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85 Mallard P30 454 Runs poor when full tank Chris 9-16-16  
85 Mallard P30 454 Runs poor when full tank.

If I DON'T fill the gas tank to the top, it runs fine.

If I fill it to the top, it will not maintain running speed. Like it loosing fuel. If I open up the secondaries it will run like a champ until i let off the gas to maintain speed and seams like it is running out of gas. Just started recently when it ran fine before.

Changed the gas cap no change. Pulled the carb apart, looks fine and changed the internal filter.

Next is to change the other fuel filter.

I think the gas tank is under a vacuum or something, is there a fuel canister or maybe the return is plugged?
Re: 85 Mallard P30 454 Runs poor when full tank Bubba The Bus Tech 9-17-16  
1st the test for your theory because it is possible. (But I doubt it)

If you have the old cap still drill a 1/8 inch hole in it and do a test run. If it runs better now you know and can look for crushed vapor canister lines, in the same system rubber lines can deteriorate internally and cause blockages. If you prove out all the vents then look at why the charcoal canister purge valve is not triggering, bad temp controlled vacuum tree, cracked line etc.

2nd If the temporary drilled vent does not fix the issue we are looking for and ignition fault. cap, rotor, wires, module, coil and Pick up coil.

To figure this out you have to understand at partial throttle and mid-range or cruse RPM with the engine @ full warmed up hot the ignition is under the highest load it will ever see.

The way I think about this is when you go full throttle the mixture goes slightly rich to provide best power and avoid ping, easiest mixture to light, at partial throttle the carb is as lean as it can go trying for low pollution and best fuel economy, a Lean mix is very hard for the ignition to ignite.

Most common problem in this era GM is cap and rotor The rotor is a real problem item if its the original GM white one., By a Blue Streak Brand premium set for best results.

PLUGS: The plugs should have the gap set at least 0.005 smaller than the GM spec. Just gives a better result when you fighting this battle.

These engines with the high under hood temperatures are high eat cheap plug wires. Buy good ones.

The coil can be weak, back in the old days we would put it on a scope to check this. Now its a change it to test kind of thing and because its a non-returnable part, an expensive test.

Finally the pick-up coil inside the distributor, they break wires and generally start showing problems when the vacuum advance has advanced the timing. To check requires removing it and checking for broken wires, and when I check I usually break a wire... OH Well

Good Luck
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