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Mercury 500 High rpm random misfires

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burner67

Member
I'm a new user to this forum but have been lurking around finding answers, how-to's, and solutions to my boating problems for some time. But now i have come to a point where i can't find an answer to a problem or a similar situation on a similar motor.

Im running a mercury 500 long shaft electric start 1974 year. It is the 4cyl side bowl carb with type 2 ignition (distributor, no points)

THE PROBLEM im having is that about half way up the rpm range the outboard starts to randomly misfire on all 4 cylinders. I checked it will a timing light. cylinder 1 is by far the one with the least misfires, and cylinder 4 is the worse, so basically 2 and 3 are worse then 1 but better then 4. But all 4 show the light cutting out at some point or another. On number 4 the light spends more time off then on.

WHAT I HAVE DONE SO FAR

New timing belt
fuel pump rebuild
carbs rebuild
new bulb
tried different plugs
tried different distributor
tried different coil
cleaned all ends of plug wires
cleaned inside distributor cap and rotor
compression test all 4 around 150
choke is out of the way
link and sync according to service manual
had to replace switch box with a used one. shorted the first one out by accident. same part #
using spark tester i get good spark more the 7/16 inch
timing belt/flywheel and distributor are lined up correctly


Motor idles great. starts right up. revs high in neutral but can't get full rpms when in gear in water. hardly any power under load

so basically high rpm random misfires and no power under load. any advice or idea is greatly appreciated. tryin to go fishin this weekend on wallowa lake in oregon

I do have the service manual.
 
Boy, that's a good one! You've already done/ eliminated everything I can think of except...

I'm thinking excessive spark advance, even though you've timed it to death. I suggest this:

Using a piston stop, verify the location of TDC on the flywheel--someone might have removed then re-installed the flywheel
incorrectly (on those 8 bolts).

If that's not it, I'd put new ignition wires on. Someone might have replaced them with the wrong type wires--they need to be steel core wires. Never seen stock wires so bad they wouldn't work, but...

Good luck, and be sure to check back with your findings.

Jeff
 
Thanks Jeff. I have already verified that when the flywheel reads tdc the #1 piston is at its highest point.

Also when i tried the new distributor and coils, it had all its own wires already on it, so i basically have tried different wires.

For some reason i think that it is the used switch box i bought is worn out. The person who sold it to me said that they either work or they don't, theres no inbetween...

When i pull the plugs there is fuel/oil on all 4. But i am guessing this is from lack of spark
 
..."the used switch box i bought is worn out. The person who sold it to me said that they either work or they don't"

Not sure if that's true. I've had automotive switch boxes do some really weird things while continuing to run (crappy).

I'd say try another one. If I had one, I'd let you borrow it for a test.

Jeff

PS: I sent this post to a guy that's selling new ones cheap.
 
Thanks. i do have one on the way. it will be here friday. ill post the outcome! and if it runs the same i'll have one for sale i guess.
 
took it out today and the boat is still not standing up as it once was. it revs good in neutral but when its in gear power and rpms are limited even tho the misfires have quit. also found out i am getting water in the lower unit some how :/
 
The limited power can be investigated (later) with plug reading, but for now that LU has to come apart for new seals. I suggest buying a sealing kit (from this site) that has all the O-rings and seals you'll need. Those LUs are not that hard to work on if you want to try it. I can coach you.

Jeff

PS: Assuming your motor has those gapless plugs, you'll have to slip in some ordinary plugs so you can read the colors (and determine fuel mixtures). But fix that LU first.
 
alright thanks. ill order that. i think there is instructions in my service manual. I think i definitely have a fuel problem now. when i go wot it will be for bore for just a sec and then bog down
 
Just an update. i am going to try a kit from the 76 and newer mercs. it looks like the part numbers match for the lower unit.

also i did fire it up again today. after running it i pulled all the plugs and it looks like all plugs besides number 2 had quite a bit of fuel/oil on them. i adjusted idle per manual with the same results.

i disconnected the plug wire goin to that spark plug and ran it with the same results. less oil on that plug then the others.

