68 340 Vibrating idle, loss of power

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I put an auto trans. on a 4 spd. motor. Took a year to fiqure out my vibration. Dowel pins on motor were incorrect. Sounds like you have different issues but had to mention it.
Good luck. Do you know how to use a piston stop to find TDC?
 
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possibly weak or broken valve spring? put a drop of oil on each header tube with it running. if it smokes of quickly, good cylinder. if it just runs down the tube without smoke, that cylinder has the problem.
 
68FishNate said:
4 speed. Its possible that the dart the motor came out of before i bought it was an automatic. Don't have that answer.
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Did you install a pilot bushing or bearing, or at least confirm that it already had one?
You didn't say, but I assume you used a new timing chain when you reassembled?
 

68FishNate said:
Could be I'm 180 out and cant time the thing properly, to much to little fuel. I'm just spit balling possibilities and trying to cross something off the list of what it could be
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No. If it's running, it's not 180 out. You need to stop guessing and diagnose it.
 
Three things you stated . Back fire through the exhaust, No power, and Vibrating. All of these symptoms can be caused by two plug wires crossed. That would be the first thing I would check.

Here is something I ran into with a 408 I bought that was in the brown Duster. It was never run in the car. Being a fresh 360 stroker and having the 360 externally balanced damper on it and the externally balanced torque converter, I assumed the engine was externally balanced.

I sold it to member fritztod and we were changing his car to 4 speed when installing it. Being it had the 360 torque converter on it we put a 360 flywheel on the engine and ran it on the run stand. The engine vibrated bad. What Steve found was the engine was internally balanced with the externally balanced 360 Damper when built at the machine shop that did the work. We installed a neutral balance flywheel and left the 360 damper on and the engine runs smooth.

So never assume the engine is not neutral balanced if it was built at a shop . They may have neutral balanced it with either damper ? You will have to try another damper. and if that don't work try another flywheel. One at a time or both if needed.

Hope its the plug wires crossed . That's the easy fix.
 
Stop over thinking and just do. Just gotta do one thing at a time over here. I did put it out there a certain way, but im not brain dead and I'll own my own stuff because other than the machine work which didn't need much thanks to the seller being truthful, thank you if hes on here, it was best to get checked, cleaned, and turn the short block with new fresh material. The rest took time, knowledge, patience, my money, time and the willingness to learn how to do it if i didn't know how. Example roller rockers and setting lash, measuring for pushrod length for that matter.

I did call the machine shop, picked brains, where to start, as I need a solution to fix all but don't know where to begin. Quit thinking. Start here

Tonight I'm gonna start it up, which it will, because I'm not 180*out. I appreciate that. And judging by the more shaking and rattling underneath which side, im gonna pull a plug until I pull a plug and nothing changes starting on the drivers side. Then I'll know which cylinder has an issue and pull the valve cover to investigate further the damage and where its located.
 
i'd pull the covers off first. the issue might be staring you right in the face. also, the motor isn't all hot from running it.

then i'd run back and check each boot and wire and make sure something didn't jump off or is getting jammed up on a cross fire.
(actually i'd probably do this first)

then i'd run it and kill each cyl systematically to determine if i have an isolated event.
 
68FishNate said:
I couldn't get throttle at that time as the screw on the linkage was sticking on that carb and not opening. I set the valve lash up for the cam spec and that moment I didn't have anymore ticks. Now I'm hearing everything, along with vibrating and shaking and definitely feel the loss of power
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Probably flattened the cam or spear'd the push rod through the rocker.

If the Machine Shop used the cast crank, they either misguided you on the dampener/balancer ...or wemt neutral balance n put metal into that crank ...or the possibility they used a lighter piston and it could very well be a cast crank from a 318 1973 and up..that had the heavier 360/340 rods to begin with.

Read that as many times as you need to.
 
Thank you. That was the opinion and advice to start with from the machine shop for severity purposes as Chrysler has an issue of pushrod through rockers, broken spring, or as far down as flattening cam lob. I'll take a look to get started hopefully tonight.
It's a steel crank, and internally balanced and had a brain dead moment.
 
If this is a manual transmission with a diaphragm clutch and you had the clutch in when it backfired............it is likely you damaged the pressure plate to clutch cover connectors. If not a manual trans, please ignore this post!
 
kenelder said:
If this is a manual transmission with a diaphragm clutch and you had the clutch in when it backfired............it is likely you damaged the pressure plate to clutch cover connectors. If not a manual trans, please ignore this post!
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How would you find that out? Push rod didn't go through the rocker. No visual sign of valve spring. Before I remove rocker shaft to see if I bent a push rod
 
Did it backfire while you had the clutch in? Was it a diaphragm clutch? If yes to both, remove bottom stamped dust cover plate from clutch housing. In neutral turn engine by had while looking at the back of the pressure plate. You’ll see the drag bars that attach the pressure plate to the clutch cover as shown in the pics. The bars should be straight and tight. If damaged they will be spread apart.

