Bucking issue under load?

[FuzzFace2]

81 F100 flare side 4x2, 300 six,NP435, Advance Adapter over drive, 2.75 gear and 235/75/15 tires.

Well I have had this bucking issue unloaded in 4th gear & over drive at about 45 to 50 MPH going up a little hill when taking the trash to the dump every week. It was not really bad so I never looked into it.

Yesterday I pulled a 20' enclosed trailer for our car club for our yearly show.

The bucking was so bad I really could not get above 45 MPH.

I pulled the vacuum line to the dist. thinking maybe a break in the wires to the pickup coil was the cause, did not help.

Both unloaded and loaded I did not see the factory tach "bounc" but it dosemove kind of slow, and the air fuel ratio gauge is about 12.5 to 13.5 a little on the rich side but is normal for this truck when under load.

It seams the more load on the motor (pulling the trailer) the worst it was. If I backed off the throttle it stopped but lost speed and when back on it bucked again.

Today I started looking in to it and so far have not found the cause., what was done.(I did 1 thing and took for a test drive, then the next ....)

1- De-greased the motor after tighten up valve cover bolts. Used shop air to blow dry.

2- Pulled the coil horse shoe connector and plug wire from coil and checked, both ok.

3- pulled cap & rotor, found a little water in side but think from the de-grease. Blew out with shop air.

Cleaned up posts in cap and rotor, they were not that bad.

4- plugged in old Motor Craft DSII box. It leaked potting when hot out and why it was changed.

The 2 plugs looked good to the box.

5- with the motor running after all the above I moved wires to the coil & DSII box and it did not skip a beat. I dont see a way to really check the pick up coil at this time.

I guess the next thing is to pull the carb and either pull it apart, I have rebuilt this when I first got it on the road, or find the Ebay carb I bought and install it.

Back to the garage to plan the attack!

Dave ----

[Gary Lewis]

In my experience a miss under load is ignition. So I'd be looking at plugs and plug wires since the cap and rotor look good.

Look for a track or even a crack down the insulation of the plug that might indicate that it has been arcing over. Or maybe throw a set of plugs at it to know that's not the problem. Then plug wires.

[85lebaront2]

One quick way to look for plug wire issues is check at night in the dark, a glow around the wires indicates weak insulation. One other item, check voltage at the coil + terminal running, I believe it should be around 6-7 volts DC due to the resistance wire in the harness. If it is low then that will cause misfiring (bucking).

On the older Fords it was made as a plug in wire built into the dash harness and was available as a service part. You bought the service wire, unplugged the bad one and plugged the new one in and secured it to the harness.

[FuzzFace2]

One quick way to look for plug wire issues is check at night in the dark, a glow around the wires indicates weak insulation. One other item, check voltage at the coil + terminal running, I believe it should be around 6-7 volts DC due to the resistance wire in the harness. If it is low then that will cause misfiring (bucking).

On the older Fords it was made as a plug in wire built into the dash harness and was available as a service part. You bought the service wire, unplugged the bad one and plugged the new one in and secured it to the harness.

I can tell you the Ebay carb did not make a change on the bucking but it feels like it has more get up and go.

It also idles smother and leaner from a 13 something to 14.5. If I tried to adjust it to that it would run ruff.

Also on the test drive it is running leaner across the board. Before was 13's to maybe 14's now 14's to 15's even saw a 16 once so the MPG should go up too (I hope).

On the test drive it was bucking but get the RPM up to 3K+ and you could still feel it up that high.

I stopped for new plugs and picked up a coil as I feel it is not a fuel issue, not running out of gas more like shutting off and on really really fast.

It did not feel like it was bucking as bad after the test so wonder if the plugs are fouled from running to rich from the old carb and now running leaner they are cleaning off?

Taking the wife out to dinner for 25 years married and tomorrow taking her for a medical procedure so cant work on the truck for a few days.

​​​​​​​Dave ----

[FuzzFace2]

I can tell you the Ebay carb did not make a change on the bucking but it feels like it has more get up and go.

It also idles smother and leaner from a 13 something to 14.5. If I tried to adjust it to that it would run ruff.

