Idle shake

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shep76

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This motor was in the car when I bought it so I’m unsure of the cam. Has decent compression little lower on number one cylinder. Brand new carb, new wires, has the pertronix conversion. I’ve searched for a vacuum leak without much luck. I’m going to pull the motor fairly soon to paint it but curious what you guys think this low vacuum/moving need issue is? It seems to want a ton of timing advance. Runs decent but the shake at idle is annoying
 
If the shaking smooths out with more rpms, engine might just have a long duration/high overlap cam. Shaking could be many other things, though....
 
TS1 miss.jpg
 
Let me guess a 360 external balanced engine mated to a neutral balance flex plate and neutral balance torque converter... 904/727.


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^^Agree.
Let's get some basic information on the build- engine, trans...
And what do you consider "a ton of timing advance"?
 
It is a 360 and 727. I don’t know much else. Rpm keeps going up as I advance past 30 and higher…. I think I have it set at 18 btdc but I’ve played around with it a lot trying to smooth it out. I’ll be honest I didn’t know there were different flywheel torque converters that can be used with this combo so I’ll have to do some research if you guys think that’s a contributing factor. Is it possible to see what flex plate and torque converter is there without pulling the transmission? I thought all 360’s were externally balanced. Sorry, I’ve never been too far inside an engine so learning.

 
I did rinse the gas tank and change the filters when I got the car but wouldn’t be the first time I’ve been accused of having bad gas….
 
360 to 727 Flex Plate

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20250105_125319.jpg


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Or this stock 360 flex plate and weighted torque converter, to go behind the external balanced 360 with a 727 trans.

s-l1200(3).jpg


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Looks good, you have the correct weighted torque converter with the stock 360 flex plate. To couple the 360 to the 727.


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it's not a balance issue. the thing would be rattling your fillings out.

i'm thinking you can tune your way to smooth-ness. or smoother-er.

it might be a "death by 1000 cuts" where you have just a little off here, a little off there and a little off elsewhere.

start with the basics: a comprehensive tune up. look for vacuum leaks. double check the mark on the balancer hasn't slipped. everything it tight, yadda yadda.
 
junkyardhero said:
it's not a balance issue. the thing would be rattling your fillings out.

i'm thinking you can tune your way to smooth-ness. or smoother-er.

it might be a "death by 1000 cuts" where you have just a little off here, a little off there and a little off elsewhere.

start with the basics: a comprehensive tune up. look for vacuum leaks. double check the mark on the balancer hasn't slipped. everything it tight, yadda yadda.
Click to expand...
Thank you. Is 12 inHG considered low? Or would that be fairly normal if it has a mild cam? Maybe I’ll remount the intake manifold.
 
shep76 said:
Thank you. Is 12 inHG considered low? Or would that be fairly normal if it has a mild cam? Maybe I’ll remount the intake manifold.
Click to expand...

depends on the cam. i wouldn't be thrilled with that, but it's more than enough to run power brakes.

speaking of... do you has? if so, check the line & fitting on that ****.

have you had the manifold off?
 
shep76 said:
Thank you. Is 12 inHG considered low? Or would that be fairly normal if it has a mild cam? Maybe I’ll remount the intake manifold.
Click to expand...
My slant 6 has 6hg at idle, so 12 is pretty good considering.
 
Yeah simply put, you have a little too much cam in there for a smooth idle. Intake and exhaust valves are open at the same time with the overlap which gives a rough idle and a low signal on the vacuum reading.

Stock grind camshafts idle great, just like the 340 grind camshaft that comes stock with the 360 4 bbl. engines. Good idle and good vacuum pull at idle.

So here is the stock 360 camshaft grind>

Lift: 430/444
Duration: 268/276

This grind is probably a little closer to what you have plus or minus:

268 degrees - intake
284 degrees - exhaust
46 degrees of overlap
.450" lift intake
.455" lift exhaust


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junkyardhero said:
depends on the cam. i wouldn't be thrilled with that, but it's more than enough to run power brakes.

speaking of... do you has? if so, check the line & fitting on that ****.

have you had the manifold off?
Click to expand...
Manual breaks. I haven’t had the manifold off but it’s dirty and nasty and it’s the wrong manifold for the 360, the ld4b so I was hoping to swap it anyways. I did confirm the 0 mark on my dampener is TDC.
 
Last edited:
George Jets said:
Yeah simply put, you have a little too much cam in there for a smooth idle. Intake and exhaust valves are open at the same time with the overlap which gives a rough idle and a low signal on the vacuum reading.

Stock grind camshafts idle great, just like the 340 grind camshaft that comes stock with the 360 4 bbl. engines. Good idle and good vacuum pull at idle.

So here is the stock 360 camshaft grind>

Lift: 430/444
Duration: 268/276

This grind is probably a little closer to what you have plus or minus:

268 degrees - intake
284 degrees - exhaust
46 degrees of overlap
.450" lift intake
.455" lift exhaust


* * * * *
Click to expand...
Ok. Curious, are you guessing my cam just by the sound?
 
I see you have an Edelbrock 650 cfm AVS 2 carburetor on your 360, that is a good match for that engine. Look down in the choke flap to the Boosters (cone funnels that mist gas into the throat of the carb), make sure they are misting evenly at idle and coming up off an idle.

Screenshot_20250105-152723_Gallery.jpg



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Trained ear, yeah you can hear the faint loap at idle.

Compared to a stock cam, the difference is night and day.

Most people over cam their engines when building them, so they don't idle well. They start making power at 1500-2000 rpm to 5000-6000 rpm... not much at idle.

Stock cams start making power right away, more low end torque.
0 rpm to 4500 rpm.


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