Engine shake/vibration 1400-1800 rpm

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ken5124

ken5124

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Winnipeg, Manitoba
My 318 seems to be misfiring. It is a professionally built motor, mild cam (supposed to get me 300 hp), mopar alum intake, edelbrock avs carb, fbo ignition system, running hooker long tube headers, 2 1/2 inch exhaust. New schumacher motor and trans mounts and a torque strap. This motor has 4200 km on it, and has done this vibration( i think it's misfiring ) since it was new.. I have replaced rear axles, driveshaft twice, torque convertor( it's the right one for the motor and my 904 transmission), and harmonic balancer already. The entire car has been re wired.
It does this whether driving or sitting in the garage. If I rev the motor up while looking at it, it starts to physically shake at about 1400 rpm and is most pronounced at 1800. While driving 1800 rpm's is where I really feel it, whether light accelerating, cruising or coasting.Under hard acceleration I can't seem to feel it.
It is driving me nuts. I have checked and replaced plugs, wires, double and triple checked timing. It doesn't seem to matter if I hook up the vacuum advance or not. I have unhooked portions of the exhaust to see if somehow that is causing the problem.

My gut feeling is that despite being a tested distributor, perhaps it is just not right..

Help!! or if you live in Manitoba.. come on over and see and feel this shaking for yourself!! Any ideas???
 
My .02 - Sounds like an issue with the a weak cylinder or bad motor mounts, or perhaps the exhaust system is touching something at that rpm and vibrating.

I'd do a cylinder balance test, some older tach dwell meters had this as a standard function, you can stop one cylinder firing at a time to see if one is particularly weak or not contributing as much power as the other cylinders, while the engine is running. If you dont have access to one of these you can do the same thing by pulling one plug at a time to see if one of the cylinders is not as strong.

Good luck with it.
 
Check the fan closely !
I had the same problem and was leaned over the fender reving the engine and scratching my head. When I found a fan blade loose at the rivets and about to fly out, I felt blessed.
 
What flex plate did you use? Stock? Let me get to the point:

Torque converter, flex plate and harmonic balancer must all have neutral balance as the 318 is internally balanced. At least it was from the factory. Contact your engine builder in regards to this.
 
Thanks for the help, harmonic balancer, flex plate and torque convertor are all correct for the motor.. I ll double check the fan blades,, i don't believe they are loose.. ( flex Fan). I also checked the rotor phasing last night and it lines up when firing...
So, I'm leaning towards plugs??
 
I have not done that.. i presumed that compression should be good ( fresh motor). I did have it in a shop last summer for them to put it on a scope and they found no problems with the motor??
 
If a LOT of work was done on the inside of the motor, it may very well not be in balance any more. I ran into the exact same problem with my 340....some Bozo stuck an externally balanced Harmonic on it (had a bolted on counterweight....an allen bolt)..........we removed the counterweight, and the motor was nearly balanced at that point.....just a little annoying vibration left.....then we discovered they had done the same thing with the torque converter.......it also had a counterweight, and was causing the rest of the vibration.

These parts were ordered for the motor in question.......but they were WRONG.

Do NOT trust what someone else has told you about the parts......get UNDER the car, and visibly check the back side of the harmonic balancer to see if it has a counterweight..........do the same with the torque converter if necessary.

The vibration you speak of was a COMMON problem when aftermarket parts came on the scene.......great care must be taken to find if your motor is indeed balanced externally, or internally.

I realize it sounds too simple, BUT you could save a LOT of time and cash by checking this out for yourself.........also, find the date-code on the block and verify if it's internally balanced or not.

Here's to GOOD Vibrations! :drinkers::shaking2:
 
If the pistons and/or rods or both were replaced with aftermarket parts weighing allot less that will throw off the balance. Do you know if they were replaced and if so what with?
 
I spoke to my engine builder yesterday,, he suggested i look at my carb at the same rpm and just as he suggested.. there is fuel dripping out of the boost venturi.. more on the right side than the left.. He says he has encountered this problem on the dyno with float levels set too high and told me to change the float height and it should correct the problem .. I will be doing that tonight and keep you posted.. Might actually be an easy fix !!
 
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