Engine misses on acceleration

C

CLawson

New member
May 28, 2017
12
0
1
Wellington, Ohio
I bought my 1975 Glenbrook about 2 years ago and have been working on it off and on.
The coach has always had poor acceleration and a very heavy thud-thud-thud when accelerating.
However the engine idles very smoothly.

Olds 455 with carb and HEI ignition.
It has 150,000 miles but the compression checks fairly well, most between 160 to 170 with one cylinder at 150.

The Rochester carb has been rebuilt
New plugs, cap, and rotor
Timing reset
New fuel filters through out
New mechanical fuel pump

The wires show no signs of leakage or sparking in the dark but I am thinking to replace them as a next step.

Also the vacuum lines have been replaced as needed but I am planning to replace all to eliminate that.

The vacuum advance moves but takes a lot of force to nudge it with a screwdriver. Should it be that stiff?

I would appreciate any and all suggestions to diagnose.
thanks! Cary

Cary Lawson
1975 Glenbrook, 455, carb, HEI
Wellington, Ohio

--
Cary Lawson
75 Glenbrook
Cleveland, Ohio
 
> The coach has always had poor acceleration and a very heavy thud-thud-thud when accelerating. However the engine idles very smoothly.

> New plugs, cap, and rotor

> The wires show no signs of leakage or sparking in the dark but I am thinking to replace them as a next step.

It is always a good idea to replace wires of unknown heritage - especially when doing an ignition overhaul as it is impossible to ascertain their electrical characteristics by visual observation or with a multimeter.

While there may be other reasons that explain your symptoms - ignition can certainly be part of it. The reason is that the dielectric constant of a rich(er) fuel/air mixture such as that present during acceleration is higher than that of a lean(er) mix as present at idle and it therefore takes a greater voltage to initiate and maintain the spark in the presence of the rich(er) mixture. Tired plug wires can reduce the voltage available at the plugs and it sounds like you have replaced or ruled out most everything else in the ignition so the wires are the next candidate for attention.

At one time I had a Honda car with 150Kmi that would idle perfectly and operate at road speed perfectly but would stumble badly during acceleration. Changing plugs, wires and cap took care of the problem immediately.


Jim Miller
1977 Eleganza
1977 Royale
Hamilton, OH
 
The advance weights should move freely and are a common trouble point after sitting.
You should be able to pull out on the wt by hand and it should snap back
If not, clean and lube them
THe problem is udually the pivot pins.
I have found a bit of anti seize prevents this, but a little goes avery long way.
Ccheck the vaccuum mechanism as well

--
76 Glenbrook
 
I bought my 1975 Glenbrook about 2 years ago and have been working on it off and on.
The coach has always had poor acceleration and a very heavy thud-thud-thud when accelerating.
However the engine idles very smoothly.

Olds 455 with carb and HEI ignition.
It has 150,000 miles but the compression checks fairly well, most between 160 to 170 with one cylinder at 150.

The Rochester carb has been rebuilt
New plugs, cap, and rotor
Timing reset
New fuel filters through out
New mechanical fuel pump

The wires show no signs of leakage or sparking in the dark but I am thinking to replace them as a next step.

Also the vacuum lines have been replaced as needed but I am planning to replace all to eliminate that.

The vacuum advance moves but takes a lot of force to nudge it with a screwdriver. Should it be that stiff?

I would appreciate any and all suggestions to diagnose.
thanks! Cary

Cary Lawson
1975 Glenbrook, 455, carb, HEI
Wellington, Ohio

--
Cary Lawson
75 Glenbrook
Cleveland, Ohio
=================
 
Not that this is the issue, but be sure to gap the plugs at .40 not any
more.

