A little Help with a 1976 Merc 500 (50 HP)

[swrredhead]

Great forum, I hope I can get a little help (well, a lot actually)

My Problem.....I have a 1976 Mercury 500 Thunderbolt that I recently inherited. When I first get on the water, this thing runs like it is brand new. It fires up with just a little bit of trying, and runs absolutely great. Powers up right away and gets me anywhere on the lake fast.

After I slow down and troll with it for a few hours of fishing, and go to head home, it does not run well at all. It runs, never stalls, but just has no power. And the scary part, is it seems like it is missing. It runs, then "chunks" or misses like it has a bad plug or is not getting proper fuel. Sometimes, after I rev up and down for a while, it will run good, but not great, and still seems to miss from time to time. It also never goes as fast as it did before (we checked with the GPS), and you can hear it struggling at times (and the boat will not plane)

The next day, when I go out, it is great again when we first hit the water, and then by the end of the day , the same thing. I changed all four spark plugs already and checked my wires and have made sure I am running new fuel and a proper mix.

Any ideas or suggestions? It is not worth spending tonnes of money on, so I am just wondering if you have any ideas or tips.

Thanks in advance.

 

[WillyBWright]

Re: A little Help with a 1976 Merc 500 (50 HP)

I think the high speed circuit in the switchbox is failing with heat. Could also be the stator. Could be a broken wire in the trigger harness. Could be crap in a carb.

I'd drain the carbs and flush some gas through the system. I'd also look at the sparkplugs to see if they are all burning dark brown and kinda damp-looking. A compression test would be good. And if the end of the carb drain screws are green or cruddy, the carbs will need a good cleaning.

Go to your public library and borrow a manual. Photocopy pages with basic specs and parts breakdowns for your records.

The safest way to check for ignition failures on the water is to hook a timing light to each plug wire in succession, then run it and watch the flash. But if it's a clog in a carb, you don't want to run it til it's fixed.

Since it's new to you, new gearlube and fresh water pump. Check the gas tank for water at the bottom. A fule pump rebuild would be a good idea too.

I always like to upgrade the water intakes on those gearcases. The old ones are just flat plastic screens. The replacements are designed much better and it's worth doing.

If you have to rebuild the carbs, you can probably just get a couple of carb gasket sets and new float valves. Way cheaper than whole kits.

 

[swrredhead]

Re: A little Help with a 1976 Merc 500 (50 HP)

Wow, thanks...
when I changed the plugs, one (the bottom one) was black and not a nice golden brown as the other three were.

Looks like I need to write down that serial number and look at some parts manuals. I will try rebuilding the carbs and do a clean up on them. Are parts pretty easy to come by for an engine like this? (I am thinking about the fuel pump and the circuits you are talking about)

 

[WillyBWright]

Re: A little Help with a 1976 Merc 500 (50 HP)

You have the choice of factory parts available through a dealer, and Sierra parts available at NAPA for a premium price, or the same parts in iboats for a lot less, but there's shipping. Look way over on the right of this page by your first post.

 

[OldMercsRule]

Re: A little Help with a 1976 Merc 500 (50 HP)

As Vice Admiral WBW stated: "A compression test would be good". One plug looking black may be a sign of your problem, (but it least it does not look like fresh metal): the compression check is prior to messing with your carbs. If the compression is good: then troubleshoot fuel or ignition issues from that point, and obviously don't run it until you fix it if one of the carbs is gummed up. Good luck. JR

 

[swrredhead]

Re: A little Help with a 1976 Merc 500 (50 HP)

Thanks, so if it is a compression issue, does that mean pulling the pistons and new rings?

 

[OldMercsRule]

Re: A little Help with a 1976 Merc 500 (50 HP)

Kinda depends on how it reads, (remember this is a thirty year old kicker). The main issue is even compression across all the holes. If one is slightly low, you may be able to decarb the engine to deal with the slightly low read, (and fix the issue that caused it if it is a plugged carb,... etc). Bottom line: verify the compression before you ponder how much to spend and how much work it is to correct the problem(s). (You would not want to fix the carbs first, then find out you had a bum hole). Good luck. JR

 

[tmcalavy]

Re: A little Help with a 1976 Merc 500 (50 HP)

And you will need to do a linc-n-synch after you pull/clean/re-install the carbs. If the compression test pans out, the black bottom plug may indicate that the ignition system is dropping that bottom cylinder after the engine heats up. A linc-n-synch used to cost around $50 in my neck of the woods. Had a 73 Merc 500...ran like a scalded ape after I redid the LU seals, replaced some of the old ignition wires, cleaned the carbs and had it tuned. Good outboards. Very solid and reliable. Hope your problem is dirty carbs or a simple ignition fix.

 

[swrredhead]

Re: A little Help with a 1976 Merc 500 (50 HP)

Thanks, is that something I can do myself (the linc and sync?) I had a carbed motocycle that i had to sync the carbs on, so I assume that is what you are talking about.

 

[tmcalavy]

Re: A little Help with a 1976 Merc 500 (50 HP)

Yeah that's it. Synchronizing the movement of the carb butterflies to the timing advance.

 

[swrredhead]

Re: A little Help with a 1976 Merc 500 (50 HP)

Great!
Thanks to all for your help, Looks like I have a fun spring project ahead of me.

Thanks again everyone. I am certain I will be back posting more questions soon enough.