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

[Zenvalo]

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

I installed a new and correct key switch. Soldered each terminal securely with good separation and further separated the terminals with 100% silicone between them which I allowed to fully cure for 48 hours before installing the switch back into the control and testing. It worked great. Off/run/start/choke all work just like new.

 

[Zenvalo]

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

Been a while since my last post. I'm about to give up on this thing completely. Got the new Switchbox put in, 2 new sets of carb kits, etc. Starts and idles just fine now, but that's it. Doesn't want to get going. With parts and labor since my last update, I've put another $600 into it. I think I'm in the $750-$850 range of money sunk into it and it's still a pile. Compression is still good (125psi x4), Spark is still great also. I'm at a loss......

 

[SilverSS07]

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

I'm right where you're at. Maybe it's a 1973 Mercury 500 thing lol. I haven't replaced switchbox yet though. Mine starts and idles fine and even plows water fine. Just can't get up on plane. If I come up with anything I'll let ya know.

 

[Zenvalo]

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

That's what my original problem was, though I could get on plane before if it was just me in the boat. Otherwise it just plowed water. Also, the switchbox wasn't part of the power issue, it's just something that died during the process of figuring this giant paperweight out. Please keep me posted if you come up with a miracle cure for the Merc 500 plague and I'll let you know if I figure anything out as well...

 

[Maxz695]

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

What size props are you guys running on this 50 HP If it is 13 pitch or more it,s way to much pitch for any weight over one person. I would try a 10 or 11 pitch prop. You should be turning a minimum of 4800 RPM up to 5500 RPM ay WOT with a loaded boat. Anything less is straining the hell out of the mottor and will not get or stay up on plane

 

[Zenvalo]

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

10 1/2" - 12p. Stock prop for the motor. It's currently in the shop and has been the past couple months. The Mech is stumped. He's talking about trying to decarbonize the reed valves next? I've already done a "dunks method" decarb, but I don't know if this is something different than a reed decarb...

 

[Maxz695]

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

If theres that much carbon on the reeds theres other issues that need to be addresed. Have you been using 50 to 1 ratio fuel mixture. Too much oil could create carbon build up fouled plugs carbon on the rings etc etc

 

[Zenvalo]

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

I don't think my reply showed up earlier. At least I can't see it. Anyway, 50:1 ratio and new plugs in it. Not sure if you've read through this whole thread or not, but I don't know what else the problem could be from. I've tested various things, cleaned some things, replaced others... I still feel it's fuel related somehow...

 

[Zenvalo]

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

Well, spring is here again... Time to start tinkering again. Anybody out there that missed this thread before and might have anything additional to add, please do so! Thanks!

 

[oldman570]

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

Any time that the carbs are worked on, you should do a Link & Sync to get the carbs and timming set correctly. Here is a simple version of how it can be done.
http://forums.iboats.com/engine-fre...q/timing-sync-merc-inlines-1988-a-168855.html
If any thing is out of adjustment, you will have troubles with the motor running above a idle. All the final adjustment need to be done with the boat in the water or in a test tank. JMO
Oldman570

 

[Zenvalo]

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

I'm pretty sure the last mechanic did that. Maybe I'll just have to pay a different mechanic to try it since I don't have access to or knowledge of the equipment needed. I've also thought of just trying to sell it as is and shop around for a newer motor. Of course I'd get it mounted and tested first. I'm not even sure what mine would be worth though.

 

[zippy83]

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

Any time that the carbs are worked on, you should do a Link & Sync to get the carbs and timming set correctly. Here is a simple version of how it can be done.
http://forums.iboats.com/engine-fre...q/timing-sync-merc-inlines-1988-a-168855.html
If any thing is out of adjustment, you will have troubles with the motor running above a idle. All the final adjustment need to be done with the boat in the water or in a test tank. JMO
Oldman570

I have the same motor as Zenavalo same year. I just went through replacing the two main fuel hoses insude the motor, both carburators, fuel pump, fuel filter, and the litle belt that sitts under the flywheel. I didint have to do any Link & Sync on this motor. Took it out this past weekend and it runns at 31.9mp/h on a 16 FT deep V LOWE. The only thing that I have left to do is adjust the idle speed it sounds a litle off.

 

[Zenvalo]

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

Were you able to find new carbs or were they used? I've had the timing belt replaced, replaced all fuel related lines inside the cowl, rebuilt the fuel pump (gaskets) and had the carbs rebuilt. Filter is good too. I do now remember that when the carbs were apart the idle tubes each had a hairline fracture in them most of the way up but I wasn't able to find replacements.

 

[zippy83]

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

Were you able to find new carbs or were they used? I've had the timing belt replaced, replaced all fuel related lines inside the cowl, rebuilt the fuel pump (gaskets) and had the carbs rebuilt. Filter is good too. I do now remember that when the carbs were apart the idle tubes each had a hairline fracture in them most of the way up but I wasn't able to find replacements.

I didint find new once, I got it rebuilt by a local mechanic in town. Just like you I got the parts from Iboats but seemd a litle off, so I made a call and the guy said he will rebuilt it for me in 2 days. I dropped it off and got it back looked like new after he was done. The guy that I went to has all the parts for older engines, he usually does work on older engines more then newer.

