1970 Mercury 500 (50hp) Ignition issue?

[riggerray]

Hi guys, newbie member fascinated by all of the info you guys make available. Thanks for the great site!

Here's my problem: I've got a '70 Mercury 500 w/Thunderbolt ignition. The first year I had it it ran great - just hummin along. However, the next year, I was unable to get out on the water at all, and then when I took it out the first time this year it fired up and ran, but just didn't seem to have any power. Seemed to idle a bit rough, but did idle, though when the throttle is put down could only get what feels like 1/3 power. Consulted with a mechanic friend of mine and proceeded to change plugs (not really fouled with the old ones), then drain fuel and replace with new mix and some fuel conditioner to help eliminate ethanol buildup, with experienced assistance checked the timing, throttle, choke, and cylinder compression, then most recently replaced fuel filter and older fuel lines between the filter and the motor which showed some minor cracking. All of these steps produced little or no result. The last time out a guy at the dock asked if I had a bad plug wire, so I started checking. When the #1 wire was pulled, the motor stalled, #2 still ran but very rough, # 3 similar to #2, but removing #4 seemed to have no effect. I could get a spark from the wire if it was right up next to a ground point, but it was no where near as strong as the spark from the other wires. The wire insulation appears in good shape and I cannot see a secondary spark happening anywhere along the wire. I've never really experienced an ignition problem that seems to manifest on only one point, but then my experience is all with automotive systems, so I'm not really sure how to proceed. Should I be looking for a new cap? Is there a rotor that can be worn? I haven't really torn into anything due to my lack of experience and the location of the distributor on this motor. It also appears that the plug wires are integral to the cap from my car experience. Is this correct and the wires are permanent with the cap or am I missing something on how to diconnect? If I do need a new cap & wires, am I looking for "vintage" aftermarket, or does someone still make something that is compatible with this distributor? Thanks for the help!

[corm]

Hi,

I'd pull the Dist and take of the cap and clean it. I am sure is carbon tracked up.
Use some disk brake cleaner to get it clean. The plug wires are held in by screws inside the cap.

The Dist is held in by the 4 3/8 head bolts. The splined dist shaft has one flat spline so you can not get it back together out of time. it drops down when the bolts are out. You will have to take off the control cable and the max throttle and spark advance stop plate off too, it is held on by 2 7/16"s head bolts MMmm maybe they are 3/8's too. I can't remember tonight.

[riggerray]

Thanks, corm. I'll give that a shot tomorrow and seewhat happens. sounds a lot better than having to find parts. If anybody else has any ideas, I'd love to hear them. thanks again!

[riggerray]

Ok, pulling the distributor was a good intro to working on this motor and there was carbon buildup, but back on the water, still no noticeable change in the power. The spark seemed to be better on #4 (again, not really using anything scientific on that, just whether it sparks when disconnected) though. running wide open in neutral the motor seems to get up good revs but kinda rough, but when in gear you make progress, but no where near where this boat used to be before sitting. Is there anything that I can do to get more detailed test results so the problem can be isolated better? If the motor seems to be running fairly smooth, could the lack of power to move be somewhere in the lower end, or would low end issues manifest as no movement?

[corm]

Hi,

I would clean the bottom carb, well both of them.
If pulling the #3 & 4 plug wire makes no difference then no fuel is
getting through to the cylinders.
If it is only #4 then you may have a bad reed valve.
This seem a little funny that it would just show up this yr
1st time out to me. So I would do the carbs.
Also check the fuel pump diaphram to make sure there
is no hole in it leaking fuel into the crankcase.

[riggerray]

Ah, corm, I was afraid we were going to get to that... does that mean "tear them down and clean thoroughly after dissassembly" or "get to the air intake of each carb and blow (insert proper product here) through them" to clean them? From what I've heard (and started to experience getting to the dist) this will be a bit of a chore. Let me know and I'll did in. Thanks!

[corm]

Hi,

It is a job. I personally always try pulling the float bowl cover and the brass bolt that covers the highspeed jet off and clean them that way. If I can't get them clean like that them I pull them off the block. I can usually get that done with the starter off.