1969 C10 Truck Gas guage, carb issue
 

Okay so on Friday night I changed out my Holley 750 carb for the new Holley demon 625 carb. Super easy install and seemed to make the truck wake up from the previous carb it had which in my opinion was too much carb or just wasn't set up right.

Anyway, the truck runs really good now with the new carb...Friday night I go fill up half tank to just test out the truck with the new carb. The truck had been sitting a couple months without starting because the previous 750 needed to be rebuilt so I went with new/refurbished one from Holley. Changed the carb and it ran quick without little to almost no fine tuning. I backed out the airmixture 2 1/4 turns as that is where the truck seemed the happiest.

After getting the truck to run, now the gas gauge seems to not be reading correctly. 1/2 tank literally turned to less than 1/4 of a tank for less than 20mile drive...1/2 tank usually lasts about 4 days for me for 20mile drives. I think the truck should be giving me better gas mileage for a newer carb, wouldn't it. Should I have to recheck the timing once again to make sure it's running at it's optimal condition?

Can't understand for the life of me why the gas gauge or the mpg's have gone haywire on me...any ideas, thoughts??

Re: 1969 C10 Truck Gas guage, carb issue
 

Maybe all the testing of the performance came with a bill for the dance?

Sitting while idling while you were tuning in the carb probably burned a lot of fuel.

Fill the truck up. Record the mileage. Run 200 miles. Take a new mileage reading Subtract the old reading from the new. Fill the truck up, and divide the mileage by the gallons.

It's winter here in NY. Gas mileage? What's that?

Re: 1969 C10 Truck Gas guage, carb issue
 

I wouldn't base too much on your experience, so far. Do like kazoocruiser says, and see what happens.

Re: 1969 C10 Truck Gas guage, carb issue
 

It may be a good idea to pop the distributor cap off and sand the contact points and rotor button. Most of the dizzy caps I have seen over the last 20 years really corrode when they sit for a few months. You may find some more power. Other than that follow the others advice.

Most of the older universal Holleys are pig rich out of the factory so going with the smaller new carb probably made a huge improvement. For FYI if the old carb was a 750 vacuum secondary it only flowed in the 600 CFM range. The vacuum system keeps it from ever going wide open on an engine. Actual CFM flow would be dictated by the spring in the diaphram. Not a bad carb but had its limitations. Considering the 3310 carb was originally a factory muscle car carb, Holley really screwed it up for an over the counter replacement. You have to know how to work on them to make them work good. I am talking more than jet changes.

Re: 1969 C10 Truck Gas guage, carb issue
 

If its been siting maybe the fuel gauge is sticking. Has varnish possibly. Check the float level or for vacuum leaks. Just some ideas

Re: 1969 C10 Truck Gas guage, carb issue
 

Need particulars of your engine in order to get help. What engine do you have? Is it stock or...? Which distributor? Whats's your timing set at? How's the fuel filter, pump, and lines? Checked for vacuum leaks? Any other work done to it recently that may affect it? Did you use a vacuum gauge when setting idle mixture screws? You more than likely need a calibration kit. Check the sending unit float inside the tank to make sure it's actually floating and not partially flooded.

Re: 1969 C10 Truck Gas guage, carb issue
 

One other thing, check your oil. If your running that much fuel thru the carb its probably washing out the cylinders. Your oil will be gassy.

Re: 1969 C10 Truck Gas guage, carb issue
 

I can help with the fuel gauge issues if needed.

Does it stay at 1/4 now even if you put more than that much gas in it?

Does it fall below 1/4 tank if there is actually less than a 1/4 tank of fuel in the tank?

Old gas if the truck has been sitting a while, will cause issues with drivability that you will never be able to tune out.

Hope this helps,
Tom

Re: 1969 C10 Truck Gas guage, carb issue
 

Thanks for all the responses guys, I'm going to start this upcoming weekend on the fuel sending unit and work to the front of the truck. Will then reset the timing, use the vacuum gauge to set the fuel/air mixture screws & possibly change out the vacuum lines just to make sure everything is up to par.

As for the truck's specs it's got a 350 crate engine that's pretty much stock but with a mild cam and the new Holley Demon 625 Carb...I'll have to double check the dizzy as my pops changed that out (when he was able to still work on it). As for the gas lines, pump & filter they were changed out within the last 3 months prior to sitting. It was only sitting 1 month due to the carb just not acting or working like I wanted it to or couldn't get it to work right.

Re: 1969 C10 Truck Gas guage, carb issue
 

Yes check oil if it smells gassy. Check fuel pressure it correct for your carb. If too high will force past the float bowl and flood into engine and mpg will go down the pan!