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I have a 73 454 4 speed that had some mods done before I bought it. One was the camshaft. I am finding the car jumpy when driving slowly at lower RPMs, thinking about going back to original cam but I have no idea how to go about the specs or getting that cam. Hoping this will make the car a little more pleasant to drive at slower speeds.
Any advice?
I have a 73 454 4 speed that had some mods done before I bought it. One was the camshaft. I am finding the car jumpy when driving slowly at lower RPMs, thinking about going back to original cam but I have no idea how to go about the specs or getting that cam. Hoping this will make the car a little more pleasant to drive at slower speeds.
Any advice?
Swapping the camshaft is a great idea, carbureted big blocks don’t like too much cam at low rpm.
Before you spend the money for a new cam, have you verified the timing is set correctly and vacuum advance is working?
If you haven’t already, contact Lar’s and get a copy of his timing papers, double check everything then buy that new cam suggest above.
Thanks, no I have not done that.
I found some of the rebuild documentation and it mentions replacing the roller cam as well as roller rockers. Probably better to just leave it be.
Tell us some more about the build of the engine and if you have the current cam specs post them as well.
Bucking isn’t good.
It can be as simple as a timing issue, poor camshaft choice or plain and simple cruising in too high of a gear.
Do you know if the vacuum advance is hooked up and if so is the vacuum line hooked up to the correct vacuum port?
This is a common mistake and can drive you nuts if you don’t know.
Post some engine pictures.
Thanks, no I have not done that.
I found some of the rebuild documentation and it mentions replacing the roller cam as well as roller rockers. Probably better to just leave it be.
Did you mean install a roller cam ? LS4 is not a roller cam.
Did you mean install a roller cam ? LS4 is not a roller cam.
It sounds like he bought the car with a rebuilt engine and a roller cam was installed.
OP, some people like to install huge cams known as thumper cams so the engine has an extreme lope at idle.
These cams are horrible for the average street car and can make the car horrible to drive.
Yes you are correct. That's how I bought it, I didn't realize everything that had done with it. Very little information on the build sheet that I have and certainly no cam specs.
I will most likely just leave it for now, I don't want to get into replacing a lot of parts right now.
You could be experiencing something nicknamed "trailer-hitching." This is caused by too much vacuum advance at low RPM's....I think this Is what OldCarBum was referring to when he was talking about timing.
Try disconnecting the vacuum line (plug it) and take your car for a spin to see if there's any difference. If so you may get away with a $15 vacuum limiting piece that fits in the distributor.
Here is what I could find for specs on the installed cam.
I put the EN-150 cam specs from ZIP Corvette on the left and the specs of the cam installed on my engine on the right for easy comparison.
I did have one mechanic adjust the timing to try and tame the bucking so I assume he would have checked the vacuum as well but I should never assume...
I did have one mechanic adjust the timing to try and tame the bucking so I assume he would have checked the vacuum as well but I should never assume...
If the mechanic isn't familiar with modified engines he would have used the stock timing for 1974 which is not advanced enough.
If those current roller cam specs are genuine, I opine they are not so big (especially for BBC) and I would leave that cam in motor.
As has been suggested, I agree OP should look at performance tuning; both ignition and carburetion. Also agree GM ign timing specs are wrong choice with current cam. And, as for assuming anything about what a mechanic may've done or not; don't assume. Instead, verify.
It is very hard to tell what size the cam is from those specs.
What I would do is measure the intake manifold vacuum level at idle, with a vacuum gauge. Say 10" of Hg, or 15", etc.
That will tell us a lot about your camshaft and we can proceed with tuning advice from there.
Both ignition timing settings and carburetor settings will vary by camshaft idle vacuum levels.
A couple of adjustments should get rid of any "bucking" issues.
LS4 camshaft
