Engine stumbling? partial throttle and only when warmed up - 87 Dodge W100


about two weeks ago i had a ton of work done to my W100: clutch, freeze plugs, valve seals, body mount bushing, headers, x-pipe dual exhaust

the shop did tell me before i picked it up that it had a lean pop and the carb needs to be rebuilt

as of now, it idles fine and drive fine when cold, as it starts to warm up it starts to become problematic - it feels like the engine stumbles any time you try to maintain a constant speed (slight throttle engagement), it accelerates fine, but that’s about it. You have to drive it in a constant state of accelerate and foot off gas because it does not like you foot on the gas pedal just a little. if you do keep your foot slightly engaged it will stumble and the exhaust will pop - not so much a backfire as there’s not at jerking

  • both coolant sensors are new
  • spark plugs are FSM spec champion copper 404/rn12yc
  • plug wires are a year old (msd cheaper line)
  • timing seems fine (around 8-10 BTDC)
  • watched at night to try to see if any arcing was happening, none that i could see or hear
  • finagled some exhaust restrictors on the exhaust tips out of rubber plumbing couplers and big lag bolt washers to try to recreate the backpressure from the old exhaust, no difference
  • went around the vacuum lines with carb cleaner and propane, couldn’t find any distinguishable leaks


  • fuel filter about a year old
  • new fuel pump installed while at shop

“the shop did tell me before i picked it up that it had a lean pop and the carb needs to be rebuilt”

Is there a reason you doubt their advice? The carb’s been around a long time, it just might need a rebuild. Jets erode, floats sink, needle valves get gummed up, accelerator pump rubbery bits wear out, etc. Perhaps a rebuild really is in order.

Along with the carburetor rebuild…I would suggest having your fuel pump pressure checked. The carburetor choke may be making up for poor fuel pressure. You may need a new fuel pump as well. BTW…check the fuel filter first because it may be clogged.

i don’t, but it was only my second time at the place. the other reason is, it was running fine before. i understand that exhaust work can change things, but just such a drastic change, all of a sudden right after all this work?

forgot to mention new fuel pump as well. EGR seems to work as well too.

bad timing of comments, fuel filter was replaced about a year ago, and had them throw on a fuel pump just because while they had it.

Then I guess you are going to get your carburetor rebuilt sometime soon.

This is a typical carburetor malfunction symptom set. These symptoms typically suggest that the feed is going lean momentarily when you hit the accelerator, and in your case when the throttle plate is in a specific range. Likely causes include gummed up jets and/or a worn out accelerator pump. A bad float or needle valve can cause this too by not allowing the bowl to stay full. Hit the throttle and the bowl lacks sufficient fuel to feed the venturi. A bad float or needle valve might not have shown up until the carb was emptied when the work was performed and then refilled.

Just as an FYI, you might want to check to see that your ignition is properly advancing too. Lagging timing advance can cause symptoms mimicking a bad carb.

Just a suggestion: my poor experience with Champion plugs caused me to stop using them years ago. I don’t think they’re a factor in your problem, but you may want to change them out for something better once you get the problem solved.

could the new exhaust and fuel pump amplify a worn out carb that much all of a sudden?

i actually have a new/rebuilt carb sitting here, just didn’t want to throw it on (in hopes of returning it) until i’ve exhausted all other possibilities (since the carb wasn’t touched)

mountain - this is the 3rd new set of plugs in it. the shop put in NGK’s and to check things off the list i took them out and put in the standard to get a baseline at least on plugs.

as for timing, i checked it myself and it’s around 8-10 BTDC that’s about all i know how to check (point and shoot) i can see that the timing does advance when i play with the throttle, as to how much or when, no idea.

No, but draining the carb to do the other work could easily cause the problems in an old carb. The float goes where it hasn’t been since new, the needle valve does too, and the rest… well, you get the idea. My guess is that is what happened.

thank you for the info, perhaps i’ll throw on the carb then. i want to rebuild the old one, but didn’t want to rebuild one for the first time on one i NEED

a 4bbl and intake are on the list as well as a new ignition system, but just not now. i know the cost of replacing this 2bbl is a waste in the long run, but i didn’t want to change a whole lot else before solving one problem and then proceeding.

I always thought that if you decreased the back pressure in an exhaust system (OP says they installed a new dual exhaust system) there was a possibility that the carburetor jets would be too lean, because more air would flow through the system, diluting the mixture from the original jets. That sounds like the situation here, and the carb might need new main jets that are just a tad bigger.

i’ve read that too. that’s why i tried to temporarily increase the back pressure by putting those restrictors on. Also, it had dual exhaust before, but only 1-5/8" diameter pipe with stock manifolds. now it have hedman headers, 2.5" x-pipe dual exhaust

Jetting carburetors is a dying art generally, except in motorcycle world. If you know how, you can replace jets with a set that are one size larger, but if you don’t feel comfortable inside a carburetor then talk to the shop that did your work, because they seem to be on the right track. Find the oldest mechanic in the shop.

i can’t find too much info on my carb, it’s a holley 2280. most people ditch it and move on. i’m just not ready to yet. i have no idea where the jets are or what size are in it. i’ve read that they’re stamped with numbers though that say what size they are. i think they’re in the front part of the carb. i don’t want to take the lid off that as i know there are some spring’d things inside. just nervous of screwing it up further.

You could search through YouTube or just find a forum online that has postings about dealing with this model carburetor. A search on Google for “Holley 2280 forum rebuild”, without the quotes, turned up a lot of info. I’ve learned by doing and making some mistakes. If you decide to try, make sure you have a large table to work at when you disassemble the carburetor, a digital camera to record how things go together, lots of containers to hold parts in some organized way, and spread out an old towel on the table top so when things slip from your control they don’t bounce or slide out of sight.

Or, since you already have a new replacement, you could just replace it. New carbs usually come with detailed instructions on how to set them up after installation. If you have any problems, just post.

i installed the reman carb today. made a difference, but it still stumbles with initial throttle engagement. i think i will try to look into putting in bigger jets. i pulled the top half off the old carb and was able to open it like a book so i’m hoping i could do the same to the reman on the truck.

the jets seems to be 3/16-32 with the number 283 stamped on them

The carb manufacturer’s website should show how to increase the size of the jets. You’ll be amazed at how much information they make available now.

Honestly, I think you’ll eventually find your problem in a carb adjustment rather than in the jets. When you hit the throttle, the accelerator pumps pump a spray of gas into the throttle body, richening the mix to compensate for the sudden increase in the air flow. The jets don’t play a part in this other than to continue the feed after the accelerator pump sprays the intake. Be sure everything is adjusted per the carb manufacturer’s recommendations.

Again, be sure to check your ignition advance too. The problem might be hiding there.

in that case then, i won’t touch the jets. i know the idle mixture screws are just for idle, but do they affect the main circuit? like, is the main circuit ran/prorated off the idle circuit?