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1975 merc 50hp 500 runs great then dies at wot

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duckhunterwa

Member
Just bought a 50 HP 1975. Fires up good compression 135 on cylinders. I run it out on the river runs great then it dies. I fire it back up it idles , put it in gear , when I get to the second part of the throttle it quits on me. I switched tanks, put an inline fuel filter in it , fuel lines are good, diaphragm in fuel pump is good looking. It only does this after it warms up and going into full throttle. Any ideas are appreciated I'm about to give this back to the guy I bought it from. Thank you.
 
Give the carbs a good cleaning and blast of air through and if it hasn't been done before a rebuild might be a good idea.If the original fuel lines check them for gunk and break down
 
Thank you , the only thing that throws me off is the thing runs great for about twenty minutes each time. (Twice now) before it craps out on me. I dont know if i have the right fuel setup. i have a plastic six gallon tank with one of those two hole merc couplers. Going to the primer bulb then to one of those quarter turn twist lock fittings on my motor. Are those two fittings compatible with each other? And is 5/16 hose big enough?
 
I have 3/8" bore on mine but that is because I put in new line and intake connection,a made up because where I live parts for the age of engine,beautiful '69 Merc800,are hard to get if you're in a hurry,and I used a smaller bore,smaller by .060th,and when I went WOT,began to starve of fuel.Two miles out in the Atlantic and one very pale ever patient wife,not good.So you could measure the bore of the coupler at the tank and see what you have.At least 5/16 is what you need.I'll be surprised if it's any less
Just a thought,what type of ignition do you have,distributor or CDI? Possible electrical problem?
Hope someone else can jump in and help 'cause I'm going off for a few days
 
I've got a brand new six gallon plastic tank. There is no vent so I'm assuming it vents through the cap? I got it from Walmart. As for the fuel line I am using the two pin quick connector on the tank then it goes to a brand new inline glass filter , to the primer bulb, then to the quarter turn mercury coupled. The fuel line is all 5/16". Will that setup give it enough fuel? So weird to me that it runs like a dream out into the river then suddenly starts acting up. Thanks for the help I gotta get out and shoot some carp.
 
duckhunterwa said:
I've got a brand new six gallon plastic tank. There is no vent so I'm assuming it vents through the cap? I got it from Walmart.
Click to expand...

I have the same tank...... the vent IS in the cap. There should be a small twist knob, it's almost a 'dial' about the size of a dime that needs to be opened counter clockwise for the vent to actually work. Hope this helps you...

Floyd
 
Ok Floyd I just went out and looked opened the cap on the plastic tank( its an atwood by the way) there is no dial inside only a little white ball inside a hole. The tank says it automatically vents. Did Walmart strike again and sell me a broken gas tank? Thanks
 
Try running it with the tank cap loose, or off altogether, just don't do this with a full tank. Or, after running it until it dies, slowly open the cap, and listen for air hissing back into the tank. Or, does the primer bulb go flat when it dies?
 
Primer bulb does not go flat after it dies. It stays firm and fuel is still visible in the glass filter. As for the tank I couldn't tell the last time. I did remove/ loosen the cap but it was after it died and it didn't make a difference.
 
The other tank is a metal tank with the two prong fitting , no manual vent on it. Both these tanks ran fine on the 35 horse mercury I had on my boat last week.
 
Sounds like you've checked everything out but the carbs. Sounds like an impropper needle valve adjustment. Take the bowls of the carbs and turn them upside down. When the needle vlaves seat, the floats should be level with the bottom of the carbs. If they aren't you need to bend the little tang that touches the tip of the needle valve. I'd blow a little air through the fuel inlet on the carbs also to make sure the passage is clear and open. That should do the trick. If you feel you are not getting enough fuel, set the gas tank on top of the motor cowling and run the fuel line directly to the fitting that splits to the two carbs. Take that vented cap off, pump the bulb until it is hard and let the elevated tank "gravity feed" to the carbs. This will eliminate the possibility of bad fuel pump and venting problems.
 
..."Primer bulb does not go flat after it dies. "

Ah-hah! Know what you problem is: Stuck check valve in the primer bulb. Had the same problem once--drove me batty! Try another primer bulb and all will be well.

Jeff
 
I just want to say this is an awesome forum I am definitely going to stick around here the help has been awesome . Hope i can help other people the way you guys are helping me.
 
Going to try another primer bulb, replace diaphragm in fuel pump. Found some hose clamps left loose on the fuel lines coming from the fuel pump to the carbs and the clamps between the carbs. Gonna spend all day on a pond sat. If I have to till I figure this out. Open to any other suggestions too. Thanks again everyone. Will update Saturday with results.
 
.."It would still run good off the bat then die ? "

Yes, and it may even run fine now and then until.... Grrr!

Jeff
 
duckhunterwa said:
Ok Floyd I just went out and looked opened the cap on the plastic tank( its an atwood by the way) there is no dial inside only a little white ball inside a hole. The tank says it automatically vents. Did Walmart strike again and sell me a broken gas tank? Thanks
Click to expand...

I know this is late - couldn't find the camera (my wife says I'm getting old) - but here's a photo of my cap. Sounds like you have a different version then I do. Sounds like Jeff is right about the check valve in the primer bulb.... I've read a LOT of his posts and he's usually right on the spot with this stuff. Good luck & let us know what happens tomorrow!


1472.jpg
Floyd
 
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Ok well I replaced the prime bulb. Blew into the old one and it fluttered a lot more than the other one. Anyways got out on the water and the thing ran great for about an hour. Then it started doing it again. Checked the bulb and it remained primed however after squezzing it hard for a bit it fired back up and drove around for another forty minutes till it started doing it again. I'm thinking fuel pump diaphragm now. Going to replace that tomorrow. What you guys think? Maybe I had more than one problem. Diaphragm and bulb? Will know more tomorrow.
 
Multiple problems at the same time do happen. It's also possible that you have an air leak at the new bulb caused by a not-tight-enough clamp. The tiniest air leak will cause what you're seeing.

Jeff

PS: Do you have a second quick disconnect at the tank? I've had to eliminate them since they tend to leak air. My hose is now connected right to the tank, which I fill with a portable tank (or at the gas dock). Not as convenient, but it runs better.
 
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Waiting for diaphragm, gonna get it this week. When I pulled the diaphragm off today I could see a small crack with daylight shining through it on the mesh part. Also figured out I wasn't gettin full throttle cause the lever was hitting the mounting bolt. Will take it out this week and update. Thanks.
 
Only other question I had was does anyone know what pitch prop I should be running on this motor? At low speeds it feels kinda like I'm "spinning my wheels" till i get on plane then it feels like it bites.
 
When you use premium gas it must not contain ethanol. If you put the cheap premium in, you now have to take it out. Put the good stuff in.
 
Ok so, replaced the pump diaphragm added some stabil and sea foam, went out ran great till I shut it off and sat a couple minutes. Then it started dying again. On a whim I pulled the tank out of its compartment low and behold it ran again after squeezing tue bulb a couple times. I think the fuel pump is having a hard time pulling the fuel up outta the compartment. Also the two prong fittings at the tank look janky as all hell. I'm gonna eliminate them tomorrow and put a straight threaded fitting with a nipple.Jeff if your still reading this any thoughts?
 
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