too much throttle kills motor...

[dOb]

... Unless I increase very slowly.

1974 Mercury 500 (50 hp) Thunderbolt ignition

Yesterday I went out with my kids. Boat in the water, new battery and plugs, starts fine. Warmed up waiting for other boats at the dock. We slowly left the dock. When we ere out in open water, I opened up to full throttle to get to a spot to fish and swim. So far so good.

I noticed that when I backed off of the throttle to idle, it felt like the control box was binding a little. I even mentioned it to my daughter.

So the next time I start up to head to another spot, everything is fine until I give it some gas. The motor hesitated and then died.

Long story short, I could get up to WOT, but only in small increments. If I moved too fast it would kill the motor.

I did a quick visual inspection this morning. I can't see anything unusual. But I noticed that the silver arm that connects to the shift cable seems very loosey goosey. Maybe it always has been and I just didn't notice.

I won't get at it until tomorrow, but I thought I would check here to see if anyone could point me it the right direction to look for a problem.

Thanks

 

[Laddies]

Re: too much throttle kills motor...

You

 

[dethalis]

Re: too much throttle kills motor...

You may have a loose linkage causing it not to advance timing fast enough or correctly... carbs dirty...

 

[dOb]

Re: too much throttle kills motor...

Is it possible that this could be my fuel pump? I went out yesterday. All I had done at this point was spray a little carb cleaner in the carbs. I didn't expect much to change. But it seemed to accelerate away from the dock just fine. I thought that maybe it had fixed itself. But after stopping in the water for a few minutes and then restarting, it was back to poor acceleration. Just for kicks I squeezed the bulb, and it seemed to pick up the rpms. I could have been a coincidence.

Is it possible to run fine at high speeds if the fuel pump is going out?

I'll probably rebuild the carbs anyway. But I thought I might try a fuel pump rebuild first.

Thanks

 

[dethalis]

Re: too much throttle kills motor...

Try putting your hand over each carb to choke it and if you get a rise or no change in rpm that carb either has some trash in it or it needs rebuilding... if it goes down in rpm then that carb is fine.. do each one that way

 

[Yesterday]

Re: too much throttle kills motor...

Poor fuel feed is my first guess. Pump or gummed up carb/jets.

 

[dOb]

Re: too much throttle kills motor...

It was the fuel pump.
I ordered the carb kits and the fuel pump kits. Since the fuel pump takes all of five minutes to rebuild, I decided to try that first. I took it to the lake for a test run and all was well. I'll hang on to the carb kits for later.

I've read that it's the ethanol in today's gas that messes up the fuel pump.

 

[ondarvr]

Re: too much throttle kills motor...

Only 39 years out that fuel pump, darn ethanol.

 

[Yesterday]

Re: too much throttle kills motor...

It was the fuel pump.
I ordered the carb kits and the fuel pump kits. Since the fuel pump takes all of five minutes to rebuild, I decided to try that first. I took it to the lake for a test run and all was well. I'll hang on to the carb kits for later.

I've read that it's the ethanol in today's gas that messes up the fuel pump.

Glad you solved your issue!

 

[dOb]

Re: too much throttle kills motor...

Thanks.

Only 39 years out that fuel pump, darn ethanol.

I should have mentioned, I rebuilt carbs and fuel pump about a year and a half back. At that time I also went with a new tank and gas lines to be sure the fuel was clean from start to finish. But still, If I only need to do this every couple of years, that's OK by me.

 

[ondarvr]

Re: too much throttle kills motor...

Parts purchased after the early 90's are made from ethanol resistant products, although even the OEM's get duped at times when purchasing products. Merc had an issue with fuel lines failing, it was from buying from the lowest bidder and not asking enough questions about what you are getting.

There is really no reason for a part to fail from E-10 use, there are known materials that have been used for decades that hold up fine.

 

[dOb]

Re: too much throttle kills motor...

I'm sure you're right. I'm no expert. That's something I read here while researching fuel problems. I'm not really sure why the pump failed. Perhaps it was from cheap materials. The corks and diaphragm were pretty out of shape when I opened it up. The replacement seems to have solved the problem.

 

[Yesterday]

Re: too much throttle kills motor...

I'm sure you're right. I'm no expert. That's something I read here while researching fuel problems. I'm not really sure why the pump failed. Perhaps it was from cheap materials. The corks and diaphragm were pretty out of shape when I opened it up. The replacement seems to have solved the problem.

That's interesting. was it an after-market pump kit? Or OEM? (I'm thinking Sierra)

 

[dOb]

Re: too much throttle kills motor...

Yes. The one that I just replaced was a Sierra International.

 

[Yesterday]

Re: too much throttle kills motor...

Hmm now I'm wondering if it was a dud, or what the scoop is. What is your choice of new? Still sierra?

 

[dOb]

Re: too much throttle kills motor...

The new carb kits are, but I don't think the new fuel pump is Sierra. I just found a replacement on line. To be honest, I never thought much about it. I'm just glad that I can get parts for a forty year old motor.

Here's a question:
When I take the boat out of the water, I always shut down the motor and disconnect the fuel line. Should I run the remaining fuel out of the motor after I remove the fuel line?

 

[mrklean]

Re: too much throttle kills motor...

The new carb kits are, but I don't think the new fuel pump is Sierra. I just found a replacement on line. To be honest, I never thought much about it. I'm just glad that I can get parts for a forty year old motor.

Here's a question:
When I take the boat out of the water, I always shut down the motor and disconnect the fuel line. Should I run the remaining fuel out of the motor after I remove the fuel line?

Personally I always pull the fuel line right as I'm about to drive it up onto the trailer. The acceleration and push needed to get it up the trailer helps burn the gas out of the bowls quicker. All of the previous generation that I have gone boating with always pulled fuel and let the motor run out of gas. (My dads generation)

I am also curious though if this is the right thing to do or not.

 

[ondarvr]

Re: too much throttle kills motor...

Here's a question:
When I take the boat out of the water, I always shut down the motor and disconnect the fuel line. Should I run the remaining fuel out of the motor after I remove the fuel line?

This opens up a whole new range of opinions, few with many facts.

Running until it stops doesn't actually drain the carbs, it does lower the fuel level in them though.

On multi carbed motors some carbs will run out sooner than others leaving those cylinders out of fuel with little or no lube, this may or may not be a problem.

The only way to actually empty the carbs is to remove the drain plug on the float bowl.

Some people run them dry every time out...others never do.... both have about the same results....experiencing no issues. This is what creates the very different opinions with both sides feeling they have it right and that the other side is wrong. You can pick whichever side makes you feel good.

 

[dOb]

Re: too much throttle kills motor...

Thanks. That's good to know. I just wondered if draining the fuel pump would help it last longer. I'll probably just stick with my normal routine.

 

[Yesterday]

Re: too much throttle kills motor...

Thanks. That's good to know. I just wondered if draining the fuel pump would help it last longer. I'll probably just stick with my normal routine.

I don't like the idea of running any part of a 2-stroke dry, even if you could. My general feeling is the oil/gas will preserve the seals which otherwise dry out, crack & leak. If sitting a while it gas will produce gum in the carbs but then I figure cleaning that up every year or two as part of routine maintenance.