How to reset TDC after flywheel has been off

[Nautique93]

1983 Mercury 70 3 cyl. 9353189
I am struggling to line up to TDC, i can just about make out the 0 on the scale on the side of flywheel, but my manual says to set no.1 piston to .491 BTDC with dial gauge then line up flywheel so pointer is in line with that mark. My flywheel is rusted and i cant read the scale very well, and cant see the 491 mark the manual speaks of. Can i simply get no. 1 piston to TDC and line up flywheel so 0 degrees on gauge is in line with pointer? If so, what is the reason for setting .491 out with the gauge?
I now realise i shouldnt of removed the 6 flywheel bolts to change the stator, and got a puller instead so i could get it off with just the main nut, then i could of lined it up with the key on the shaft. I think i put it back on where it came off, but i am about to do a linc and sync and want to be sure.
If someone has a link to a picture showing the scale on the side of the flywheel that would be very helpful.
Thanks, Peter

 

[Yepblaze]

Re: How to reset TDC after flywheel has been off

bring the top piston to the top of it's travel and set the flywheel at zero.

The .491 refers to the point before top dead center that the maximum spark advance happens.

 

[Nautique93]

Re: How to reset TDC after flywheel has been off

The manual says 0-4 BTDC for idle timing, and 23 BTDC for maximum spark advance, which is 27 BTDC if set with engine not running. It says to line up fly wheel i need the pointer pointing at the .491 mark which i cant see. It shows a picture of the mark, which looks like its to the left of the scale, as you can just make out a couple of lines and a 2 to the right of it. I cant see a reason why i cant line up TDC to 0 degrees but if there was no reason, why the .491?

 

[Jeff_G]

Re: How to reset TDC after flywheel has been off

Because O degrees, when the piston is at TDC has a duration.
By giving the the timing mark .464 or .491 that is a precise position of the piston in relation to the crank.
Do this, put a dial indicator in the #1 spark plug hole or even use a pencil.
Rotate the flywheel by hand all plugs out, until the pencil or dial indicator stops moving. Make a pencil mark on the flywheel across from the timer pointer. Now continue moving the flywheel until the dial indicator or pencil starts to move again. Make another mark. See the difference between the two marks? Half way is actual tdc. But you can see how far off it could be.
As for taking your flywheel off like you did that is a no, no.
Use red loctite on the bolts.
If you didn't move the timer pointer just move the flywheel to about TDC using the pencil or dial indicator and install the flywheel where the tdc mark aligns roughly with it.

 

[j_martin]

Re: How to reset TDC after flywheel has been off

bring the top piston to the top of it's travel and set the flywheel at zero.

The .491 refers to the point before top dead center that the maximum spark advance happens.

Man oh man. Loose lips sink ships. Misinformation grenades powerheads.

.491 BTDC would grenade it immediately, if it started at all. Max advance is around .150", but varies from engine to engine a little.
The .491", or another measurement in the .4xx range on other engines is a convenient setup point to perfectly zero the timing. The logic is this. At TDC, the delta change with rotation is extremely low, approaching zero. At 45 - 90 degrees timing, the delta change with rotation is high. Therefore, a small error at .4xx is just that, a small error. A small error at TDC could be quite significant.

If you do, and understand the procedure, it makes sense, and it is very accurate.

hope it helps
John

 

[Nautique93]

Re: How to reset TDC after flywheel has been off

Thanks everyone, turns out my flywheel is spot on- using the pencil trick and marking either side of TDC, the pointer is bang on 0 degrees. Turns out i got lucky! The pointer is fixed and cannot be adjusted as far as i know. Started up and pointed timing light at the pointer, is showing 2 degrees BTDC. I will take to the lake to check maximum with it in gear and strapped to trailer.
After all that i have spotted the .464 on the flywheel which showed up after sanding it a little.
I put the 6 bolts in with blue lok tite, and it has run about 10 hours since then, do i need to re-do with red? is it stronger?
The reason i took the 6 out in the first place was to change the stator without having to buy a puller. Stator cured the problem for a couple of trips then the trigger gave up, so i had to pull the flywheel hub in the end anyway. With the old stator and trigger reading well out of spec, im hoping i have cured the problem. ill let you know by the weekend!
Thanks, peter

 

[j_martin]

Re: How to reset TDC after flywheel has been off

No link and sync procedure I know of needs the boat underway for timing. Set the WOT timing to spec while cranking with the throttle at max and the spark plugs out, and the plug wires shorted to ground.

If the trigger's been changed, it needs to be checked.

hope it helps
John

 

[Nautique93]

Re: How to reset TDC after flywheel has been off

my manual says

'if initial timing adjustments are made without engine running, then final timing checks should be made with engine running due to timing advance charicteristics of ignition system. minimum engine rpm required to check maximum timing advance is 3000 rpms'

Due to this i had decided not to bother removing and grounding plugs 2 and 3, just put it in the water and check it once.
If this is not the case i will just do it on the drive, then take it for a blat round the lake with fingers crossed!

