Shake Shake Shake

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Detroit Iron

Detroit Iron

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My 340 has a shake that I can't seem to tune out. I wonder if anyone has input after watching the video. The engine actually runs really well despite the way it sounds in the video. It's an intermittent shake that you can clearly see by looking at the air cleaner.

The motor is pretty low compression 9:1, cylinders cut .040 over (built 20 years ago low miles), Comp Cams 270H, Mopar Performance electronic ignition kit, Carter 4966 on an Edelbrock LD340 intake with Edelbrock RPM heads.

I've eliminated the carb as the problem as far as I can tell by tuning and even trying different carbs. There are no vacuum leaks at the intake or carb base.

The problem seems to remain, regardless of where I put the timing and advance. Currently the base timing is at about 15 degrees, and read about 35 total at 3k rpm.

https://youtu.be/ep-v-J5DJVY
 
If it's hitting on all 8 the crank is out of balance. Seen it many times.

As easy as it is to balance a crank, it can still get screwed up.
 
To me it looks like a misfire, idling fine then a shake accompanied by a noise.

Look for spark in the dark.
 
yellow rose said:
If it's hitting on all 8 the crank is out of balance. Seen it many times.

As easy as it is to balance a crank, it can still get screwed up.
Click to expand...

You may be on to something here. The engine has had this shake (at idle only) since I built it years ago. It actually runs killer, but just at idle I get that intermittent shake.
Dana67Dart said:
To me it looks like a misfire, idling fine then a shake accompanied by a noise.

Look for spark in the dark.
Click to expand...
No misfires. It hits on all eight and runs very smooth while I drive it. Crank my be the source of it.
 
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Is that just valve over lap causing reversion in the intake? Not a stock cam. Maybe normal for the cam you have?
 
Sounds like the valves are way out of adjustment, solid lifters ?
 
Still looks like misfire. At higher rpm misfire would be smoothed out.
 
Put the timming light on all cyl one at a time and look for the light to not fire when the shake happens.
 
What rockers are you using the 340 style or the 273 stuff.
 
A temp gun on the pipes might also give a clue.
A balance problem isn't intermittent, but a miss can be, so I also have to go with a miss.
Keep in mind that a miss can cause valve train noises to change, so we can't assume it's a valve train problem causing the miss just because their sound changes when it misses.
 
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Sorry I see you said comp rockers so I'll assume they are the stamped 340 style. I have to agree with Brian6pac the valves sound WAY OUT of adj. or you have very low oil pressure at idle. Have you pulled the valve covers and checked anything? Look and see if all the pushrods look ok, you can just crank it and look. Also check to see if any are protruding thru the rockers.
 
LO23M8B said:
Sorry I see you said comp rockers so I'll assume they are the stamped 340 style. I have to agree with Brian6pac the valves sound WAY OUT of adj. or you have very low oil pressure at idle. Have you pulled the valve covers and checked anything? Look and see if all the pushrods look ok, you can just crank it and look. Also check to see if any are protruding thru the rockers.
Click to expand...
I just had the heads off, and everything looks great. Rockers, pushrods etc. When I made the video, I should have walked around back, because the valves don't sound out, nor does it sound like there's a misfire. Idles really nice. The sound in the video is not a good representation of reality. Actually it sounds like hell in the video. Haha.
 
When you put it back together you should check lifter preload just as a precaution.
 
I would not think this shake would come and go if the crank was out of balance.

I'm trying to think what parts of the engine and/or trans could be out of sync then in-sync. I can only think of the timing chain whipping around, which would move the cam and ignition timing around too.
 
Upon further investigation.... I may have a valve problem after all. I have a knock up high on the passenger side. Like maybe I have a failing valve spring. This would be weird though since these are brand new Edelbrock heads and the problem predates these heads.
 
Detroit Iron said:
You may be on to something here. The engine has had this shake (at idle only) since I built it years ago. It actually runs killer, but just at idle I get that intermittent shake.
No misfires. It hits on all eight and runs very smooth while I drive it. Crank my be the source of it.
Click to expand...

No, if it's an imbalance, it's shaking all the time. You only feel it at idle. Is it a steel crank of cast crank engine? Does it have the correct corresponding balancer?
 
sounds like a knock to me but cell phone audio sux. start pulling plug wires one at a time to see if any DONT make a difference pulled, ie a dead cylinder. Laser thermometer on manifolds would not be accurate as IR header pipes. 18436572? Easy to cross 5 and 7. Forged crank 340s (what year you got?) were neutral balanced and .040 over replacement pistons would be weight matched to the STD ones. If you stuck with stock or OEM weight pistons (not Icon, Probe, JE, etc) I doubt the crank is out of balance. Unless your running a 360 balanced converter? Cast 340 balancers (big cast in warning on them) look unlike a symetrical neutral balancer, you'd know. Sometimes at idle, the advance plate will be pulled out of gap spec, then it will hit on 8 off idle as the plate moves. Just double check the advance plate arc while looking at it while applying vacuum to the pot.
 
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