TOPIC: 72' 50hp Merc 500, What is single #1 most cause of

Ok you've all been a great help on this odessy( corroided wire harnesses, bad sw. box/rectifyer,stuck impellers,clogged pee hole). I've fixed all these.
Yesterday I took the boat and motor to a local lake and power tested it on the launch ramp( I don't trust this motor to take it out on the lake yet!).
Ok it starts and idles(purrs) just fine. Shifts in/out of gear fine, pumps water fine.BUT.......
At full throttle it does not seem to have the sound of a healthy all out great running machine. I don't have a tach for it, but I can tell she's just not winding out!

I'm about to quit and sell this whole thing "As Is"
Two "marine repair?<more like paying a traffic fine!>shops haven't helped in the past so I'm not putting more hard$$ into this thing.

What Is the "SINGLE" most cause for this? I'll try one more session of tinkering w/it if I get a consensus>

Seems to me that these old Mercs are just a big hole in the H20 to trhow $$ into!

Wondering what you mean by power tested. If you mean somehow throttling it up while still tied to the dock or something like that, it's not likely it would reach normal rpms.

Yes, I powered it up to full throttle while it was still on trailer w/whole lower unit in H20. Why would that make a difference? It's better than a test tank (the way ashop would test it),at least it's under a full load.If it can't max out this way, having it running free and maybe getting stuck out on the lake won't help a bit.

Any way, thats not the issue.

Good fresh gas,good spark,good H20 output (no overheating).Why won't it achive top RPM?
I'll try "one"more simple? fix if it's in my relm of skill and tools to fix..... then this motor becomes an anchor!

Yes and are you sure you have the correct prop for the boat, didn't read previous threads:( Sounds like you've done alot to get it right, don't give up, just the right pro's haven't been on forum yet. With a small boat I'd think you'd get into the high 30's but don't get crazy with over reving. We've seen a g3 with a 500 get well over 40mph but thats with lots of trickery:)

DelVel,

I am not a engine or prop expert, but I think the RPMs may be different if the boat is moving through the water. As I take off in my boat, I go to full throttle. It takes time for the boat to plane out, and gain speed. The RPMs continue to climb during this time. It does not go right to full RPMs. Sure the RMPs jump up quickly at full throttle, but they do not reach higher levels until the boat is moving faster through the water. I think each “bite” of the prop is pushing the boat forward and allowing it to spin more. Without the boat moving, it seems like it is more effort for each bite of the prop and more work, which is less RPMs.

Your motor runs and does not stall, the water pump works and the engine cools right?…So why are you keeping it on the trailer? The other boaters will help you if you stall out. Don’t venture off too far, but idle out, throttle up and see what it will do. All the fun is out on the water. I would give up too if the boat never got off the trailer.

take the leap and go!. you will not reach full rpm on the trailer unless your using a factory test wheel. you are pushing water, if you could reach full rpms,You would need a prop with LOTS more pitch


Of course you could go into the wind(if any) that way you get blown back to where you started, or up stream if its a river.



I am a certified OMC and Mercury Tech!

Without moving it will probably have about half the rpm it should at WOT. Thats why they make test wheels for running the engine at the correct rpm when ran in a test tank. Go for a test run, what is the worst that could happen? Paddle back?

the only way to check full throttle in a static position (on a trailer) is with a test wheel that is the purpose for them a good 500 with a test wheel on is 5800-6000 RPM.

Post the results of a compression test.---Then check to see if you have spark at each lead that will jump a gap of 3/8" or more.--This could be as simple as a bad sparkplug / water in the fuel / partially plugged jets / sprak jumping out of a plug wire / dirty distributor cap.

Thanks all of you for the infor and advice.
it's not a spark/fuel problem,
best bet is that it could be a timing issue?
I may get time to run it again next week(and take a paddle & tow line)when I can afford to get stuck out on H20 !