Also on the test drive it is running leaner across the board. Before was 13's to maybe 14's now 14's to 15's even saw a 16 once so the MPG should go up too (I hope).

On the test drive it was bucking but get the RPM up to 3K+ and you could still feel it up that high.

I stopped for new plugs and picked up a coil as I feel it is not a fuel issue, not running out of gas more like shutting off and on really really fast.

It did not feel like it was bucking as bad after the test so wonder if the plugs are fouled from running to rich from the old carb and now running leaner they are cleaning off?

Taking the wife out to dinner for 25 years married and tomorrow taking her for a medical procedure so cant work on the truck for a few days.

​​​​​​​Dave ----

Both have good information thanks.

I should point out the wires and plugs were replaced with the cap & rotor when I first got the truck.

The motor had a miss, a compression check showed all good there so I replaced the cap, rotor, plugs and wires and no more miss.

I then took the whole truck apart for the 4 year rebuild.

I will check the coil in put volts just to make sure it is good. Garbage in garbage out right LOL

As you can see from the post above got plugs and coil not that I have ever seen a coil go bad.

Also for leaking plug wires in the dark spray water from a squirt bottle on them and if they arc they are bad.

BTW after the de-greasing the motor started right up and ran smooth as I had to get it back to my garage & tools.

Dave ----

[85lebaront2]

I can tell you the Ebay carb did not make a change on the bucking but it feels like it has more get up and go.

It also idles smother and leaner from a 13 something to 14.5. If I tried to adjust it to that it would run ruff.

Also on the test drive it is running leaner across the board. Before was 13's to maybe 14's now 14's to 15's even saw a 16 once so the MPG should go up too (I hope).

On the test drive it was bucking but get the RPM up to 3K+ and you could still feel it up that high.

I stopped for new plugs and picked up a coil as I feel it is not a fuel issue, not running out of gas more like shutting off and on really really fast.

It did not feel like it was bucking as bad after the test so wonder if the plugs are fouled from running to rich from the old carb and now running leaner they are cleaning off?

Taking the wife out to dinner for 25 years married and tomorrow taking her for a medical procedure so cant work on the truck for a few days.

​​​​​​​Dave ----

16:1 is quite lean, the old 2 barrel 390s would run that lean with no problems and that was with breaker points. Have you taken any of the plugs out and examined them for carbon build up on the insulator and for burned electrodes?

On coil failure, I have seen a few, I have also seen carbon tracking on the coil tower leading to arcing to the can or one of the primary wires.

[FuzzFace2]

16:1 is quite lean, the old 2 barrel 390s would run that lean with no problems and that was with breaker points. Have you taken any of the plugs out and examined them for carbon build up on the insulator and for burned electrodes?

On coil failure, I have seen a few, I have also seen carbon tracking on the coil tower leading to arcing to the can or one of the primary wires.

No I have not pulled the plugs yet and have a new set I just picked up and coil at same time.

I should say I have seen a coil go bad like you said, arcing from tower to 1 of the wire leads as spark was looking for the easy path to ground.

This was caused by bad plug wires where the spark did not want to go to the plugs.

This happened when I was just starting out to repair cars so it kind of stuck with me LOLO.

When the coil went bad it stayed bad.

Again this only happens under load, heavier the load the worst the bucking.

I got a feeling the new plugs will not fix it because it would have to be ALL THE PLUGS as it kills the power / bucks.

If 1 or 2 plugs failed you could feel that as a miss not shut off like is happening.

As for the 16 on the AFR I only seen it pop up.

Once I can get it to run with out bucking I will look at the AFR closer and adjust as needed.

I also remembered the old carb was missing a part for the needle / jet and think why that carb ran rich.

I also seen fine rust in the bowl of the old carb.

The new fuel filter on the carb did not have rust come out before I put it on the new carb.

Thanks for the help

Dave ----

[Gary Lewis]

16:1 is quite lean, the old 2 barrel 390s would run that lean with no problems and that was with breaker points. Have you taken any of the plugs out and examined them for carbon build up on the insulator and for burned electrodes?

