cracks, then you have found
your problem. The manifolds crack because of the hot exhaust passing
through the crossover passage below the carb. If yours are blocked off,
then all you have to contend with is the vacuum leak. Some GMC guys go so
far as to remove the intake manifold and pour the crossover passage full of
melted pistons or other molten metal. Some just block the gasket surfaces
with stainless steel plates. Some replace the manifolds with Rockwell
manifolds. Some with Edelbrock performer manifolds. The latter requires
raising the hatch cover.
I have fitted several coaches with the Rockwell manifolds. They solve the
cracking problem, but they require very careful installation to prevent
passage leaks. I have not installed a Rockwell in a couple of years, and
they might have changed their castings to make them easier to line up with
the ports. Jim K. can tell you for sure, he sells them. Hope this helps.
Jim Hupy
On Sep 30, 2016 11:38 AM, "Steve Adams" wrote:
> Thanks Jim,
> I hope not. Is there a common location? How best to test. I also
> thought about other catastrophic items like worn out cam lobe for fuel pump?
> --
> 1978 GMC Royal
> Eastern Pennslyvania
> 1968 Chevrolet C20 396 Camper Special
> 1969 Chevrolet C20 Camper Special
> 1985 Buick Electra Park Avenue
> 1992 Camaro 25th Anniversary Heretage Edition Black
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
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Re: Loss of Power - Need your thoughts [message #308077 is a reply to message #308048] |
Sat, 01 October 2016 10:19 |
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SeanKidd
Messages: 747 Registered: June 2012 Location: Northern Neck Virginia
Karma: 4
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Steve, check vacuum, a cracked manifold will reflect low vacuum, also a stuck needle (not seating)or float in carb will create same effect.
I had to add a fuel pressure regulator to my Carter electric pumps...my Rochester doesn't like anything over 4psi, was pushing past needle.
Hope this isn't effecting your Bar Harbor trip...
Sean and Stephanie
73 Ex-CanyonLands 26' #317 "Oliver"
Hubler 1-Ton, Quad-Bags, Rear Disc, Reaction Arms, P.Huber TBs, 3.70:1 LSD Honda 6500 inverter gen.
Colonial Travelers
[Updated on: Sat, 01 October 2016 10:22] Report message to a moderator
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Re: Loss of Power - Need your thoughts [message #308206 is a reply to message #308048] |
Mon, 03 October 2016 19:04 |
Steve
Messages: 506 Registered: September 2013 Location: East Greenville, Pa
Karma: 1
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Update & Resolution.
First, thanks to everyone for your input. Jim Hupy, thanks for the very detailed response. Fortunately I was able to resolve the issue with looking to a possible cracked manifold. Sean, I did not make Bar Harbor as planned, mostly due to rain. I also tried to get to GMCI in Ohio but did not make that either. I did get a shorter trip to Shenandoah National Park in Virginia.
Sean, your suggestion was correct. It was a combination of vacuum and timing. Vacuum gauge showed weak vacuum so I checked all connections and replaced some dried cracked hoses. After resolving the vacuum leak the gauge was indicating "Late Ignition Timing". I advanced the timing until the gauge was in the center of the "Normal". After that adjustment power was back to normal. I had put in a new Patterson Distributor and had set it in the exact position of the old distributor and vacuum advance as a starting point. I should have got a timing light on it but it was very late and I had to be to work early, my BAD. I still need to get the light on it, but it ran beautifully down to Virginia. It cruised easily beteen 65 and 70 MPH (403 with stock 3.07 gear) with no sign of knock. I will get the base timing set to the suggested 10 degrees this week.
The power was really good. It made me wonder what it would be like to have the 455 or Cadillac 500 and a lower gear set?
Happy to be back on the road again. This week I changed the fuel pump and distributor, both of which were in my spare parts inventory.
