My 1978 F-100, 302, started so easily and ran so well. But I could not leave well enough alone, and since it had been many years on the same spark plugs I decided to replace them. I chose Bosch Platinum Preset Gap Plugs. I also replaced the fuel filter. Now I have to pump, pump, pump, crank, crank, crank to get it started - and then it runs rough. I replaced the fuel filter again - but still no change.

What do I need to do? Is it the plugs? Please give me some advice. Thanks!
CB

 

If it was me doing it, I would wonder if I switched the plug wires.

After you check that, if it wasnt it, put a timing light on each wire to make sure it is sparking on each cylinder.

If you are wondering about the fuel filter, put a fuel hose on it and make sure fuel comes out into a gas can while cranking


Good luck

 

I jumped in my pickup the other day and crank, crank, crank, and no fire. hmmm
My first inclination was accelerator pump not working. Couldn't buy a holley pink umbrella valve locally, so ordered a two-pack from eBay vendor. Upon arrival and install, I had to remove the float bowl of course. Thank heavens for the newer blue re-usable holley bowl gaskets now, and made the replacement of the pink umbrella valve (inlet check valve) a breeze. No help on start-up.
Ok, must be sucking air on outlet check valve on accelarator pump cirsuit. This is a real PIA requiring carb removal, so as to not drop the squirter nozzle down into the throat of the carb. Holley 4bbl. Underneath the sqirter is a check needle. Only thing i could figure is that metal needle check valve was sucking air on inlet charge of the accel pump. I carefully removed the squirter and check-needle and blew out the passage with brake cleaner. Upon re-assembly, the engine fires up after two pumps on throttle.

Make sure you don't have two wires crossed up which of course would create a rough idle.
You could try re-installing your old plugs after a cleaning with acetone and fine wire brush, and see if any improvement to single out the new plugs.

Another thought: you may have pulled a wire from the plug clip under the spark plug boot causing a miss. ?

Kinda hard to grasp these symptoms after your engine rans fine one day and the following day, a "no-go" !
I'm supposing the engine was fine the day prior and was running normal.

 

Difficult to tell as each situation is different. I can tell you that my 77 only likes the correct Motorcraft copper plugs. I decided to try Motorcraft platinum replacements some years ago and those caused problems.

 

Thanks for these suggestions. I threw the old plus away. So I think I will try some plain ol' Motorcraft copper plugs.
CB

 

I'd be looking into the wires themselves- either crossed up a couple of wires during the plug replacement effort (easy enough to validate the correct firing order) or damaged a wire at the plug boot whilst removing the wire off of the plug.
Replace the wires with a quality set and check and validate your firing order.
Tbruz

 

WE HAVE A WINNER!
You only made 2 changes and you went and changed one of them again and still had the issue what not put the normal copper plugs back in the motor?
I have seen it posted of people using the new $$$ plugs only to have it rum poorly.
Something about the old IGN systems just dont like the new plugs for some reason.
Then again other have run the new $$$ plug with no issues but me old school plug on my old school motor with old school IGN system.

Save the money and use copper plugs and I bet it will work as it did before.
Dave ----

 

Thanks again. Since I was a Do0Do and threw the old plugs away, my next step is to order some Motorcraft copper plugs and replace these expensive Bosch Platinums. I only pulled 1 plug and 1 wire at a time so I don't think I got anything crossed. I will check the spark plug wires caps to ensure they are tight. After the new plugs arrive and are installed I will post again as to the outcome.
CB

 

Maybe also check the hoses running to the fuel filter. Not sure if you replaced the filter on the mechanical pump or not but if it is inline it is entirely possible one of the lines cracked or is hard and not sealing well and its sucking in air.