79 k20 400 running rough, can't figure it out.

It wasn't running up to par so I started with plugs, cap, coil, rotor and ignition control module, but even though the cap and rotor needed replaced badly it didn't help. I got new plug wires and tried to run it and it fell on it's face, so I changed the fuel pump. I thought that fixed it but no, it runs fine on reasonably level ground at about 45 mph, but still rough at low speeds and very weak on a hill. Any suggestions? I have had the carburetor rebuilt a couple of times in the not too distant past, neither rebuild fixed the problem of needing to prime it to start when it's cold. I don't know if that has something to do with the way it's running now?

Check fuel pressure and volume even if a new pump was installed.

Rough at low speeds, runs good on reasonably level ground, be more specific.

Prime when cold, what brand of carb? Is choke operational?

fast 99 said:

Check fuel pressure and volume even if a new pump was installed.

Rough at low speeds, runs good on reasonably level ground, be more specific.

Prime when cold, what brand of carb? Is choke operational?

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Okay I need to learn how to check the fuel pressure. I'll figure that out.
When I take it out on the road, it is rough running at low speeds even on pretty flat ground, smooths out as I accelerate, at about 40-45mph it runs fine, not sure what you mean by more specific, it has a hard time on my driveway which is a definite hill.
99% it's a Rochester, doesn't say carter anywhere on it. Numbers are 17059226 and 3018CHC, choke seems functional to the best of my knowledge, which isn't a lot.

Check fuel pressure and volume first. Most of what you describe could be caused by a fuel related issue. No reason to go any further until that is eliminated. Numbers are for a Q-jet. Has the filter been checked or replaced? It's behind the inlet fitting on the carb.

Yes I replaced the filter and the fuel pump, lines to the tanks were replaced by a professional 2000-3000 miles ago, I have a fuel pressure gauge, where do i connect? Not sure how to test volume

I have an idea how to build a hose that I can do these tests with, but I don't know what to ask for. Do they make short ends that you can screw into the fuel pump and fuel filter housing to put a hose on them? If so what size do I ask for?

yellowdog5 said:

I have an idea how to build a hose that I can do these tests with, but I don't know what to ask for. Do they make short ends that you can screw into the fuel pump and fuel filter housing to put a hose on them?

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Where are you at in Oregon? If you are close to Salem, it's probably easier for us to just meet up and I can run through your Q-jet and show you haw to troubleshoot and adjust everything. You can rent a fuel pressure test kit from autozone or oreilly that has the T-fitting to put it in-line between the pump and carb. If you still have the Q-jet, it isn't likely to be a pressure problem. You can just disconnect the line from the carb and stick it in a plastic bottle, crank the engine over for a few seconds and see how much fuel you get in the bottle. A 20oz soda bottle should be filled completely if you crank the engine for ~15 seconds. I would recommend you crank it for 5 seconds and see if you have roughly 1/3 full bottle or more. If so... your pump all the way back to the tank is likely just fine and its going to be a carb or ignition issue. There's an old saying that ~90% of carburetor issues are ignition related, but since you replaced all of that and it didn't change anything... my next guess would be that the person who rebuilt your carb set it back to factory default specs and you never adjusted it from there.

I'm near Creswell, pretty far for you to come, thank you for being willing to help. I will do the test the way you said. I do have a pressure test gauge, for vacuum and fuel but I will do your method and let you know what happens. The outfit that rebuilt the carb adjusted it, it worked fine except you have to manually prime it if it's cold, takes about 80 pumps otherwise. Ihad it rebuilt once before and the need for priming got worse after that first rebuild, second one didn't fix it. That's been a while, the current problem just started recently.

I thought I had fuel problems. I tried everything in my experience. Even tried some other stuff that didn't work. Turned out it was wrong plugs that were gapped way too wide. Truck is running great now. In my case it was ignition related, not fuel or carb. @Bextreme04 has pointed you in the right direction, and if you are able to meet up, take him up on it.

yellowdog5 said:

I'm near Creswell, pretty far for you to come, thank you for being willing to help. I will do the test the way you said. I do have a pressure test gauge, for vacuum and fuel but I will do your method and let you know what happens. The outfit that rebuilt the carb adjusted it, it worked fine except you have to manually prime it if it's cold, takes about 80 pumps otherwise. Ihad it rebuilt once before and the need for priming got worse after that first rebuild, second one didn't fix it. That's been a while, the current problem just started recently.

