Terrible mpg. Need help

Just recently bought an 86 b2000 with plans of getting good mpg. After a week or two of normal driving in the city and highway I noticed it gets 10 mpg average. Now the previous owner installed a weber carb and pacesetter header, stripped all emissions and kept half butting the exhaust and getting it re-welded at repair shops. Now there is a hole in the muffler and the last section of pipe after muffler is rotted off at muffler so it hangs free and any exhaust that goes through hits my fender. Is a lack of back pressure killing the mpg? Or does the carb need adjusting? It makes a sucking sound when revved. Air filter is dirty so maybe its that? Any advice would be awesome. Thanks everybody.

The first thing you might want to do is a tune up. Cap, rotor plugs, Then you might want to do a compression test. That might tell you if the engine is capable of developing good power. It might be worn out otherwise.

Okay the previous owner rebuilt the engine 4 or 5 years ago. I mean i saw a video of him doing it himself with friends so he might have screwed something up but he claimed to get great mpg when i went to buy it. But i don't know if he did that. Thank you I'm definately going to try the tune up idea regardless so i know its done. Oh and would you recommend or go against putting some gumout fuel system cleaner in next full tank? Just to try and help clean things up a bit.

That being the cap rotor and wires.

check for vacuum leaks as well. Spray a little carb cleaner around each port. Sometimes the plugs get brittle. also check your egr if it is still there. If it isn't make sure the block off plate is sealed properly. I have a Weber and average about 25-28. still have the factory manifold and pre-cat then aftermarket exhaust. yes You can run some gumout threw but I recommend Seafoam. Also can do a seafoam drip threw a vacuum port. helps clear carbon out of the intake runners.

He could have told you anything as far as mileage.

I see you asked whether to keep the weber in another forum thread and it's probably better to work with what you have right now then try to reconstruct what may have been destroyed in getting to this point.

I agree. I'll do my best to fix what's there; new air filter, check for vac leaks, clean fuel system, and do a tune up. But if the carb needs rebuilding i might go back to OEM since id be taking the carb off anyways.

Make sure you don't have platinum plugs installed. Our ignition systems don't provide enough power to get any benefit from platinum. They actually are a detriment to efficiency.

Sorry I meant vacuum port plugs, not spark plugs.

Sounds like you might have a massive vacuum leak that shows up when you step on the gas. That would be where your bad mileage is coming from.