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Shaky idle -help!

V12T

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There only a few thing it could be. I'm just bouncing somethings back. This might get a little long and drawn out but I'm drawing on my personal experience and others.You are working on narrowing it down. What is mentioned above is very good. I always look at the mechanical first. Mostly it can be seen or checked and if it's bad your done. A few things to consider that I'm sure you did or will. Compression and or leak down test. This is a place of concern. Floating of vale train. Is it a sticking valve bad guide or seat or a weak spring. Cam more than likely ok, Lifter?

Then there is the vacuum portion of that. Gasket intake,carbs gaskets x 3, head gasket. Distributor vacuum, vacuum advance and hoses. Seen this happen. A shipmate had 70 454. It would acted up time to time. At different RPMs it would act up. Stumble miss hesitate. Changed the points and condenser the same parts it had , basic tune up. After going through it for a day or two. found it. It was the wrong condenser both times. As the vacuum advanced and retard it was it was hitting the inside of distributor. Vacuum advance working? This may not be you problem just sharing it to be considered.
Then on to the air and fuel. you seem to be on top of that. There are experienced ones here. Myself I would consider the source. Meaning the fuel tank in good shape no rust. No pinched lines,fuel pump presser ok. What size line do you have? I have seen crazy things happen. I would purge the fuel system. Fuel filter? Something might be obstructing as pressure changes. Something floating around there. AS far as the 6 pak That is a different critter. It's goes from almost nothing to balls to the wall!
As mentioned a base line is a necessity. Right now I would discount that until you make sure the mechanical and others are correct. There are some very well versed members here to help with that.
Transmission problem not even going to go there but a possibility. Toque converter problems you never know. Keep it mind. Is there a check vale in the cooler line?
I see that you have worked on diagnosing the issue. This might help trigger something or someone with a solution.
Next for my area of mistakes. I know you covered that. The electrical. I'll be back.
Wow - thanks for the detail. I think I have covered most of this. Compression all about 190 - not done leakdown. New fuel tank. I bypassed the fuel filter and Mopar fuel return thing. Have a simple pump inline which delivers 5 1/2psi. (I use this to prime before a cold start). Not measured mechanical pressure but am getting a new high volume pump anyway. Get shake with either or both running. Heads, cam, followers all new. Did smoke test for vacuum leaks. Seemed OK but might do it again. Electronic ign so no condenser. You mentioned transmission which got me thinking - When I rebuilt my 440 to 512 I went from externally balanced to internally. I changed the front damper but used the same flex plate and converter with the 727. Should I have used a different "externally balanced" flex plate and/or converter. I'm OK with engines but never learnt about transmissions. Performance is awesome except for this annoying shake.
 

Challenger RTA

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There might be a trans guy respond to clear any questions up. (That doesn't sound right) Some dampers are not to be used on certain motors. Hope this all helps. I'll get back if the electrical seems to be one of the problems. Just posting thought to see if something comes to light.
 

V12T

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There might be a trans guy respond to clear any questions up. (That doesn't sound right) Some dampers are not to be used on certain motors. Hope this all helps. I'll get back if the electrical seems to be one of the problems. Just posting thought to see if something comes to light.
Regarding electrical I haven't found anything wrong so far but have decided to get new distributor/ecu/coil anyway. I won't get it until July when I visit US due to punishing freight cost and import taxes here in UK. Plugs and leads new.
 

Xcudame

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If your flex plate or torque converter where balanced (asymmetrical design or added weight) for an external big block which you had originally and you didn't change to a neutral balance flex plate and/or torque converter going to an internal balanced engine, it will cause shaking! And wear things out much faster! The good thing is if the weight in on the torque converter, you can probably take it off from under the car. If the flex plate was for external balance, you're taking the engine and transmission apart.
 

V12T

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If your flex plate or torque converter where balanced (asymmetrical design or added weight) for an external big block which you had originally and you didn't change to a neutral balance flex plate and/or torque converter going to an internal balanced engine, it will cause shaking! And wear things out much faster! The good thing is if the weight in on the torque converter, you can probably take it off from under the car. If the flex plate was for external balance, you're taking the engine and transmission apart.
As I didn't change either. It sounds like this could be the cause of all my problems. So I need remove any weights from the converter (would they all be on the front which I can access). I will try this first (at the end of the week) and hope that the existing flex is not contributing to the problem. If it is I'll have to get a neutral flex plate and take the transmission off. Good to have something to investigate - thanks.
 

