Engine shakes BADLY!!

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340Duster247

340Duster247

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My newly build 340 has a nasty shake/rough idle at idle.

Details of rebuild:
Balanced .40 over 340
Comp muthr thumpr camshaft
Eddy heads
Air gap
Holley 750 hp dp

All new ignition including wires and msd billet distributor

I have the timing set at 19 initial. + the lowest 18 advance in the new msd distributor

Now I've checked for vacuum leaks and can't find any. I checked the motor mounts and all good.
Firing order is correct.
I'm only pulling 5" of vacuum in park at around 850 rpm

Any ideas on why this engine runs so rough or is this just because of the cam.
 
The Thumpr I had in my 318 shook pretty bad. Took it out ad went with another cam(only change) and everything settled down.
 
340Duster247 said:
Any ideas on why this engine runs so rough or is this just because of the cam.
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First find out if the darn thing is actually running on all 16

You hear guys talk about "pulling plug wires."

Don't do that for a couple of reasons.

1....if you have electronic ignition, it can damage the ECU and even the coil

2....Pulling plug wires creates a sort of miniature lightning strike, looking for someplace to go. It can crossfire to other cylinders and give you FALSE conclusions

So instead..........short out each cylinder. Either pull the dist boots up and poke a small grounded probe down beside the wire, or else pull each wire with insulated pliers and simultaneously insert a grounded probe in it's place.

You should easily be able to tell if you have a dead cylinder(s)

Other factors are carb and idle adjustment, initial timing and intake manifold design.
 
does the engine smooth out when the rpm is increased?

If not is the balancer and torque converter right for the crankshaft?



As 340 came with both forged and cast cranks , forged cranks internally balanced , cast cranks externally balanced.

If you have them mismatched they will shake really bad
 
GEO said:
does the engine smooth out when the rpm is increased?

If not is the balancer and torque converter right for the crankshaft?



As 340 came with both forged and cast cranks , forged cranks internally balanced , cast cranks externally balanced.

If you have them mismatched they will shake really bad
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Hey thanks for the reply.
My engine builder(machine shop) did all the internal balancing. I know we kept the stock crank but he made sure everything was balanced correctly together.

As far as the convertor I have yet to put my new 3500 stall in. My current stall is around 2300 foot brake stall (supposedly 2800 from previous owner) and may be a lil shy of stall for this cam.
 
Is it just running rough, or is something out of balance. Does it get worse if you give it a bit of throttle?
 
Make sure you have the correct balancer on it.Some engines are internally balanced and some are externally balanced
 
If it's one of the two I'm thinking of, the LSA is 107* and the Lobe centerline is 102* meaning the install should be 5* advanced. Was that cam degreed in properly? That 107 is there to make some chop, and if it's not dialed in right, you'll think your driving a Pro Stocker...lol.
 
The whole cam is designed to chop. Period. You're also never going to have a good vacuum signal. I think you're doing good to have what you do.
 
OldmanRick said:
If it's one of the two I'm thinking of, the LSA is 107* and the Lobe centerline is 102* meaning the install should be 5* advanced. Was that cam degreed in properly? That 107 is there to make some chop, and if it's not dialed in right, you'll think your driving a Pro Stocker...lol.
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The shop that built my engine is one of the best where I'm from. Nothing but good stuff said about them and was referred over and over again , so I'm sure they installed it correctly. And they build some big powerful badass engines.

But even if they did install it incorrect. I've personally never installed a cam and would not known how to tell if it was installed right or wrong.
 
If it's rough at idle and smooths out with rpm, it's cam and carb. Lumpy cams need more are in the idle circuit. There is only so much you can do and if the cam isn't properly degreed, it only aggravates the problem. I've had an out of balance problem before. It tends to get worse with rpm. Your initial timing is higher than I like but if it starts fine I bet it's responsive.
 
I'll go with the cam is installed properly. I wont doubt their work as you know them way better then I do.

Now my question is, when you're idling in gear, does the engine seem to be loading up like it's rich??? That idle vacuum you're pulling with a stock spec carb could be dumping fuel with only 5". Even the HP carbs come stock with a 6.5" Power valve at the lowest if I recall.
 
I did have problems getting the carb dialed in before. It's gotten much better. Used to run very rich. Now it's pretty good. I have a 2.5. Power valve. Idle mixture screws are 1 turn out. I checked a plug today and it wasn't wet fouled like before but now dry fouled ( black carbon suit). I have a feeling my plugs might be a little to cold temp.

On another note been having trouble starting the engine once warm. I really have to pump the gas and hold the pedal as the engine spins, and spins till she flashes.

This set up has way better ignition then my last duster and my old duster would flash instantly. Not sure if I'm just having carburetor problems.
 
Large cams or cams that act large, like yours, can cause some plug issues. There's a lot going on in the chamber at a low idle. I can't recommend a particular plug, but a step or two may help with the low end idle and performance.

The other issue regarding the converter.......Is that your current stall or from the previous combo? I can only think that the new replacement that has yet to be installed will make some difference in the character of how it idles.
 
OldmanRick said:
Large cams or cams that act large, like yours, can cause some plug issues. There's a lot going on in the chamber at a low idle. I can't recommend a particular plug, but a step or two may help with the low end idle and performance.

The other issue regarding the converter.......Is that your current stall or from the previous combo? I can only think that the new replacement that has yet to be installed will make some difference in the character of how it idles.
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A step or two hotter correct? My current plugs are the ngk race 7 heat range. Don't know the exact part number right now.
Yes my current stall is foot breaking at 2300. I have a 3500 stall that's yet to be installed. I'm sure it will make a difference

Thanks a lot for the great replys
 
Yea, I was thinking a step or two warmer on plugs.

Just to go one step out of the norm, since your mounts are good and not solid/metal, did you have the K frame out during the engine work? Could there be something loose or even a header or exhaust banging on something causing the shake/vibration feeling?

I'll step out and see how things progress. If I have any other thoughts, i'll chime in. Good luck, sounds like you have really good potential with the combo you put together. :thumleft:
 
Thanks old man. No never touch k frame. I'll keep everyone posted. Can't wait to see what this lil engine can do down the track when finished and dialed in.
 
I think I try getting the initial timing back down to 12 degrees or so and work from there. And set the total timing to around 32 degrees
 
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