My 300 was just rebuilt about 5,000 miles with a comp 268 cam, brand new 4 barrel, and heddman headers. I dont think it has been running right at all. It doesnt idle like any other 300 with the 268 cam. It doesnt have the choppy idle like it should it just runs rough like it wants to die without any rythm. When i drive it it always smells like gas really bad and the tail pipes are always black. It has power but no where what i think it should be with all the modifications done to it. Can anybody identify the problem to me?

Thanks, John

 

manifold leak
mal-adjusted carb
blown out power valve
sticky choke
leaky needle / seat
defective float
excessive fuel pressure
bad gas
wiped cam lobe

 

Yikes! Lets hope it isnt that last one. Is there a way I could test for it?

 

I had a wiped cam lobe after installed a Comp 260. There was a very very definite and odd "knocking" sound that would increase in volume the harder I pressed on the gas. It sounded like someone under the hood with two wooden dowels and the harder I pressed the gas, the harder they'd smack the dowels together. So that's a definite symptom.


However, easiest way to tell is to pop the valve cover and rotate the engine and watch the rocker arms. If one doesn't move like the others, it's got an issue.



But start with FTF's list, and go from the easiest to check first.

Your symptoms sound a lot like when I blew a powervalve. Have you put a vacuum gauge on the engine?

 

I have put a vacuum gauge on it and it idles at about 16-17. Its a brand new carb but I geuss anything can happen.

 

The carb is brand new? Or it's newly rebuilt?
A new carb has blowout protection, so you can "probably" rule that out.

16 - 17 isn't that great for a newly rebuilt engine. It should be around 20.
I'm assuming you've checked the simple things like timing?

Another idea is that the cam marks weren't lined up right when the cam was put in.



Otherwise, yeah, comb the engine over for the simple things.