Sad stat of affairs when a person can't find a repuatable shop that will give a sound estimate w/o doing more damage to a motor than when you bring it in,in the first place( and still have the nerve to charge you for it!).
See 4 page saga of woe on I-boats link. I'm experienceing the same thing!

forums.iboats.com/mercury-mariner-outboards/1973-mercury-thunderbolt-500-50hp-no-power-horrible-fuel-economy-563374-4.html

Maybe it is that the lower crankshaft seals are leaking and thus the motor is running on 3 cylinders.--Could be the exhaust tube plugged with carbon too.---These are easy things to check out !!

Revving the Merc up tied to the trailer is not only dangerous without a test prop, it can undermine the boat ramp and upset other folks that use the ramp.
Here is a picture of what the professor is referring to.
doc F

Reving the engine to WOT while still on the trailer? You have to be kidding me. Aside from the above stated obvious about not being able to reach any kind of revs without a test prop, if that trailer ball hitch got loose or the boat decided to climb over the winch..... NEVER do anything like that again, besides being dangerous you can wreck the engine as well loading it up like that without allowing it to reach rated rpm. Surprised you did not hole a piston.

Time to find a grey haired mechanic who will take the time to do a " proper diagnoses " instead of them young fellers who are good at changing parts and recommending " why not buy a new motor like everybody does "

My 1967 Merc 500 is running good, but, like you, I just cannot trust an engine that old so I take a Minn Kota trolling motor along just in case.

I agree with others here - take it out on the water to test it. If it starts and idles it should get you back to shore.

Good luck,
Bill

To be honest with you, I would rather go offshore with a vintage engine that I built myself than trust a new one. The electronics and fuel system components are built to a price, not a standard and the lack of quality is appalling. At least with the older systems you can usually jury rig a repair to get home in the event of trouble, but the new blackbox engines? Lets just say, well, not so much.

Up-date:
Took it out on the lake:
@ WOT she wnt about 4 kts.No more.
Engine maxed out at about 1000 rpms(hand held tach.
No sputtering,slight mis occassionally:
Can't access carbs w/o removing whole front cover plate where sw. box and rectifiers are held on(so how on earth can one adjust them w/ engine running?) but it looks like trottle shaft is full open<hard to tell>.
Back at dock, we put a timing light on it(all cycl sparking).
Pulled plugs ,<no fouling>
Total run time puttsing around about 20 min(least we didn't get stuck!

So where do I go from here?

P.s.
New seperate issue, Now eng won't shift into reverse(it still runs in forward when put into reverse! and when it should be in netural, it idels but prop is still engaged (in forward)!.

Clean the carburetors !-Sounds like one may be plugged and you risk major damage to the engine.---No fuel going through one carburetor will starve 2 cylinders of oil as well.

Knowing the 500 like I do, it could be that the exhaust is plugged. The tuner has a bad habit of plugging up if cheap oil has been used at the wrong ratio and will choke the engine. Take off the lower unit and look up the tuner with a flashlight to see if this is the case, they can plug right over in some cases.As far as the shifter goes make sure it is not a busted cable in the box, you should be able to see it move back at the engine. Shift shafts have been known to rust through at the waterline as well killing the shift.

Shame your half the country away from me , i would love to come help with your merc. I am working on two merc 500's right now while restoring my traveler,this time I'm not using wal-mart spray paint!

Hello Delev,
You made an interesting comment about the lower unit not shifting of of forward. Have you checked the gear case for lube? I have experenised this first hand where gear cse will seize (per-say) and slow the engine, also stop the ability to shift the engine. Have you looked at this?

