TOPIC: Mark 75 problems

Hey all,

I've been up vacationing in northern Michigan with the boat all week and have been having a weird problem with the mark 75.


My problem is that after about 15 or 20 minutes of running the engine will start to missfire and it seems to lose about 1/3 of its power. When this happens it will not start in reverse and if I shut it off it does not want to restart. When the engine is cold it runs excellent and will start very easily in forward and reverse. If I wait 30 minutes or so the engine will fire right back up and run excellent again.

I thought it was coils so I replaced both of them with no change. I also made sure that the vent was not closed on my tank. I wondering if this could be condensors breaking down? When I set the dwell I dressed the point but did not change condensors. Anyone have any other ideas? This thing runs sooooo good until it runs for 15 or 20 minutes and It has me stumped.

what color is the dis cap?

The cap is yellow

try cleaning the cap you have the better cap.

Thanks dave. I'll give it a try. I took it off and there looks to be some shavings of something floating around in there.

Do you have a different rotor? If so, try installing a different rotor and see if the problem repeats. One section of the rotor might be breaking down under heat/load and killing 3 cylinders.

Any fuel running out of any carbs? Did you check/replace/adjust the floats? Sometimes an engine, running rich, will run good until it gets really warmed up. What do the plugs look like?

Condensers rarely go out, but I had a 78 Camaro once, that I bought dirt cheap because no one could get it running right. It would quit intermittantly, always when warm. A new condenser did the trick. Plus they are cheap.

Ed- yup I do have a different rotor off my parts distributor. I can pull it and see what it does.


Gregg- I have replaced all three floats along with needles and seats in all carbs. The only thing I didn't replace was the check valves in the fuel pumps. If one of them is sticking could it cause a problem like this?

I just wanted to add that I am not 100% sure the problem is ignition. The way it missfires reminds me of how my ice race motorcycle misses when the battery is low. (It is a dead loss battery ignition system), which is why I'm thinking ignition. At first I actually thought the alternator wasn't charging the battery, but I put my multimeter on it while it was running and it is putting out 13 volts and the battery is not draining.

If you haven't already checked,

Turn on the ignition and measure the voltage at the coil primaries with a digital volt meter. the negative lead to the meter connected to the engine block. Then run the engine until it is misfiring and measure the primary voltages again. Keep in mind that the ignition will need to be on in both situations. the ballast resistors the run the coils could be failing.

Also, the condensers could very well be breaking down. Unfortunately, getting to them is a pain. Also check the wiring inside the disto body for any areas that could be getting shorted

Quick note NEVER leave the key on without the motor running . it will burn the points.

I agree on the ballast resistors, check what you get for voltage at each coil. It does sound like you are dropping three cylinders. Rotor and the side port ignition lead could also be suspect, check the carbon plunger on it. Could be getting stuck as temps rise from running.
Potentially a set of points is overheating also due to resistor issue. Are you sure wiring is correct? one set of points to one resistor to one coil. I have had a point breaker arm crack that caused three cyls to be erratic but that was from the get go not temp or run time related to show up.
Randy

Oops also ballast issue could cause points to burn from not suppressing the arcing when the points open so that's a possible too. At least you won't effect the points timing set up to replace those.