Interpreting Mercury Timing Decal

thunderbolt!

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Hello everybody,

First I have to apologize for my not so perfect English. I'm from Germany, but I will do my very best :)

I own a Mercury 500 Thunderbolt from 1977 (serial# 9136259) Originally a manual start model, it was upgraded to electric start a few years ago.

I think my timing is not optimal. One reason: my butterflies are at full throttle only 50% open...

TDC, which I have found by the dead stop methode, is pointing to the marker. So the new flywheel was correct installed. On the old flywheel there is a decal (see picture 1) with a bold line at 1? BTDC.

My first question is, is that the "real" TDC or has this mark another purpose?

Furthermore there is another decal on the engine (picture 2) which says ".172 B.T.DC. 30?" I Think the 30? is the max. spark, but what does .172 mean?

Thanks in advance! And greetings from Germany!

Hendrik

Pictures:

2.jpg

1.jpg
 
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flyingscott

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I think you are going to need to do a link and sync carbs not opening all the way is more of a throttle issue than timing. I would check your throttle cable and carb linkages. Timing very seldom ever goes out on them I would look to the throttle assembly not timing.
 
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thunderbolt!

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Hello Scott,

thanks for answering! The carbs are in sync with each other. I think it is a timing issue because none of the screws at the throttle lever are contacting the stops. they are al completly turned out. So I have definately to do the idle and max spark things.
 

Chris1956

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I am not sure why the TDC mark is 1* BTDC, but expect mercury knew what it was doing. On american Mercs the .377 mark is used to calibrate the timing pointer. However, American models use .464 inches. I am not sure what unit your .377 mark uses. CM doesn't seem to calibrate with .464 inches. Any idea if the motor is from Belgium?

Anyway we normally set the top piston to TDC and then measure from spark plug opening to top of piston. Add that to .377 and roll the motor back to that setting. Now adjust the timing pointer to match the .377 mark.

I am not exactly familiar with a '77 Merc 500. That timing spec sounds like the older ignition system. You have points and a distributor?

You need to find out your idle pickup timing spec. You need to set the carbs to start to open when the motor reaches idle pickup timing spec. The max spark advance screw needs to keep the max timing at 30*. Throttle stop should allow the carbs to open all the way. Adjusting the throttle cable may be necessary. idle stop screw should set minimum timing to achieve idle speed of 900 RPM or so.
 

thunderbolt!

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Hello Chris,

Thanks for your answer! Yes indeed, the motor is Belgium made. Also I've seen on this forum only pictures with the .464 mark on it. Maybe it depends on different production techniques in Belgium.

Your idea with adjusting the timing pointer to the .377 mark sounds good. I will do it to double check the TDC which I have found with my "piston stop method".

My motor has already CDI- ignition, so I have no distributor.

Can you imagine what the decal ".172 B.T.D.C. 30?" means? okay, the 30? is probably the max spark advance, but why .172?

Is it possible to set the max spark advance with the motor in the water with the boot on the trailer? Because the motor was originally hand start, the specs on the decal are not for max spark advance at cranking speed (am I correct?)

Thanks again!

Hendrik
 

Chris1956

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Hendrik, You need both the idle pickup timing spec and the max spark advance. I looked at an old non-OEM manual and found the following, for motors made in USA and about your age. Looks like Idle pickup timing is 7-9*BTDC and max timing is 30* BTDC, but not sure if I have the right model. See link

boatinfo.no/lib/mercury/manuals/mercury_1965-1989_40-115.html#/230

I always remove all but spark plug #1, and ground their bases. Next I set the idle and max timing, with the motor cranking. The throttle stop can be set with the motor off.

I would have expected your motor to be ADI (4 coils), but the timing spec of 30* caused me to question that. If you can figure out he unit for the .377 and the ,172, you can do some division and see if .172 is 30 degrees advance on the flywheel. You will need to measure the diameter of the flywheel closely, somehow.

Best of luck.
 

thunderbolt!

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Hi Chris,

Now you made me unsure about my statement that the motor is a CDI one. I have no distributor, but I have 4 coils (see picture). So do you think my motor is an ADI-one? Maybe not so important, but nice to know. I learn everyday more about my motor.

Well, I think I'll do it your way and do the Idle and max spark advance configurations at cranking speed. Also I will do a little more reasearch about the .172 decal; or maybe somebody else here do know what this mysterious mark mean :joyous:

Thank you very much for your help - I really appreciated it!

Kind regards
Hendrik

3.JPG
 
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Chris1956

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Hendrik, To be a little more clear. Capacitive Discharge Ignition (CDI) refers to an ignition system where the output of some capacitors is used to power the ignition coil. Instead of about 8VDC powering the coil as in a battery/coil/points ignition, the capacitors discharge 300V into the coil raising the output of the ign coil from 20KV to 500Kv. CDI is independent of the type of ign system be it 1 coil and distributor or 4 coils, as you have.

Merc's brand of CDI on it's 4 cylinder motors after 1975 is called Alternator Driven Ignition (ADI). ADI has 1 coil per cylinder and the stator supplies all the power necessary to run it. Merc's brand of battery powered CDI ( 1 coil+distributor) was used on 4 cylinder Mercs prior to 1976.

Your decals look to be 76/77, but since the motor was from Belgium, I was not exactly sure what you had.

Best of luck
 
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