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

[Zenvalo]

Hi all!

Just got my first Merc motor ever, it's a 1973 500EL Thunderbolt 50hp. I have it on the back of a 14' Glassmaster Bassmate IV. Dry weight of the boat is approx. 650 lbs.

Took it out for the first time today, total of 2 adults and 1 kid, approx total people weight 550 lbs. It would not plane out, seemed extremely underpowered and burned through about 3 gallons of fuel in 30 minutes. It also didn't like to stay running every so often at low idle. Compression tested out at 125psi across all 4 cylinders. Lower unit was just rebuilt and new impeller was installed, so it's something up top aside from compression.

After pulling the boat out and tilting the motor most of the way up, I noticed gas dripping out of the motor housing also. I think a carb rebuild is in order, but is there anything else I should look in to? Like I said, this is my first Merc, so I hope it's something easy to rectify.

Thank you for any and all suggestions and if there is anything I may have failed to mention as far as descriptions, let me know!

*Edit* I put brand new plugs in before taking it out and I visually inspected them when I got home. It appears all 4 are firing. The top three had a normal amount of burnt fuel/oil mix on them, but the bottom one was only slightly "burnt". Also, see my post below about the fuel lines(?) that I noticed when I looked a little closer on the port side.

Thanks again!

~Zen

 

[Zenvalo]

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

I noticed that these hoses are pretty brittle and one them is actually missing the rubber that surrounds the fabric liner. Whether this is part of the issue or not, is there a replacement kit for all these lines?

 

[Zenvalo]

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

So nobody has any tips, hints or suggestions? How about part numbers for a carb rebuild kit or a hose kit? Any positive response would be greatly appreciated!

 

[Texasmark]

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

I noticed that these hoses are pretty brittle and one them is actually missing the rubber that surrounds the fabric liner. Whether this is part of the issue or not, is there a replacement kit for all these lines?


View attachment 156921

The missing rubber is just there to protect the reinforcement. If you had a problem there you would see some fuel oil mix wet spotting. The inside is neoprene and that is what holds the fuel in.

That is a cross flow engine but is a very efficient for the breed. The picture you shot looks to me like the rig ought to perform nicely with the 3 of you in there.

You can get parts here and other places. Get your serial number before attempting to order your carb kits. It's on the right side of your transom bracket to engine interface mount on a metal plate. Might as well get a fuel pump kit while you are there.

Back then Merc went nuts with their fuel fittings. Nowadays, they just use a hose barb, a piece of fuel line, and a nylon tie wrap to do the fuel interconnecting so if you choose to re do your fuel lines, then you can go to your auto parts store and get fuel line and hose barbs....just take a hose with you to get the right size thread....probably 1/8" pipe thread with a 5/16 ID hose.

If you can't find what you want on here and have no dealer nearby, my NAPA auto parts dealer used to supply me with Merc OB parts, usually with next day delivery.

HTH,
Mark

 

[Zenvalo]

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

Just went out and checked again and the one is wet/sticky/gooey. I also noticed that the larger vertical hose fittings are both loose going into the #1 and #2 cylinders as well as #3 and #4. It looks like there are 5 hoses total of at least 2 different diameters. Sound correct? The carbs look like they might have been recently redone. From the outside view, they are a much brighter metal and are much cleaner looking than everything else inside.

Would I be better off getting a whole fuel pump or just a kit? It seems as if it's working though considering the 3 gallons of fuel used in 30 minutes...

My serial # is 3722026

I found a fuel pump diaphram kit without any problems, but I'm unsure on the carb kit. I'll be doing the feul pump kit and hoses first to see if that changes anything, but if I have to do carbs, what part number would I need for a kit? One set of numbers I found on the carbs is 1399-4664 but that doesn't show up in the parts section here on iBoats. It only shows a general range of years 1961 through 1975 in the 50 hp models. Did the carbs not change at all during that time?

