1977 merc 850 timing sync questions

I discovered that only cylinders #3&4 had spark. I could get #1&2 to get spark but had to rev the motor. The motor would not idle below 1200rmp. I performed resistance testing on the both sectors of the stator and they both checked out. I read on the CDI website that I could change the blue/bluewhite and the red/redwhite around and see if the problem reversed. However, when I swithed the leads I had spark on all four and the engine purred like a kitten and idled at 850rpm. Reversed them back and ran like a dog. I?m perplexed where the problem is. Could one stator sector be weak or could it be the switchbox? Magnets loose?

I took the boat out on the lake and it ran fine. Good smooth power with no noticeable hesitation. A little rough at idle but I can work on that. I found a good write up on the web on how to time this ?77 merc 850. When I hooked up the induction timing light the timing was way out. I had to move the light three inches to the right of the tab to see the timing sticker. What is up with that? Since the is the solid state type V ignition ( coil packs) there is very little as far as timing adjustments, except pickup advance, that can be off provided everything is installed properly. This is my first year with this motor so I have no prior history. The engine runs smooth so the fundamental timing must be correct. I?m aware that significant damage can occur if left unchecked, so I was to get it right. Any ideas?

Finally, I could only get 4500 rpm in the water. That is 1000rpm off from the manufacture specs. Not sure about the prop size so I will investigate that. I just wanted to add that as it may be part of the timing/ignition.

Re: 1977 merc 850 timing sync questions

Las, That motor has a two piece flywheel. Set the top cylinder to top dead center and see where the timing decal is. It should be very close to the timing pointer says TDC. If not. remove the 8 5/16 bolts around the center of the flywheel and rotate the outer flywheel a bolt hole or two to set the decal to TDC.

The other issue with the stator, I cannot assist with. As for the prop, you might post the type of boat and the prop pitch. Maybe you will have a 21" pitch on an 18 footer, and it will be obvious.

Re: 1977 merc 850 timing sync questions

The numbers stamped on the prop are 1520 and some other letters. Based on the research I have done that would make it 15" diameter and a 20 pitch. It is on a 20' smokercraft pontoon. Thanks

I will check out the flywheel and adjust if possible.

Re: 1977 merc 850 timing sync questions

I am not sure you are reading the prop pitch properly. A 15" diameter prop would likely not work on that motor. a 20" pitch prop is also likely too tall for that boat/motor combo.

The prop will be more likely a 12 1/4 X 17 pitch, or perhaps 12 1/2 x 15 pitch.

Re: 1977 merc 850 timing sync questions

I measured the prop from the center of the nut to the outer blade which is 6 7/8". That would make the prop 13 3/4" . Thanks for taking the time on this.