Here is my problem....I have a 73' F100 w/ a 302. About a week an half ago my truck started to slightly backfire out pipes and through the carb. But since then it has gotten worse. As I apply the gas and I get to about 2000 RPM I can feel the pedal vibrate and I hear and feel the motor stutter and RPM's jump all over the place. When truck is in park it runs nice and smooth and when light pressure is applied fine but 0once i hit the spot its runs like crap.
I have since tried the following to fix the problem but with no success.
1st. I changed out all vacuum lines.
2nd. I ran a vacuum test. got a reading of a steady 20hg.
3rd. I ran a compression test. lowest cylinder 123 psi highest cylinder 135 psi.
4th. I changed out plugs. ( Bosch platinum .035 gap ) all old plugs looked really good.
5th. I reset the timeing to 6 degrees BTC. Idel to 650RPM. ( Points adjusted to .017)
6th. I changed out the Fuel Filter. ( looked ok )
7th. I cleaned out the fuel bowls and piston rod housings. ( the fuel in bowls was dirty )
Any help would be great. Going to take a sample of fuel from tank when I get home tomorrow night.
Thanks Ya'll
Tailgater_73F100
73 F100 Long Bed
Edelbrock 600cfm #1406
Edelbrock Intake #2121
Dynomax Long Tube Headers
Dynomax Super Turbo Mufflers
K&N Filtercharger 14"x3" Air Filter
Accel 8mm Plug wires
Accel Super Stock Coil + Cap and rotor
Friday New Mallory unilite breakerless Electronic Ignition convertion kit

 

hi
there are three more things that i can think of to cause that.
1) bushings for the throttle valve on the carb
2) choke miss adjusted
3) possibly the vacume advance or mech. advance not working right

maybe it's not any of these but you could sure try them and see.
just trying to help!

 

Timing chain? Sounds like it jumped time.

 

I concur about possible timing chain & gears.
You can check for that manually.

Also,the vacuum advance's are known to seize also.

You never mentioned a distributor check.
Possible Worn shaft?
Have that checked


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How many miles on the engine? Is it original? Most OEM setups used a plastic cam gear that wears out and falls apart.

Check the mechanical advance mechanism also.

 

I had the distributor go out on mine and it did this same thing. Of course with my luck two days latter the cam gear fell a part. Three days after that the fuel pump went south by southeast, and yep, the next week the carb puked.

Then things got better for six days in a row I didn't have to replace anything, until the spark brain went insane. I don't know how ford did it, tied the brain into the clutch, but that weekend the clutch showed me how much it missed the brain and fried also. The alt felt bad for the clutch missing the brain and become emotionally detached from the battery. Working only when it felt like it, and it only felt like working when it was installed on the local autoparts store alt tester. When I finialy figured out that the alt didn't love me any more I got a divorce from it and dropped in a c***y one wire unit.


Welp the one wire has been working great for the last three months, but the power brake booster noticed that the alt was bigger then he was and got upset. It got so upset that it wanted to end its days on my truck, so off it came. After replacing that (with a stock ford unit) on Tuesday the truck showed me the error of my ways for using a GM alt by sending smoke signals out the back. I think it's warning other ford trucks that I use non-ford parts. After testing the cylinders today found that the rings are gone on the number eight cylinder (of course I didn't need to waste my time because the plug had more oil on it then the Fonz's hair). So now my toy burns enough oil that I am waiting for the city to offer me money to drive it at night and kill skitter's to pervent the west-nile virus from making it here.

In short, dont forget to look at the distributor.

 

[updated:LAST EDITED ON 07-Aug-02 AT 01:18 PM (EST)]Mine did that with the cause being a bad plug wire. It was burned off inside the connector in the dist. cap. You didn't say you changed plug wires, or did you?

I just re-read your post. I also used Accel 8mm wires. I checked mine with a voltmeter and checked the resistance through them. They should all be fairly close, if one is way out of whack with the others, that would be your bad wire.

Good luck

 

Ok tried a few test when I got home.

first thing its always a good idea when taking a sample of fuel from the pump to have a big enough bucket to catch the fuel. because the amount of fuel in the bowls went a long way.

1st. fuel pump fine.
2nd. no water in tank.
3rd. when I ran the motor with the vacuum advance line attached it did the whole popcorn maker, backfire sounds. Tach would just jump all over. When given gas.
4th. when I pluged the vacuum line and gave it gas the needle on my tach was real smooth and no backfire.

BTW not sure how to tell if motor is OEM. I do know that the truck was taken care of. And only had a couple of owners. I did find the Line Ticket under the seat what a great find.

Thanks for the advise.
Tailgater_73f100

 

Check the power valve in your carb. With the truck off, pull the air cleaner off and look down the carb. If it looks like smoke in the carb when you work the throttle, I would change it b4 you spend any more money on other things. If I remember correctly, you have to pull the carb off and turn it upside down to change it. Its been over twenty years since I had a simular problem with a '75 F100
To check it is free, and it's not that expensive to fix.

 

Think about it............you plugged off the vacuum advance line, and the problem went away. So, the vacuum leak you have must be in the diaphragm of the vacuum advance (little silver fying saucer shaped thingy on the side of your distributor).The rubber diaphragm in there can get a hole it that too much air into your carb, and engine backfires being too lean.

If that isn't it, maybe timing chain is and throwing your timing off. Only bad timing or too lean fuel mixture going to cause the popping you hear.

 

I hope this isn't too late. With the tach jumping around (signal from the points dropping out) when the vacuum advance is unhooked it sounds like the pivot point on the breaker plate is worn. The vacuum advance pulls the breaker plate and the point gap changes. For a quick fix try setting the points a little closer or farther apart and see if it makes a difference. At least it will get you by 'til you can replace the breaker plate. Those were the same symptoms I had on my '71 Mach 1 with a 429 CJ and the factory dizzy. I had to set the points very close to get it to run and it rapidly went downhill until I fixed it right. Hope this helps.

 

As it stands I have not fixed the problem yet. I'm waiting on my new vacuum advance to come in. I have everything else and waiting to get my ride back on the road. Going to change over to electronic ignition as soon as it gets in ( dont want to do twice the work ) But as soon as I change it over and get the vacuum advance on. I will report back in with a update.

Thanks for all the advise guys!

Tailgater_73F100

 

Well I have great news for everyone.

Update on the FORD... After waiting only two weeks for all the correct parts.
I was able to get outside friday morning and reattach my coil to the front of the head on the drivers side. I had it on a bracket I made from some thin aluminum. It shook to damn much so I had to toss that.

Once done with that and some cleaning around the fuel pump. (ewww)

I started on the installation of the new Cran Cams vacuum advance and Mallory unilite breakerless electronic ignition conversion kit. (phew)
The total installation time was about 4 hours for everything even cleaning. I would have to say that it was 6 on a scale of 1 to 10 for difficulty being it was the first time I have ever attempted this type of task. Next time I would say a 3 at most.
After all was said and done, and I double and triple checked all the connections. I turned the motor over and noticed a difference right off. I reset the timing and was off on a test run.

Wow is all I can say, The performance difference is really there. Plus no backfire or popcorn maker sounds and has some get up and go.


Thanks for all your support guys.

Tailgater73_F100
Savannah, Georgia

1973 F100 Long Bed
302cid
Edelbrock 600cfm #1406
Edelbrock Intake #2121
Dynomax Long Tube Headers
Dynomax Super Turbo Mufflers
K&N Filtercharger 14"x3" Air Filter
Accel 8mm Plug wires
Accel Super Stock Coil + Cap
Mallory unilite breakerless Electronic Ignition convertion kit
Cran Cams Vacuum Advance