1978 Mercury 800 timing question

[Tony22msu]

Hello all,
I have a 78 Mercury 800 4 cylinder with coil per plug ignition.

I've been trying to set the timing in neutral and I'm not sure I fully understand the process. If I make adjustments that move my spark arm away from the hull, my Rpms increase. I then adjust my idle stop screw to bring the Rpms down and recheck timing with a strobe and my timing is right back where I started. I can get the timing within spec but every time I do I have to lower Rpms and it puts the timing too low again. Can some one please help me with what I'm missing here. Also how critical is timing at idle?

 

[Texasmark]

Ping Chris1956. Some engines have idle set by timing advance, and others by opening the carb throttle linkage. Don't know which is used on your engine.

 

[GA_Boater]

First question - Why are you trying to adjust the timing? The motor isn't running well or what?

 

[Tony22msu]

I have been having issues with idle/ stalling during take off. I have done a lot of things to the boat but I'm not sure if I have found the root cause. Anyhow I should have never messed with the timing but I did at the end of last year. Now I'm trying to get it corrected. I'm just not confident that I understand the process.

 

[racerone]

Simply put , the timing on these engines does not go out of adjustment.----Find out what the real problem is first.---Perhaps have a shop re-adjust to spec.

 

[Tony22msu]

I believe I may have fixed real problem. I had rebuilt carbs twice but did not change gasket between carbs and manifold. Last weekend I replaced these gaskets. However I have already tampered with the timing so I need to get it back. I've been having trouble finding a mechanic willing to work on a motor this old so I figured i should just do it myself.

 

[Chris1956]

There are two timing settings. First you set a specific idle Pickup (primary) timing when the carbs are ready to open. Secondly, you set the max timing advance. A manual is best but here is the reader's Digest version. Assuming the timing pointer is set properly.....

Remove spark plugs 2-4, install them into their wires and ground their bases. Remove Prop. Put timing light on #1 spark plug wire. Advance throttle in gear until carbs are ready to open. With ign on, jumper solenoid and read timing with timing light. Adjust throttle and primary pickup screw to make the idle pickup timing correct. You probably want an idle pickup timing of 0-9* ATDC. The primary pickup screw should be just above the bolt on the throttle arm.

Next advance throttle fully. Crank motor and adjust max spark advance screw to read 21* BTDC. The max spark advance screw should be the topmost screw on the throttle arm.

Now adjust the idle speed by adjusting the isle stop screw (next to where throttle cable attaches) and the throttle to push the throttle closed.

 

[Tony22msu]

I do have the manual, but the process described in the manual assumes the throttle is set in the idle position (if I am interpreting it correctly). Chris I like this method because it relies on a fixed position of the carbs. I think I'll give it a shot this afternoon.

Thank you all for your help thus far.

 

[Faztbullet]

Biggest thing left out ...engine needs to be in a barrel/lake and have back pressure as idle RPM will change if set on muffs and then dumped in lake. I myself only set max timing and let engine tell me want it wants on low end, a engine with internal wear/low compression likes a lot more low end timing.

 

[Tony22msu]

Thank you faztbullet, I have been doing all the tweeking in a bucket. I wanted to take it to the lake today to try out but too many things came up. I messed with it yesterday and it's sounding better than before in the bucket. My tach is not working but I've been able to achieve a lower idle than I have in the past. However there sounds like a miss to me at idle Rpms. I'm not familiar enough with misfire to know if I'm correctly interpreting what it's doing or not though. Seems to go away if I increase the throttle a bit.