Thanks

 

[Zenvalo]

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

I got all the hoses off and am about to run and get new fittings and hoses. Below is a photo with all the lines off. There are two plastic white fittings as seen in the photo that are somewhat loose, but I can't seem to get a wrench or socket in there to snug them up. Obviously being plastic I wouldn't tighten them too much, but a little snuggness is needed I'm assuming. Thoughts?

 

[Zenvalo]

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

I must be doing something wrong... Lot of views here, but no comments. Hmmmm.... Can anyone help me figure out what the OEM part number for carb kits are? There are numbers in various places on the carbs, so let me know which one I need to post to figure it out. I've already ordered a fuel pump rebuild kit and replaced all the fuel lines inside the housing as well as the main feed from the tank.

 

[Texasmark]

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

Just went out and checked again and the one is wet/sticky/gooey. I also noticed that the larger vertical hose fittings are both loose going into the #1 and #2 cylinders as well as #3 and #4. It looks like there are 5 hoses total of at least 2 different diameters. Sound correct? The carbs look like they might have been recently redone. From the outside view, they are a much brighter metal and are much cleaner looking than everything else inside.

Would I be better off getting a whole fuel pump or just a kit? It seems as if it's working though considering the 3 gallons of fuel used in 30 minutes...

My serial # is 3722026

I found a fuel pump diaphram kit without any problems, but I'm unsure on the carb kit. I'll be doing the feul pump kit and hoses first to see if that changes anything, but if I have to do carbs, what part number would I need for a kit? One set of numbers I found on the carbs is 1399-4664 but that doesn't show up in the parts section here on iBoats. It only shows a general range of years 1961 through 1975 in the 50 hp models. Did the carbs not change at all during that time?

Thanks

A pair of long nose pliers should work. Just be real careful that you don't slip and mess up the threads. Agree a nylon insert doesn't need to be all that tight.

Why don't you get a can of Sea Foam at your auto parts dealer, most carry it or ww and dump the whole thing in your tank of 50:1 fuel oil mix. Run through the whole tank before you decide on any further action. It may clean things up adequately to make a difference. Otherwise contact Chris1956 or Steelspike on here. Both are mechanics and should be able to help your.

Mark

 

[oldman570]

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

Using a timming light as a spark tester on all the plug wires will tell if all the plugs are getting spark. Look for the light to flash and move the plug wire sensor from one plug to the next on all of the plug wires. If one will not light the timming light you have trouble with the ingnition system on that cyclender. As Texas states replace all the fuel lines . On the older models the SR# will be found on a plate between the transon brackets on the tilt tube is.

The missing rubber is just there to protect the reinforcement. If you had a problem there you would see some fuel oil mix wet spotting. The inside is neoprene and that is what holds the fuel in.

That is a cross flow engine but is a very efficient for the breed. The picture you shot looks to me like the rig ought to perform nicely with the 3 of you in there.

You can get parts here and other places. Get your serial number before attempting to order your carb kits. It's on the right side of your transom bracket to engine interface mount on a metal plate. Might as well get a fuel pump kit while you are there.

Back then Merc went nuts with their fuel fittings. Nowadays, they just use a hose barb, a piece of fuel line, and a nylon tie wrap to do the fuel interconnecting so if you choose to re do your fuel lines, then you can go to your auto parts store and get fuel line and hose barbs....just take a hose with you to get the right size thread....probably 1/8" pipe thread with a 5/16 ID hose.

If you can't find what you want on here and have no dealer nearby, my NAPA auto parts dealer used to supply me with Merc OB parts, usually with next day delivery.

HTH,
Mark

 

[Texasmark]

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

Using a timming light as a spark tester on all the plug wires will tell if all the plugs are getting spark. Look for the light to flash and move the plug wire sensor from one plug to the next on all of the plug wires. If one will not light the timming light you have trouble with the ingnition system on that cyclender. As Texas states replace all the fuel lines . On the older models the SR# will be found on a plate between the transon brackets on the tilt tube is.

