Mercury thunderbolt 300 fuel delivery issue

[Lucas300]

I have a late 50s early 60 Mercury thunderbolt 300, it has a fuel issue. I bought a new fuel pump kit, fuel line and primer bulb and cleaned the carbs. I went to start it and it wouldn't start at all so I squirted some fuel in the carbs. It would run for a bit and then die instantly. I then proceeded to check to see if it was pumping fuel to the carbs. I loosened the cap on top of the bottom carb. (Tiltson carbs) it was pumping fuel to the carbs so I proceeded to leave the cap slightly loose to see how much it was pumping out. As soon as I pulled it over it started an ran fine and was pumping fuel fine, the moment I snugged the cap back down again it would die again. So is it possible the screen type material inside the cap is plugged or do I have another issue present? Thank you for any help

 

[alldodge]

Howdy

Looks like you posted this in the wrong area, not going to get much input here, should be in the Merc outboard area. GA_Boater can you assist?

I'm not an outboard guy but it sounds to me that the fuel bowl vent is clogged. That is if your talking about a part on the carb and not the fuel tank

 

[southkogs]

Moved ya' to the Mercury forum. That should help things along.

 

[Lucas300]

Okay thank you, it took me a half hour before I could figure out how to post I will try putting it there instead. The cap I'm talking about is the inlet for the fuel on the carb

 

[racerone]

On older outboards the carburetors should be taken apart for cleaning.-------Have a couple of those 300 models.----Excellent / smooth motors.------Distributor should be split to clean / set points too.

 

[alldodge]

I will try putting it there instead

No need to move or repost, your thread has been moved by the moderator.

 

[GA_Boater]

I was asleep at the switch on the move, but a valiant effort by southkogs and AllDodge saved the day. :smile:

Welcome to iboats, Lucas.

On first reading, I thought you loosened the fuel tank vent. But I'm a little confused by "I loosened the cap on top of the bottom carb". Referring to the diagram below, is this small bolt #25. holding #23 or another "cap"?

I think you need to rebuilt the carbs, not just clean them.

The carb diagram below came from the iboats OEM diagrams for your Merc 300. The complete set of parts diagrams is right here. Your have an old and distinguished motor like me, so don't be worried about so many parts being marked as "unavailable". I've found OEM parts or aftermarket substitutes here on iboats. I prefer OEM, but what are ya gonna do with motors over half a century old?

 

[Lucas300]

Yes I loosened the screw (#25) on top of the fuel inlet cap, I would assume it would be the carb vent is plugged causing it not to suck fuel in which I didn't think of at the time until someone had mentioned it.

 

[Chinewalker]

No Title

Pull the carbs off and clean them. Kind of a pain if it's an electric start model, but doable. Well worth the effort, though. Here's mine. Haven't done anything to it, but I have run some of its siblings (Mark 55s, Mark 30s, Mark 30Hs, etc.). Any of the old inline 30-45 cube 4s were solid motors. Enjoy!