1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

[Moody Blue]

Re: 1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

It is not uncommon for the battery voltage to run up to 15 or 16 volts untill the rectifier shuts the charge down. thats the way they work. The rectifier will start charging again anytime that it see a drop in the voltage from running lights or any DC powered add on ACC.
Take it out and run it as if you own it. Its more fun that way. The new manual will come in handy more than you first thought.
Oldman570

Nope that's not how a rectifier works. A rectifier merely converts the AC voltage from the stator, to DC voltage for the battery. It does not switch on or off and has no ability to alter the voltage. The higher the RPM of the motor, the higher the voltage to the battery will be. A regulator (commonly combined with a rectifier) will maintain a specified output (typically around 14V) as long as the input voltage to it is more than 14V, regardless of the motor RPM.

The high voltage will tend to boil off the water in the battery with extended running time so be sure to monitor the water level in the battery. The maintenance free type batteries are not recommended for use with these older motors but probably all of them on the water today use the maintenance free type.

I ended up installing an after-market regulator/rectifier because of the excessively high voltage. The onboard electronics don't like the voltage that high. I was measuring 17 volts at WOT on a fully charged battery.

 

[aussieflash]

Re: 1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

Yes,as moody said keep an eye on the battery water for those old motors.Most were only equipped with rectifier(no regulator).
My V6 also peaks at 17V.

 

[Zenvalo]

Re: 1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

Thanks guys! I didn't get notified of any additional responses after Oldmans. I'll definitely keep an eye on the water as it's a brand spankin' new size 29. Don't really want to fry it anytime soon! I'm planning to get a 2nd battery eventually. I wonder if I have them both hooked up either in tandem or via a battery switch if that would help disperse the voltage and not boil up as easily. Something for me to read about I suppose.

So that aside, I did want to post a little update. I received my spark gap tester today and was able to get a whopping 5/8" bright blue spark on all 4 wires. Tried to step it up to 3/4", but no luck. What is considered great spark? I thought I read somewhere that 7/16" is, so I'm sitting pretty good if that's the case.

Still waiting on the 2 carb kits and the manual. I was wondering though, does anyone know how much "play" should be in the lever (Item 11 in the photo below) when the throttle lever is at WOT? While the motor was off, I put it WOT and noticed there seems to be a lot more room for that lever (as it's described in the full diagram) to go. I'm sure the manual will tell me what I need to know, but I'm just curious as I wait.

 

[Moody Blue]

Re: 1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

From your description it sounds as thought your ignition system is working properly. If you can achieve a good blue spark at 7/16" gap then you are golden.

I think you've found you lack of power problem. On the carb body there is a mechanical stop at which the throttle lever (#11) comes to rest against at WOT. In reality, you want to adjust the throttle stop screw so that the throttle lever stops JUST BARELY before it hits the mechanical stop. If the throttle butterflies are not opening all the way, then you are only running at about 3/4 throttle.

 

[Zenvalo]

Re: 1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

Just took a couple photos quick because I believe I seen the mechanical stop you spoke of and it's a good 1/4 to 1/2 inch + from touching it at WOT. Let me apologize right away for the photos. I don't want to start taking things apart until I get the carb kits. I'd lose parts knowing me. First pic is in neutral and second is WOT.

Does it look like the throttle lever is in the right position? If it's too hard to tell, that's ok. It seems like the part that moves when pushing the throttle forward and causes the throttle lever to to turn might be in the wrong spot. Should it be in between the 2 throttle lever arms or on the outside of one or the other? (if that makes sense).

At WOT, the arm you see on the right can be moved an additional 2" + before the short end hits the mechanical stop. Sorry for my lack of terminology, I'll get the lingo down eventually! ;-P

 

[Zenvalo]

Re: 1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

Just peeked in through a little hole in the front while at WOT and the butterflys are hardly open at all. If full open would be considered the edge of the butterfly facing straight forward, I'd say it's open between 1/4 and 1/2 way... Not good I'm assuming... Hurry up and get here manual!!!

*EDIT* Would the RPM's still run high where they should even if the butterflies aren't open all the way? I ask because audibly, when on the water, RPM's sound about where they should be and also considering the amount of volts it's putting out would make me think the same.

I'm excited to figure out how to get those butterflies to open fully so I can take it for test run #3!

 

[oldman570]

Re: 1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

The first picture shows the throtal linkage where it should be at the first start of the opening of the carb. The second is showing that the linkage is only 1/2 the way open. The OME manual will walk you thru the steps as to set the link & sync of the carbs and should have pics of the link stops. There should be a small screw, on the curved plastic plate in the first picture, that has to be adjusted sometimes also. I always disconect the cable to the throtal linage when doing a link and sync then adjust the cable barrel to fit as it should when done.
JMO
Oldman570

 

[davidmerc50]

Re: 1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

i am having trouble i broke the black plastic lickage that opens the 2nd carb and i cant find one anywhere any ideals where i could get one?

