1970 Merc 500 Thunderbolt idles somewhat but stalls when put in gear and given throttle. Ugh!

I got stuck out on one of the biggest inland lakes in Michigan today. My Merc 500 is still having issues and will not run!

I can get it started but as soon as I try to go, it shuts right down. I can put it into gear fine and it spins the prop and still runs but as soon as I advance the throttle, it shuts down.

Oddly enough, I can start it. Put it into gear and then push the fast idle up and get it moving but if I give it much more throttle then that, it shuts right down.

Here is a video of what I'm hearing. You can hear me put it into gear and advance the throttle right before it dies. As you can see I can push the idle lever up and it runs OK'ish, it's only when I try to actually go that I have this issue.

Where do I start? Electrical issues? Rectifier or distributor? Timing issues? Carbs? Fuel pump? Set the boat on fire and let it burn til it sinks?

Does anyone have any factory service manuals in PDF for working on these things? Anyone have a guide to rebuilding the carbs, fuel pump etc? Anything advice and any how-to's would be GREAT!

Here is a video of what I was dealing with today. As you can hear its idling weird but it is idling. Stalls everytime I try to give it any real throttle though.


This is a video of a run two weeks ago. In my opinion, it ran GOOD. Then I took it out to a big lake the following weekend and it ran about the same, ran good. We buzzed the whole lake back and forth without issues. Then the past two weeks I have what you see in the top video happening. I've gotten stuck on the lake twice now trying to trouble shoot.

Anyway here is a video of it running good just two weeks ago:


This is my first boat EVER and I just picked it up a couple weeks ago from a family friend. Ran good the whole time he had it but he only have it for about two years. He got it from his cousin and it ran good for him too. I'm totally lost as what to do first.

Open the idle mixture screws 1/2 turn OUT (CCW).

Jeff

Why would it run OK then start to run like it is but only have it be an adjustment?

I'm asking because I really don't know much about outboard motors. This is my first boat ever!

Also where are these screws located? Are those the brass screws ON the carbs? I can't get to those without pulling the motor half away apart.

The idle mixture screws at slotted screws in the left side of the carbs. You'll need a loooong slotted screw driver.

Jeff

fastjeff said:

The idle mixture screws at slotted screws in the left side of the carbs. You'll need a loooong slotted screw driver.

Jeff

Click to expand...

How long are we talking? I can pick something up on the way home tonight.

Could someone give me instructions for adjusting idle, WOT, etc? I have no idea how to adjust anything on this engine, as I said I'm new to boats.

Are there any links for a manual that might help me?

1. Warm motor with boat on the trailer or tied to a dock

2. Put it in gear at the lowest idle without stalling--use the idle up lever.

3. One at a time. SLOWLY turn the idle mixture screws in (CW) until the motor starts to slow down, then quickly turn it OUT (CCW) a half turn.

4. Go the the other carb and repeat a few times.

5. Add 1/4 turn OUT (CCW) to each.

Next, see if it will throttle up without bogging. If not, add another 1/4 turn OUT (CCW).

Jeff

fastjeff said:

1. Warm motor with boat on the trailer or tied to a dock

2. Put it in gear at the lowest idle without stalling--use the idle up lever.

3. One at a time. SLOWLY turn the idle mixture screws in (CW) until the motor starts to slow down, then quickly turn it OUT (CCW) a half turn.

4. Go the the other carb and repeat a few times.

5. Add 1/4 turn OUT (CCW) to each.

Next, see if it will throttle up without bogging. If not, add another 1/4 turn OUT (CCW).

Jeff

Click to expand...

I just found a copy of the actual factory service manual and your instructions while basically the same are easier to understand!

Going to go try this tonight.

I have one more question. Something my friend pointed out to me that I did not meantion. OK so we get the engine running and back down the idle lever. We put the thing into gear and it goes right into gear. Then we roll the throttle almost all the way forward to what I would call the WOT position before we notice any change in the engine at which point it stalls.

I checked and the distributor is moving as I roll the throttle forward but no change to the running condition until it gets to that point of where it wants to stall out.

Would you still say adjust the idle first?

Are the fuel tank vents open?

guyjg said:

Are the fuel tank vents open?

Click to expand...

Yes the vents open all the way.

Borrow a different tank and fuel line to rule out yours as a problem.

guyjg said:

Borrow a different tank and fuel line to rule out yours as a problem.

Click to expand...

Um I don't know anyone that owns a boat that has a tank like this. They either all just have electric trolling motors OR large capacity built in tanks.

One thing I was thinking about though. So my bulb and lines were OLD and the bulb was dry rotting on the outside. It worked fine though. I took the boat to the lake on that setup a half dozen times or so. The line was clearly 1/4 inch line but the fitting appears to be bigger then that.

I reused the fittings because they were metal and in decent shape but I replaced the fuel line with 5/16 automotive grade fuel line which is ethanol safe and I replaced the bulb with a OEM Quicksilver 5/16 bulb.

I took the boat out on a big lake and had a good run ALL day. Started right up etc.

I run the boat WOT across the lake and back which was a good 20~25 minutes. Used about 3 gallons of fuel.

I checked the engine repeatedly to make sure it was pissing water out of the spout thingy, it has a steady flow.

Then we took the boat out of the water for a week, put it back in and this is when all my issues started.

Could there be some engine damage somewhere from running it like that for such a long period of time? I never tuned the motor when I got the boat as I didn't know what to do with it. I mean it sounded OK. It always sounds like that above video I posted where is was running good. I have no idea if it was too lean or not though.

I wanna try to get to a lake tonight to try adjusting the idle but right now I don't know where I would get another tank or lines. I will have to ask around.

Well I found a screw driver! Picked up a 1/8 inch 10 inch long Craftmen Professional screw driver just for doing this.

I happened to check the plugs and do a compression test tonight. How does this look? These plugs were brand new ummmm maybe 2 or so runtime hours ago? I've had the boat out 5 or 6 times and most of the time was just moving to a fishing spot.

However, I should mention the last two times out due to the engine running problems I had to limp the boat back to shore restarting it and moving a little at a time in almost the lowest speed I could go so I would think that might account for the oily wet carbon fowling?

Plugs, top to bottom:










Compression test, top to bottom with all plugs out:

For anyone who doesn't know much about reading compression testers, from top to bottom that was roughly 140, 140, 135, 140 +- 1~2psi...

Anyway, I've got my 10 inch long screw driver and can reach the idle screws now so off to the water I go tomorrow. Hopefully this does the trick.

Did it work for you?

Did this work for you?