Mercury Ignition timing troubles

jorenac1 · Dec 22, 2002

This 90 HP Mercury 78 Thunderbolt 6 cyl in line came separated from the boat. The timing is off. The spark plugs are non gap type. The pointer is a wire. the wire aligns to TDC when cylinder is at top nearest the plug TDC is at the wire, and the distributor pulley to cyl # 1. Compression is 115 to 120 PSI. It cranks but no fire. at end of cranking like a pop. Carburators were rebuilt. The book says to set the distributor pulley in line to the dot in the flywheel. There are three that look like balancing marks. At .640 there is also a dot on flywheel. On top of the flywheel also there is like an arrow mark. Can any wizard outhere can help me dcipher this enigma? Mixture screw set at 2 turns.

12Footer · Dec 22, 2002

Re: Mercury Ignition timing troubles

I'll try to help long-distance...too windy, can't fish... First, take the dist cap off,and inspect the rotor. They have a tendancy to strip-out,or break-loose from the dist shaft sometimes. When this happens, it's time to replace the dist shaft assembly in total. I have often thought about JB-Weld or MarineTex to re-cement then in place, but I is too chicken to trust such a patch-job.. With the dist cap off, try to rotate the rotor with your finger.Not too hard, as you dont wan't to break it if it's ok. But it should not turn at all with one finger. If it's secured to the shaft as it should be,rotate the engine to TDC by hand (or better yet, with a breaker bar on the flywheel nut,cause it's easier). Observe where the rotor is pointing, and it should be ballpark pointing to number one. If not, your timing belt jumped a cog or two. If it is ballpark, the engine should start,albeit, a tad rough. Now it's time to drag-out the timing light and set the timing to idle-specs (2deg ATDC,and 27deg BTDC with advace linkage maxed out). These engines rarely "jump-time" in the linkage , so your orginal advance linkage settings should not need to be changed.

jorenac1 · Dec 22, 2002

Re: Mercury Ignition timing troubles

Thanks for your reply; I will try an inspection first thing tomorrow. Yup is cold ( high 50's)and windy here in Daytona too.

Gold Bear · Dec 23, 2002

Re: Mercury Ignition timing troubles

xyphrx 12 footer has given you good advice. If you have trouble with his suggestions, try this. Pull the cap off the top of the distributor pulley so you can see the arrow cast in to the top of the pulley. Turn the engine around until the three marks on the fly wheel are pointed straight in line with the shaft / middle of the distributor shaft / centerline on the top of the pulley. The arrow on the pulley should be pointing at the three marks on the flywheel. (Make sure the distributor is set where the engine would be at idle - not at an advanced throttle setting). At this point, if you find the arrow and three marks not aligned, the belt has slipped a cog - or has been set up wrong. You will find that the arrow may be pointing slightly the the left of the three marks, that is OK. When the engine runs, tension on the belt will "take up slack". Slip the belt off and re-align it if needed. Look at the cogs on the belt to see if they are badly worn - they are probably OK. If you are still having problems, get a dial indicator, set number one cylinder at top dead center. At that point the three marks should align with the centerline of the distributor pulley. The arrow on top of the pulley should align with the three marks (point at them), and the rotor should be pointing at the terminal for the number one plug wire. This should get you close enough to use the timing light for fine tuning the timing. Good luck, Gold Bear

jorenac1 · Dec 23, 2002

Re: Mercury Ignition timing troubles

Just discovered there are three vertical lines on the timing decal. one of them with blue paint on it. I'll try that. Thanks.