If you find them spread apart, like the picture Img_3120.jpg, you’ll need a new pressure plate and clutch cover.

Note this only applies to diaphragm clutches, the factory 3 finger clutches don’t have this problem. Also the bent drag bars make the pressure plate off center so it will vibrate like an out of balance engine. Or like an engine with a bad valve, etc.
IMG_3120.jpeg


IMG_3119.jpeg
 
kenelder said:
Did it backfire while you had the clutch in? Was it a diaphragm clutch? If yes to both, remove bottom stamped dust cover plate from clutch housing. In neutral turn engine by had while looking at the back of the pressure plate. You’ll see the drag bars that attach the pressure plate to the clutch cover as shown in the pics. The bars should be straight and tight. If damaged they will be spread apart.

If you find them spread apart, like the picture Img_3120.jpg, you’ll need a new pressure plate and clutch cover.

Note this only applies to diaphragm clutches, the factory 3 finger clutches don’t have this problem. Also the bent drag bars make the pressure plate off center so it will vibrate like an out of balance engine. Or like an engine with a bad valve, etc.View attachment 1716460685

View attachment 1716460684
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I dont remember exactly when it backfired at that time. I do remember that everything changed sound and performance wise after getting back on the highway, and with the clutch, it makes sense. Practically nursing it back 2 miles up the highway to my exit and thinking i was just completely out of time with exhaust practically blowing out under my door. I'm gonna remove the dust cover and hand turn over the motor and check this thing out. Not in favor of dropping the transmission from under the car, but if I find that issue with the plates, I hope that summit will replace the clutch kit. It is what it is at that point and I'll definitely breathe a sigh of relief on the motor.
 
I had a big block that I put plug wires on the way I did a small block. Shook and backfired. Small block and big block distributors rotate in opposite directions (clockwise vs counter clockwise). They will start and run but shake and backfire if rotation is opposite to what it should be. I also have the harmonic balancer listed that you need for cast crank 340. They sat "cast crank 340 right on them.
 
Thank you for that information as well as the balancer in case. However, it is an internally balanced motor. The car had been running and driving and once I had fixed the throttle linkage problem I had because I went from a single plane short intake to an rpm air gap intake which was mounted higher, I had just screwed the linkage onto the carburetor with the screw catching on the carburetor linkage. The night I figured that out, I wanted to drive it and find out how it was going to feel with throttle pedal capability. Not timed exactly where it needed to be, the rest of it is what it is and here I am. Im gonna check out the clutch plate tomorrow and figure out the plan. I hope
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If it is a manual transmission, flywheels are different for external and internal balanced engines as well. With automatic trans the torque converter has a weight on it for external balanced engine and no weights for internal balanced engine fyi.
 
68FishNate said:
How would you find that out? Push rod didn't go through the rocker. No visual sign of valve spring. Before I remove rocker shaft to see if I bent a push rod
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lean of the valve springs. if one is broken, there will be give to it. warning! do this when the engine is cold.:)
 
This past weekend after long last just trying to find a solution for what the issue of no power and shaking violently, I've been able and grateful this group exists. Grateful I can pick up the phone and ask for help from others and the machine shop. No matter the ideas I kept putting in our out there, gotta start basics and cross things off the list. Have to walk away scratching my head sometimes, sleeping on it, etc before tearing everything apart or throwing the hands up.

I checked the rockers, intact. I checked underneath pulling the bell housing cover off and turning and everything in tact. I picked up a compression tester from oreillys and said my number 7 cylinder was reading 0. Verified it by checking #1 which read 135. It wasn't running when I did this check. So, I called the machine shop stating about was about to pull the head off and bring it to him. He advised that possibly the valve were not closing/opening as the lash could be to tight. Friday night, I checked all the lash and had finally noticed my number 7 pull wire was arching through the boot and so I replaced 5 & 7 plug wires as they both sit pretty close to the header. Waited to Saturday as trying to be a good neighbor at 1:30 in the morning and probably more for my sanity. Fired it up and everything is normal again. Spent Saturday evening and Sunday ripping up and down 35 and feeling way to good about the car running and driving as best it has been. Just wanted to stop by and say thank you everyone.

20251018_195948_02.jpg
 
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