# Question regarding engine shake



## cnemike (Apr 12, 2018)

We recently rebuilt the 389 in our 65 GTO and it appears we have a problem. The engine idles normally but raising the rpm to say 1500 to 2500 and holding it at a constant rpm the engine shakes. My thought after talking to the guy who did the machine work that because he resurfaced the heads (and we have no record and he can't remember how much was removed) perhaps the valves are not closing all the way. Does anyone have any idea as to what would cause the shake?


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## PontiacJim (Dec 29, 2012)

cnemike said:


> We recently rebuilt the 389 in our 65 GTO and it appears we have a problem. The engine idles normally but raising the rpm to say 1500 to 2500 and holding it at a constant rpm the engine shakes. My thought after talking to the guy who did the machine work that because he resurfaced the heads (and we have no record and he can't remember how much was removed) perhaps the valves are not closing all the way. Does anyone have any idea as to what would cause the shake?


Engine rebalanced during the rebuild?

Rocker arm nut has backed off.

Dead cylinder - rocker arm fell off or pushrod bent not activating a rocker/valve.

Worn/rounded cam shaft lobe and valves not opening as they should.

Could be an ignition/distributor problem. 

Could be a fuel problem - bad gas/water in fuel, carb problem.

If valve problem - ie not closing fully, you would most likely hear popping out the exhaust or back through the carb - unless a valve is simply closed due to rocker arm off.

Typically resurfacing the head only takes off a minimum of material and is not the same as milling the head to raise compression.

These are just a few thoughts as it could be something else completely different. You could also have a very major problem as well. So very hard to be specific on such a general question and unknown. :thumbsup:


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## bigD (Jul 21, 2016)

Lots of guys have reversed the #5 & #7 plug wires. It's easy to do, unless you have some very nicely arranged wires, with dividers. I've been in a hurry and done it a few times myself.

On my race cars, I ran long wires, because I took the valve covers off frequently. Never ran short wires with nice neat dividers. Didn't wanna have to remove all the wires every time i removed a VC. So, when hooking up the longer wires, it was easy to reverse #5 & #7, if you were in a big hurry.

Some of these guys have probably never made this mistake. But I've made this one & LOTS of others.


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## cij911 (Oct 25, 2017)

Bad motor mount ?


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## cnemike (Apr 12, 2018)

Engine not re-balanced during rebuild. Same connecting rods, pistons, crank, flywheel and dampener. It was basically a .030" bore, bearings and a new cam (same profile as stock). Checked plug wires for #5 and #7 swap. Clean fuel in a new tank. I spoke to a guy who builds Pontiac race motors (Carl from Carl's Tin & Chassis in Oakdale, Ca.) and he was leaning heavily on valves not closing completely. He suggested ARP 7/16" rocker studs with ARP Perma-Loc nuts and manually adjusting the valves. I ordered these from Summit today.

Thank you everyone for your responses. I should probably start another thread for this question but does anyone happen to know what the correct vacuum configuration is for a 1965 GTO 389 with the 4-barrel carburetor as it relates to the vacuum advance and the brake booster? There is one port on the back of the carburetor and as you know the only two ports in the manifold are in front for the PCV valve. It would seem that a T-fitting on the back of the carburetor would work. I have looked through several restoration books and online and cannot find a definitive diagram or picture of this set up.


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## GTO44 (Apr 11, 2016)

Still using the stock harmonic balancer?


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## cnemike (Apr 12, 2018)

Yes we are still using the stock balancer. The balancer appeared to be in good condition.


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## cnemike (Apr 12, 2018)

The motor mounts are brand new.


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## cnemike (Apr 12, 2018)

In response to PontiacJim, we believe the problem is related to the material removed from the heads during resurfacing. I keep reading stories about people replacing the rocker studs with 7/16" studs which screw into heads with screw-in studs. Our heads have the old style, pressed-in, hollow studs that provide oil to the rocker balls. We can tap the heads but I'm looking for an easier fix. I realize that due to the machining done to the heads our push rods are now a bit too long. Since the rocker nuts tighten against the shoulder on the stud, does anyone know if there is a different nut that doesn't push the rocker ball down so far, compensating for the extra push rod length? The original nuts look like they could have material machined off of them to fix this issue. Has anyone ever dealt with this issue?


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## BLK69JUDGE (Jun 10, 2010)

would a vacuum gauge give you any clues ? ??


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## geeteeohguy (Feb 2, 2008)

Take about 2 minutes and hook up a vacuum gauge to the intake manifold. A fluttering needle means valve issues (tight or burnt valves). First thing to do.


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## Machinest-guy (Jul 19, 2019)

May I suggest that looking into the valve train might uncover your problem. A cam swap was mentioned. Unless the cam profile is checked there is no way to know how much difference there is in base circle diameter from OEM spec. Hence your push rod length may be incorrect. Reduced base circle cams are the norm when getting reground cams, and sometimes when buying a new one. Effects of reduced base circle cams are negated by resurfacing the heads to some degree. But that combined with gaskets and replaced tappets can introduce errors that sum to being problematic rather than cancel into no big deal. A fast way to trouble shoot a part throttle misfire is to find a shop with an old SUN Engine analyzer that can do a cylinder balance test. - Shorting out one cylinder at a time to reveal which cylinder is weak or not producing power. That test can be faked by simply removing the plug wires one at a time to see which cylinder makes no difference. 
You can test valve action with a feeler gauge. Remove the valve covers, block off squirting oil when you run the engine. Insert about a .025 feeler gauge between the valve stem tip and rocker arm. Just quickly jam it in there. The engine should stumble and shake a bit as the hydraulic tappet readjusts itself. If it doesn't change then that lifter may be plugged up and cannot maintain constant contact or cannot relax to allow seating. A broken or weak valve spring can product the effect you are describing too. Just because springs are new they may be defective or out of spec. The machine shop should have checked installed height and spring tension when rebuilding your heads. Many shops don't. Good luck.


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## integrity6987 (May 10, 2017)

kinda interested in how this story turned out. Would the OP please update this?


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