Shaking 360 LA


Senior Member
Mar 21, 2013
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Florence, Oregon
I need some guidance on what could cause my 360 to shake pretty decent at idle. Its the same in gear or park and sort of new ish. The engine always had a little shake but not like that.

Two things, I am running efi and have tried to adjust the idle to the most stable solid vacuum and such but the motor has a pretty strong consistent shake. It does not appear like its dropping a cylinder or misfires, I can tell because the efi works like a saw tooth pattern at idle. I have the AFR set to 13.8:1 (leaner and it gets unstable). The timing is commanded at 18° at idle and I do have the option for "idle stability spark which I have currently turned off.

So if I set it to 13.5 afr at idle, the efi fluctuates between 13 and 14.2 to keep it there. At 13.8 the fluctuation is less. However, when it goes lean it starts stumbling for a sec and then goes back commands richer to 13 and stumbles again, its subtl but noticeable. This does not happen as the efi keeps the values closer together at 13.8.

Vacuum relatively stable. Around 7 in Gear and 9 to 10 in Park. Fluctuates maybe 0.5. I dont think this is the cause as thats always been around those numbers.

The motor mounts are rubber and are bouncing the engine quite decently... I have a feeling that I always had the wrong balancer on it maybe or something I was told that the previous owner installed a "cam" but I dont know the values and the engine runs otherwise pretty good and strong. I had some issues with adjusting the crank and it would kick back on the starter and looseened the flexplate bolts, i have tightened them back up but i need a little higher stall anyway... oh I run a CDI box too!

Anyway, what should I investigate, it feels like when you screw the idle way down and it just shakes along evenly. Or like when you lean out the carb idle screws and just leave it shaking. I normally have the idle set at 800rpm. 850 makes little difference. Same with 750 (makes the shake worse i think).

I'm a little clueless..
Vacuum seems low. Manifold Idle Vacuum in most street motors I've owned is 15-17HG. Vacuum leak?

Engine mounts could be weak?
For the CAM in the motor, not too bad. Needs MORE exhaust manifold. Obviously the cam is the reason for the low vac at idle AND even lower in gear. Especially with the elevated idle speed. Probably needs a little looser torque converter, too. The obvious reason it works as well as it does is the EFI system.

What spark plugs are in the motor?

Just some thoughts and reactions.
Yep currently a stock converter. Needs like at least an 1800 to 2000 stall. I am looking for some headers.
Oh sparkplugs I run some MSD platinum. A small bit colder as NGK GR4. The motor mount does seem pretty soft, as the rest of the car runs on Poly and it loud and rattly anyway, should I put some solid or polyloc motormounts on it? Id rather not....
I would aim more for a stall speed of about 2200 rpm for a bit less load on the motor at idle.

Solid motor mounts are "race only", to me. The unexpected issue is that Chrysler recommended the front "elephant ears" mounts like dragsters used back then. That put the solid mounts, and related forces, on a stronger part of the engine block. I knew of some Chevy racers who went to solid side mounts and cracked the side of the engine blocks due to "torque reaction".

Put the normal rubber engine mounts back in, BUT also do the modification of putting one or two bolts through them for retention purposes. Seems like @Big_John has a thread response on how to do that? Or you can build a torque strap motor movement restrainer device. Polyurethane can be a good transmission mount material, though.

Just some thoguhts,
And let me point out that the bolt(s) are NOT to be tightened at all, which would squish the rubber so that you end up with a solid-ish mount. You leave the nut(s) slightly loose (30-40 thousandths) and either use Loctite or self-locking nuts. This allows the mount to properly act as a vibration isolator under acceleration and deceleration but is impossible to break.

And I do think that one good, strong 3/8ths bolt will do the job for 99 percent of the cars out there.
I was thinking of eventually going with the Hughes Performance Torque Converters 24-20. Thats a 2000 Stall. I will see if they have a 2200.
I have absolutely 0 idea. The owner I bought it from got it from someone who passed away and all I was to was that it had "a cam in it". I can say that the engine does really start waking up once you pass 1800rpm. I find myself manually keeping it in 2nd often to keep the rpms up a little more in town.
Oh I added the EFI. I could never tune the Carter AFB right, then went to a Holley with modded annular discharge boosters and that ran fantastic even got better mileage than the efi, however in my climate I was tired of constantly had to fiddle with the carb... had the efi for a good 8 years now or so.
So, what changed since it started to idle roughly? It was running good with a smoother idle and now it is shaking. Is it possible you have a broken mount. In the replies above there are some ideas on how to stiffen the mounts, maybe one is broken. If it is, it might not be that easy to detect. Other than the shaky idle it is running good.
A broken motor mount, as in the LH side, will not cause any difference in idle feel, but as soon as you throttle into the motor from idle, the rear a/c line will contact the inner hood structure. When you let of the gas when that happens, THEN you'll feel a "bump" as the engine settles back down. This can also be verified by loading the engine against a firmly-applied foot brake. Just takes a second.

I agree, but with an aggressive cam you might feel it at an idle. It is worth looking into. He just needs to do what you said about checking it.
You know, now that I think about it, the air cleaner nut started hitting the hood (before I recently cut that area out). That was an unusual thing because normally hadn't happen before, I moved and am now just getting to have the space and time to look at some stuff that I mentally noted.