Loss of Power - Need your thoughts

[steve adams]

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

 

[Jim Hupy]

Cracked intake manifold.
Jim Hupy
Salem, Or
78 GMC ROYALE 403

> 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
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[steve adams]

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

 

[LQQKatJon]

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/

 

[Jim Hupy]

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

> 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:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[Jim Hupy]

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

> 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
>

 

[sean kidd]

Steve, check vacuum, a cracked manifold will reflect low vacuum, also a stuck needle 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 Harbir 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

 

[steve adams]

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