UPDATE: Credit to Chris1956, the symptom of excessive smoke doesn't mean a bad VRO, but a tell that there is fuel restriction.
not sure if you have the System Check or not...but the following are the only two reasons that the horn stays on continuously.
1 - overheat
2 - fuel restriction
1 - you'll need to test each head's temp sensor for proper functionality. Remove each one, you're checking for lack of continuity between tan wire to the sensor...then boil each of them and quickly test for continuity it should be present at boiling for sure....if you have a thermometer, test when the lack of continuity returns, the sensors should open up I would expect around 160 degrees maybe less (I don't actually know)
2 - test the vacuum switch that is near the VRO pump (I assume you have one for this year)
Part #26 in this diagram (it looks like this forum doesn't like competeing links)
wwwDOTboatsDOT.net
then cut/paste this
/parts/search/BRP/EVINRUDE/1997/BE150NXEUE/FUEL%20BRACKET%20&%20COMPONENTS/parts.html
to test that you can disconnect it from the fuel line, attach a vacuum gauge/pump , turn the ignition to the ON position and squeeze the vacuum pump....the horn should sound at 5hg I think.
if the tests for #1 come out fine, then you either have a bad vacuum switch or fuel line blockage....just systematically walk through each fuel line run and test for blockage.
I had varnish granules in my tank...walked through the fuel line runs and got some out of the lines, but not really blocked...it wasn't until I got to the pickup tube at the tank that I found the blockage. I hooked up a piece of fuel line to the pickup tube, then a primer bulb, then another piece of line, into a clear filter, then another fuel line into a portable tank...pumped the bulb until it wouldn't suck anymore(sucked flat)....then worked on getting that blockage cleared various ways....finally did..primer bulb would fill up again...changed out filter and pumped some more until I got clean gas..
Not a VRO pump problem.