1978 GMC Royal
Eastern Pennslyvania
1968 Chevrolet C20 396 Camper Special
1969 Chevrolet C20 Camper Special
1985 Buick Electra Park Avenue
1992 Camaro 25th Anniversary Heretage Edition Black
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Re: Loss of Power - Need your thoughts [message #311640 is a reply to message #308048] |
Wed, 28 December 2016 13:08 |
Steve
Messages: 506 Registered: September 2013 Location: East Greenville, Pa
Karma: 1
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I am posting a further update on this in hopes it may help someone who might experience the same issue. I have continued to have issues with no power coming off of idle. I had replaced both the fuel pump and the distributor but continued to have issues. Engine starts fine including the choke operation and fast idle setting. Everything normal until you put it in gear and try to move off idle and the engine would stumble with no power. Top speed was 15 to 20 mph and I was afraid I would not make it home if I took it out to test drive. On test drive I discovered that if I put the pedal to the floor it would accelerate to 65 mph but could not cruise or maintain any speed. Only accelerate at wide open throttle. My driveway has a slight incline and sure enough I could not make it up the driveway. In desperation I put the pedal to the floor, it stumbled and then came alive and I did a burn out half way up the driveway. With idle good and appearent power from the secondaries I now assumed it had to be carb primary circuit related.
I called Ken Frey to see if he could look at it. Although he was trying to wrap up and get out of town for the holidays, he said to take the carb off and bring it over. I got a good education on Q-jet. We disassembled, inspected, cleaned, and reassembled. Nothing was drastically wrong but it was dirty and gunked up inside. Carb was fully rebuilt by Ken in fall of 2014. I have two filters, one in the carb and one before the pump. I also keep tanks full, treat with stabill, and run every 30 days.
Anyway, I installed the carb with a new base gasket and torqued the 4 mounting bolts to 10 lbs. After making all connections it started fine and I went out for a test drive. I had good power across all RPM ranges including climbing some pretty good local hills.
I hope this carb cleaning is the final fix on my loss of power issues. I suspect ethanol gas may have something to do with it.
Many thanks Ken and to all who provided ideas and assistance. I am back on the road again.
1978 GMC Royal
Eastern Pennslyvania
1968 Chevrolet C20 396 Camper Special
1969 Chevrolet C20 Camper Special
1985 Buick Electra Park Avenue
1992 Camaro 25th Anniversary Heretage Edition Black
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Re: Loss of Power - Need your thoughts [message #311641 is a reply to message #311640] |
Wed, 28 December 2016 14:06 |
lqqkatjon
Messages: 2324 Registered: October 2010 Location: St. Cloud, MN
Karma: 5
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Thank you for the update. Glad you are up and running!
Jon Roche
75 palm beach
EBL EFI, manny headers, Micro Level, rebuilt most of coach now.
St. Cloud, MN
http://lqqkatjon.blogspot.com/
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Re: Loss of Power - Need your thoughts [message #311643 is a reply to message #311640] |
Wed, 28 December 2016 15:13 |
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Matt Colie
Messages: 8547 Registered: March 2007 Location: S.E. Michigan
Karma: 7
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Steve wrote on Wed, 28 December 2016 14:08I am posting a further update on this in hopes it may help someone who might experience the same issue. I have continued to have issues with no power coming off of idle. I had replaced both the fuel pump and the distributor but continued to have issues. Engine starts fine including the choke operation and fast idle setting. Everything normal until you put it in gear and try to move off idle and the engine would stumble with no power. Top speed was 15 to 20 mph and I was afraid I would not make it home if I took it out to test drive. On test drive I discovered that if I put the pedal to the floor it would accelerate to 65 mph but could not cruise or maintain any speed. Only accelerate at wide open throttle. My driveway has a slight incline and sure enough I could not make it up the driveway. In desperation I put the pedal to the floor, it stumbled and then came alive and I did a burn out half way up the driveway. With idle good and appearent power from the secondaries I now assumed it had to be carb primary circuit related.
I called Ken Frey to see if he could look at it. Although he was trying to wrap up and get out of town for the holidays, he said to take the carb off and bring it over. I got a good education on Q-jet. We disassembled, inspected, cleaned, and reassembled. Nothing was drastically wrong but it was dirty and gunked up inside. Carb was fully rebuilt by Ken in fall of 2014. I have two filters, one in the carb and one before the pump. I also keep tanks full, treat with stabill, and run every 30 days.
Anyway, I installed the carb with a new base gasket and torqued the 4 mounting bolts to 10 lbs. After making all connections it started fine and I went out for a test drive. I had good power across all RPM ranges including climbing some pretty good local hills.