On Fri, May 17, 2019 at 6:25 AM Gerald Work via Gmclist <

> Check the advance weights in the distributor. If they are stiff or frozen
> your timing can be correct at idle but way off while driving. With that
> many miles check the timing chain. It may have stretched to the point the
> cam timing is way off also. Pull the fuel pump and you can check the
> timing chain for slack.
>
> Jerry
> Jerry Work
> The Dovetail Joint
> Fine furniture designed and hand crafted in the 1907 former Masonic Temple
> building in historic Kerby, OR
>
> glwork
> http://jerrywork.com
> ===============
> Message: 11
> Date: Fri, 17 May 2019 04:05:26 -0600
> From: Cary Lawson
> To: gmclist
> Subject: [GMCnet] Engine misses on acceleration
> Message-ID:
> Content-Type: text/plain; charset="utf-8"
>
> I bought my 1975 Glenbrook about 2 years ago and have been working on it
> off and on.
> The coach has always had poor acceleration and a very heavy thud-thud-thud
> when accelerating.
> However the engine idles very smoothly.
>
> Olds 455 with carb and HEI ignition.
> It has 150,000 miles but the compression checks fairly well, most between
> 160 to 170 with one cylinder at 150.
>
> The Rochester carb has been rebuilt
> New plugs, cap, and rotor
> Timing reset
> New fuel filters through out
> New mechanical fuel pump
>
> The wires show no signs of leakage or sparking in the dark but I am
> thinking to replace them as a next step.
>
> Also the vacuum lines have been replaced as needed but I am planning to
> replace all to eliminate that.
>
> The vacuum advance moves but takes a lot of force to nudge it with a
> screwdriver. Should it be that stiff?
>
> I would appreciate any and all suggestions to diagnose.
> thanks! Cary
>
> Cary Lawson
> 1975 Glenbrook, 455, carb, HEI
> Wellington, Ohio
>
> --
> Cary Lawson
> 75 Glenbrook
> Cleveland, Ohio
> =================
>
>
>
>
>
>
>
>
> _______________________________________________
> 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
http://www.appliedgmc.com
1-800-752-7502
 
> I bought my 1975 Glenbrook about 2 years ago and have been working on it off and on.
> The coach has always had poor acceleration and a very heavy thud-thud-thud when accelerating.
> However the engine idles very smoothly.
>
> Olds 455 with carb and HEI ignition.
> It has 150,000 miles but the compression checks fairly well, most between 160 to 170 with one cylinder at 150.
>
> The Rochester carb has been rebuilt
> New plugs, cap, and rotor
> Timing reset
> New fuel filters through out
> New mechanical fuel pump
>
> The wires show no signs of leakage or sparking in the dark but I am thinking to replace them as a next step.
>
> Also the vacuum lines have been replaced as needed but I am planning to replace all to eliminate that.
>
> The vacuum advance moves but takes a lot of force to nudge it with a screwdriver. Should it be that stiff?
>
> I would appreciate any and all suggestions to diagnose.
> thanks! Cary
>
> Cary Lawson
> 1975 Glenbrook, 455, carb, HEI
> Wellington, Ohio

A "thud" under acceleration sounds more mechanical than ignition. "loading - unloading the drive axles" I would consider checking your
axle/differential flange bolts and u-joints.
--
Patti & Jerry Burt Fresno, CA.
73 Gmc 26' Canyon Lands
77 Palm Beach - Parts Coach
Members: FMCA - GMCMI - GMCWS
A truly happy person is one who can enjoy the scenery on a detour.
 
Check those motor mounts and CV axle joints. The advance is usually difficult to move unless you disconnect the vacuum canister. Check it with an
inexpensive hand vacuum pump:

https://www.harborfreight.com/brake-bleeder-and-vacuum-pump-kit-69328.html?_br_psugg_q=vacuum+pump
As others have said, make sure the advance weights are working properly and substitute or replace the wires. Make sure they are compatible with your
ignition system! I'm using the Taylor 8mm wires, and I've found 1 or 2 hundred extra idle RPM after installation on every vehicle where I used them
and they stopped the stumble on the manual transmission Bronco when engaging the clutch.
--
Terry Kelpien

ASE Master Technician

73 Glacier 260

Smithfield, Va.
 
If the distributor has not been disassembled, cleaned and synthetic greased it needs to be. Lubing the weight pivots is just the tip of the iceberg.
40 year old grease is now glue. Also a good time to clean off and replace the white heat sink compound under the module and use something like D5 on
all the push on connections. The high ozone environment under the cap is murder on connections.
Also be sure your vac hose routing is correct and ported vac is going through the TVS and getting to the distributor. Confirm vac snd mech advance
with timing light.
I think you have more than one issue and the other could be as simple as lose support bearing bolts on pass side.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
I wanted to thank everyone for all the expert advice. I am overwhelmed with all the support!

