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I like your idea of routing VST vapors to outside the cowling to see if too much vapors into the air intake are what is causing the problem.
On your motor there is one cowl mounted trim switch installed on the right hand side. However, there is an opening for mounting that switch on the other side of the cowl. Route the hose through that opening into ambient air and then see if the problem persists or goes away.
When you do this check the orifice air vent to see if air can be blown through it in both directions.
Instead of cracking the throttle, it could be opened halfway to more quickly purge the intake of excessive fuel fumes, if that is the issue. Just be prepared to quickly retard the throttle when it lights off.
See illustration below.
Another thought is that perhaps a fuel cooler is not doing its job and the fuel is getting too hot. It will boil more easily if so. Another thought is perhaps the pressure relief valve for bypassing low pressure lift pump fuel is not doing its job in which case the pump is working more than it should which can also heat up the gasoline.
On your motor there is one cowl mounted trim switch installed on the right hand side. However, there is an opening for mounting that switch on the other side of the cowl. Route the hose through that opening into ambient air and then see if the problem persists or goes away.
When you do this check the orifice air vent to see if air can be blown through it in both directions.
Instead of cracking the throttle, it could be opened halfway to more quickly purge the intake of excessive fuel fumes, if that is the issue. Just be prepared to quickly retard the throttle when it lights off.
See illustration below.
Another thought is that perhaps a fuel cooler is not doing its job and the fuel is getting too hot. It will boil more easily if so. Another thought is perhaps the pressure relief valve for bypassing low pressure lift pump fuel is not doing its job in which case the pump is working more than it should which can also heat up the gasoline.
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