Robinr01

Up-date:
Took it out on the lake:
@ WOT she wnt about 4 kts.No more.
Engine maxed out at about 1000 rpms(hand held tach.
No sputtering,slight mis occassionally:
Can't access carbs w/o removing whole front cover plate where sw. box and rectifiers are held on(so how on earth can one adjust them w/ engine running?) but it looks like trottle shaft is full open<hard to tell>.
Back at dock, we put a timing light on it(all cycl sparking).
Pulled plugs ,<no fouling>

THE SAGA CONTINUES:
Based on all your comments I took motoer into shop about a month ago. They said it wpuld be one week before they got to it. OK. TOLD THEM THROTTLE WOULD NOT GO PAsT 2/3s (open butterfly was hitting stop on dist. Advance?
That I wanted them to adjyust throttle so as to get full open and set timing and see if engine would get more power. Pretty clear right?
Week two:
Shop" we changed the spark plugs and replaced a cracked fuel line($80.oo). we still cant'figure out why it's not geeting power, maybe it's in distributor or coil? but it has great compression and it runswe'll go ahead and adjust timing and throttle and see(Dah I asked you to do that in the first place: why the new plugs $)
Week three:
Shop "your coil is no good/(it runs why is it no good? ) it's intermittant spark." ME "Did you adjust throttle and timing? SHOP "no not yet we're dealing w/ all these other issues first?)
Ok new coil($$).
Week 3.5:
Shop: "Well the throttle wasn't opening full and the timeing was way out(DAH/ DAH?...**$#*!!!). but it's running a little better but it should be just screaming, we think the carbs are starving for fuel? Ok 2(rebuild carb kits + labor more $$ more time).We can have it for you tomorrow........BUT..... we need to order the rebuild kits and we're closed Sundays & Mondays so we won't get the kits in till Tuesday.
Week 4.5
Shop..." Hi this is Tim, give me a call.
Me" is it ready?
Shop: Wow man your motor must be possesed by demons! It should be just screaming....but we can't get it to go much better.
That will be $485.00 sorry but we don't know whats wrong with it must be in the Reeds. This isn't worth fixing?

OK GUYS SO I'M THE SHMUCK> MY GUT INTHE FIRST PALCE ONCE IT WAS RUNNING WAS TO JUST SELL THE BOAT,TRAILER MOTOR COMBO "AS IS" LET THEM SEE THE MOTOR RUNS, PUMPS AND SHIFTS BUT DOSN'T COME TO FULL THROTTLE AND AT LEAST GET $600-700 for it. NOW I CNA'T EVEN DO THAT!

i need advise; My inclination is to just walk away and let them keep the whole package as a LEIN" BECAUSE BASICALLY THEY NEVER DID WHAT i ASKED THEM TO DO IN THE FIRST PALACE(ADJUST THE THROTTLE AND TIMING 1st! And the motor dosn't run any better then when it was brought in!!

Shop: IS CLAIMING THAT "ALL THESE OTHER ISSUSE HAD TO BE DEALT W/ FIRST!

bottom line is engine still won't peak throttle i'm out best part of $500.00 bucks and I'VE LOST THE ABILITY TO SELL BOAT COMBO SINCE IT'S NOW FAST APPROACHING LABOR DAY AND UP HERE IN Pacific N.W. everyone bails and sells their boats befor winter. I'll be lucky to get $500. for it if that!

P.S.

And as far as the shop using a "POWER WHEEL" They don't have one and never heard of one so they don't know how it will now run under load>


Is there a sane compromise to end this all (basicly the motor is a $500.00 anchor)
Moral of the story is never buy a used outboard w/o seeing it run on a boat in the H2o under load and for a full day.

We'll thanks forlistening to my Rant.

The one 50 HP I did last year was a simple fix.----Lower crankshaft seals were completely shot.----Exhaust tuner was 2/3 plugged.

---

Owner said thanks for helping me as the shop just took my money ( lots of it ) and did not do a thorough diagnoses.---This happens all the time! ----Note , it is hard to find a 25 year old mechanic with 40 + years of experience.

You need to check local law i think. they cant hold it for fees i think esp if they did work you didn't ask for, go to small claims court. I think usually any work exceeding what you requested in writting cant go over 10% of that amount unless you signed it off. If nothings in writting your in even better shape with a judge imo.?