HELP Mercury 850 timing

[tompowers85]

I have a 1975 mercury 850 that I have rebuilt with the help of this forum. I have replaced everything. The engine has 150 psi on each cylinder, great spark and great fuel pressure. I have even replaced the reeds, but I cannot get the engine to idle at 800 rpms and still drop into gear. I do not have timing marks on the flywheel showing TDC to 10 degrees. I have marked TDC with a dial indicator and I have the first pickup set to halfway between the 10 degrees that my sticker does show and the TDC mark I made. I have been adjusting the carbs in a large drum I got and it works very well; the rpms in the drum are the as when I am on the lake. I was wonding if my first pickup maybe to high? Should I possibly need to have th butterfly opened a little more and that will help the engine run when I drop it in gear due to more air, or is it better to have the valve closed as much as possible when running at 800 rpms. I have been trying to get the RPMS down and the best I can do is around 1100 - 1000 and I do not want to ruin the lower unit. I have been using clams post to do the timing, but I just need some more help please..... This is driving me crazy.

 

[Chris1956]

Re: HELP Mercury 850 timing

You need to have the correct idle pickup timing spec. On my old dissy-ignition 1973 850, it was 4-6 degrees BTDC. I presume you motor has the ADI ignition? Is so, you need the spec.

BTW - those carb's idle mixture needs to be set on the water. The method I use is to set the idle mixture until the engine runs smoothest and fastest in gear at idle or in neutral. Now when you try to accelerate the motor will likely die. If it dies, open the top carb idle mixture screw 1/8 turn and try to accelerate again. If it still dies, open the bottom carb 1/8 turn and try again. Repeat if necessary.

 

[tompowers85]

Re: HELP Mercury 850 timing

I have been trying to tune the engine in a 50 gallon drum. I can run the engine in gear without water shooting every where, but you are saying the only way to tune the carbs the right way is to tune them on the water. Should I have a friend steer while I lay across the back and adjust the carbs. When adjusting them i should set the timing at 4-6 degrees like my manual says and try to tune them for the right rpms from there. I should not change to timing once i get it set i should just adjust the RPMS using the fuel air mixture screws on the carbs. I also just realized that I had the timing set really low; it was at 25 degrees when the throttle all the way down and the carbs opened all of the way. My max spark was set around 15 degrees. I thought i had it timed right unit I got a dial indicator. How much will that affect my RPM's I am at 4800 with a 15 pitch prop i have a 21 that I could go to? Will my timing being advanced that much really speed the engine up? Thanks for replying to my post.

 

[Chris1956]

Re: HELP Mercury 850 timing

OK - My Merc Service Manual says that a 1975 Merc 850 is a distributor model. It shows throttle primary pickup as 3-5 degrees BTDC and max spark advance to be 27 degrees BTDC.

You want to set these two advances with the spark plugs removed, inserted into their wires and grounded. Timing light goes on #1. Crank motor and set timing to idle pickup spec. Loosen two bolts (5/16"?) on dissy and rotate the actuator plate to just touch the carb yoke. You have just set idle pickup. Now advance throttle to max and check timing. Turn the max advance screw to set the max advance. Reinstall the plugs. Do not change these two settings again. You can only adjust the idle mixture and throttle cable from now on. Make sure the carbs are all the way open at full throttle and adjust the throttle stop to achieve this.

It makes sense to do the rest of this on the lake. You can do this alone, but a friende makes it better, if he doesn't interrupt. Idle out to a place where you are alone. Now adjust the idle mixture (in gear is best but not essential) to get the fastest, smoothest idle. Turn down the throttle stop (or adjust the throttle cable) until the RPM is 1200 RPM or less in neutral. Now try to accelerate in gear. If the motor stumbles or dies, open the top carb mixture screw 1/8 turn and try again. If the motor stumbles or dies, open the bottom carb 1/8 turn and try again. Repeat as necessary. You want to have smooth acceleration with a smooth idle. These two items are opposite, so you get a compromise.

BTW - If you go from the 15" prop to a 21" prop you will lose 1200 RPM off the WOT RPM. This is likely to be bad for the motor. I ran my 850 on a light 16 footer with a 17" prop, and performance was good.

 

[tompowers85]

Re: HELP Mercury 850 timing

Sorry I was at work when I posted back and I was mistaken on the first timing pickup numbers. In my manual it does say 3-5 degrees. Even with the new reeds and everything I should be happy with engine running at 600-650 rpms in gear and around 1000-1050 rpms in neutral. I do not want to trash my lower unit. I just cannot get it to 800 rpms in neutral and 600 in gear without stalling. I get really close but it does not run real consitant in gear it will stall after about 30 seconds. Thanks alot for you help. This is the first outboard I have ever worked on.