My truck has a pretty crummy. weak, and jumpy idle (78 f100, with 300), and the timing seems to jump all over the place when idling. been to several different mechanics, and they always point to the carb. Well, been through three carbs, new egr plate, replaced intake/exhaust gasket, and have replaced all vacuum lines. Oh, did plugs, wires, cap, rotor too. Runs great with the foot on the gas, but at idle it makes me miserable.

I'm thinking that my issue is the distributor itself, and If I replace it my deisling issues, weak vacuum when braking issue with go away.

Does anyone think that I am on the right track? And if I am, how hard its it to do myself. I am not bad with a wrench, I've done brakes, suspension, and even replaced a rear end, but some of the more tricky repairs may be beyond me.

Any guides out there? I do have the haynes manual.

 

The timing changes around when it's at idle? You mean, you put a timing light on it and the timing keeps changing? If that's the case, I would say that your distributor is toast and any mechanic that saw that your timing kept changing at idle and pointed at your carb needs to be strung up.

As for replacing the distributor, it isn't difficult. I did it in the parking lot of the parts store. On the 300, there is ONE bolt holding the distributor in. Take the cap off, have your new distributor right next to the old one and light the internals and externals up to eachother, take the old one out and switch the plastic casing and shaft from the old one to the new one taking care not to turn anything, and pop the new one in and bolt it down. Make sure to time it when you're done.

IF you mess up the settings on the distributor and need to redo it from scratch, that's not that difficult either if you make sure to follow ALL the steps in your repair manual. There's an alignment inside and every 1/4" is like 7* timing off, so make sure it's accurate. I think you set your number one cylinder to TDC, line up the distributor how it says to do it, and slide it back in.

Either way, it's a piece of cake. The only difficulties you might have is getting the teeth to slide all the way in and keeping it aligned at the same time. That just takes some finagling.

 

Yup, timing is jumping around, it jumps in tune with the rough idle. My first mechanic saw it, and apparently never considered that the distributor was the cause of it. I think he thought that the timing was a symptom of something fuel/air related.

I bought a timing light and witnessed it myself.

The last guy, when I had my tranny rebuild, said he looked at the timing but I doubt that he did. And guess what, my tranny is leaking now.

 

I imagine a new distributor will take care of that. I can't imagine much else that would make your timing keep changing before your eyes.

As for the transmission, where's it leaking from?

 

Don't know yet. Need to get down there and take a look.

 

89 5 sp 4x4, I have a similar problem,although my problem comes and goes, one day the timing is perfect the next day it is not. But my timing issues only arise when my engine is hot. I replaced my dist. when my motor was rebuilt but it did not change. also after engine was rebuilt timing mark is now 180 degrees off. I don't know if any of this helped, but good luck.

 

question if you have the time
i have 94 ford f150 6 cylnd runs very rough. i think my distributor is bad based off of the following symptoms
while driving truck will start acting like its running out of gas

i used to get almost 400 mi to a tank now its closer to 200 mi
when i put it in reverse she sputters and coughs and sometimes see white smoke
it seems to idle fine
replaced cap,rotor,plugs, wires 6 months ago ran great for 2-3 days started acting up again
what do you think or anyone else out there?

 

did you ever find out what caused the 180* out? my 4.9 has this issue, so I've had to time it by ear. just want to know what has caused this issue.

 

It very well could be that your harmonic balancer has slipped and the mark is no longer pointing at TDC. If it's moved THAT much, I'd say it's time for a new harmonic balancer.

 

From the front of the engine, using chaulk or crayon, draw a line across the face of the harmonic balance and then drive the truck. After driving, if the line is not still straight across, then the balancer has shifted due to the rubber middle layer having broken loose or bond failure.

 

You said a '78, right? That means the DuraSpark ignition, right? You might want to try to time it using the old style timing light with the spring between plug and wire, rather than the induction type. Sometimes the reading will jump around with the induction type.

I was thinking high mileage wear on your t. gears, but start with the dizzy. If that doesn't solve the problem, and it is the t.gears, you can always return the dizzy. I don't think it's ignition though when all is well when pedal is down.

I wonder if the vacuum problem is related to poor idle.

 

i do think that you should check the bushings in the distributor itself. Liok for any plat where the armature can move away from the internal mag. Any distance than what it is suposed to be will make it run like it is misding, or carb problems. Does it get worse after it warms uo?