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mercury 70hp idling erratically!!

J

johnbatman520

New member
Hi. Hope you can help. I have a 1987 mercury 70hp 3 cyl 2 stroke. Last year while on the water It just died on me and I discovered it had no spark. I didnt do anything with it until recently when I bought a donor 1987 Mariner 75hp 3cyl 2 stroke for parts. I swapped the stator and trigger and still had no spark, I then swapped the switchbox and got spark back. I now have the stator, trigger and switchbox from the donor engine on mine. I fired it up today and it started immediately. I brought it out for a spin and it took off like a scalded cat! When I came back to the harbour and put it into neutral I noticed it was idling very bad. It's popping a bit and seems to be revving up erratically. I brought it for a spin again and it seemed fine but when back in neutral the bad idling continued. Some additional info... I was afraid the auto lube might be blocked due to sitting for almost a year so I mixed 2stroke oil at 50:1 in the fuel tank incase the autolube wouldnt work... Also after a couple of minutes on the water I noticed the tilt and trim wouldnt work so I checked the battery and the positive lead had come loose, I re-attached it and continued on my test drive. I just wondering could this erratic idling be caused by gunk in the carbs or xould it be something to do with the replacement electrical parts? And help would be much appreciated. Thanks in advance, John.
 
I would clean cars first and check that engine is charging as the loose cable could have damaged rectifier/regulator and causing stator to overheat..
 
Hi. Thanks for reply. My plan is to clean the carbs tomorrow. If the engine is charging, is that an indication that the rectifier is ok? Could it be that the stator is now damaged as a result?
thanks.
 
Hi. I didnt get cleaning the carbs yet as just didnt have the time but I did get checking if it is charging and it doesnt seem to be. How many volts should I be expecting if it is charging?
thanks
 
One other thing. The stator that was on the engine was a red stator with serial number 398-832075A3 and the one I replaced it with is CDI 174-8778K1. Just wondering if that is ok?
Many thanks
John
 
Hi. Thanks for reply. The battery is reading 13volts before I start the engine and doesnt increase when I start it! I'm guessing I have a rectifier problem? Could the stator now be damaged?
 
When you get that issue resolved, and want to make it idle, let me know.

I have the method!

Jeff (AKA: Mr. Merc Triples)
 
Hi. Cleaned out carbs and checked rectifier/regulator and it seems fine! Interested in your method to get it idling good, any advise is much appreciated!
Thanks,
John.
 
When you get that issue resolved, and want to make it idle, let me know.

I have the method!
Click to expand...
Dont follow his method as you will end up buying new throttle plates for carbs, you have a looper motor and his so called "fix" is for crossflows. To get a good idle this engine must be in barrel or lake and have backpressure ,it will idle rough on a hose and still not be real smooth as its the small bore looper with 1-2-3 firing crank and the newer ones are 1-3-2 so they idle better
 
Hi guys.
Thanks for your input and advice. Have carbs cleaNed out and found that the plastic linkage to the top carb was broken. It did actually operate the carb but didnt seem to be opening it as much as the others, would that be a cause of the erratic idling and would it affect performance by much? I have it now repaired but am unsure if all carbs are now operating as they should. I heard of a drop off test, only thing I'm concerned about is will it damage the ignition system if I disconnect a plug while engine is running?
 
Drilling the throttle plates cures bogging, not bad idle; and these engines don't need it.

You get the idle smooth with an infrared heat gun. Shoot the metal shell of the plug and dial the idle mixtures until the plugs are all about 88 to 90 degrees.

Jeff
 
I call bovine scatology as to many variables and never seen a plug run that cold as most engines run at 120 degrees plus. The plugs "heat" is dissipated thru threads and "shell" into the cylinder jug. Spark plug cooling efficiency is largely determined by the material the cylinder head is constructed of and the design of the water passages around the spark plugs in the cylinder head. Some cylinder heads are better at cooling the spark plugs than others. Some even have the capability of directly plumbing coolant to the center of the head. Basically, a cylinder head that does a good job of taking heat away from the spark plugs may tolerate a slightly “hotter” spark plug. Conversely an engine that is run with a dry cooling system will require a “colder” plug. Another variable is the quality of the connection between the spark plug and the cylinder head. Heli-coiled spark plug threads may need a “colder” spark plug. Improperly tightened spark plugs will have a poor thermal connection and will run hot but this is not to be confused with needing a “colder” spark plug(C&P) from NGK.
 
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