I hope this carb cleaning is the final fix on my loss of power issues. I suspect ethanol gas may have something to do with it.
Many thanks Ken and to all who provided ideas and assistance. I am back on the road again.
Steve,
Glad to hear that the problem is handled. I missed this thread because I was at Mansfield and did not follow the forum closely during that.
I can almost guarantee that the problem was some restriction of the primary power circuit. Of course, Ken did the right thing. I have had these same "mystery fixes", but it was only with the third of the same unit that disassembling the carburetor over clean white paper found the debris. If you do the normal knock down and blow out, it is easy tp fix it without ever finding the actual problem. Yes, this can be "engine temperature" dependent because the flow problem can get changed by the carburetor temperature.
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
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Re: [GMCnet] Loss of Power - Need your thoughts [message #311645 is a reply to message #308052] |
Wed, 28 December 2016 16:08 |
jimk
Messages: 6734 Registered: July 2006 Location: Belmont, CA
Karma: 9
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Also check all the vac lines and snug the 4 carb mounting bolts.
This is not a car/pickup, and there is lot of load on the engine that most
of you do not realize.
Therefor you need to learn these things .
Fuel consumption is twice that of a car/pick up, so the fuel filter will
load up with particulates.
Also keep in mind, the new ethanol fuel is washing your old gummed up tank
and that mini fuel filter is loading up.
On Wed, Dec 28, 2016 at 2:01 PM, Jim Kanomata wrote:
> Your fuel filter must be plugged, blame yourself for not changing it
> frequently.
>
> On Wed, Dec 28, 2016 at 12:43 PM, Billy Massey
> wrote:
>
>> Yeah! Thanks for finishing the thread. Disheartening that a carb
>> overhaul
>> only lasted less than 3 years.
>>
>> bdub
>>
>>
>> On Dec 28, 2016 1:15 PM, "Steve Adams" wrote:
>>
>> I am posting a further update on this in hopes it may be ... Snip ...
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>
>
>
>
> --
> Jim Kanomata
> Applied/GMC, Fremont,CA
> jimk@appliedairfilters.com
> http://www.appliedgmc.com
> 1-800-752-7502
>
--
Jim Kanomata
Applied/GMC, Fremont,CA
jimk@appliedairfilters.com
http://www.appliedgmc.com
1-800-752-7502
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Applied/GMC
jimk@appliedairfilters.com
www.appliedgmc.com
1-800-752-7502
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Re: [GMCnet] Loss of Power - Need your thoughts [message #311646 is a reply to message #311642] |
Wed, 28 December 2016 16:01 |
jimk
Messages: 6734 Registered: July 2006 Location: Belmont, CA
Karma: 9
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Your fuel filter must be plugged, blame yourself for not changing it
frequently.
On Wed, Dec 28, 2016 at 12:43 PM, Billy Massey wrote:
> Yeah! Thanks for finishing the thread. Disheartening that a carb overhaul
> only lasted less than 3 years.
>
> bdub
>
>
> On Dec 28, 2016 1:15 PM, "Steve Adams" wrote:
>
> I am posting a further update on this in hopes it may be ... Snip ...
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
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>
--
Jim Kanomata
Applied/GMC, Fremont,CA
jimk@appliedairfilters.com
http://www.appliedgmc.com
1-800-752-7502
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Jim Kanomata
Applied/GMC
jimk@appliedairfilters.com
www.appliedgmc.com
1-800-752-7502
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Re: [GMCnet] Loss of Power - Need your thoughts [message #311647 is a reply to message #311645] |
Wed, 28 December 2016 16:11 |
jimk
Messages: 6734 Registered: July 2006 Location: Belmont, CA
Karma: 9
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Since I only monitor Gmail, I did not see the complaint, maybe one of you
can relay it so I can read it first hand.
On Wed, Dec 28, 2016 at 2:08 PM, Jim Kanomata wrote:
> Also check all the vac lines and snug the 4 carb mounting bolts.
> This is not a car/pickup, and there is lot of load on the engine that most
> of you do not realize.
> Therefor you need to learn these things .