On coil failure, I have seen a few, I have also seen carbon tracking on the coil tower leading to arcing to the can or one of the primary wires.

No I have not pulled the plugs yet and have a new set I just picked up and coil at same time.

I should say I have seen a coil go bad like you said, arcing from tower to 1 of the wire leads as spark was looking for the easy path to ground.

This was caused by bad plug wires where the spark did not want to go to the plugs.

This happened when I was just starting out to repair cars so it kind of stuck with me LOLO.

When the coil went bad it stayed bad.

Again this only happens under load, heavier the load the worst the bucking.

I got a feeling the new plugs will not fix it because it would have to be ALL THE PLUGS as it kills the power / bucks.

If 1 or 2 plugs failed you could feel that as a miss not shut off like is happening.

As for the 16 on the AFR I only seen it pop up.

Once I can get it to run with out bucking I will look at the AFR closer and adjust as needed.

I also remembered the old carb was missing a part for the needle / jet and think why that carb ran rich.

I also seen fine rust in the bowl of the old carb.

The new fuel filter on the carb did not have rust come out before I put it on the new carb.

Thanks for the help

Dave ----

When running the carb I saw 16:1 on Big Blue under light loads with no problems. Tip into it and the metering rods popped up and the AFR came down.

As for the bucking, the way you are describing it makes me rethink things. If it is as if the engine shuts off and then comes back instead of just one or two cylinders dropping out then I don't think it is plugs or wires. That sounds like something in common with all cylinders, such as the cap, rotor, coil, coil wire, ignition module, or power supply to the coil. But you've inspected the cap and rotor and replaced the ignition module. So that leaves the coil, coil wire, or the power supply to the coil.

If replacing the coil doesn't help then I'd consider getting a Chrysler ballast resistor and bypassing the resistance wire in the harness to see if voltage is a problem. Start the engine and then jumper from the battery to the coil with a wire that includes the ballast resistor.

But what if it is fuel? Like too little fuel volume or pressure? I'd think you'd see the AFR go wonky in that case when the bucking starts, and since you aren't I still think it is ignition.

[FuzzFace2]

When running the carb I saw 16:1 on Big Blue under light loads with no problems. Tip into it and the metering rods popped up and the AFR came down.

As for the bucking, the way you are describing it makes me rethink things. If it is as if the engine shuts off and then comes back instead of just one or two cylinders dropping out then I don't think it is plugs or wires. That sounds like something in common with all cylinders, such as the cap, rotor, coil, coil wire, ignition module, or power supply to the coil. But you've inspected the cap and rotor and replaced the ignition module. So that leaves the coil, coil wire, or the power supply to the coil.

If replacing the coil doesn't help then I'd consider getting a Chrysler ballast resistor and bypassing the resistance wire in the harness to see if voltage is a problem. Start the engine and then jumper from the battery to the coil with a wire that includes the ballast resistor.

But what if it is fuel? Like too little fuel volume or pressure? I'd think you'd see the AFR go wonky in that case when the bucking starts, and since you aren't I still think it is ignition.

I also think it is something common to the ignition system and why I got the coil.

I got plugs as I want to pull them to see what they look like so might throw plugs at it.

The coil wire looked good at both ends but did not measure it. But being it started right after the water bath I ruled it out. Same on the other plug wires.

I will check volt to coil and hot wire and or replace it to see if that helps.

But gut feeling is pickup coil in the distributor is starting to go bad under load.

More testing needed.

Dave ----

[AmericanSavage]

I also think it is something common to the ignition system and why I got the coil.

I got plugs as I want to pull them to see what they look like so might throw plugs at it.

The coil wire looked good at both ends but did not measure it. But being it started right after the water bath I ruled it out. Same on the other plug wires.

I will check volt to coil and hot wire and or replace it to see if that helps.

But gut feeling is pickup coil in the distributor is starting to go bad under load.

More testing needed.

Dave ----

Hey Dave,

Are you SURE this is ignition related? I only ask because once I had this problem--in a 1973 chevy. Turned out the splines between the engine and trans were almost completely chewed through. Anyway, FWIW.

I am interested if this is a coil issue though.