My Idle tubes seemd fine from what I could tell. Maybe I should have replaced that as well now that I think back

 

[Bosunsmate]

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

Put a line of paint across prop and bushing and then put your girlfriend etc back on if the line doesnt match up afterwards then its slipping.

Mine had this exact same issue with the carbs. It had horrible fuel economy and then soon started surging. Tilt it up a bit and pump the bulb if it flows out the carb it is an incorrrect seating float/valve. Mine happened in the lower carb too. Dont tilt it up to much as the fuel will flow from the forward facing needles anyway

That broken part id stick together with superstrength epoxy putty, after backing out the seat.

 

[Texasmark]

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

Administrator, this thread is from last summer. No wonder I couldn't remember having said what I said.

Mark

 

[Zenvalo]

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

Mark, I didn't want to start a new thread and have to repeat everything that has already been gone over. Sorry for any confusion and thanks again for your help before.

I just got back from dropping it off at a new mechs place and from the quick tests he did, lower unit is good, compression is good and electronics are good. It idles great and said it sounds like an extremely strong and clean sounding motor. He's going to try the carbs over again, namely the high speed jet since from the get go it had no hole shot, top speed or power it seemed. If that doesn't work, he wants to try the reeds. The nice thing is, is if he suggests a part, he let's me know which one I need and I can search for the best price. If it doesn't fix the problem, he doesn't charge for his labor. I hope this works out better than last time around.

 

[WoodOnWater]

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

Good to hear you're making headway. Sounds like your new mech is going down the road I was going to suggest.

When figuring out engine problem, the are three roads to go down. Every internal combustion engine is the same in this regard. You need AIR, You need Fuel and You need Fire(Spark).

Air with this engine is going to consist will be checking for full open and full closed throttle setting, checking for vacuum leaks and with this engine, ensuring that the reed valves are functioning properly.

Fuel will be to ensure that you are getting adequate fuel to the engine (carburetors) and that the carburetors are functioning properly.

Fire is ensuring that your ignition system is working properly.

From the thread discussions, it sounds like your spark and timing is good. The carburetors opening have been adjusted. You are getting fuel to the carburetors. This leaves two suspects. Carb function (fuel) and reed function (air). Your mech has already said it, but I got online this morning to say you should investigate carb function further and if that doesn't fix the problem, the only item left IMO would be your reed valves. I would lean heavily on the carburetor side of things. I hope this helps. If anything, maybe it will ease your mind about the direction your mechanic is going in.

Good Luck,

 

[Zenvalo]

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

Thanks Wood! But, now he is suggesting that I just try to sell the motor or boat/motor combo. He was saying I shouldn't put any more money into it because parts are so hard to find that if something else goes wrong, there's a chance I'll be out of luck.

The carbs were rebuilt once already, though I don't know how well. I'm tempted to pick up a pair of used carbs on ebay and maybe some reeds too. I don't know anything about reeds, but the guy said they were cheap. Hopefully easy to do.

Anyway, Of the 2 mechs I found that were willing to look at it, both say basically the same thing, it's a great, strong motor, but in the end can't/won't figure it out. None of the actual marine shops will even bat an eye at it because of its age and I don't know a whole lot about outboards. If I lived on a lake, life would be a little easier I suppose. It's a hassle to fix something, drag it to the lake, break down, load it up, haul it home, fix something, drag it to the lake... Well, you get the idea.

I officially hate this motor! I love the boat though. Not sure if the motor is even worth anything anymore even though it starts, idles and runs great with the muffs on. Maybe it's worth something in parts. At any rate, I might try some different carbs and maybe reeds. If that doesn't work, I think I'm just done with boating until I can afford a brand new setup.

 

[WoodOnWater]

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

Zen,

It's unfortunate that you can't verify engine RPM at WOT. It's about the only way to tell if the engine is producing power while on the water.

Before you do anything with the carbs again, other than verifying that all of the parts are installed and adjusted correctly, I'd suggest waiting until you have the boat on the water again. I just cleaned my carburetors and caught myself about to leave out a main jet. That would have had me stumped for a while and I probably would have lost the jet before figuring out my problem.

I don't think the reeds are an easy fix, but I can't say for sure. I think that if you had a problem with your reeds, the engine would show other signs; like being hard to start, maybe. I would think a vacuum check would be revealing in this area, but I can't say for sure. The question is what are the symptoms of bad reeds and do you have them? Maybe someone here can say.

In an earlier post, you mentioned "plowing" through the water. Is this bow up or bow down? You could simply have a motor trim adjustment problem. There is a manual adjustment for the angle that the motor sits on the boat. The motor has to be up and there is a bar down low on the transom bracket that can be pulled out and reinserted in a different location to change motor trim angle. Raising the engine will raise the bow and lowering the engine will lower the bow coming out of the hole and while on plane. I hope I'm not being too basic here, but a trim problem is easy to remedy. You may also need to change where people sit while coming of of the hole.

Going in the other direction, if you decide to sell, I've been told my engine is worth $500-$1000. I have the same engine, '72 model, and used those values as a guide for determining what kind of money I would sink into it as I brought it to life. I got mine dirt cheap, but was told it ran when pulled from the boat it was on and then it sat for 3-4 years while I finished the building the boat. I'm still sorting a carburetion problem (go figure) and am eagerly awaiting some warm weather to get it on the water to get it adjusted. Your engine might be a tough sell knowing that it has unreconciled problems, but parted out on ebay, you might recoup some of your expenses.