So Merc has their cool fuel systems, is there a real advantage to cooling the fuel? I have never heard it discussed here. One reason I ask is that I had vapor lock issues on one motor last year, have not quite figured that out.

My application is supercharged with chillers.

When fuel is not returned to the tank, fuel coolers are very helpful...yes, because of vapor lock.

I do not have return lines

Just return your fuel back to the tank and vapor lock will be gone. There are usually empty fittings on top of the tank or you can buy an adapter that dumps it back in the fuel filler hose. Very easily done and not much cost to it. Had the Vapor lock problem with my stock 502 mag for years. Jeff wurl

If no return fittings are available you add a Tee fitting on the tank fill hose for the return.

this is how i did it and it works fine.

Looks like I do have return lines, the little lines out of the regulators. Now the question is what is causing the vapor lock in the port motor. After a run and a short shut down the starboard motor wont fire, let it sit for 30 minutes and it immediately fires.

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That doesn't sound like vapor lock to me, sounds more like, loose connection, something getting hot, or the distributor cap. Those kind of caps (crap cap) go bad and they look fine

I can switch the caps from side to side and check that out, also going to replace the ignition on that side to match the other motor.

ignition problem and or --- possible coil ---- next time put an ice pack on the coil or blow a fan over it to cool it down - if the engine starts right back up - u most likely found your problem.

I have been out on lake test drives for service reasons - then had no restart of the engine --- I have taken the blower hose loose and routed it to the coil and other ignition components to cool them down -- basically a layman test and for a possible restart of the engine. --- some worked for restarting the engine and some did not for other failed reasons. Just passing that along as possible tip for whomever.