Gold Bear · Dec 23, 2002

Re: Mercury Ignition timing troubles

xyphrx I believe the three vertical lines you see are marks for the max. timing setting. Further to the right you should see three "punch marks" vertically on the side of the flywheel. These are the three marks that I was refering to. If they are not there you will have to find top dead center, set it there, then point the distributor rotor at the number one terminal inside the distributor. This should get you running, then you can use the timing light. Good Luck, Gold Bear

jorenac1 · Dec 23, 2002

Re: Mercury Ignition timing troubles

Hafta handit to you guys. Yup the three dots were exactly were you said. I set the flywheel three dots in line with the cast arrow in the distributor pulley next notch to the right, since the belt would not fit in. The darn thing starts every time, except is like 3000 rpm. I shot it down; looks like is going to take off the ground. I have it on a 55 ga drum. There are three screws on the front of the engine. The Idle stop is screwed all in but 1/4"; the other one is the spark advance scre, and is screwed all in except 1"; the throttle screw is missing. Could you give me an idea how to do a basic screw setup so I don't blow it up ? Shall I reset the distributor arrow a notch to the left ? I really appreciate your expertise in this matter. we are going to have thunderstorms at Christmas Eve in Daytona they say...

12Footer · Dec 23, 2002

Re: Mercury Ignition timing troubles

The timing light will sort everything out for you, providing, the throttle advance linkage has not been adjusted. I'm werkin on a pic of what I think you are refering to, but in the meantime, if you have not turned anything,don't. It should track from idle timing setup just fine. If you have, all bets are off, and you'll need a test tank,or a good pilot, so you can set it WOT. No biggie, but a logistics hastle, for sure. I'll be kinda scarce over he folowing week (for obvious reasons), so in case I don't get back before,Merry Christmas to ya.

jorenac1 · Dec 23, 2002

Re: Mercury Ignition timing troubles

You guys are great. Thanks a million; puts my faith back on mankind.Have a merry one. My wife says: close up the windows when you fire that thing again !!

Gold Bear · Dec 24, 2002

Re: Mercury Ignition timing troubles

xyphrx As you noticed, the cogged belt doesn't always line straight up. As a reminder, when you slip the belt in place, the arrow should point just to the LEFT of the three dots. As 12Fotter mentioned, you can correct the timing with a timing light, but you will find out that with the timing correct, the throttle cable / barrel will have the barrel located in a position that it is not practical to connect into its fixture. You should buy a Mercury Service manual. It has a detailed section on Timing & Synch. of the carbs / distributor. Basically, you can do the following. Remove the throttle cable / barrel at the engine connection / fixture. Set the idle screw so the carb butterflys are just closed / touching. (Look inside with a flashlight and make sure the butterflys are just barely closed). Do not leave so much slack in the adjustment that they jam. Adjust the throttle cable barrel so it slips back into its connector with a little bit of tension towards closing. That way the throttle cable / carbs always close completely. With the engine NOT running move the throttle to wide open - looking at the butterflys and see if they are straight inline the the carb bore. This should give you the best speed when running. If the butterflys are not inline with the bore at this point, someone has been adjusting without the service manual and there is no telling if your other settings are correct. As 12Footer suggested, do not adjust the Spark Advance, or Throttle Stop screws until you have a Service Manual to take you through the step by step process of Timing & Synch. You can adjust the idle as I suggested above because it is apparent that those orginal settings have been lost. I strongly recommend you don't go further until you have a service manual. In addition to the adjusting screws mentioned above, near the upper part of the distributor there is a flexible metal band with another piece of carb linkage. The linkage has another adjusting screw on it. This linkage, when adjusted properly contacts the carb linkage and opens it from about 3/4 to full throttle. This is just another reason to have the service manual to adjust correctly. I noticed you said earlier that the throttle screw is missing. I expect that you will have to correct this problem before you can go further towards setting up things correctly. Good Luck, Gold Bear

jorenac1 · Dec 24, 2002

Re: Mercury Ignition timing troubles

Thanks GBear for the words of wisdom. See ya after X-mas. My wife wants to keep the windows open. Have a nice Holiday.

jorenac1 · Dec 27, 2002

Re: Mercury Ignition timing troubles

well that is the beginning. Did not startup again. Went got the book; took out and tested the coil, and shows continuity between the tower and tboth terminals when tested separately. Is this enough reason for no starting up ?. Going to figure a way to test the "black box" (has only one) So far the black output shows continuity when tested with the white or the brown. I have a coil coming, I will let you know what happen. Please let me know if I am in the right track.Thanks.

