1978 Mercury 800 - Ignition Problem?

[kb67]

I have a 1978 Mercury 800 four cylinder. Picked it up a couple years ago after it sat for 5 years. Rebuild carbs, fuel pump, new wiring harness. It starts great, but it only runs "good" half the time. It will run at about 3/4 speed at full throttle, then instantly get up to full revs (like someone flipped a switch). It seems like electrical to me. I've done all the coil, stator, trigger tests as descrbed in the Seloc manual and all the resistances seem good. The other test I've done is uses a standard timing light. When it is running "good" - the #2 and #3 cylinders appear to be firing less often than #1 and #4. When it is running "bad" the #1 cylinder also seems to be firing less often. Any idea or recommended steps?
Thanks.

 

[Texasmark]

Re: 1978 Mercury 800 - Ignition Problem?

CharlieB, Chris 1956, and Faztbullet, are at least some of your marine mechanics that have a lot of experience and can probably help. If you can't get their attention in the normal manner, PM them.

Mark

 

[Maxz695]

Re: 1978 Mercury 800 - Ignition Problem?

Not expert but first check the butterflies on the carbs to be sure there opening at the same time as the upper carb or carbs secondly readjust the mixture screws to 1/8th best idol starting wth the top then the next carb. Plugs are always in view of problem performance. wires grounds etc. Old dried up gaskets would be next in line after sitting for some time. Water intrusion hopefully not the issue is always an issue that needs to be addressed. Fuel restriction old hoses leaking air and restricting intake is another as well as a clogged tank vent, Just a few suggestions until the pros show up.

 

[Faztbullet]

Re: 1978 Mercury 800 - Ignition Problem?

Cylinders 1 & 3 are a pair on trigger as are 2 & 4, what you posted does not indicate a trigger problem, check the grounding of coils and ground straps to ignition mounting plate. You need a DVA adapter to test stator as ohm readings can be misleading.

 

[kb67]

Re: 1978 Mercury 800 - Ignition Problem?

Thanks for the tips. Ended up putting the boat shrink wrap on the boat so troubleshooting will have to wait until spring.

One more question for when I go to troubleshoot in the spring, can I use my Actron Dwell meter (set to RPM mode) for a tachometer for each cylinder? All I have is a standard timing light.

Thanks, Keith

 

[Texasmark]

Re: 1978 Mercury 800 - Ignition Problem?

Thanks for the tips. Ended up putting the boat shrink wrap on the boat so troubleshooting will have to wait until spring.

One more question for when I go to troubleshoot in the spring, can I use my Actron Dwell meter (set to RPM mode) for a tachometer for each cylinder? All I have is a standard timing light.

Thanks, Keith

I guess. Didn't have that function on my old dwell meter. Question is, where do you hook it to read trigger pulses? Since dwell measures the period that the points are closed/open, forget which, and since the dwell is measured across the points where 12v is applied and shorted out, then this would also be a good spot for a tach. Certainly don't want it clipped onto a plug wire like your timing light. But the question is, what is the tach setup to read. If reading across the points on a 6 cyl 2 cycle engine, it will get a pulse 6 times per rev. If 4 cycle, half that.

Probably easy enough to extrapolate. At idle you engine should be running around 600 to 1000 rpm. So if the rpm scale reads that at idle fine, you are good to go. If something else do the math and go for it. Whatever the ratio you come up with at idle will also apply at WOT.

Mark

 

[Chris1956]

Re: 1978 Mercury 800 - Ignition Problem?

Since that motor does not have points, I am not sure where you would connect the dwell/tach. The closest place to connect it would be the primary of one of the ign coils. Of course the CDI ignition will pulse that coil with some high voltage. Not sure what the voltage would be, but certainly a lot higher than the 8VDC in an automotive point/coil ign system. If it does work, w/o burning something out, you will need to multiply the RPM times 4 to get true RPM.

 

[kb67]

Re: 1978 Mercury 800 - Ignition Problem?

Good thoughts on frying the dwell meter, hate to ruin it in the event I get myself another old motor...probably invest in a DVA in the spring and attack the problem properly. Thanks again. Keith

 

[Texasmark]

Re: 1978 Mercury 800 - Ignition Problem?

Good thoughts on frying the dwell meter, hate to ruin it in the event I get myself another old motor...probably invest in a DVA in the spring and attack the problem properly. Thanks again. Keith

You don't need to invest in one. Build your own with about $3 worth of Radio Shack parts. Takes a diode, capacitor (condenser) and a resistor. Circuit is on here. Just go to archives and plug in the abbreviation DVA. Scroll through them and you will get the circuit. Just keep in mind that the high voltage input is connected to the anode of the diode and the cathode, high side of the cap and one side of the resistor go to the meter +. Ground is the other side for everything.

Mark

 

[kb67]

Re: 1978 Mercury 800 - Ignition Problem?

Thanks Mark, sounds like a good winter project...