Need help with very bad sputter with 83 k20 bbc 454.

[Tanner_Shany24]

Hey guys, recently bought a 1983 k20 with a bbc 454 in it. I like the truck a ton, but it has its flaws, and a lot of them haha. I am 18, and while I have worked on motors my whole life, this is the first carbed motor I have owned, so I am kind of new to the carb game and just want to get some peoples opinions.
The truck was running pretty poorly, so I just swapped the carb for an edelbrock 750. We had one sitting around and the old was was crusty as heck and I didnt want to take the time to rebuild it when we had a new carb already just sitting there. It definitely runs a little better after trying to get the carb tuned up, but it still has one big problem which it had before the carb swap. It sputters very very badly when the engine isnt at full operating temp. Like so bad that it would take the truck at least 20 seconds to get to 50 mph, and acts like it wants to die the entire time as it seems to be missing more than not. However it will idle fine and it will rev fine in neutral. Once it hits full operating temp it runs absolutely amazing, pulls hard, no backfires or sputtering, and it will idle all day and if its warm it starts with one bump of the key.
One thing that may be important to mention is that the fuel is provided by an electronic pump, hardwired to run all the time the key is on (I know this is the wrong way to do it, I plan on changing it back to mechanical, but it was like this when I bought it and cant afford to change it as of right now). I have a couple of ideas but want your opinions on what you all think it could be.

Is it possible that the fuel pump could be providing too much fuel, and the motor simply cant burn it all until it is nice and hot?

I have also read that the stock bbc intake is designed poorly and does not allow the fuel to atomize correctly, causing the motor to miss when cold. Surely it cant make the motor run as bad as mine does could it?

Is it possible that it is timing related? I don't have a timing light but my buddy does so it probably wouldn't hurt to check it.

I have not yet tried to run it without the air filter now that I think of it, so I am not going to dismiss that as a possible option but the filter looks clean. The distributor, plugs, and plug wires were pretty new when I bought it according to the PO, and they look brand new.

Overall it isn't a huge deal, as it runs perfect once it reaches full operating temp. But the nature of a clutch fan spinning at all times means that the coolant takes a while to warm up, so it has to sit there and idle for around 15 minutes before I can really even drive it, and for a big block that means probably at least a gallon of gas... good thing I have a car for driving back and forth to college XD.

Thanks to anyone that comments!

 

[1987 GMC Jimmy]

You asked for opinions, and I have plenty. One, that electric pump setup is not good, as you know, but it’s also dangerous to your vehicle and you if, worst case scenario, you’re in an accident, and fuel keeps pumping, and it’s dangerous to your motor if you leave the pump running by accident, and try to start it up with watery oil without thinking. That’s something that I could see happening to me if I was in that situation. A Delphi mechanical pump shouldn’t cost much at all. Anything extra would be for some fuel hose. A carb only needs 3-5 psi of pressure. If it was getting an inordinate amount of fuel, you’d see black smoke. Check a plug to see if it’s wet and/or fouled.

Two, was this carb brand new, or had it been used before? What’s the choke situation, electric? It would be good to know that all that is working.

Three, a timing light is a rudimentary tool when dealing with something like this. You gotta have it. It’d be nice to see what your base timing and total timing are while watching to make sure the timing mark isn’t doing anything fishy. I’d also like to know what the vacuum is when it’s running both poorly and well.

No way the intake is causing problems to the extent that you can’t use the truck till it warms up.

I would be more suspicious of anything the PO claims. I’d like to know these aren’t cheapie wires that aren’t arcing through the insulation. I’d check to make sure that every cylinder is getting the spark it’s supposed to. You’re probably fine on the air filter.

 

[Tanner_Shany24]

You asked for opinions, and I have plenty. One, that electric pump setup is not good, as you know, but it’s also dangerous to your vehicle and you if, worst case scenario, you’re in an accident, and fuel keeps pumping, and it’s dangerous to your motor if you leave the pump running by accident, and try to start it up with watery oil without thinking. That’s something that I could see happening to me if I was in that situation. A Delphi mechanical pump shouldn’t cost much at all. Anything extra would be for some fuel hose. A carb only needs 3-5 psi of pressure. If it was getting an inordinate amount of fuel, you’d see black smoke. Check a plug to see if it’s wet and/or fouled.

