850 merc won't start in cooler temps

it's that time again, I have an 850 merc 1974 and it refuses to start in cooler weather. Starts great in warm / hot weather when engine block is warm. no problem, starts right up, runs good. its now 75 degrees and cooler at night, when i feel the engine block its cool, figured I'd have a problem and sure enough, turns over, spins, but won't fire up. In the past (just didn;t do it today as I'm now in a marina without access to electric) i'd put a heat gun blowing on the motor to heat up the engine block, took anywhere from 15 minutes to 30 minutes, once warm, turn the key and bang, starts on first crank after trying for many long attempts just before that. Any ideas? someone sugested the magneto needs to be warm to expand contacts to fire it? much appreciate any help, it's been 10 years this way, no problem before as I would pre heat motor in cooler / cold weather but now in a marina with no access to electric. thanks, Bill

Re: 850 merc won't start in cooler temps

check compression spark


when were your carb rebuilt and cleaned last? make sure u use the choke all the way until it starts warm up lever all the way up and may need a dash of choke once started to warm up

Re: 850 merc won't start in cooler temps

thank you, the carbs have never been rebuilt nor cleaned that I know of, so thats probably 10 - 15 years, I've had it 10+ and the prior owner was elderly and got sick, didn't use it for 5 years before me.

Re: 850 merc won't start in cooler temps

I used to have the same problem as you, after cleaning and adjusting my carbs and timing the problem was sovled. If you not ready to tear you carbs apart start with the timeing and idle mixture screw should be around 1 1/4-
1 1/2 turns out from lightly seated. This is what I have my 850 set at.

Just recently I purchased a mariner 8hp that was hard to start, finally after
tearing the carbs apart and cleaning them and setting them back as they were ( since I had no manual) still had the same problem. I finally thought
to adjust the idle mixture screw out to 1 1/4 from seated and it fired up on the 2nd pull and has every time since. the screw was set at 3/4 out when it wasn't starting.

Also make sure your choke is working and the high speed idle lever is advancing the timeing. good luck

Re: 850 merc won't start in cooler temps

yup time for an carb clean

Re: 850 merc won't start in cooler temps

I agree with the udder guys about getting your carbs done and btw what prop works best for you? let us know how it runs good luck