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

[Roadblock]

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

I know this thread is over 90 days old but my god it's filled with SUCH good information. I have a 1970's but it's basically same motor.

Tell you what I learned, want a factory manual and a TACH as I'm having some of the same issues!

 

[Zenvalo]

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

I had a manual, but the mech that originally made it run worse than when I first got it, never gave it back. Now it's just been sitting. Still don't know what to do with it. Found a place last year in Wisconsin that said they'd look at it, but it's kind of a long haul for potentially no results.

 

[highsider989]

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

Zenvalo

I'm old enough to have a lot of experience with motorcycle carbs, both two stroke and four. so some of this is very fimiliar.

Did you replace the float and set the level properly? It sounds like a fueling issue to me. You should get your manual from the tech.

As the motor does run and idle, you dont own a complete POS. Often tuning is difficult, tedious and costly. Keep thinking and keep going!

I think I'll own one of these motors shortly so this thread has been great.

 

[Zenvalo]

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

I'm 99% sure it's something with the carbs, but even when I had the manual, I couldn't figure out adjustments and such. On top of that, they are not very easy to get at since there is so much stuff packed into a tiny engine compartment. I'll either try to sell the whole setup or just sit on it until I can afford to have it worked on by someone who is competent. Parts are becoming increasingly rare, so it's hard to say what's next...

 

[bietau85]

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

I'm 99% sure it's something with the carbs, but even when I had the manual, I couldn't figure out adjustments and such. On top of that, they are not very easy to get at since there is so much stuff packed into a tiny engine compartment. I'll either try to sell the whole setup or just sit on it until I can afford to have it worked on by someone who is competent. Parts are becoming increasingly rare, so it's hard to say what's next...

Where are you looking for parts? I have a motor older than yours and share alot of parts with your motor and I can find almost anything I need in 10 min or less browsing the web. If you take part numbers and google them 99% of the time u will get what u need. I also noticed alot of old part numbers are replaced with revised ones yet the same part.

Also I believe your boat mechanics are one of 2 things,... 1 is incompetent, 2 if they sell motors they just want u to spend money on a new one from them rather than them fix it.

I live by a few marinas that work on these motors all day. If a mechanic says everything is diagnosticly checking out great but I cant fix ur issue hes not a real outboard mechanic, hes no different than u and I but maybe with more trial and error experience. I personally think your letting people lighten your wallet (im not trying to come off as a jerk at all) and you need to find a real certified mechanic. $100 an hour though makes you cringe, I know, ive been there, and ive been swindled by a marina before. It wont happen again. I bet ur issue is a stupid adjustment. I will tell ya bud I have issues of my own with my 70 mercury 650 specifically on throttle issues like you. A black plastic cam was out of adjustment and prevented my carbs from closing all the way. Took me 3 days of staring at the distributor and the linkage to figure it out.

Long story short if ur carbs are in spec than stare at ur linkage and distributor and make sure everything is turning sequentially and fully. There should be minimal play on anything at wot and I mean like .008mm play if I recall from manual. Make sure your throttle screw is pushing that lever on top of carbs all the way, if not adjust it. That lever on top of carbs should be pinned and not move at wot granting also that your distributer is in proper advance also. JMO bud, hope you get that thing going soon.

edit? also send me ur email through private message, I have a pdf file for you that has all the spec's and how to on your throttle adjustments that you say u dont have. I bet 100 to 1 I have what you need to get u on ur way, I got it from Carholme off here.

 

[racerone]

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

The reed valves on these engines are internal and inside the block, around the cranshaft.----Requires major work to get them out if bad.

 

[oldman570]

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

Make a mark on the prop and the center metal sleeve of the prop so the are lined up. Then take the boat out for a ride and after your done look at the marks made and see if they are still lined up. If they are not then the center of the prop is slipping and needs to be replace or a new prop. JMO
Oldman570

 

[Texasmark]

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

I guess I'm not "sure", but it does get on plane and going pretty well with just me in the boat. Not as good as I know it can though. Should I pull it and take a look at the teeth?

*Edit*

I just went out and pulled it. Teeth on the shaft look good and squared off and same for inside the prop. Anything else to look for?

Take the prop nut off and with a magic marker or other permanent marker, scribe a line from the prop shaft straight out to the outside of the prop hub. Run the engine with the load that causes the problem. Pull the nut off and look at your lines. If no longer a straight line, you have a spun hub. You probably have the rubber hub prop and they will spin when aged or if you hit too many obstructions causing it to spin frequently. The brass part of the prop is connected to the rubber and it friction fits against the prop proper such that if you hit an obstruction, the hub will slip momentarily inside the prop shell is how they are designed to operate.

Mark