Just trying to learn by following, but I'm not understanding how venting a tank will cause starting issues.
The pump pumps with or without a vented tank, doesn't it?
If the tank has no vent, the pump suction will build vacuum, which OP describes, but the pump still delivered gas to the engine. OP says it ran well without a vent.
I can imagine the pump dropping flow without the tank vented, but it obviously was still moving gas when the tank was not vented. Doesn't this suggest the tank is getting some air to replace the gas pumped? Maybe the cap has a vent that will hold some vacuum, but still let air into the tank when the vacuum builds?
Is it possible the fuel pressure at the engine increases when the tank is vented? If so, how would this cause starting issues?
I am also confused by "I'm running... with no vapor line", then later "It doesn't stall when the vapor line is capped".
Might be helpful to get more details?
For example, mechanical pump? Carb or FI? Does the pump have a vapor return that is blocked, then opened? If original tank, how is it now sealed? What cap is on the tank? Are you sure it isn't allowing air into the tank?
Sorry, but I'm not following all this but it's interesting!
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