| Engine misses on acceleration [message #343406] |
Fri, 17 May 2019 05:05  |
CLawson
Messages: 11
Registered: July 2016
Location: Wellington, OH
Karma: 1
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Junior Member |
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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
1975 Glenbrook, 455, Rochester carb
Wellington, Ohio
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| Re: [GMCnet] Engine misses on acceleration [message #343409 is a reply to message #343406] |
Fri, 17 May 2019 06:55  |
Jim Miller
Messages: 501
Registered: March 2008
Karma: 10
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Senior Member |
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On May 17, 2019, at 6:05 AM, Cary Lawson via Gmclist wrote:
> 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
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Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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| Re: Engine misses on acceleration [message #343411 is a reply to message #343406] |
Fri, 17 May 2019 08:19 |
Chris Tyler
Messages: 458
Registered: September 2013
Location: Odessa FL
Karma: 7
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Senior Member |
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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
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| Re: Engine misses on acceleration [message #343418 is a reply to message #343406] |
Fri, 17 May 2019 11:36 |
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pjburt
Messages: 436
Registered: February 2016
Location: Fresno, California
Karma: 1
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Senior Member |
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CLawson wrote on Fri, 17 May 2019 03:05I 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.
Jerry Burt Fresno, CA.
73 Gmc 26' Canyon Lands
Members: FMCA - GMCMI - GMCWS
A truly happy person is one who can enjoy the scenery on a detour.
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| Re: Engine misses on acceleration [message #343432 is a reply to message #343406] |
Fri, 17 May 2019 18:46 |
Bullitthead
Messages: 1411
Registered: November 2013
Karma: 5
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Senior Member |
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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.
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| Re: Engine misses on acceleration [message #343433 is a reply to message #343406] |
Fri, 17 May 2019 19:27 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Senior Member |
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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
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| Re: Engine misses on acceleration [message #343461 is a reply to message #343406] |
Sat, 18 May 2019 19:41 |
CLawson
Messages: 11
Registered: July 2016
Location: Wellington, OH
Karma: 1
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Junior Member |
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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
1975 Glenbrook, 455, Rochester carb
Wellington, Ohio
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| Re: [GMCnet] Engine misses on acceleration [message #343462 is a reply to message #343461] |
Sat, 18 May 2019 20:07 |
emerystora
Messages: 4442
Registered: January 2004
Karma: 13
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Senior Member |
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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
> On May 18, 2019, at 7:41 PM, Cary Lawson via Gmclist wrote:
>
> 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
>
>
>
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> GMCnet mailing list
> Unsubscribe or Change List Options:
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| Re: Engine misses on acceleration [message #343469 is a reply to message #343406] |
Sun, 19 May 2019 07:35 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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Senior Member |
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Fry's Electronics, DigiKey>dot<com, any electronic supply house will have it. Used the stuff regularly before I retired. It's called heat sink compound or heat sink grease.
--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
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| Re: Engine misses on acceleration [message #343475 is a reply to message #343406] |
Sun, 19 May 2019 09:11 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Senior Member |
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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
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| Re: [GMCnet] Engine misses on acceleration [message #343478 is a reply to message #343475] |
Sun, 19 May 2019 09:33 |
emerystora
Messages: 4442
Registered: January 2004
Karma: 13
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Senior Member |
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That’s what Autozone sells. A small box with a few of the rear open pouches.
Emery
> On May 19, 2019, at 9:11 AM, John R. Lebetski via Gmclist wrote:
>
> 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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