OK i did a stupid thing and now am paying for it. I needed to change the fuel tank sensor on my 78 Ann Model as the gauge was not working. I put gasket sealant on both sides of the gasket when installing the sensor. As anyone who has changed this component knows, this is not a fun task. Well i put the gasket on the top of the tank and started the process of installing the new sensor. While installing, you have to twist and move the unit to get it into the tank while also attempting to align the 3 tubes on the existing hoses. When i got it into place i lifted the rim of the new unit and noticed the gasket was not there. i took the new unit out and discovered it had fallen into my 80% full tank. I retrieved the gasket but noticed alot of the sealant had come of the gasket and was in the gas. I didnt know what to do so i took the unit out, figured out a way to keep the gasket on the unit when installing, and put the new sensor in the tank. So long story short, now my car is missing a little and backfiring when first giving it gas. Tromping on the accelerator and it isnt backfiring. Today i removed the Holly carb filter for inspection and it looked clean. Should i just keep driving until the tank is empty or is there something else i can do. I just got this vehicle 2 weeks ago. HELP!!! Thanks

 

First I have never heard of anybody putting sealant on a gas tank float sensor gasket.
There is a possibility it is coating the sock in the tank and may dissolve shortly. Can you see the sock in the tank using a flashlight?

 

Go back and remove the sealant you put on tank and gasker befor it fully cures,, you will be glad you get it off if you ever need to open it up i down the road,,,,, no sealant ever needed in that location,,
,
,
. Then address the running issues after that,

 

Well i ruled out the sealant in the tank as being my problem with the engine missing and back firing through the carb. I removed the gas sensor today and completely drained and cleaned the entire tank to where it looks brand new. I put the sensor back in, put gas in the tank started the engine and it is still running rough and backfiring. I have had this car for 3 weeks and it ran good until last Saturday. I replaced the plugs, replaced the gas tank sensor, changed the oil and replaced the air filter. All of this was done before the running rough and backfiring. This started 3 days after all the above was done. The car has HEI so no points of course. So my problem must be with timing or carb issues. I did remove, inspect, and clean the Holly carb filter. The filter looked good and i blew out with air but nothing came out. I believe with HEI there is no points or distributer. So how do you check timing and how do you adjust it? If it is and can be adjusted, how do you do it and for a 78 L48 what should to timing be set to? Any other suggestions on what i can do?

 

5 and 7 plug wires are easy to switch.

 

You say it ran fine for 3 days after the work you performed, any chance you cracked one of the spark plugs when you tightened them or had one of the spark plug wires accidentally laying against the exhaust manifold and is now shorting out!? I would double check firing order (as mentioned above) 18436572-clockwise. You could always use the old fashioned method of disconnecting the plug wires one at a time at the distributor (be careful, sucks to get an HEI zap through your arm) and see if there is a wire that when you unhook it there is no change in how the engine runs. With the hood up and engine running in the DARK check around the heads and see if you can see spark grounding out anywhere.

 

When i changed the plugs I did one at a time so I did one at a time as they were difficult to change with the headers. Also it ran fine after i changed the plugs. The problem surfaced about ten days after they were changed. But i am open to any suggestion so i will look at it. Any and all suggestions i will take action. Anybody else got any ideas?

 

There is a chance that one of the plugs is cracked or that one of the wires is shorted as they are resting on or near the headers. I will check that out. Right now it is running so rough i can barely keep it running to get our of the car.