12Footer · Dec 27, 2002

Re: Mercury Ignition timing troubles

Before you do that, disconnect the orange wire from the side harness,as close to stator as is practical. This is the "kill" wire. To kill the ignition,this wire is switched to ground. When it is floating, you should have spark. IF so, check out the ignition and kill circuit (could even be this wire touching metal grounded part.

jorenac1 · Dec 27, 2002

Re: Mercury Ignition timing troubles

Thanks for the update. The orange wire is actually tied, not attached to anything. The stator ( under flywheel ?) has only two yellow with red stripe lines, and lead to the rectifier. The wiring appears to match the book of the 90 HP thunderbolt 78. There are only three trigger wires in the distributor; they are black, white and tan. My ohm meter does not have the 100x setting. I took the cd box, and the coil to my local dealer for testing this morning, and they said I had to get the boat in to do the test properly. They did not have a 78 coil("too old")

Gold Bear · Dec 27, 2002

Re: Mercury Ignition timing troubles

xyphrx I am confused. On 23 December you posted that the engine was starting, but running at 3,000 rpm's. I doubt if there is anything wrong with the ignition system - coil, black box or distributor. Think about the steps you have taken since the engine was running, and consider if there was anything you may have done to affect the ignition. Try to "go back" to where the engine was running last. Pull a spark plug and clamp / ground its casing to the block so you can see if you are getting spark. (You can also use your timing light to see if you have spark.) Your engine has a "battery powered" ignition so make sure the battery has a good charge. It is possible for the engine to turn over and not have any spark if the battery is low. Try these things then get back to us. Good Luck, Gold Bear

jorenac1 · Dec 27, 2002

Re: Mercury Ignition timing troubles

Makes sense. I will. Thanks for the patience.

jorenac1 · Dec 27, 2002

Re: Mercury Ignition timing troubles

yup, plugs have little or no spark. Will recharge battery again tonite. I remeber that when it did start, I did jiggle the ignition switch kinda harder than usual. Also I did notice before that all the wiring connections to switch had been redone. I will check that too.

jorenac1 · Dec 28, 2002

Re: Mercury Ignition timing troubles

Guess what guys. I Did read all of your advice again. Tripple checked timing setting. The last time it fired, I had tinkered with installing the old CD box. Today, I re-installed the one I got on the net, and... you guessed right; I got SPAARKS !!! at the plugs that is. With a full charge that was not enough, and the carburators began to flood. In the realm of hopes for a fish in the boat in the Summer, I came-up in finding a way to fit the camel thru the eye,before I started to plan on ditching the whole thing in the backyard, and removing bone by bone until it was really dead. ughh. 1.)I took out the distributor coil wire, cleaned it, sanded the contacts with emery cloth. 2.) Looked inside of the distributor center grafite electrode, and noticed it was flat on the contact surface, took out the spring and noticed there was a plstic edge around the bottom, wich I diligently cleaned up. 3.) Then I looked at the rotor middle, and also noticed that the contact surface was also recessed, and had plastic above the recess. Must have been a temporary molding design fault. 4.) Then I cleaned the brass contact on the rotor. 5.) I followed to file in a conically fashion the carbon electrode to leave a 1/8 diameter on the center, since I figured the electrode was not making contact with the rotor. 6.) I immediately opened the boat throttle control box and cut out fried wires leading to the tachometer, and tightened the batch job done by others. There were posibilities that they could have been making contact. 7).- I turned the ignition key and guess what... You guessed right. Full start blast every time. You guys were right all along. Thanks for giving me the push that I needed to keep going on this project. At times I wondered of the little things I forgot about in life, and of the many engines that did not get started...

God bless.. Oh by the way, is the red wire going to the CD box supossed to be hot when the key is not even on the ignition switch ???

thanks again.

jorenac1 · Dec 28, 2002

Re: Mercury Ignition timing troubles

Oh I forgot: I filed also the rotor outside edges to make them smooth and wide. Only sligthly.

Gold Bear · Dec 28, 2002

Re: Mercury Ignition timing troubles

xyphrx Congradulations!!! Yes, the red wire is hot all the time. Best Regards, Gold Bear

Mercury Ignition timing troubles

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