Mercury 50 hp thunderbolt issues

[Outdoorsman321]

I bought a 14ft flat bottom with a 50 horse mercury thunderbolt jet (not sure of the exact year) a few weeks ago. I've taken it out a number of times and ran great. Yesterday I took it out and was cruising along at top speed, and it started to lose rpm then it would pick back up, and then it did it again and it died and I was having trouble starting it. Is there something I should look into before I start tearing into it? Was thinking maybe new spark plugs and fuel pump?

 

[Mercurylips]

Make sure the valve on top of your gas tank is open. If it was start checking your gasoline for age and quality. Then your carbs and fuel pumps. While you are at it give us your serial number so we can tell you the age of your motor.

 

[racerone]

One of the first things to be done is a new water pump impeller on a " new to you motor ".-------Now do a compression test on it.

 

[Outdoorsman321]

So I compression tested it when I bought it. All of the cylinders read at 135 if I remember correctly. There is no serial number plate on the motor. I've gotten the numbers

 

[Outdoorsman321]

So I already did a compression test and all of the cylinders read 135 if I remember correctly. There is no serial number plate on the motor and the numbers I could find on it we're 27935, and 831-3719

 

[Outdoorsman321]

One thing I did notice was when I tilted the motor up to search for a serial number I noticed fuel dripped out of the carb when I tilted it. Not sure if it was fuel coming out of the top end of the float area or what.

 

[racerone]

Those are part numbers and NOT a serial number !

 

[Outdoorsman321]

I took it out last saturday, started up find idled fine but it wouldn't go past half throttle without falling on its face. I'm going to start with new plugs and fuel pump. Any other ideas?

 

[Mercurylips]

I borrowed the picture from another thread so you could see that you don't raise the motor to find the serial number. I have a 1977 50 HP and had some real ups and downs with it. Started with the fuel system then into the wiring. The insulation on all the internal wiring was crumbling and falling off. Common with these and others from the year class and near year classes. I used and use Sea Foam and Regular unleaded non ethanol gas.. You may find what motor year you have printed inside your front cover. Mine had a date and serial number there as well in a couple other places. If you will, please remove your flexible cowl and take pictures of each side of the motor. this will give me a good indication of what you are dealing with and the correct model and year class. Thanks.

 

[Outdoorsman321]

Sorry I haven't gotten back to you. Serial number is 2518908. I worked on the motor all day today. Got it running at my house to try and diagnose what's wrong with it. It starts right up and runs for a minute or two and then it starts to stumble and then dies. I've narrowed it down to the fuel system. If I pump the fuel bulb when it starts to stumble it catches back up and runs fine for abother minute or so.

 

[Outdoorsman321]

I rebuilt the fuel pump and replaced the bulb, as well as new fuel line from the tank to the motor. Not sure where to start on next..

 

[Outdoorsman321]

Also hooked up a seperate 5 gallon tank with a much shorter fuel line and ran it, same issue. So between the fuel tank and bulb is not the issue.

 

[Mercurylips]

I would replace the fuel lines running to the fuel pump from the motor's hookup to your tank. Very inexpensive to do. Same as auto fuel line. Sounds like it is sucking in too much air thru the fuel lines . If I found the correct motor using your serial number , it doesn't look like yours is anywhere near the same as mine. Looks to be a pricey fix on yours. Hoping you will be lucky and able to make your parts work just by replacing the rubber hose on existing fittings using clamps.

 

[Outdoorsman321]

What is the pricey fix? So I pulled the line coming from the pump to the carb and cranked the engine and nothing comes out, I pump the bulb and fuel comes out of the pump. Im goin to have dig a little deeper into the fuel pump and hopefully find a diagram for it to make sure I'm not missing something. I did forget to mention that the old diaphragm that came out of the fuel pump had nice size slits and cracks in it.

 

[Mercurylips]

I was looking at a parts diagram and the price list for the lines. Over 110.00. Fuel line for mine was about 2.00. Looks like you are getting it narrowed down tho.

 

[merc850]

1969 Merc 500 parts are cheap and available from Mercury, if you replaced the check valves one goes in the valve body and one sticks up from the casting.

 

[Outdoorsman321]

Yep I got it back together last night, fired right up and stayed running without stumbling so we'll see how it runs out on the river today. Fingers crossed! Thanks for the help I apprecite it. Where is a good place to find partsh for the motor?;

 

[Mercurylips]

Honestly. The best place I have found is with our host parts department, here, at I Boats. I try to dance with the one that brought me (so to speak).The prices seem to average out with just a few bucks either way with competitors. Overall, I-Boats is less expensive and the forum "followers are incredible".

 

[Outdoorsman321]

Well.. turned out to be the fuel diaphragm. Have about 5 or 6 trips back out on the water with it running great. Take it out on sunday were heading back to the boat dock full throttle and it's like I turned the throttle down to idle. Won't rev out and it just sounds weird when I give it some gas. Doesn't sound right... has great spark and good compression.

 

[Outdoorsman321]

Any ideas?