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

[Zenvalo]

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

There is no gas spraying or leaking with cowl off. no smell of gas either. Only when I put it back on the trailer at the lake and tilt the motor up to delaunch a small amount drips out from the front. Less than half an ounce I'd say. Floats are plastic. I'll look into it more later. Thanks!

 

[Moody Blue]

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

Can't offer much advice in the way of your fuel consumption other than the general rule of thumb is that a motor running at WOT will consume approx 10% of its HP (in gallons) per hour. So in your case, you could expect to consume approx 5 gallons of gas per hour when running at WOT. Lots of factors involved, but it is a general rule of thumb.

I had a similar sized boat a few years back with a 40HP Mariner (Merc clone) and I ran a 15" pitch prop. Boat performed awesome. Great holeshot, fast (for its size) and able to pull young adult skiers out of the water no problem. You might want to post a question in the PROPS section for more input. Your motor MIGHT be underpropped resulting too high of RPM (could damage motor) and poor efficiency, IE prop is screaming fast but providing little forward thrust. Just my 2 cents. You must have a tach to prop the boat correctly.

 

[Zenvalo]

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

It does scream quite loud when trying to get on plane there's no doubt there! After I'm planed out it sounds like I imagine it should at the speed I'm moving. I'll check with the "Prop Dept." and get their input...

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...

Also, I've been looking online for OEM manuals and think I may have found the correct one. Does the pic below look like what an old 70's OEM one should look like? I don't want to get the wrong one. Sometimes eBay sellers don't know what their selling and list items incorrectly. The info on the auction just says it was printed in 1973. Manual # C-90-68647. I sent a message to the seller asking if it covers my serial/make/model/year but haven't heard back...

Here's a pic.





As always, thanks for everybody's input!

 

[BonairII]

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

It does scream quite loud when trying to get on plane there's no doubt there! !

You sure you don't have a spun prop?

 

[Zenvalo]

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?

 

[BonairII]

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

The teeth(splines) have nothing to do with a spun prop....the rubber bushing(the hub) is what starts slipping.

Since you mentioned that your motor will scream, I was just guessing that the hub is slipping and power is not being transferred to the prop.

 

[Zenvalo]

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

Oh, gotcha! I just edited my previous post. I checked the splines before reading your new reply and they are good. To check the hub, I should be able to turn fairly easy one way in gear and not at all the other?

In forward, I can turn it and it clicks. Turning it the other way it's not nearly as easy, but can be done. I do hear a "rubbery" like squeak when I do it that way.

Should it move at all? Am I just really strong? lol! Never even thought about this being an issue...

 

[BonairII]

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

You shouldn't be able to make the hub slip "by hand", but if you are able to...then your prop is def shot.

 

[Zenvalo]

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

When in forward, it's easy to turn clockwise and clicks every 1/6 turn. When trying to go counter-clockwise (still in forward), it sounds like I'm moving the pistons. I hear a little "poof" from the exhaust, like it would sound if it were being very slowly pull started. It's not easy to do, so I only did it twice.

 

[ckangaroo70]

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

With the additional information provided regarding the "engine screaming"....well that also makes me think prop. If you have power out of the hole, but think you are down in top speed...then you should probably step up your prop pitch. On a boat like yours..you are probably looking for as small as a 13 pitch and as big as a 15 pitch. The 13 should give you a good combo of power and speed and the 15 pitch will make you a little slower out of the hole,but should give you a bit more on the top end. I run a 10 1/2" X 13pitch on my 1979 Mercury Thunderbolt 50hp. My Lowe Hustler is a 16ft aluminum boat and may be a bit lighter then your fiberglass boat, but my top speed is around 32 mph with a couple people in the boat and around 36 mph with just myself in the boat. Our boats are not the same, but close enough in length and size that I would think that if your 50hp Thunderbolt was setup the same as mine that your fiberglass boat should be hitting at least in the high 20's or maybe even low 30's in speed.

 

[Zenvalo]

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

Current prop is a 10 1/2" 12P which is a stock prop to that motor as I found out by searching the numbers on the prop itself. As is, top end seems good enough if/when I can get there. If I add 300lbs of people after myself, it doesn't want to get out of the hole. That amount of people weight does not exceed the limits of the boat, by the way. Being a fairly flat bottom tri-hull bass boat, it likes to plow for quite a while and if I have to turn, forget about planing at all.

I'll probably just bring it in for a diagnostic to see if I can narrow down the poor performance. Still do the work myself, but at least I would know where to focus my attention and money...

 

[Zenvalo]

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

Ok, enough people twisted my arm that I just ordered an OEM manual. I called a couple marine shops in my area just to see about getting a diagnostic and nobody will touch a motor of this age, so I'm on my own...

I'm going to order a spark gap tester just to verify the spark is strong enough. There is spark on all 4 as is, but maybe some or all are weak?

I'm also going to order carb kits for it, so I need opinions... I would prefer to go OEM on those, but they are about $10 more than the Sierra's and Mallory's. So my question is, is one better than the other? I looked at both kits here on iboats and they have different parts included so I don't know which to choose...

The OEM kit I believe I need is 1399-4218 (previously 1395-92621). Can anyone confirm or deny and if I'm wrong, help me with the OEM kit number?

I'll get this slug going eventually! Thanks again!

 

[oldman570]

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

OME carb kit for the motor is 1395-92621 as you show. Depending on manufacture, Serria or Mallery, numbers will be diffrent as shown on some websites. One has to remmber that parts in some kits will work on several other motor carbs.

 

[Zenvalo]

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

Thanks oldman, I was looking at the kits here on iboats as I stated and for example, one of the kits comes with the screen filter that sits on top of the float bowl cover and one doesn't. One looks like it comes with a needle and seat while the other doesn't show a needle in the photo. I figure If I'm going to have it apart, I might as well get the kit that has the most available pieces to replace.

The OEM ones are $30 a pop, but I think I'll go with them as I'm hoping they have the most pieces that are compatible. Thanks for your help! I'll be ordering them up in a little while!

 

[davidmerc50]

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

Thanks oldman, I was looking at the kits here on iboats as I stated and for example, one of the kits comes with the screen filter that sits on top of the float bowl cover and one doesn't. One looks like it comes with a needle and seat while the other doesn't show a needle in the photo. I figure If I'm going to have it apart, I might as well get the kit that has the most available pieces to replace.

The OEM ones are $30 a pop, but I think I'll go with them as I'm hoping they have the most pieces that are compatible. Thanks for your help! I'll be ordering them up in a little while!

hey let me know if you fix it im haveing the same problem my merc runs good starts good but acts like its under powered

 

[BonairII]

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

hey let me know if you fix it im haveing the same problem my merc runs good starts good but acts like its under powered

Start a thread about your motor, so we can help you. No sense in waiting around to see what happen's with the OP's motor...to try and diagnose your motor.

You may motor may have similar symptoms, but the cause may well be something completely different.

 

[turtles11756]

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

used carbs are on ebay often for the one that broke

 

[Zenvalo]

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

I ordered the spark gap tester and OEM carb rebuild kits last night, so it will probably be a week or two before the next update. On a side note, I was able to test the voltage output for battery charging at WOT and at it's highest point reached 16.0V, but on average was around 15.5V. Does that seem a little high to anyone? I thought somewhere in the 14V range was ideal. At idle it shows mid to high 12's.

It could have just been the meter is off too. I have one of the digital made in China type that plugs into the 12V cigarette receptical.

 

[RepairGuy]

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

www.mygreenmanuals.com if you dont see it listed just ask them and they'll list within 24hrs .... enjoy

 

[oldman570]

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