Xcudame

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Yes, the weights are on the front of the torque converter. If present, you'll need to remove the dust cover and you should be able to see the weight(s) similar to the ones in the photo below. I've had the best luck with a really sharp small chisel and BFH. I suppose an air chisel would work too, but I kinda like whacking on the car when I'm doing a necessary rework. Gets rid of frustration! 😁

068A9754-0CC1-4B48-BE1B-1FD44D70FF66.jpeg
 

V12T

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Yes, the weights are on the front of the torque converter. If present, you'll need to remove the dust cover and you should be able to see the weight(s) similar to the ones in the photo below. I've had the best luck with a really sharp small chisel and BFH. I suppose an air chisel would work too, but I kinda like whacking on the car when I'm doing a necessary rework. Gets rid of frustration! 😁

View attachment 124211
This is new to me. I remember that my converter had weight(s) on it. To be clear before I start hacking do you mean ALL 6 of the weights in the pic or just the two at 4 and 5 o'clock? Is the "pin" at 4.30 the flexplate locator?
 

Xcudame

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Just the ones that don't have bolts from the flex plate going through them. It will be obvious when you see them. No the "pin" is a drain plug on certain years and models. The weights are on either side of the drain plug in the photo above.
 

Deathproofcuda

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Have you done the test where you have the car idling and methodically disconnect the plug wires one by one? If the idle changes when you disconnect the plug wire then, you know that cylinder was firing correctly. If the idle doesn't change, then that cylinder was not firing.
 

V12T

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Just the ones that don't have bolts from the flex plate going through them. It will be obvious when you see them. No the "pin" is a drain plug on certain years and models. The weights are on either side of the drain plug in the photo above.
Thanks. I'll get to it at the end of the week.
 

V12T

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Have you done the test where you have the car idling and methodically disconnect the plug wires one by one? If the idle changes when you disconnect the plug wire then, you know that cylinder was firing correctly. If the idle doesn't change, then that cylinder was not firing.
Yes, did that - couldn't notice any change -thanks.
 

V12T

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Update. Hacked off 2 x 1 oz weights from converter. Flex plate is a neutral one. Although I'm sure it was the right thing to do there is no noticeable difference in the shake. So, back to the six pack. I've had a couple of backfires and fuel spits when starting so I've taken them off (again!) and checked everything. Power valve is a 7.5 and holds vacuum. My usual idle vacuum is 16 so that should be OK. Centre carb has the correct gasket for the metering block. Re-checked float levels - all just up to the sight hole. Throttle blades all moving freely. To see if there is a fuel leak I put newspaper under the carbs and ran the electric pump - no leaks. Replaced the fuel lines with AN ptfe fittings and braided hose for ease of carb removal and reduced leaks. Anybody know what the inch by half inch plate on the centre carb body is? - and which way up the thing inside goes - it fits either way - I suspect the little round button goes down towards the hole but want to be sure - googled but can't find any reference to it. Not run it yet - out of time. Before doing all this I noticed that the idle increases significantly when the crankcase breather hose is removed. Does this mean it's over fueling? - my wife says the exhaust smells of petrol but it seems OK to me - perhaps I'm immune to it!
 

V12T

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Thanks Challenger RTA. Very useful info. I may have missed it but can't see any ref. to the little plate that is on my centre carb which is a 4144 -see pic from the Holley website. Can't see it in any of the illustrations. No idea what it's called or what it does. The little mechanism under the plate will fit either way up so want to be sure I've got it right.

Centre carb 4144.jpg
 

V12T

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Carbs back on and no difference. Shakes at idle - sounds great when I punch the throttle. When running (on choke) no fuel is being released above the throttle blades on all carbs at idle. When I stop the engine, disconnect the rods and look down into the manifold it is WET with gas. Confirmed again by running pump and looking down through carbs - no fuel leaks. Where is all that gas coming from ???????
 

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Just in general had a problem with wife's car. It would do the same as what you described.
The check engine light is always on Right, that a common thing. Read codes same old stuff o2 bla bla. A week goes by and it gets worse said it was now blinking. OK how much. She said just when it starts and off and on. Ok I'll get to it. An other week goes by. Now she said it's flashing all the time. So Read codes. change spark plugs and wires. seem good for a day. Codes still the same.
So I dig deeper into it and go over it. changed coil, air filter, mass air flow sensor and fuel filter. No difference.
Getting to the point! There is only one thing left other than tearing the motor down. Electrical. I cleaned every ground in the engine compartment including the battery terminals. It now runs perfect!
Now I know something is wrong even the engine light is out!
 
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