2500 RPM stumble partial throttle 2.0 -4 Options

[SVG223]

2400-2600 rpm stumble with partial throttle, stumble goes away under full throttle. Runs smooth at all other RPMs. Pulled the plugs, a little lean maybe?



Edit 3-17-25 I found a possible cause; worn distributor bushings causing a variation in the points gap.

[emerygt350]

That is looking very lean as well. Is this stock? Have you checked the resistance on the CHT? Has your MPS been played with?

My new tps board went bad in less than a year and resulted in that stumble, however, I think you may be asking for other problems if you don't richen that up a hair.

When the TPS develops a wear issue I found that the 'whole' thing shuts down, not just a miss on a cylinder, the computer basically tells all injectors to stop firing. Strange considering the job of the TPS (accelerator pump) but that is what I found.

[SVG223]

That is looking very lean as well. Is this stock? Have you checked the resistance on the CHT? Has your MPS been played with?

My new tps board went bad in less than a year and resulted in that stumble, however, I think you may be asking for other problems if you don't richen that up a hair.

When the TPS develops a wear issue I found that the 'whole' thing shuts down, not just a miss on a cylinder, the computer basically tells all injectors to stop firing. Strange considering the job of the TPS (accelerator pump) but that is what I found.

I had these plugs under a strong LED shop light which may have made them look more white.
It is stock. I just pulled the MPS off and was eye-balling the seal. I have heard of inserting a potentiometer in the circuit to tune, which sounds interesting.
I adjusted the fuel pressure from 27 to 34 psi to see if the miss would go away but it didn't. The car did seem to run a bit stronger at 34 psi or it was just later in the day when the air was cooler.
This engine may have been rebuilt. I'm wondering if resistance should be added to the mps after a slight increase in displacement. More to read. I think no.

[Superhawk996]

I had these plugs under a strong LED shop light which may have made them look more white.
It is stock. I just pulled the MPS off and was eye-balling the seal. I have heard of inserting a potentiometer in the circuit to tune, which sounds interesting.
I adjusted the fuel pressure from 27 to 34 psi to see if the miss would go away but it didn't. The car did seem to run a bit stronger at 34 psi or it was just later in the day when the air was cooler.
This engine may have been rebuilt. I'm wondering if resistance should be added to the mps after a slight increase in displacement. More to read. I think no.

Let me introduce myself - I’m the guy the just says (IMG:style_emoticons/default/stromberg.gif) like I see it. I try not to be too harsh but sometimes fail.

Stop trying all the internet “easy” fix rumors that were proven to be BS decades ago.

Putting a potentiometer in the MPS circuit most definitely isn’t a solution. The MPS works by varying inductance - not resistance so adding resistance into the circuit as a troubleshooting technique or a solution is a no-go.

Raising fuel pressure isn’t going to address an intermittent stumble as you’ve now discovered and have verified.

Things you should be doing:

Step#1. Replace those spark plugs - the glazing they have developed can cause misfire. Glazing isn’t easily cleaned off. Spark plugs are cheap. Trying to diagnose an intermittent stumble with questionable plugs only makes the task 10x harder. Look at pic #3 - you have enough build up on the center electrode that it’s thick enough to be flaking off. That stuff is not conductive and impairs the spark from jumping the gap. Do this before doing anything else. Go for a brief drive - you may find that alone cures the stumble. However if it does fix it - you still need to address what is causing the glazing.

Verify, clean, and tighten all grounds. Intermittent grounds cause havoc with FI in general and are a common cause of intermittent issues like stumble and misfire.

Verify the TPS - it can be bench tested. Guesswork not required. Replace the TPS board if it is defective. Testing procedure is well documented. The TPS acts like an “accelerator pump” as you tip into the throttle. Make sure it’s working via testing not guessing.

Verify your Ignition system. I don’t know why this hasn’t come up yet. Ignition problems are very often mistaken for fueling problems.

Ignition to do:

Inspect / clean / replace distributor cap if necessary. Corrosion build up and wear on the internal terminals is a common cause of misfires.