 

[oldman570]

Re: 1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

Go to Crowlleymarine.com, Seawaymarine.com, or oldmercs.com, and they might have that link if a search here on Iboats. You might need the small key that goes in that link if you have lost it. Vary easy to do if you did not know it was there.
Oldman570

 

[ckangaroo70]

Re: 1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

Might have better luck if you open your own thread asking your question.

 

[Zenvalo]

Re: 1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

New update. I am able to get the butterflies wide open, that may have just been stupidity on my part... rebuilt the carbs, but I'm pretty sure it was the wrong carbs kits. I got the ones I mentioned earlier. Most of the parts were slightly different. Motor won't even idle now. The OEM manual doesn't explain anything hardly either. Ready to bring it in to a specialist.

 

[chum1]

Re: 1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

[QUOTE=Zenvalo;3901441
Speaking of Tachs, how well do the 'Tiny Tachs' or generic versions perform? The kind that you wrap a lead around the #1 plug wire... Anybody ever used one and compared results to a permanent one? My controller has the plug on front for an actual tach, but I don't really have much space to put in a 2" gauge...



These things work pretty good for the price, just wrap the wire around the lead and walla digital tach reading, the chinese instructions were wrong on the 4 settings for setup but after some trial and error found the right one.

 

[Zenvalo]

Re: 1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

Thanks Chum! I've read both good and bad reviews on Amazon about the Chinese ones, so maybe they're hit and miss as far as getting one that works. Couldn't find any user reviews on the actual Tiny Tach brand, but I'd prefer not to spend $50+ on one anyway...

 

[chum1]

Re: 1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

That price seems to high for what it is, these things are great because u can hook em up temporarily just to find rpm, once u have the right prop pitch/ motor tuned, etc. just pull it off and its ready for another engine. good luck with your project

 

[silentwatch]

Re: 1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

Okay I am brad new to all this but I will share what I just found on my motor Evinrude 50 which was causing similar issues. I don't even know what its called, Choke housing maybe. The fuel line goes in and there is a little plastic valve with a small plastic lever. Anyway this is how the problem was found. With the cowl removed and fuel line and tank attached slowly squeeze the fuel line bulb and look and listen very closley in that area. The cap of that (choke Housing?) was cracked and you could barely hear a small pressure release. What little fuel that escaped ran down against the motor and I was lucky to ever see it and only after the sound made me look even closer. So the motor would heat up and the crack would open up. It was terrible on gas consumption and fought to get on plane. When it was cold in ran faster. $12 fix. That was when I discoverd my lower was locked up as well. No dobt the cracked housing was killing power and fuel consumption though.

 

[davidmerc50]

Re: 1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

mine did the same thing when i rebuilt my carbs the book said to turn the idle screws till snug then turn back 3/4 turn well mine wouldnt idle so i turn the screw till snug then urned back about a turn and a half and it started idleling enough to where i could tweek it the i put it in a tub of water and put it in gear and tweeked it till it idled good i done both carbs like that

 

[davidmerc50]

Re: 1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

New update. I am able to get the butterflies wide open, that may have just been stupidity on my part... rebuilt the carbs, but I'm pretty sure it was the wrong carbs kits. I got the ones I mentioned earlier. Most of the parts were slightly different. Motor won't even idle now. The OEM manual doesn't explain anything hardly either. Ready to bring it in to a specialist.

mine did the same thing when i rebuilt my carbs the book said to turn the idle screws till snug then turn back 3/4 turn well mine wouldnt idle so i turn the screw till snug then urned back about a turn and a half and it started idleling enough to where i could tweek it the i put it in a tub of water and put it in gear and tweeked it till it idled good i done both carbs like that

 

[Zenvalo]

Re: 1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

It turns out that the kits that I got, even though I asked here first, were in fact the wrong kits. I replaced the idle needles with the original ones and now it will idle for a while. The originals were quite a bit longer than the new ones I got. Either way, I'm going to have both carbs rebuilt by someone else and then tuned up to spec. I've put too much time and effort into this thing. My brain is starting to hurt.

 

[Zenvalo]

Re: 1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

Ok, next update. There is a guy down the road from me that works on outboards. Started as a hobby and now it's a home business. Anyway, he said timing was way off and the belt was somewhat stretched and obviously old. Fine, easy enough, replace the belt and get the timing right and we should be good to go!

Well, he just called and left a voicemail saying that he got it on and timed, it turned over a couple times and now it won't turn over at all. He said in the message it could be the switch box? I don't really know what that is/does, but it's something to do with ignition I'm assuming. It all seemed to work fine previously. Good spark, strong cranking etc etc.

I tried looking up a new switch box, but couldn't find one for my model year. The ones I did find were $300+. This is getting ridiculous...

Any thoughts?

 

[davidmerc50]

Re: 1973 Mercury Thunderbolt 500 50hp no power and horrible fuel economy!

wouldnt a key switch do that? the key switch cuts the spark on and off so that might be whats wrong i would ask him if it was still getting spark and gas