1970 Merc 500 50HP Thunderbolt now stalls when I give it throttle. Carb issues?

[Roadblock]

First off excuse my grammar, I'm sending this from Google Chrome on my iPad on my lunch break!

I just picked up an old 1970 Merc 500 Thunderbolt with electronic ignition. It started right up when I got it, ran OK with only minor hesitation that passed as soon as you opened up the throttle. In fact, here I'll post a video from a run just two weeks ago. As you can see, it ran well just two weeks ago.

So things I know are issues.

1. Wiring is hit. All of the wiring threw out the engine is cracked from being old and heat damaged. I plan to put in a new CDI wiring harness this winter while I'm doing some other stuff.

2. Water gets into the lower unit. Not to much that I have standing water that drains but just enough to contaminate the gear case oil. No big deal there, I know how to fix that I just haven't yet.

OK so that said I took the boat out this weekend twice and had issues both days.

On Saturday I had it stall out several times and when I would give it throttle it would stall. It sounded sickly when it was running too even at idle. Then I couldn't restart it and my battery started sparking at the terminal and the starter would not turn over. Tried to disconnect the battery and burned myself. The wiring just about melted. I removed the starting battery and connected my trolling motor battery and it took RIGHT off. In fact it idled the best it ever had. I applied throttle and it hesitated for a second and then took right off. We fished for about 3 hours and then restarted and it took right off.

I went out Sunday, I used the trolling motor battery all day. I had a hell of a time getting it started and every time I gave it gas, it stalled. Went threw this for over 30 minutes, started, gave throttle and it stalled. Finally I gave it gas and it hesitated bad and then went. It was moving pretty good but no where near the power you see in the above Youtube video. We shut it off, fished for about an hour and then went threw ALL the same hassle getting it restarted. This time I never got it to actually go when applying throttle. I had to put the motor into gear and chug it back to shore at idle speeds. Any throttle what so ever and it shut right down.

This is my first boat. I know nothing about motors but I don't really want to pay a Merc mechanic to work on a 1970's motor with hard to find parts! I have so little in vested, I want to learn to do it myself.

Reading other threads here on iBoats and via Google search it sounds like my issues is carbs.

Engine compression tested at 125psi across the board which I'm told is close to spec?

Also I've replaced the gas bulb with a brand new 5/16 OEM Quicksilver bulb and 5/16 gas line. Looks like I had a 1/4 bulb and lines on 5/16 or 3/8's barbs so I upgraded to the 5/15 (it's all my shop had in-stock that was OEM) BUT I DID NOT install an inline gas filter or water separator. I do have a factory OEM 5/16 Quicksilver filter here, I will definitely be putting in tonight.

OK so all of that said, um so do I clean the carbs or rebuild them? If I take them apart to clean them wouldn't I technically need to rebuild and replace seals?

Does anyone know of any videos (Youtube etc) that show the process of removing the carbs? If not can someone please give me the steps and any tips, tricks or suggestions on how to do it and what I need to be careful of? I noticed the link between the carbs is plastic and it looks brittle as hell.

 

[Roadblock]

Are there any PDF's out there of the factory service manual for this thing? Anyone have a link to one?

 

[racerone]

They idle really nice when low speed adjustment is lean.-----If they stall on acceleration you open each low speed needle 1/8 turn at a time and see if there is an improvement.

 

[Roadblock]

Yeah but based on how well it was running and now it doesn't, couldn't I assume something is plugged or just not right now? Like more so than just adjustment. I mean it really SHUTS right down, not just runs like crap. Did you see how well it was running two weeks ago in my Youtube video? It ran like that the previous 3 or 4 times I took it out too and now, nothing.

 

[jigngrub]

Taking a new to you used motor and putting it on a boat and expecting it to be reliable without doing anything to it... it's going to give you problems and your expectations will be shattered.

Things you need to do to make your motor reliable and won't wait 'til winter:
1. Rebuild carbs
2. Rebuild fuel pump
3. Install new impeller
4. Fix lower unit leak
5. Install new wiring

All the items above are ticking time bombs just waiting to strand you on the water. If you continue to ignore the 5 items you will continue to have problems... but sooner or later you'll tire of unreliability and fix the motor right.

 

[Roadblock]

Yep you are right and I plan to address all of that stuff as soon as I can afford to. I spent a small fortune on the transom and deck rebuild so I'm pretty tapped the rest of the summer.

That said my co-worker thinks the main issue right now is related to water in the bowls?