 

[sschefer]

Re: How to reset TDC after flywheel has been off

my manual says

'if initial timing adjustments are made without engine running, then final timing checks should be made with engine running due to timing advance charicteristics of ignition system. minimum engine rpm required to check maximum timing advance is 3000 rpms'

Due to this i had decided not to bother removing and grounding plugs 2 and 3, just put it in the water and check it once.
If this is not the case i will just do it on the drive, then take it for a blat round the lake with fingers crossed!

Most Merc techs will tell you the same thing that John did. You can set your timing in the driveway with the plugs out at cranking speed and the switch box will take it from there. The 3000 rpm check is usually done in a tank with a wheel on the prop shaft. Merc doesn't expect you to hang off the back of the boat and time your engine. Although, admitedly, some of us do it.

 

[j_martin]

Re: How to reset TDC after flywheel has been off

Most Merc techs will tell you the same thing that John did. You can set your timing in the driveway with the plugs out at cranking speed and the switch box will take it from there. The 3000 rpm check is usually done in a tank with a wheel on the prop shaft. Merc doesn't expect you to hang off the back of the boat and time your engine. Although, admitedly, some of us do it.

Not me, I'm getting too old for that chit.

John

 

[sschefer]

Re: How to reset TDC after flywheel has been off

Not me, I'm getting too old for that chit.

John

LOL,, with the way I drop stuff it would cost me a fortune in timing lights.

 

[Nautique93]

Re: How to reset TDC after flywheel has been off

ok i'll check it on drive, but i think im still going to double check in the water, i cant see it being that difficult: Boat strapped to trailer, engine cover off and timing light rigged up, one person advancing the timing, and the other pointing the timing light. Both well clear of the prop obviously. Im sure it will be spot on , but just doing it for peace of mind.
If it did require adjustment i can see the potential for dropped tools/ bolts!
Someone suggested i use red loc tite on flywheel bolts, is this different to the blue i used? Is it neccessary to re-do with the red?

 

[sschefer]

Re: How to reset TDC after flywheel has been off

Red loctite hardens, blue doesn't. Blue is for the bolts you're gonna want to remove often but still need to stay in place when you torque them. Red is for places where you rarely need to remove things and definitely don't want them comming loose. Rod caps, flywheel center hub bolts, etc. Neither of them are permanent like welding, (don't do that).

 

[Jeff_G]

Re: How to reset TDC after flywheel has been off

Most Merc techs will tell you the same thing that John did. You can set your timing in the driveway with the plugs out at cranking speed and the switch box will take it from there. The 3000 rpm check is usually done in a tank with a wheel on the prop shaft. Merc doesn't expect you to hang off the back of the boat and time your engine. Although, admitedly, some of us do it.

No, not correct. You should always recheck the timing under load at rpm.
Depending on the ignition system will depend on where the timing is at WOT and hom much. Each system varies, even from engine to engine.
Most of the common Mercury ignitions using the 7778 switchboxes retard the timing about 2* at WOT.
Most of the common OMC powerpacks advance the timing about 3* but I have seen as high as 7*.
If one does not check the timing you risk serious damage to your engine.
On the Mercury timing can be set on the hose initially, but must be checked at WOT for final timing.
Don't let the switchbox "take it from there" a blown engine can result. And yes Mercury does expect you to do a final timing under load.

Also never try to run the engine under load in a test tank with the prop on, you must use a test wheel or you will throw the water out of the tank, cavitate and possibly overheat long before you can set the timing. Once you reach WOT timing you need to hold it there for a few seconds to stabilize to ensure correct timing.

 

[sschefer]

Re: How to reset TDC after flywheel has been off

Mercury does not expect you, the consumer, to hang of the back of your boat and check your timing at wide open throttle. Serious injury could and likely has occured. Be smart and take it to a good shop to have the final timing set. You'll find it really inexpensive compared to a hospital bill.

 

[Jeff_G]

Re: How to reset TDC after flywheel has been off

Nautique93 had the right idea. It can be safely done without going out running the boat.
Check the timing then SHUT OFF the engine before making any adjustments, restart. Put the boat into the water until the split between the prop and midsection is covered by water. Run the engine under load, prop on in gear boat secured to the trailer. You only need to hold it until the timing stabilizes a few seconds at most.

 

[Nautique93]

Re: How to reset TDC after flywheel has been off

Thanks for the replies, i checked max adavance today on the drive, with 2 of the plugs out and grounded, and it is spot on- 27 degrees BTDC as per the note on the air box cover. It says when checked at 5500 rpm under load it should be 23 degrees. This is slightly different to the manual but i guess the manual covers so many models (all the 3 cyl mercs) that some are slightly different.
I will double check it in the water strapped to the trailer just for peace of mind. In theory it shouldnt require any adjustment. Is it common to not have to adjust timing after new stator and trigger or was i just lucky?

 

[Jeff_G]

Re: How to reset TDC after flywheel has been off

Pretty common to not have to change timing. The problem comes when you get a switchbox that changes the timing in an unexpected way.