Vibration under acceleration, troubleshooting - The 1947 - Present Chevrolet & GMC Truck Message Board Network

pofo · 08-10-2018, 11:00 AM

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!

geezer#99 · 08-10-2018, 01:05 PM

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.

pofo · 08-10-2018, 01:27 PM

Quote:

Originally Posted by geezer#99

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.

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.

geezer#99 · 08-10-2018, 01:37 PM

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!

SunSoaked · 08-10-2018, 04:59 PM

[QUOTE=Hard way is to wiggle them when it’s running.[/QUOTE]

Learned that lesson the hard way.

Justins76 · 08-10-2018, 08:14 PM

Does it have fresh fuel in it? Check your distributor cap inside for corrosion on the terminals.

87Flh · 08-10-2018, 08:58 PM

Motor/trans mounts maybe?

pofo · 08-11-2018, 10:34 AM

Quote:

Originally Posted by Justins76

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

pofo · 08-12-2018, 11:38 AM

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.

geezer#99 · 08-12-2018, 11:58 AM

Gap an hei at .045.

Pontiac1976 · 08-12-2018, 12:34 PM

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.

pofo · 08-12-2018, 02:02 PM

Quote:

Originally Posted by Pontiac1976

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?

geezer#99 · 08-12-2018, 02:48 PM

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.

Pontiac1976 · 08-12-2018, 02:51 PM

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

pofo · 08-12-2018, 06:33 PM

Quote:

Originally Posted by geezer#99

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.

Allright, thanks! So with HEI, the gap should be .045”? The plugs Ive used have .035”
Posted via Mobile Device

geezer#99 · 08-12-2018, 06:39 PM

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

pofo · 09-06-2018, 05:34 PM

Thought I should do a follow up.

I finally got new plugs, gapped them at .045" and it runs much better and smoother, no vibration!

Really appreciate the help, guys.

My idle is running a bit higher, but thats probably because the fresh, new plugs.

08-10-2018, 11:00 AM	#1
pofo Registered User Join Date: May 2018 Location: Egersund Posts: 13	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!

08-10-2018, 01:05 PM	#2
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,508	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.

08-10-2018, 01:37 PM	#4
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,508	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!

08-10-2018, 04:59 PM	#5
SunSoaked Registered User Join Date: Dec 2011 Location: Yuma Arizona Posts: 1,524	Re: Vibration under acceleration, troubleshooting [QUOTE=Hard way is to wiggle them when it’s running.[/QUOTE] Learned that lesson the hard way.

08-10-2018, 08:14 PM	#6
Justins76 OutOfMyMind & I'llNeverKnowWhy Join Date: Apr 2004 Location: Another world, some other time Posts: 672	Re: Vibration under acceleration, troubleshooting Does it have fresh fuel in it? Check your distributor cap inside for corrosion on the terminals. __________________ -Justin-Out of my mind and I'll never know why 1976 Chevrolet Silverado C10 350 4bbl / TH350 Big10 longbed 1986 Pontiac Firebird T-Top 5.0L TPI 700R4 WS6 1992 Pontiac Firebird T-Top 5.0L TBI 700R4 My cardomain page I don't like to save, it's more fun to spend!!

08-10-2018, 08:58 PM	#7
87Flh Registered User Join Date: Mar 2014 Location: Mt Dora FL Posts: 68	Re: Vibration under acceleration, troubleshooting Motor/trans mounts maybe?

08-12-2018, 11:38 AM	#9
pofo Registered User Join Date: May 2018 Location: Egersund Posts: 13	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.

08-12-2018, 11:58 AM	#10
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,508	Re: Vibration under acceleration, troubleshooting Gap an hei at .045.

08-12-2018, 12:34 PM	#11
Pontiac1976 Registered User Join Date: Oct 2015 Location: Ottawa, Ontario, Canada Posts: 750	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. __________________ 1977 REG cab Long Bed with a 1984 Front Clip, 82 305 .60 over 9.0.1 flat top pistons, 4bbl, TH350, 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.

08-12-2018, 02:48 PM	#13
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,508	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.

08-12-2018, 02:51 PM	#14
Pontiac1976 Registered User Join Date: Oct 2015 Location: Ottawa, Ontario, Canada Posts: 750	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 __________________ 1977 REG cab Long Bed with a 1984 Front Clip, 82 305 .60 over 9.0.1 flat top pistons, 4bbl, TH350, 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.

08-12-2018, 06:39 PM	#16
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,508	Re: Vibration under acceleration, troubleshooting Yes! Most plugs are gapped at .035 new. You need to check and gap them.

09-06-2018, 05:34 PM	#17
pofo Registered User Join Date: May 2018 Location: Egersund Posts: 13	Re: Vibration under acceleration, troubleshooting Thought I should do a follow up. I finally got new plugs, gapped them at .045" and it runs much better and smoother, no vibration! Really appreciate the help, guys. My idle is running a bit higher, but thats probably because the fresh, new plugs. Last edited by pofo; 09-06-2018 at 05:34 PM. Reason: spellcheck

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