Misfiring...progressive loss of power again
 

After about 500 miles of use the state of tune of my 302 powered Aurora slabside falls off in a steep decline to the point were it runs and idles so rough that it's hard to keep her alive.

I've had the car for about a year and the first time I replaced the carb, plugs,
wires,cap and rotor, coil in addition to an entire new fuel system from the tank foward and all was well.

Then the slow progressive decline began again. A new cap and rotor from MSD with brass contacts the last time and all was fine again.

Yesterday after an extended ride and about three weeks of trouble free running I thought I felt a drop off of power but thought that maybe it was just me but today when I fired her up it was more of the same...low rough idle and poor performance. Took it around the block and at a steady constant amount of throttle the power would come on and off almost like a switch being turned on and off.

Fuel pressure is good, plugs were good, no smoke of any color or any increase in exhaust fumes. I can't believe that I'm having so much trouble with stone simple equipment. Any thoughts as to why just replacing a cap and rotor makes a huge change and the progressive loss of tune afterwards.

Thanks,

Dale

PS I did have to replace the almost new Holley electric fuel pump which burnt out after about 1000 miles of use which seems to be unrelated to the loss of tune.

Electrical, you replaced the fuel pump and now it still demonstrates the same tendancies a thousand (plus or minus) miles later. You said it yourself, like someone turning a switch on and off. Thats the same as a bad electrical contact, sometimes you have current and sometimes you don't.
Nothing like a intermitent electrical problem. Distributor drive gear?(slow and progressive due to wearing down) Loose ground? Belt too tight on the alternator(burned bearings). To me it sounds like your distributor drive gear is wearing out/down, what kind of cam are you running? A lot of places to look, take your time, one step at a time.

Mark

Mark,

I purchased the car from the widow of a Nascar engine builder, Eddie Graves and only have some receipts for parts purchased for the build. He purchased the block from Raceparts Distribution Inc. and one of the labor items was for oversize cam bearing bore and then there is a line item that states-
Bearing,cam,5 ID,2.215"OD. Comp Cams supplied the cam and as stated on the receipt specs are for cam installed at 106 intake centerline:

Duration at .050 intake 245 exhaust 252
Lobe lift .3610 .3600

I should have stated that this was not the stock motor in the car and makes brutal power and the cam is quite radical. The distributor appears to be a stock Ford unit. As per the receipts the engine was built in 03 and used for one season.

If the distributor gear is wearing down would the cam have to be changed?

Dale

Probably not a cam change unless you want to alter the nature/behavior of the engines performance via camshaft performance/type. You may need ot take a look at the distributor drive gear and how it relates to the type of cam you are running...solid lifter....hydraulic lifter...solid roller lifter...etc. etc. etc. A bronze drive gear may be in order.

Sounds like ignition to me. Heres the clue: Replaced cap, rotor, plugs etc and all was fine for awhile. With BRAND new components AND a 'weak spark' there would 'just enough' spark to make things work OK. But as soon as the new components start to get worn in, the spark plug threshold voltage drops to low and once again causes a 'weak spark' condition.

I'd check the coil and the ignition 'box' that fires the coil. As well as primary voltage to the coil\box.

Again, overall your symptoms sound more electrical than fuel related, with spark being a prime suspect. Generally speaking the FIRST question to ask and needs to be answered is:
Is the problem FUEL or SPARK? A weak spark will leave the plugs fouled, maybe wet, leading one to the WRONG conclusion of a 'rich mixture', for instance. Diagnostics, as much 'art' as it is 'science'.

I had an intermittant ignition failure after I put in an MSD 6AL. Replaced the unit, and a week or two later, same problem.

Turned out to be an intermittant connection from my alternator (single wire regulator) to the battery.
I put an 8 guage wire in between the alternator and the battery disconnect, and never another problem.

Whether it was lower voltage (it never really got below 12 volts, and they claim it will run on 5 volts) or more likely the intermittant output from the alternator I don't know, but the new wire solved the issue.

Just a thought.

Doesn't hurt to check all the grounds for the ignition either

You said that after you replaced the cap and rotor it was fine the last time this happened.....do you think it may be a good idea to try this again?

The MSD caps are notorious for wearing out the center carbon contact......using one of the high tower MSD coils seems to help this.

I picked up a new MSD cap and rotor but didn't have time to install and try.

Interesting, I have an MSD 6AL that came installed on the car and I replaced the alternator with a single wire unit as part of my sweeping upgrades and I noticed that when the car started to run rough I am only reading 12 volts.

Thanks for all the imput.

Dale