50HP Mercury Thunderbolt Stalling

[Bennes]

I have a 1970's 50HP Mercury Thunderbolt on my boat recently purchased, old engine but only done just over 350hrs and had regular services, first time in the water starts first time every time, idles fine etc, when putting into gear its going fine just above idle as soon as I open the throttle the engine stalls, if I pull the throttle back to idle or neutral before the engine dies it goes back to idle normally, since then had new plugs, carbs cleaned and overhauled, fuel lines changed, compression checked etc. back in the water same thing can cruise at low speed for hours no issue, just when I accelerate it looses revs and wants to cut out, if I don't pull throttle back engine dies out completely and will start after that straight away no issues, it does sound like when I'm trying to accelerate as if there's like a more noticeable rattle / metal noise from the engine (almost like a car with noisy tappets/lifters) I'm out of options and don't know what to do, I'm relatively new to boating and need help
** NOTES **
1. When the boat had new paint done the throttle control was removed opened, cleaned and refitted - but had the same issue before this was done.
2. The propeller does have a small chip out of one of the blades
3. I've used fuel that's a couple weeks old, after carb clean was new fuel (50:1 Mixture ratio)

Any help would be appreciated

Thanks

 

[The Force power]

I have a 1970's 50HP Mercury Thunderbolt on my boat recently purchased, old engine but only done just over 350hrs and had regular services, first time in the water starts first time every time, idles fine etc, when putting into gear its going fine just above idle as soon as I open the throttle the engine stalls, if I pull the throttle back to idle or neutral before the engine dies it goes back to idle normally, since then had new plugs, carbs cleaned and overhauled, fuel lines changed, compression checked etc. back in the water same thing can cruise at low speed for hours no issue, just when I accelerate it looses revs and wants to cut out, if I don't pull throttle back engine dies out completely and will start after that straight away no issues, it does sound like when I'm trying to accelerate as if there's like a more noticeable rattle / metal noise from the engine (almost like a car with noisy tappets/lifters) I'm out of options and don't know what to do, I'm relatively new to boating and need help
** NOTES **
1. When the boat had new paint done the throttle control was removed opened, cleaned and refitted - but had the same issue before this was done.
2. The propeller does have a small chip out of one of the blades
3. I've used fuel that's a couple weeks old, after carb clean was new fuel (50:1 Mixture ratio)

Any help would be appreciated

Thanks

Welcome to IBoats
1. Verify if the engine is running on all Cylinders, by pulling the cable off the spark-plug & listen for RPM drop on that cylinder (no drop = no fire)
2. Inspect/Replace the fuel-pump if not done already

What is the year/serial number of the motor?
What are the compression readings?
What Rpms is the engine running at idle?

 

[Chris1956]

I have not worked on your exact engine, however, Mercury's of that vintage need a richer idle mixture to allow them to accelerate. Modern marine mechanics do not know this, so they set the idle mixture to give a smooth idle. That setting is too lean.

The next time you are out on the water, warm the motor and while running at idle, open the top carb idle mixture 1/8 turn (CCW). Try to accelerate. if she still dies/stumbles, open the bottom carb idle mixture screw 1/8 turn and try to accelerate. Repeat top carb then bottom carb idle mixture richening, test acceleration after each adjustment.

It will take a few trials to get a rich enough mixture to allow acceleration and still idle reasonably well.