Motor breaking up...making me nuts!

Too much carb is the downfall of many a nice running engine not going to its full potential. Most of the time you will not need as big a carb as you think. Take for instance the 480 CFM 2BBLs on ford 429s and 460s. They ran like hell and had lots of torque. The modern HO 302s and 351s had a vacuum secondary 600 CFM carb. You need to look at the overlap and the valve events on your cam. Is it a modern grind cam or an old grind? The leaps and bounds technology in cam grinds has made it easier to get more HP out of an engine with less fuel and lower octane.

It may be valve float as well. What is the lift and duration of the cam and what is the overlap? Also look at the center line cut of your cam. All this has to do with the max your engine will produce.

The symptoms you are describing could be a number of things. Ignition, fuel, valve float, valve springs and the list goes on.

One thing though to remember, ONLY CHANGE ONE THING AT A TIME!!!!!!!!!!. I cannot stress this enough. You will track it down in time but do it in a structured manner.

Good luck.

 

Thanks man. Right on, on the on thing at a time.

 

http://www.4secondsflat.com/Carb_CFM_Calculator.html
play with that calculator.
the z303 cam is advertised to have a power band up to 6500 rpm on a 302. so on a 347 that power band will be even lower. a 650 would be really close to the right size.

 

Do you have a full 12 volts going to the coil ? My buddies 351w was skipping at higher RPMs and he wasnt getting the full 12v,fixed it now it runs like the scalded dog it is.

 

I was shocked at how much better mine ran with the 6AL, the only other times I had upper rpm cutout was wires once, low fuel pressure once and over carbureted once but this was with 2-450cfm holleys on a 302 with an F cam and gt-40 heads, very mild, it sounds like you have the wires already I think the Duraspark box is your weakest link, but I would sure take a look at fuel pressure it shouldnt ever fall below 5 1/2-6 psi, By the way my little mild motors fastest pass was with a 750dp,and I've tried many.

 

Thats kind of a cool calculator too bad they dont take into account for air density at higher elevations.

Here is a site that explains air density, its primarily based around snowmobiles and 2 strokes but the equations are all applicable

http://www.2-stroke-porting.com/Air_Density.html

And yes I know Supelead76 is in Tampa Fl(48 ft above sea level) so this data is not applicable to him and the calculator will suit him very well. But to us up here in the hills (4,436 ft above sea level where i live, or somewhere like Amarillo TX which is 3,605 ft above sea level) there are definitely a little different carburetor requirements.

 

I'm gonna buy a $30.00 ignition box from Napa and see if there is any improvement. What the hell right....Haaaaaaa....

 

Update: It was the DuraSpark! I got a replacement and it pulls all the way to 6500. I see an MSD 6al in my future! Thanks for all the input guys!

 

Good deal, glad you found it so fast.

 

Thanks!

 

I decided several years ago that they just dont make em like they use to, I could find oem boxes at the junk yard that would run better than new ones, I just got tired of hunting good ones so I bought an MSD box, your car should be pretty nasty on the street, I'll bet it pulls great.

 

Ah SH*^ the HVAC guy nailed it.

Guess I should get rid of my Duraspark??

 

Yeah...I hear ya. It' pretty gnarly!!! Haaaaaaaa!!! I need some cal-tracs now because it's rotating bad! It lays the rear quarter over on the drag radial. The battery is back there too.

 

I had same issue in my truck. It was the distributor. I could ease my way up sometimes but if I got on it it sounded like it wanted to kill itself. Either distributor or the coil bad.