Hey all,

Here's something I've been fighting on the side for far too long and I'm at the point where it's gotta get fixed or the truck's gonna get sold. Truck in question: 1982 F350 dump truck with a 400/6.6L and a reproduction 2150 two-barrel carb.

I bought this truck a while back, it was an old beater yard truck when I got it but it ran pretty good. I put it to work for a couple years without doing really much of anything to it and then one day all of a sudden it stopped working very well. It was still driveable but it would idle at unbelievably low engine speeds. The day it happened, I fired it up, it idled great, I was headed about three miles down the road to pick something up, and it started doing it. While I waited to get the item loaded, the truck was idling at what sounded like 200RPM or less, which is insane. It chugged slowly but never shut off, and when I would get on the throttle and hold it, it would increase the RPM like normal and drive fine, but at stop signs it was a nightmare.

Anyway, fast forward a couple more years. I figured it must be a carb issue, the carburetor that was on it was mismatched and old. I pulled it apart, it was full of dirt and mud dauber nests (and had probably been even as I had been using the truck). I cleaned it out and rebuilt it. I had never really had a functioning accelerator pump, and the rebuild absolutely fixed that, but the strange issue persisted. I could fire it up and it would idle beautifully, but a blip of the throttle would all but kill the engine.

Next step, I bought a cheapo reproduction carburetor off of Amazon and tuned it close enough and I threw a manual choke cable on (mostly because I felt like it). Identical issue. I tooled around with a few more things, I replaced all of the fuel lines and the filter, a cap and rotor, stuff it needed anyway (worth noting that for as beat up as it was when I got it, it had a brand new fuel tank. It also doesn't seem like fuel is the issue anyway). Same thing, no solution. I even tried the old ignition module, also not the issue.

So here I am today messing around with it again - my dad wants a load of mulch and if I had a functioning dump truck that would make it a lot easier, but I don't. Same thing happened this morning like any other time I've tried. I cranked the engine to get fuel to the carb, pumped the pedal once, and it fired off right away. Warmed up enough in maybe thirty seconds for me to slide the choke open and it sat there and idled exactly like it is supposed to. The engine runs really well, actually, for as abused as it was and as cobbled together as it is. I let it sit there for ten minutes running and the entire time it was as smooth as could be. I reached in and blipped the throttle lever on the carb and immediately it dropped drastically in RPM, chugged a bit, and then sputtered itself out.

It's also worth noting, once I go through this routine, it won't start again. Shutting the truck off does not "reset" whatever is wrong with it. I can get it started by holding the pedal to the floor and cranking but it won't idle and just shuts itself off. If I let it sit for a couple of days (or maybe less, I haven't really timed it), it goes back to its usual pattern where it will idle fantastically but die. So maybe there's a clue in there.

It has to be something simple, right? What am I missing here? I drove the truck quite a bit with zero issue, and then one day, this weird issue. It doesn't seem to be fuel, it doesn't seem to be spark, it doesn't seem to be air intake... any ideas? Could it have jumped timing or something?

 

check timing and fuel mixture. timing first

 

This problem replicated through several carb changes. I'm wondering if you have a temperature related vacuum leak. Like through the manifold or EGR or????

Be sure your choke is operating and your hot idle is set right. Most have a stepped cam for the cold idle.

Pictures will help.

 

yea a vacuum leak i didn't think of. will it idle up cold w choke on or fall on its ***

 

When it won't idle very fast, I would get out and turn up the idle stop screw and see if that helped it. If it didn't, it may help you figure out what is really wrong with it. Sometimes you have to play with different things till you figure it out. It almost seems like when you blip the throttle, you are getting the throttle off the last fast idle step on the choke, and it's dropping back to the idle stop screw, which is set too low.

 

If the old carb was full of crap, did you replace the fuel filter ? Make sure your not sending more junk from a dirty fuel system into the carb.

 

I was also thinking the same, the blip kicks it off the last step of the fast idle cam and now the idle is set to low.
Also easy to check to see where the 2 idle speed screws are resting.
The fast / choke idle with motor up to temp the screw should not be on the steps.
Up to temp the normal idle speed screw should just be touching the carb body but may need a little more adjusting.

I would also recheck the timing and maybe bump it up a few deg's.
If the factory wants 6* BTDC (before top dead center) bump it up to 10* BTDC.
If you get piping or kick back on a hot restart kick it back to say 8* BTDC but I think you should be ok at 10*.

BTW what are you running for an air filter if you had mud dubbers in the carb?
Dave ----

 

Oh just a thought this motor I bet runs a EGR valve right
If so I would pull the valve off, 2 bolts, the base or intake and make a plate to block it off.
It the EGR is held open just a little the fast idle can mask this but once off fast idle it will run a little ruff or low RPM and also cause running issues when under power or more so on take off from a stop.

BTW this dose work off motor temp, only when up to temp.
It could of been it got up to temp and carbon is holding the valve open now and causing the issues.
Dave ----

 

Thank you everyone, the more information I can get, the better. I finally had time to tinker with this truck this afternoon. I started with adjusting the curb idle speed screw, and that did in fact work... until it didn't. Actually when I started on it today, the truck wouldn't even idle (sort of like I described above, but it's been sitting a week). I adjusted the curb idle screw to give it a little more juice and it started up, idled fine, and I could even rev it up and it would return to the preordained idle speed. I thought I was in the clear. I went to move the truck and after a solid 15 feet it started sputtering and dying all over again. Progress was made, but there is still an issue.

It's worth noting that I found a few resources on the carburetor, and one excellent one in particular. I quickly realized that they're not going to apply to me in their fullest capacity though.

- The truck has a manual choke conversion, so there are no pots or vacuum levers or doodabs or whosawatsits. Just me with my pull **** sitting in the seat. Do any of those things on the choke side of the carb still do anything? Should they? I can't figure out how to adjust them relative to the manual choke, everything I see shows how to do it with the automated chokes.

- The previous owner rather unceremoniously deleted the primitive emissions from the vehicle, so on the throttle side of the carb, there are also no pots or vacuum assists or whatever is supposed to be there. In fact, it doesn't even have all the springs where they're supposed to be, which seems like it would be a problem, but it never had those springs there since at least my time of ownership (or at least, if they were there, they dangled and did nothing). This is basically a "modern" 1980s carburetor that has been forced back into a 1960s unit.

The thing that still stymies me about it all is the sudden onset. Like I said in the original post, I bought this truck in not-too-great condition, but it ran fine and I drove it quite a bit. Then one day this all began, and the problems have persisted through a carb rebuild and a carb swap. Did something snap, or blow out? Whatever it was seemed to be something that happened acutely. That's the part that is weird to me. Unless a carburetor can get very suddenly un-adjusted, I don't know if that's what it really would be. But I'm also struggling to find any vacuum leaks as well, and when I can get the thing to run, the engine sounds totally fine. The mystery is thickening, and those other old dump trucks on Marketplace are starting to look pretty good, as is the offer from my friend to buy this one

 

Ok so carb rebuild and a carb swap and it dose the same thing, so not a fuel thing that I would say.

Remove the vacuum hose to the vacuum advance on the dist. and plug it so you dont have a vacuum leak.
Test if it still happens.
You have swapped out the module on the inner fender?
What color is the plastic holding the wires to the box?
I would also pull the 2 plugs on the box and make sure they are clean and same with the coil connections.
Do each test at a time so if you have a change, good or bad, you know where the problem could be.
Do all of them at once and got a problem where do you go to start looking?
Dave ----