You don’t mention whether you’re running points or electronic ignition - inspect verify it’s all working. Especially if you’re running points.

If using something other than a 123 ignition you need to make sure the advance plate is clean, lightly greased, and smoothly moving allowing timing advance smoothly and consistently.

Verify timing. You need to verify timing advance is smooth and consistent. Especially in the RPM range your having the stumble. Personally - I’d put the timing light on every wire and verify the advance is smooth and consistent and that all wires are firing OK.

Replace wires would be the bottom of my list. It might move up a notch or two if they are ancient.

Do these steps before even contemplating messing with the MPS.

[SVG223]

I had these plugs under a strong LED shop light which may have made them look more white.
It is stock. I just pulled the MPS off and was eye-balling the seal. I have heard of inserting a potentiometer in the circuit to tune, which sounds interesting.
I adjusted the fuel pressure from 27 to 34 psi to see if the miss would go away but it didn't. The car did seem to run a bit stronger at 34 psi or it was just later in the day when the air was cooler.
This engine may have been rebuilt. I'm wondering if resistance should be added to the mps after a slight increase in displacement. More to read. I think no.

Let me introduce myself - I’m the guy the just says (IMG:style_emoticons/default/stromberg.gif) like I see it. I try not to be too harsh but sometimes fail.

Stop trying all the internet “easy” fix rumors that were proven to be BS decades ago.

Putting a potentiometer in the MPS circuit most definitely isn’t a solution. The MPS works by varying inductance - not resistance so adding resistance into the circuit as a troubleshooting technique or a solution is a no-go.

Raising fuel pressure isn’t going to address an intermittent stumble as you’ve now discovered and have verified.

Things you should be doing:

Step#1. Replace those spark plugs - the glazing they have developed can cause misfire. Glazing isn’t easily cleaned off. Spark plugs are cheap. Trying to diagnose an intermittent stumble with questionable plugs only makes the task 10x harder. Look at pic #3 - you have enough build up on the center electrode that it’s thick enough to be flaking off. That stuff is not conductive and impairs the spark from jumping the gap. Do this before doing anything else. Go for a brief drive - you may find that alone cures the stumble. However if it does fix it - you still need to address what is causing the glazing.

Verify, clean, and tighten all grounds. Intermittent grounds cause havoc with FI in general and are a common cause of intermittent issues like stumble and misfire.

Verify the TPS - it can be bench tested. Guesswork not required. Replace the TPS board if it is defective. Testing procedure is well documented. The TPS acts like an “accelerator pump” as you tip into the throttle. Make sure it’s working via testing not guessing.

Verify your Ignition system. I don’t know why this hasn’t come up yet. Ignition problems are very often mistaken for fueling problems.

Ignition to do:

Inspect / clean / replace distributor cap if necessary. Corrosion build up and wear on the internal terminals is a common cause of misfires.

You don’t mention whether you’re running points or electronic ignition - inspect verify it’s all working. Especially if you’re running points.

If using something other than a 123 ignition you need to make sure the advance plate is clean, lightly greased, and smoothly moving allowing timing advance smoothly and consistently.

Verify timing. You need to verify timing advance is smooth and consistent. Especially in the RPM range your having the stumble. Personally - I’d put the timing light on every wire and verify the advance is smooth and consistent and that all wires are firing OK.

Replace wires would be the bottom of my list. It might move up a notch or two if they are ancient.

Do these steps before even contemplating messing with the MPS.

Great info, thanks! I will try to do all of this.
I'm running stock ignition. I think I have a leaky seal on the distributor shaft, I will be ordering a new one. When I replace the seal, I will do my best to clean and inspect the distributor components. I had the valves adjusted about 4,000 miles ago but the guy said to come back at 3,000 miles because he wasn't confident in something, it does sound a little clattery. Eventually, I would like to adjust the valves myself. I noticed a little fuel puddle in the plenum I thought maybe the cold temp valve was leaking but I pulled it out and it isn't. I'm just slowly going through everything.