Two, was this carb brand new, or had it been used before? What’s the choke situation, electric? It would be good to know that all that is working.

Three, a timing light is a rudimentary tool when dealing with something like this. You gotta have it. It’d be nice to see what your base timing and total timing are while watching to make sure the timing mark isn’t doing anything fishy. I’d also like to know what the vacuum is when it’s running both poorly and well.

No way the intake is causing problems to the extent that you can’t use the truck till it warms up.

I would be more suspicious of anything the PO claims. I’d like to know these aren’t cheapie wires that aren’t arcing through the insulation. I’d check to make sure that every cylinder is getting the spark it’s supposed to. You’re probably fine on the air filter.

Sorry about getting back so late, I was waiting for an email saying someone replied but appearently you don’t always get one haha.

Yes the carb is basically brand new. My father bought it and it didn’t work for his application the way he wanted so he got a different one and I bought it from him. It’s a manual choke, which I don’t yet have set up. But it runs the exact same way as it did with the other carb, except a little cleaner when it is warm. This makes me think the carb doesn’t have anything to do with it.

I haven’t pulled my plugs out yet to look at them, but the truck doesn’t smoke at all. It wouldn’t hurt to do so, so I probably will here next weekend when I’m home.

As far as plug wires, we have spares at the house that I could hook up and test to see if that is the issue.

And as for the fuel pump setup, I will be changing that soon. I need to check the casting number on the motor to find out what gen it is. But does that really matter for the fuel pump or are they all the same for the big block?

Lastly, I do need to check my timing and I hadn’t thought of the vacuum changing when the motor is cold vs hot but that very well could be so I will check that too. Also that’s a very good point about getting fuel in the oil by accidentally leaving the pump on, I want to fix that ASAP. Anyways, thanks for the help! I will try to keep updated when I get more stuff figured out. I currently have the suspension all torn apart due to installing a lift so can’t do much until I get that finished up, which I should this weekend!

 

[1987 GMC Jimmy]

Better late than never! It happens. I’d get the engine year on it simply because it’s good info to have. A pre-‘80 motor uses a pump universal to old 454s, and 1980-89 use a different one. I’ve only dealt with the latter, but having done a side by side comparison, they look a tad different, and the flow rates are different. I don’t know if the arms or the flange are different between the two styles, but the latter version is more expensive so I’m assuming there’s something.

There’s a saying that goes something like 99% of carb problems aren’t related to the carb. It’s very heavily misdiagnosed on older vehicles. With what you’ve told me, barring some freak problem, I agree that the carb probably isn’t the issue but something else.

Keep us updated, and if you run into something, you’ve got the digits.

 

[Craig 85]

I ran into a problem on my truck when it cam time to install a fuel pump. From what I can tell, 4WD 454 trucks had a specific pump. When I bought my truck, the PO wired up an electric fuel pump similar to your's because the parts stores kept giving him the wrong pump. I found out, Federal Mogul is no longer the supplier for the original pump, Carter is. They are marketing the 2WD pump as the 4WD pump. I notified them of the issue.

As you can see in the photo, the Carter pump is too long. I found these part numbers based on an old Napa number. I bought a couple of them on eBay as NOS so I had a spare. Airtex 41615 & BWD 41615.

Also, I could not find that formed fuel feed hose. I came up with a solution. Eckler's Corvette had a hose that's close for a BBC. I had the cut it in the middle and added a small section of metal fuel line as the curves were just a hair different.

Eckler’s #6271610 Formed “S” Hose, Frame Fuel Line to Fuel Pump
1970-1981 Corvette - 454

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[4WDKC]

while you have the plug out checking it, also write the part number down and make sure its the right one for the truck, to cold of a plug will make it sputter and not burn all the fuel and make it run poorly. Went through this with my car and the heads, changed the plugs to a hotter plug and never have an issue.