I am working through the list and already showing good improvements:

Replaced the wires-they 'looked' okay but who knew their age and they had been tortured with a lot of heat
Replaced about 80% of the vacuum hoses with totally new stuff. (I hadn't realized a couple were loose)
The weights were moving but I thoroughly cleaned them and put a bit of anti-seize on each pivot
Bought a hand vacuum pump and found the vacuum advance is working - most likely helped with new hoses
The plugs were gapped at 40 thousands so good there
Cleaned the electronic module and applied heat sink compound

FYI-tracking down heat sink grease was a bit of an adventure. Around here O'Reilly's didn't know what I was talking about, NAPA didn't carry it, but
Auto Zone had it in stock.
I was told to put just a very thin layer on the module plate. The purpose was to create a good contact with the distributor plate in order to conduct
heat so the module doesn't burn out. Putting too much coating on would act as insulation and defeat the purpose. Seemed like good advise.

The engine now has much more power and smoother acceleration.
Still more to debug and I will now study up for replacing the timing chain, water pump, and front seal.

Thanks to all, Cary
--
Cary Lawson
75 Glenbrook
Cleveland, Ohio
 
When I was looking for some of the heat conductive grease last fall most auto parts stores didn’t know what it was. NAPA tried to sell me dielectric grease and insisted it was the same thing. I told them it wasn’t and left the store.
On the Internet the only place I could find it was at Autozone but when I went to their store they said they didn’t know what it was. They also tried to sell me dielectric grease and said it was made for that application. I showed them the page I had printed from their web site and they then found it on a shelf in the back room. They told me their inventory sheet showed that they hadn’t sold any in 10 years. I said it’s no wonder since nobody in the store knew what it was.

Emery Stora
77 Kingsley
Frederick CO

>
> I wanted to thank everyone for all the expert advice. I am overwhelmed with all the support!
>
> I am working through the list and already showing good improvements:
>
> Replaced the wires-they 'looked' okay but who knew their age and they had been tortured with a lot of heat
> Replaced about 80% of the vacuum hoses with totally new stuff. (I hadn't realized a couple were loose)
> The weights were moving but I thoroughly cleaned them and put a bit of anti-seize on each pivot
> Bought a hand vacuum pump and found the vacuum advance is working - most likely helped with new hoses
> The plugs were gapped at 40 thousands so good there
> Cleaned the electronic module and applied heat sink compound
>
> FYI-tracking down heat sink grease was a bit of an adventure. Around here O'Reilly's didn't know what I was talking about, NAPA didn't carry it, but
> Auto Zone had it in stock.
> I was told to put just a very thin layer on the module plate. The purpose was to create a good contact with the distributor plate in order to conduct
> heat so the module doesn't burn out. Putting too much coating on would act as insulation and defeat the purpose. Seemed like good advise.
>
> The engine now has much more power and smoother acceleration.
> Still more to debug and I will now study up for replacing the timing chain, water pump, and front seal.
>
> Thanks to all, Cary
> --
> Cary Lawson
> 75 Glenbrook
> Cleveland, Ohio
>
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
I used to buy it at Radio Shack but there aren’t any Radio Shacks near me. They probably wouldn’t carry it anyway since there is no telephone attached to it! :)

Emery Stora
77 Kingsley
Frederick CO

>
> Fry's Electronics, DigiKey>dot Braselton, Ga.
> I forgive them all, save those who hurt the dogs. They must answer to me in hell
>
>
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> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
Emery ditto on Radio Shack for a quick grab no waiting-- but mine closed as well , to become a phone store! I think you need to apply enough to fill
the concave void on bottom of module. I have been driving HEI cars since 1976 (Pontiac) and never had a GM module failure. If you ever see the GM
supplied soft one use tube, that is the ammount to use.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
That’s what Autozone sells. A small box with a few of the rear open pouches.

Emery

>
> Emery ditto on Radio Shack for a quick grab no waiting-- but mine closed as well , to become a phone store! I think you need to apply enough to fill
> the concave void on bottom of module. I have been driving HEI cars since 1976 (Pontiac) and never had a GM module failure. If you ever see the GM
> supplied soft one use tube, that is the ammount to use.
> --
> 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
 
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