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Old 08-10-2018, 11:00 AM   #1
pofo
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Vibration under acceleration, troubleshooting

Hi guys, pardon my english, I am from Norway.


I did a compression check today.

The engine (as far as I know):
Stock 350 with the original Rochester Quadrajet carb, no modifications.

The truck:
1976 C20 Silverado with a TH350.

It was stored outside in the norwegian winter, it was around -17C at the coldest. A buddy of mine started it up and let it idle till it got warm 3-4 times during the winter.

In springtime I drove it 260 miles without problems. After that I drove it around town for a couple of days and noticed a vibration while accelerating. It has some uneveness/shaking while in P and some throttle, maybe 1200-1600 rpm.

I got the following results:

Sylinder 1: 137psi
Sylinder 2: 150 psi
Sylinder 3: 137 psi
Sylinder 4: 150 psi
Sylinder 5: 135 psi
Sylinder 6: 140 psi
Sylinder 7: 138 psi
Sylinder 8: 150 psi

The lowest was 135 psi and the highest was 150 psi.

Are the compression OK? Whats the next thing for me to check out/do? I changed the fuel filter without effect.


Thanks for tips and answers!
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Old 08-10-2018, 01:05 PM   #2
geezer#99
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Re: Vibration under acceleration, troubleshooting

Compression’s good.
10% between the high and low reading is good.

Have you checked your spark plug wire or spark Plugs?
That will cause your problem.
Easy way to check is look at them in the dark. Hard way is to wiggle them when it’s running.
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Old 08-10-2018, 01:27 PM   #3
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Re: Vibration under acceleration, troubleshooting

Quote:
Originally Posted by geezer#99 View Post
Compressionís good.
10% between the high and low reading is good.

Have you checked your spark plug wire or spark Plugs?
That will cause your problem.
Easy way to check is look at them in the dark. Hard way is to wiggle them when itís running.
Wires and plugs are 2 years old. The plugs do have a lot of rust on them, and when I pulled them, some of them are a little sotty and a couple are light grey..

Should probably just change the plugs and wires, pretty easy, and doesnt cost a fortune.
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Old 08-10-2018, 01:37 PM   #4
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Re: Vibration under acceleration, troubleshooting

Do the plugs first.
If you do one thing at a time you’ll find the problem easier.

And it doesn’t cost much to check your timing!
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Old 08-10-2018, 04:59 PM   #5
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Re: Vibration under acceleration, troubleshooting

[QUOTE=Hard way is to wiggle them when itís running.[/QUOTE]

Learned that lesson the hard way.
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Old 08-10-2018, 08:14 PM   #6
Justins76
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Re: Vibration under acceleration, troubleshooting

Does it have fresh fuel in it? Check your distributor cap inside for corrosion on the terminals.
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Old 08-10-2018, 08:58 PM   #7
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Re: Vibration under acceleration, troubleshooting

Motor/trans mounts maybe?
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Old 08-11-2018, 10:34 AM   #8
pofo
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Re: Vibration under acceleration, troubleshooting

Quote:
Originally Posted by Justins76 View Post
Does it have fresh fuel in it? Check your distributor cap inside for corrosion on the terminals.
Yes, fresh fuel and I checket the distributor cap (forgot to mention that), everything looked good.

Quote:
Motor/trans mounts maybe?
Could worn/broken motor mounts give the impression that the engine runs a little rough? I will have a look

Last edited by pofo; 08-11-2018 at 10:35 AM. Reason: spellcheck
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Old 08-12-2018, 11:38 AM   #9
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Re: Vibration under acceleration, troubleshooting

I am trying to figure out whats the correct plugs for the engine. Now I got ACDelco R44T with a .035" gap (according to Rockauto). Is this the correct gap for my truck?
If I look up my truck, engine and year on Rockauto I get lots of different plugs with different gaps.
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Old 08-12-2018, 11:58 AM   #10
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Re: Vibration under acceleration, troubleshooting

Gap an hei at .045.
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Old 08-12-2018, 12:34 PM   #11
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Re: Vibration under acceleration, troubleshooting

Does your 76 have HEI in now or points.
Maybe check the points and gap them if needed.( clean if that all they need or replace)
after doing might be good to check timing.
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Was a 77 454 w/308 gear. Taken out 550 lbs. up front with motor clip change.
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Old 08-12-2018, 02:02 PM   #12
pofo
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Re: Vibration under acceleration, troubleshooting

Quote:
Originally Posted by Pontiac1976 View Post
Does your 76 have HEI in now or points.
Maybe check the points and gap them if needed.( clean if that all they need or replace)
after doing might be good to check timing.
Not sure actually. I think it got points. Should the gap be different than with HEI?

And whats the easiest way to tell the difference between points and HEI?
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Old 08-12-2018, 02:48 PM   #13
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Re: Vibration under acceleration, troubleshooting

Yours would be hei.
Hei was put in trucks starting in 1975.
Easy to see difference.
Hei doesn’t have a coil wire in the centre of the cap, points does.
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Old 08-12-2018, 02:51 PM   #14
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Re: Vibration under acceleration, troubleshooting

The easy way is the cap and coil

https://www.google.ca/url?sa=t&rct=j...15GPas00x-M8Bm

Sorry 76 my mistake 75 up I must be getting old
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1977 REG cab Long Bed with a 1984 Front Clip, 82 305 .60 over 9.0.1 flat top pistons, 4bbl , w/84 SS-intake & cam , w/ 1977 big block rad, 1975 gear box, 1984 master w/ metric fittings top & SAE fitting @ portion block, 1975 3.42 gear open diff. Duel 77-87 Factory air cleaner, duel inlet from rad support . Down sized 9' to new 8' Steel flat bed
Was a 77 454 w/308 gear. Taken out 550 lbs. up front with motor clip change.
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Old 08-12-2018, 06:33 PM   #15
pofo
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Quote:
Originally Posted by geezer#99 View Post
Yours would be hei.
Hei was put in trucks starting in 1975.
Easy to see difference.
Hei doesnít have a coil wire in the centre of the cap, points does.
Allright, thanks! So with HEI, the gap should be .045Ē? The plugs Ive used have .035Ē
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Old 08-12-2018, 06:39 PM   #16
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Re: Vibration under acceleration, troubleshooting

Yes!
Most plugs are gapped at .035 new.
You need to check and gap them.
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