That's even better....your timing light suggestion. I had forgotten about that.
----------------------------
In case you don't know how to do that, power your light from the battery or plug into an AC receptacle and put the sense clip on each spark plug wire, one at a time, to test. You aren't checking the timing, you are looking for a good solid flashing of the light every time the plug is smacked with high voltage. If it doesn't fire, or fires erratically, considering that some timing lights are erratic anyway, then you have your smoking gun. Change the plug first and if the problem continues then suspect your CDM.

Mark

 

[Zenvalo]

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

Thanks for the response guys! I was starting to think I was the only one able to see my posts! Good suggestions too! So far I've replaced all the fuel lines inside the motor housing, just waiting on the "T" fitting to connect the upper and lower carbs. I'm also waiting on an OEM fuel pump rebuild kit. Both should be here mid week I'm hoping. I also have a new tank, feed hose and primer bulb.

I replaced the plugs before I took it out for its first run and checked when I got home and they all look like they were firing, though the bottom one wasn't quite as dark as the upper 3. I was thinking about getting a spark gap tester as I've read that they are the easiest way to check the actual strength of the spark. 7/16" gap if I remember right is considered excellent...

I also picked up 2 cans of sea foam. I plan to do a decarb using "Dunk's Method" and after that, use a full can in my fuel tank for the next first run.

I'm hoping I won't have to do carb kits. I priced them last night and it looks like it'll be around $100 including new floats. Eek!

All that aside, what is a CDM? Is there an individual for each cylinder or one part that powers them all? And finally, I know nothing about props, but the one I have on there is a 12P I believe. Is that a good pitch for my setup? I'll do a little searching around and try to learn a little more about them, but any suggestions are always appreciated. A prop would be the last thing I would get anyway. I want to get the motor running like a top first!

Thanks again all!

 

[oldman570]

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

The CDM mod should be mounted on the front motor bracket and supplys power to the coil and plugs. The motor only has one coil and it sounds as tho both are good. The distribuater cap and rotor, plug wire or plug will be where you will find the trouble with the plug not preforming as the rest. If you do not have a OMC manual for the motor and have never worked on electical systems as this before you might want to take it to a shop and have them replace the cap and rotor as they can be a SOB to get off and replaced at times.
The 12 pitched prop should be fine on a boat for power, but it really depends on what you have for a boat and what you want to do with it.
Running Sea Foam in the gas oil mix might do the trick on the carbs. I use a can of Power Tune from a marine dealer , cost less than $15.00, it dose a great job on the carbs also. Check the timming belt for any cracks and bad wear while you have the motor being worked on. Timming and sync of the carbs can be hard to do on these motors. If you take the carbs off the motor be very watchful for a small tab on the link between the carbs as it can be dropped and lost easily. I would go with the Sea Foam and or Power Tune before worrying about the carbs as it has been run lately. Rebuilding the fuel pump sure wont hurt and dose not cost that much.
Oldman570

 

[BonairII]

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

Re: 1973 Mercury 500EL Thunderbolt 50hp issues?!?

I'm hoping I won't have to do carb kits. I priced them last night and it looks like it'll be around $100 including new floats. Eek!

You probably won't need carb kits. A majority of dirty carbs just need a cleaning and/or float adjustment. If you need new base gaskets you can make them out of gasket material sold at any auto parts store.

To find the correct prop you'll need a tach(there's no way around it). You'll also need to find out what your motor's optimum rpm range is. This info is in an OEM service manual....and something that every outboard owner should have. DON'T buy a Seloc or other aftermarket manual.

 

[Zenvalo]

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

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

Definitely plan to get an OEM manual eventually. For now I'm just doing the common sense stuff first. One thing I'm wondering about, the screen filters on top of the carbs are in good condition after I cleaned all the white crusties off, but I noticed there was only one gasket thing on the top side of each and looking at the kits and a rebuild thread on here there should be one on the top and bottom made of cork? I could be mistaken though. Sound familiar to anyone? Still crossing fingers I don't have to do any electrical work...