I forgot to mention that with the issue on Saturday when I got the motor running I did a hole shot with the engine covers OFF, with the transom mounted trolling motor in the water. I had water flying up and into the Merc 500. It was literally pouring into the motor.

Based on how well it ran previous to this, what happened on Saturday and how it ran on Sunday finally, my co-worker things my bowls have more water in them than gas. He said water would totally cause this exact issue, when you throttle up the engine tries to pull in more gas than is there to pull in so it gets all that water and boom, on her ass she falls.

Sounds like a place to start at least?

Also I've got one of those adaptors you connect a outboard to when you run it off a garden hose. Is it safe to use one of those with higher throttle? I would like to attach the motor to a water source, clear these bowls and try running the motor in the driveway before going back to a lake if possible. I would like to run the motor up a bit and see if it stalls.

 

[Roadblock]

OK lets go this route. OK so I know what I need to do to get this engine into tip-top shape.

For right now though, does anyone have a link to a factory service manual in PDF format for this motor?

Also I'm told there is a drain screw on the fuel bowls and that I should drain the bowls and check for water. Where is this located? everyone I've spoken to at work says it should be like a small bolt on the bottom of the carb? Not seeing anything like that. I do see something like that on top of the um I believe it's called a float bowl cover? There is a housing over that with a bolt in it and I see a fuel line attached to that.

Help? =D

 

[racerone]

Your friends are giving you excellent advice.--------Look for a brass screw near the bottom of the carburetor bowl !!!-----No point examining the patient anywhere else.

 

[Roadblock]

OK well good news I guess. The carbs were filled with water which would explain the working great and then not.

I took the starter off to get in there a little easier and the wiring is no where near as bad as I thought. The main bad wire is the one that the kill switch thingy is on. That appears to be factory wiring as where MOST of the wiring in the harness has been rebuilt. The biggest issue I see is the terminals on the electronics are rusted to hell. I would almost worry about trying to take off a nut for fear of having a terminal on the control box break off.

The solenoid isn't as bad nor is the rectifier or however you spell it.

I took off the fuel filter covers, the ones that go over the float bowl lids. I had an almost rubber like black build up of crap in there. I'm sure it was rubber from the hose and fuel ball. I cleaned the covers out with carb cleaner and flushed the filters with gas. They look brand new now. I Do not have new seals but the seals are intact so I think I can get away with putting the covers back on until I get seals.

I feel way more comfortable taking the carbs out now. There isn't much to them once you get in there. I think I may just rebuild those sooner than later. The pump looks easy too.

For right now though, I'm going to assume its going to run again now that the water is gone. I drained the top carb into a plastic cup, I could clearly see a ton of water. I wasn't really able to drain the bottom carb into anything but I had a TON of water coming down into the engine so I'm sure it was full of water too. I tasted a little of the gas that came out on my hand when I was taking out the nut with my fingers, it didn't taste like gas at all. Tasted like bad water so again, I assume the bottom one had a ton of water in it too.

I just got a raise at work today, wasn't expecting that so yeah it freed up some cash to play with to rebuild the carbs and pumps now.

 

[merc850]

Water shouldn't get into the carbs from spray (if water gets in gas gets out) I think the fuel tank is the source of the water.

 

[Roadblock]

Which is possible I suppose, I do not have a water/fuel separator. Maybe it was building up in there, no idea how that works. They definitely had water in them though, I drained them into a plastic cup and could see the water droplets in the fuel.

Also found an air leak in my fuel system so I suppose water could have gotten in threw there too?

I put some new seals in the mercury connectors, the one inside of the female tank side connector and the one on the female end of the fuel line that connects to the motor. Fuel picks up MUCH better now. I can prime the bulb in just a couple pumps as where before it took like 20~30 pumps.

 

[Roadblock]

OK well that did not help I mean the bulb picks up fuel correctly now but I still can not get it running.

Got stuck out on the lake today!

I did NOT notice this weird noise while I was on the lake but listen to the way the boat is idling. Ugh...

So I guess I start with the carbs and the fuel pump then? Any guides on how to rebuild these things?

 

[Roadblock]

OK!

I'm being told before I start ripping into anything I should try adjusting the carbs. I've had two people tell me to adjust idle and then adjust for WOT.

How do I do this? Any guides? Instruction manuals etc?

*Update*

Just found the full 506 page 1966~1974 factory service manual for this engine online in PDF format! WOOT!