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You can't adjust it without it being installed on the vehicle. I have the original quadrajet on mine and I rebuilt it myself a few years ago. They should have set the float height on the vehicle and then done all of the linkage and choke adjustments at that time too. It should take a single pump of the gas all the way to the floor and slowly released, then it should fire right up within 2-3 cranks and run on the high idle cam until warm. Warm it should fire up without touching anything within 1-2 cranks. You have to get the linkages and everything properly set after a rebuild, then make sure timing is spot on, then do the mixture adjustment and hot idle adjustment on the warm engine. Then you have to wait for everything to cool down and do the high idle adjustment on the choke first start up in the morning after all those adjustments. Once all that is done, it should run great.

There is a fairly big squarebody group there in Eugene that I'm a part of and they do meets throughout the year. I drive my K25 down for the meets a few times a year and there are also some really knowledgeable folks that live in that area and could help you out. If you aren't already a member, I suggest getting involved in the Eugene/Springfield Square Bodies group on facebook(or whatever it's called now).

DoubleDingo said:

I thought I had fuel problems. I tried everything in my experience. Even tried some other stuff that didn't work. Turned out it was wrong plugs that were gapped way too wide. Truck is running great now. In my case it was ignition related, not fuel or carb. @Bextreme04 has pointed you in the right direction, and if you are able to meet up, take him up on it.

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That's a good point on having the RIGHT plugs. I run AC Delco R44TS or R45TS gapped at .045" on my non-vortec SBCs that are using HEI. Same plug in my 70 Buick 455

Bextreme04 said:

That's a good point on having the RIGHT plugs. I run AC Delco R44TS or R45TS gapped at .045" on my non-vortec SBCs that are using HEI. Same plug in my 70 Buick 455

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Yeah, I learned a valuable lesson recently on the spark plug issue. What plug do you use for the vortec'd sbc?

The plugs are AC Delco 10 platinum 0.35 gap, from Rock auto for a 400 engine is what they have.

yellowdog5 said:

The plugs are AC Delco 10 platinum 0.35 gap, from Rock auto for a 400 engine is what they have.

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Those are a stock-ish replacement as far as thread and length... but they are a MUCH colder plug. I don't think it is your actual problem, but it could be making your actual problem more visible. The R44TS or R45TS are the correct factory plug with a heat range of 4 or 5 respectively. The Rapidfire platinums have a heat range of 12.

DoubleDingo said:

Yeah, I learned a valuable lesson recently on the spark plug issue. What plug do you use for the vortec'd sbc?

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The correct plug for a Vortec head with an HEI ignition would be an AC Delco R44LTS or R45LTS.. i is basically the same plug, but with a longer tip on it due to the chamber shape and plug location in the vortec heads. A factory Vortec motor(L31) would have come from the factory with the newer Platinum or Iridium equivalent 41-932 plug.

Bextreme04 said:

Where are you at in Oregon? If you are close to Salem, it's probably easier for us to just meet up and I can run through your Q-jet and show you haw to troubleshoot and adjust everything. You can rent a fuel pressure test kit from autozone or oreilly that has the T-fitting to put it in-line between the pump and carb. If you still have the Q-jet, it isn't likely to be a pressure problem. You can just disconnect the line from the carb and stick it in a plastic bottle, crank the engine over for a few seconds and see how much fuel you get in the bottle. A 20oz soda bottle should be filled completely if you crank the engine for ~15 seconds. I would recommend you crank it for 5 seconds and see if you have roughly 1/3 full bottle or more. If so... your pump all the way back to the tank is likely just fine and its going to be a carb or ignition issue. There's an old saying that ~90% of carburetor issues are ignition related, but since you replaced all of that and it didn't change anything... my next guess would be that the person who rebuilt your carb set it back to factory default specs and you never adjusted it from there.

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10 seconds of cranking got me 6 ounces, I don't know if it matters but I think it may need a battery, cranking seems a little slow on a full charge.