Primer bulb strangeness

[nipperdog]

Hello all, and thank you for letting me be a part of this great forum. I've been a long time lurker, and finally registered a short while back. Anyway, I have what seems to me to be a strange issue on a Mercury M-500 (1978 4 cylinder 2-stroke). I live on a small lake and am a retired pipefitter who was in industrial HVAC service for 30 some odd years, so people think I can fix boat motors? I do have a bit of luck now and then, and I do enjoy working on them, it kind of gives me something to do. This little Mercury had the bad luck of being worked on by someone who should probably never work on outboards, it was pretty messed up. One of the main issues I found was a wire harness clamp tab pinched between the crab. and the intake causing one heck of a vacuum leak and crazy high idle as you can imagine. I finally got things going my way, and did a basic tune up including changing the lower unit lube, spark plugs, wires, and the seafoam treatment. I filled the tank with fresh high octane non-ethanol gas and synthetic Mercury 2 stroke oil. The motor starts and runs very nicely, but when it gets close to WOT, it pretty much falls on it's face. It runs fine up to just below WOT. The other strange thing is, the primer bulb never gets hard. I changed the bulb and fuel line, and even stuck the new line right down into the tank to bypass the pick up line and anti-siphon valve, but it still doesn't get hard. It won't even get hard if I disconnect it from the motor? If I pump the heck out of the primer then start it, it does smoke an awful lot, like I flooded it some. I'm wondering about the carburetors as it's obvious this other fella had at least one of them off due to the pinched clamp tab. I guess if the float height was wrong, it could go lean at WOT. Maybe I should do a high speed kill, then read the plugs? Any ideas are appreciated.

 

[Mercurylips]

That is possibly a fuel line leak (internal) or floats on the carbs needing readjusted. The first thing I did was replace the external fuel line and bulb and got rid of my fuel filter and just use non ethanol gas. After all that I had to replace the Stator and the wiring harnesses . I have the same motor in 1977 vintage. Now I am redoing the lower unit. (but it sure runs nice).

 

[GA_Boater]

Welcome aboard, nipper.

If I pump the heck out of the primer then start it, it does smoke an awful lot, like I flooded it some.

Primer bulb not firming up is usually a sign of either carb float setting or bad float needles, which does tend to flood the motor. So suspecting the carbs is spot on.

 

[nipperdog]

Thank you gentlemen, this little motor does run good. It's kind of amazing how well it starts from cold and just idles like a dream. I'm more used to working with Evinrudes than Mercs, but this one has been awfully agreeable with me. I think I'll pull the carbs off and take a look, set the floats, and replace the fuel plumbing inside. Speaking of lower units, I hope this one will be ok. A lot of boats here don't come out of the water very often, and this seems to be one of those. I drained about a cup of black oil out of the lower unit after I took the plug out with an impact driver, yikes. I flushed it out and filled it with that nice blue stuff so hopefully it will survive the neglect. It's kind of funny, I work on several outboards every year, but I run a 20 H.P. Elco electric on my pontoon. Other than changing the lower unit lube every year, there's not a lot of maintenance on those. I have to get some better batteries next time though. I'll keep you all posted on the Mercury and thanks again.

 

[QBhoy]

You are no doubt on the right track...GA has the most likely solution.
In addition, I'd be tempted to check the fuel lines for security. Possible they could be drawing air somewhere at high rpm. Most likely around the fuel lines and filter at engine.

 

[nipperdog]

Argghhh! Well, it's getting to the spot where I'm hopefully going to learn something about these rascals. I have everything fairly well ironed out as far as new plugs, solid copper wires that I made up, and new fuel lines throughout. This was all stuff it needed, so I did it as a semi-shotgun approach to fixing this thing, as I don't know a heap about it. The end result is; it will rev up just great in neutral, and even in reverse, but falls flat on it's "bow" in forward gear. I saw a youtube where a fella was using an inductive timing light to check the trigger? I can't seem to find it again, but he basically showed that it was firing more than once by marking the flywheel in 4 spots with chalk and running the motor (you get the idea). Is that a good test, or do you all have a better one. I don't have any special Mercury tools or diagnostic stuff. Oh, for what it's worth, I even pulled out the anit-syphon fitting on the built in fuel tank and replaced it with a straight through test fitting I made up, and no difference. I will of course re-install the safety fitting once I get it sussed out. Thanks a heap for the help in advance.

 

[Renn90]

Welcome aboard, nipper.



Primer bulb not firming up is usually a sign of either carb float setting or bad float needles, which does tend to flood the motor. So suspecting the carbs is spot on.

Good advice.

Dog, have you checked the needles and floats yet? Bad floats and/or float settings, and/or worn needles can cause the problem you describe.
If you already don't have one, you can get a free manual at boatinfo.no.
Most manuals explain how to disassemble and reassemble carbs with appropriate float settings. If there is any degradation to the cone of the needle, or if they don't easily fall out when the float is removed, they need to be replaced.

 

[Fgrand54]

Good advice.

Dog, have you checked the needles and floats yet? Bad floats and/or float settings, and/or worn needles can cause the problem you describe.
If you already don't have one, you can get a free manual at boatinfo.no.
Most manuals explain how to disassemble and reassemble carbs with appropriate float settings. If there is any degradation to the cone of the needle, or if they don't easily fall out when the float is removed, they need to be replaced.

Did boatinfo.no change?

 

[jbcurt00]

Did boatinfo.no change?

Yes, to a no longer available website,
Avoid hijacking old inactive topics

Closed