> Fuel consumption is twice that of a car/pick up, so the fuel filter will
> load up with particulates.
> Also keep in mind, the new ethanol fuel is washing your old gummed up tank
> and that mini fuel filter is loading up.
>
> On Wed, Dec 28, 2016 at 2:01 PM, Jim Kanomata
> wrote:
>
>> Your fuel filter must be plugged, blame yourself for not changing it
>> frequently.
>>
>> On Wed, Dec 28, 2016 at 12:43 PM, Billy Massey
>> wrote:
>>
>>> Yeah! Thanks for finishing the thread. Disheartening that a carb
>>> overhaul
>>> only lasted less than 3 years.
>>>
>>> bdub
>>>
>>>
>>> On Dec 28, 2016 1:15 PM, "Steve Adams" wrote:
>>>
>>> I am posting a further update on this in hopes it may be ... Snip ...
>>> _______________________________________________
>>> GMCnet mailing list
>>> Unsubscribe or Change List Options:
>>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>>
>>
>>
>>
>> --
>> Jim Kanomata
>> Applied/GMC, Fremont,CA
>> jimk@appliedairfilters.com
>> http://www.appliedgmc.com
>> 1-800-752-7502
>>
>
>
>
> --
> Jim Kanomata
> Applied/GMC, Fremont,CA
> jimk@appliedairfilters.com
> http://www.appliedgmc.com
> 1-800-752-7502
>
--
Jim Kanomata
Applied/GMC, Fremont,CA
jimk@appliedairfilters.com
http://www.appliedgmc.com
1-800-752-7502
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Jim Kanomata
Applied/GMC
jimk@appliedairfilters.com
www.appliedgmc.com
1-800-752-7502
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Re: Loss of Power - Need your thoughts [message #311648 is a reply to message #308048] |
Wed, 28 December 2016 16:36 |
Steve
Messages: 506 Registered: September 2013 Location: East Greenville, Pa
Karma: 1
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Jim K,
Thanks for the advice on the fuel filters. I failed to mention that we also replaced the fuel filter in the carb inlet housing. It was the correct filter with the check valve/ball. We opened up the old one and it was dirty but not plugged, nor was it water logged or water damaged. One interesting thing about my carb is that it has a metal tag installed over the fuel inlet that says "No Filter inside Carb, Check filter near fuel pump". Someone along the line had put that tag there to steer others away from the fuel inlet filter. I had not opened the inlet up, but have changed the in-line filter before the fuel pump each year. I'm glad the filter was there because it caught a bunch of junk the in-line filter missed. I'm thinking I should ditch the cheap Autozone in-line filter for a WIX or other higher quality unit. I watched a lot of you tube videos on Q'jets. I have a much better understanding of them now. Simple float and needle valve, idle circuit, enrichment circuit, primary circuit, and secondary circuit. I am just happy they are all working and transitioning nicely.
Ok, what's next on my list?
1978 GMC Royal
Eastern Pennslyvania
1968 Chevrolet C20 396 Camper Special
1969 Chevrolet C20 Camper Special
1985 Buick Electra Park Avenue
1992 Camaro 25th Anniversary Heretage Edition Black
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Re: Loss of Power - Need your thoughts [message #311650 is a reply to message #308048] |
Wed, 28 December 2016 17:25 |
Larry
Messages: 2875 Registered: January 2004 Location: Menomonie, WI
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One of the things that Ken Frey likely did is free up the power piston in the Q jet. That power piston is physically connected to metering rods and controlled by manifold vacuum. As the piston is lifted by spring pressure against lesser manifold vacuum, the metering rods lift in the high speed jets allowing for a richer mixture and increased power. Over the years of operation the operation of that piston can be compromised by carbon and/or varnish buildup and can become stuck in the full down position which does not allow for more fuel when needed for acceleration. Symptoms of this condition are exhibited by low power, bogging, and backfire because of a lean condition. Often you can "unstick" that piston temporarily by lifting the power piston with a needle nose pliers and working it a little. Then take it apart when you have time and clean it with carb cleaner. JWIT
Larry
78 Royale w/500 Caddy
Menomonie, WI.
[Updated on: Wed, 28 December 2016 17:27] Report message to a moderator
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