1976 Mercury 500 only runs for a few seconds

[OCboat]

Any advice is appreciated. My previously running 1976 Merc 500 will not stay running more than a few seconds at a time. If I give it some choke as it's dying, I can keep it running for a little while longer but as soon as don't choke it, it will die. I took it out to the river a few days ago and it ran fairly well for around 20 minutes until it just died. According to my tach, it was running around 4800 rpm at full throttle. It did not like to idle very well and died a couple of times earlier but restarted before finally failing to start. Here is everything I have done since this issue has started:

Checked compression, 125, 122, 123,121. Checked spark on all plugs and it will jump 7/16 gap on all of them. Changed out fuel line from tank to engine including primer bulb and filter and bayonette. Disassembled both carbs, cleaned with carb cleaner and blow out with compressed air. Changed the built in fuel filter diaphragm and gaskets. Set the float on both carbs according to the service manual. Replaced spark plugs with Champion l76v. I have disconnected the kill switch wires from the switch box.

The primer bulb does get hard but squeezing it while running does not keep motor from dying. I have checked for bad or not connected ground wires but haven't found any. I have bypassed the tank pickup by placing the end of the line directly in the fuel. I have bypassed the motor fuel inlet by connecting the fuel line directly to the tee between the carbs. It sure seems like a fuel delivery issue but I can't think of anything else to try.

 

[jimmbo]

I have bypassed the motor fuel inlet by connecting the fuel line directly to the tee between the carbs. It sure seems like a fuel delivery issue but I can't think of anything else to try.

So are you also bypassing the Fuel Pump? Or does this motor have 2 Fuel Pumps, one for each Carb?

 

[OCboat]

There are two fuel pumps integrated into the sides of each carb. I just bypassed the male connector on the front of the motor in case that connector was causing the problem. The fuel is still going through the fuel pumps and the line is now connected back to the front of the motor since it didn't help the problem.

 

[jimmbo]

Is the Primer Bulb installed in the correct direction? If it is and it is not moving Fuel, one or more of the Check Valves in it are stuck open
Have you tried spraying a bit of Premix into the Carbs?

 

[OCboat]

Primer bulb is installed in the correct direction and it is moving fuel. It will fill the carbs until the needle valves close and will stay hard. I have not sprayed any premix but the engine will run for a couple of seconds on what little fuel gets into the crankcase when the choke is activated. I'll try the premix tomorrow but I'm sure it will run until it uses it up. It just seems like the fuel is not leaving the carb bowl. I've removed the carbs one more time and am cleaning them thoroughly once again and will try again tomorrow.

 

[jimmbo]

To look up the correct Parts list, I would need a Serial #.


When you cleaned the Carbs, did you remove what look like little Frost Plugs, and get the area behind them, all cleaned out? New Discs come with the Carb Kits.

 

[OCboat]

Serial number is 4357748. I have not removed the frost plugs since the kits that include them is a complete rebuild kit and are close to $100.00 each. Maybe I can buy them separately if I can find a part number for them. These are Tillotson ba3 carbs. I appreciate the help so far.

 

[OCboat]

Progress! I realized that I didn't change the short fuel hose from the engine side fuel bayonet to the tee as well as the two 2" hoses from the tee to the carb inputs. It will now stay running for longer than 2 seconds but it will not idle well or run for long with any neutral throttle advance. Now I'm not sure if it is still not getting enough fuel, too much fuel or possibly an air leak somewhere. I'm sure I'll figure it out but at least I'm making progress.

 

[Faztbullet]

Sound like carbs and fuel pump diaphragms needs to be addressed again. If installed incorrectly or wrong type the lil flapper ears do not seal the passages in carb body.

 

[OCboat]

Sound like carbs and fuel pump diaphragms needs to be addressed again. If installed incorrectly or wrong type the lil flapper ears do not seal the passages in carb body.

I agree. I've got it running well as long as I pump the bulb when the bowl gets empty. I have another set of diaphragms on the way. The kit contains two "flapper ear" gaskets, one yellow nylon an one thinner rubber one. Do you know which one I should use or should I use both and in what order?

 

[racerone]

Is the gasket between the carburetor and block the correct one ?----Some have 1 wee hole for the crankcase PRESSURE pulse.----It must line up correctly !

 

[OCboat]

Yes. Small hole in gasket lines up with small hole in crankcase.

 

[Lammie]

Is the carburetor link between the carbs adjusted so that both throttles are closed at idle. The linkage has a slot. There is a screw in the slot that allows adjustment of the bottom throttle valve in relation to the top throttle valve. The top throttle valve is spring loaded and should automatically close the top throttle valve. The carburetor link synchronizes the bottom carb so it is also closed at idle. Make sure that the idle adjustment screw is backed out before making this test. Also is the fast idle setting correct. On my 500 the fast idle lever needs to be straight up and down at start-up. That is when the fast idle stop has a 3/8 inch gap between the stop and the stop lug. Seems each Merc control is a little different but I believe the 3/8 inch gap is important to the start and running until the engine is warmed up. On my engine it needs to run at fast idle for about 30 seconds before the fast idle setting is no longer needed. From the content of your posts it sounds as if you have enough savvy that it seems unlikely that you did something wrong so I would go back and re-check the mechanical linkage setting between the carbs and the setting of the fast idle adjustment. Those 500's with thunderbolt ignition can be cold runners until warmed up. That's my 2c woth.

 

[OCboat]

Thanks for the detailed post. Yes the carbs are synchronized and the fast idle is working correctly. I can get it to start easily now and it will run at all throttle positions but only until the fuel bowls run out of fuel. If I pump the bulb periodically it appears that it would run all day although this is just on the muffs. I'm guessing that something happened to at least one of the fuel pump diaphragms when I was having the issues with the deteriorated fuel lines. I have another set on the way along with new needle valves and will report back when they are installed.

 

[OCboat]

Definitely the fuel pumps. I put the gas tank above the engine and it ran by gravity feed for 10 minutes. I then put the tank back in the boat and it ran out of fuel in 1.5 minutes.

 

[Lammie]

I'm 83 years old and trained in auto mechanics and diesel as well as small engines. I say that not to brag but to say that I have been around the block on engines. When I was in my late thirties the mercury 500 thunderbolt was the hottest thing in town. In my previous post I covered the common overlooked symptoms for the problem you are having. Two other things that had me pulling out my hairs were bent spring on the needle valve. The needle valves on these carbs are for some reason very small. In the case I experienced during disassembly the springs got deformed so that the needle valve was not centered in the hole and would bind up. The second issue was an air leak in the hose connection of the small carburetor to carburetor fuel line. In your case the fact the motor will continue to run when on gravity does suggest a problem in the fuel pump system.

 

[OCboat]

So I installed the new fuel pump diaphragms and gaskets and the new needle valves. It will now run without running out of fuel at fast idle but it still wanted to stall or run very rough at normal idle. During the time that I could get it to run at normal idle, I sprayed some seafoam in the top carb and it ran normal until it burned off. When sprayed into the bottom carb, the engine started to flood out and die. This tells me that the top fuel pump is still not working correctly. After running it at fast idle for several minutes I was able to return to normal idle and it ran fine without having to add Seafoam to the top carb.

Could this be a reed valve problem? When the carbs were off, I put a piece of paper over the carb openings on the the block and cranked the engine. The bottom carb really sucked the paper inward while the top did a little but not with as much suction as the bottom one.