So its still the same problem that when i open it up for the first second it runs top then bogs down the RPM and stays there. not super low just not where it should be.

I am curious if this could be a reed problem. im not sure how to check the reeds as the manual i have doesn't really say where or how to get to them
 
Uh-oh. Sounds to me like you're getting water in that cylinder. That's the only way it will run clean like that.

Jeff

PS: It's NOT reed valves! Never seen a bad reed on a Merc in 50 years.
 
If water is getting in there you might need a new block.

I had a customer with a triple that ran lousy on the top cylinder. If I puulled its plug wire off, the motor ran much smoother. Water getting in from a cracked block. You might try this test.

Note: Water in the LOWER cylinder can mean a bad lower seal, but water getting in any other ones is trouble. That particular motor is NOT famous for water ingestion, so if there's water there it's a cracked block--it happens.

Good luck!

Jeff
 
i did pull the plug and it ran the same. odd thing is that this just started recently. im gonna double check tomorrow and see for sure if that's what it looks like. i hope not. if it does for sure look like water i guess i'll be tearing her down.
 
PS: It's NOT reed valves! Never seen a bad reed on a Merc in 50 years.
Click to expand...
I have see hundreds of cracked /chipped/broken reeds on Mercs..........
Note: Water in the LOWER cylinder can mean a bad lower seal
Click to expand...
Change these first but it will require you to remove powerhead...if water is getting in cylinder the plug will have water dropplets on it when removed and will rust around electrode when it air drys..
 
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:/ ok thanks guys im rebuilding the lower unit at the moment and installing a new fuel line as well. the upper half of the fuel line was cracked and leaking a bit
 
Any good advice on rebuilding the lower unit would be great. im following the service manual and i notice it keeps saying "special tool" hopefully nothing i can't improvise for or just fabricate at home
 
haha oh man. i got it done yesterday just before i took it out. nearly beat the hell out of the bearing carrier thinking is was threaded on there. but i did get it completed.

im going to post a video of how the boat ran today.

basically anything over 60 or 70% throttle does not speed up the boat. it may actually slow it down. it seems to be right when the "economizer collar kicks in and the distributor runs into the timing stop. (@35 degrees of course)

I feel like it is maybe getting to much gas.

also i only had 75w-90 gear oil to put in the gear case. it that ok? or could that be bogging it out. because when i do go full throttle it sounds like when you get seaweed in the prop and it bogs out.
'
 
Hummm... Sounds like a major bogging problem, but I'm not totally sure.

The carbs are supposed to start opening when the spark is nearly fully advanced (unlike a car engine, which is the opposite). You might take a look at that. (On my little boat with its custom engines, I open the spark ALL the way before the carbs start to open.)

If that's not it, it might be water intrusion on that cylinder. Hope not!

Jeff
 
no i don't think im getting water. after running it today i looked at the plugs and they all had oil on them. the manual says to have the carbs open at 7-9 degrees of timing and that's how i have it set. this is frustrating
 
If anyone can verify float specs on this carb for me that would be great. my service manual has different specs then the sheet the carb kit came with
 
ok i tore down the carbs. soaked them for 10+ hours. blew them with compressed air. and assembled again paying extreme attention to float settings and still bogging down and losing rpm at full throttle. timing is perfect and idle fuel adjustments are dialed in.

when im holding full throttle and pump the bulb it bogs down even worse. when im full throttle and engage the choke a bit it bogs down worse. I don't understand at all how this motor could be getting so much fuel at full throttle that it bogs down. There is no high speed adjustment or needle. there is just idle and main jet. so basically these are all fixed settings that cannot be changed. Besides it was running at top rpm at one point.

This pretty well leads me to believe i have bad gas and or a bad gas tank. I have been using penzoil 2 cycle oil to mix with my 91 octane unleaded non ethanol gas in my old metal 6 gallon fuel tank.
 
ok tried just a new gas can with new gas and it didn't help. but when i unplugged the fuel line and held it wide open for a few seconds when it would start going lean she gained lots of RPMs and sounded like the beast she should be! of course before she died.
 
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