 

[ckangaroo70]

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

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

Zen

From reading your opening post it really just sounds to me like your float needs adjusted and possibly a couple new float needles while you are in there. I have a 1979 Mercury Thunderbolt 50hp and have experienced what you described in your opening post. Basicly just flooding the engine with to much raw fuel due to an improperly set float or worn float needle. Simple fix and not very costly if you do it yourself. When my engine began dumping fuel out the front it really lost its top end and became a real slug. Two float needles later and a proper readjustment of the floats and she runs real nice and will get up on plane in a matter of a few seconds. Once you get it running good...then I would also suggest running sea foam in your fuel always.

 

[Zenvalo]

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

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

Thanks ckangaroo70! I will try that idea next, but first...


I'm in a bit of a pickle (as they say)...

This (pic below) broke when I was putting the new gas lines on. I must have bumped into it the wrong way or something, but it shouldn't have just snapped off. I'm guessing it was already cracked to begin with.

I know what the part is for, but I don't know it's actual name or specific part number. I thought I seen an exploded picture of my carb setup on here somewhere, but can't seem to find it. If anyone has it, feel free to post within this thread. If not, anybody know the number for this specific peice? it has a 1399-4664 on it, but I don't think that narrows it down from what I can tell.

Someone save me!! I want to order a new one a.s.a.p. The longer the boat is land, the grumpier I get! ;-P

Thanks for the help thus far everyone!

 

[Zenvalo]

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

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

Well, I figured out the name of the part, but WOW! The prices for new and used are ridiculous! I found an old thread from last year where someone had the same issue. I replied on it hoping someone has one to sell at a reasonable price. If that's you, let me know.

Thanks again all!

 

[Zenvalo]

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

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

Ok, so I got antsy after receiving and installing the float bowl cover and figured since I just replaced all the fuel lines, plugs, fuel pump kit, etc. I'd take it out and see if it was any better...

Nope! Exactly the same! Slow out of the hole, sucks fuel like no tomorrow and with the girlfriend and her kid in the boat takes close to 5 minutes to plane out and get kind of reasonable speed. Keep in mind, the dry weight of the boat is 650lbs, which I don't believe is that heavy being a fiberglass tri-hull..

I remember my Dads old 70's Glastron Tri-hull with a 70's 50hp Johnson would have 4 people in the boat and still pull a skier no prob. I'm not trying to pull skiers, but I just want it to perform how it should...

So, I guess it's time to purchase a couple float needles as Kangaroo mentioned. Are those the ones that move up and down in the float bowl cover? How do I "properly" adjust the floats?

Yes, I know I need a manual eventually, but like I said in an earlier post, I want to try the inexpensive, simple stuff first.

Thanks again!

 

[ckangaroo70]

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

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

If it is dumping fuel out the front of a carb or both carbs...then yes I would replace the needles. If the float needles or seats are bad...then this will allow fuel to pour into the carbs. The floats are basicly trying to shut the fuel flow off when they reach there shut off level, but if the float needles are unable to seal the passage....then the engine will be flodded with fuel. If you just have the carbs set to an overly rich condition.....my question is "have you tried to lean the lower and upper carb while the boat is in the water and running"? Clockwise to lean and counter clockwise to richen. You will need a helper to drive the boat while you lean the lower carb while idling in gear and then do the upper carb at full throttle. Like I said though...if the float needles are bad..then you are not going to be able to regulate the fuel no matter how much tinkering you do with the settings.

 

[aussieflash]

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

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

You don't understand....Getting a Mercury Service manual and servicing the carbs is the cheap easy way out.
Can you smell fuel while running?Have you removed cowl while running on hose (idle only) and check for any visible leaking or spraying fuel?
Floats if plastic will probably be good,needle/seats should be replaced,all gaskets replaced,all metal parts sprayed with carb cleaner and blown out with compressor.
Your manual will walk you through all this and provide exact specs for things like setting float heights etc.