Merc 500, Carburettor trouble

[ZandoR]

Hi,

I am having trouble to get a 1984ish Mercury 500 started. Here is what i found:

-Spark on all cylinders
-Don't have the number, but compression is okay
-Engine fired up when I dumped premix straight in the cylinder

Now to me that suggests a carb problem. So I cleaned the carbs thoroughly twice, removing some TINY gas deposits.
Still nothing. Not even a pop or something. Spark plugs are completely dry.

I cleaned all the visible jets, float chamber etc. with a whole can of carb cleaner and a needle.
The only thing that might be a problem is that I didn't take the welch plug off. Do I have to drill it out in order to clean out the idle circuit?

Are there any other typical problems these engines have that I should have a look for? Any other ideas regarding not getting gas to the cylinders?

I would really like to have everything figured out before putting it back together again, the way this motor is crammed just sucks. LOL

Appreciate your help!

 

[GA_Boater]

Got a serial number? "1984ish" doesn't help.

Was it running before the work?

Float adjustment, fuel pump and idle screw setting are good places to start.

 

[harringtondav]

I'd try using aerosol carb cleaner with a straw to flush all passages and circuits. Then follow with compressed air. Both will make the turn under the welch plug. Look for a stream of the carb cleaner at each of the bore passages.

My first guess is you have a fuel delivery problem to your carb. With the float bowl off, connect the fuel tank to the motor and squeeze the primer bulb and see if gas dribbles from the carb. Then button up the carb and try running the motor off the primer bulb's pressure, or with your gas tank set higher than the motor for gravity feed If it runs, but dies when you stop squeezing or siphoning fuel, it could be your fuel pump.

My 4 hp "Gnat" had similar problems. My fuel pump was contaminated with fuel hose crumbs. Later I had another starting problem. I removed the reed valve assy and cleaned it up with carb cleaner. The motor had been idle for a couple years, and the reed valves were sticky. It started fine after that.

I did have to turn the motor over with my cordless drill and a socket on the flywheel nut the first time. After that the starter cord worked fine.

 

[racerone]

Look at the choke / primer circuit.------And are you pushing the key in and holding it in while cranking it over ??

 

[ZandoR]

@GA: I was told it would be running, however this was not the case. I will have a look at the serial Nr. tomorrow.

@harringon: Alright, will try it again with the carb cleaner. This engine doesn't really have a fuel pump. The pumps are built into the carburetors. They are the ones with the blue plastic cover on the side. Is there a way to clean up those rotary reed valves on this motor without disassembling the whole thing?

 

[ZandoR]

@racerone: So far I was cranking it with just the pull start. I choked it via the manual choke. No Primer solenoid on this one.

 

[racerone]

But those are the fuel pumps !!---And they work the same as any other outboard fuel pump.---Does the choke or primer enricher work on this motor , yes or no ???-----If you need to clean the reed valves you have major problems.-----They are just a reed valve and NOT a rotary valve.----The reeds on these rarely cause problems.------Is the motor cranking fast enough to produce good spark ??

 

[ZandoR]

Okay, after getting distracted by some other stuff I'm back on it again

I'm getting fuel in the bowls, so the way up to the float chambers are clear.

Started the third disassembly and carbs look pretty clean, though there was minor debris in the bowl again.

I cant remove the main jet, as the head is stripped. Upon closer inspection it looked like there was some

debris around the jet. Not sure how to get it out.

Does anyone know what the idle circuit is on those carbs? I Will soak everything in carb cleaner tonight and

do some more cleaning tomorrow. Last time I didn't remove the floats, which I would like to do this time. I'm wondering how to get the pin out that the float pivots on. Just tap it out?


Thanks for your help!

Cheers, Leo

 

[ZandoR]

Little update if someone encounters a similar problem:

When you clean the carbs make sure you don't miss out on that little tube next to the main jet. I guess it is part of the idle circuit.

What almost brought me to cleaning the carbs a fourth time was a wrong starting procedure. I've had OMC's, Yamis and two Force which would give me at least a little pop when started with choke engaged and no throttle at all.

It seems that you have to open the throttle at least a bit for it to pick up some gas with this motor. Otherwise you'll get absolutely nothing.

To get it started for the very first time I also had to pour some gas down the carbs. It now starts fine and idles very smothly on the muffs.

Have a nice one!