Finding TDC on number 1....advice please..merc 120

[libellav15]

Hi all

Engine details:- 1972, Mercruiser 120, GM in Line 4.

story so far....i have established that the balancer has slipped slightly and the timing notch is now miles out from the timing mark range on the block.

if i get the mark anywhere near retarded enough to the 6 deg btdc mark with the timing light it stalls.

i can get the engine running fairly sweetly by ear/feel but i want to get it spot on to eliminate any bogging, pinking etc etc

so i need to get a new mark made at TDC on the balancer to get the timing right.

i have set up a local ignition switch to jump the engine round but its far too jerky to set up the precise tdc of compression stroke on pot 1.

i took all the plugs out thinking i could turn the engine with a wrench as you would with a car - but the balancer is behind the engine mount and the other option above it has 3 bolt heads in a triangle.

any tips/tricks advice on finding the precise tdc would be greatly appreciated please.

i have also read on a post somewhere that getting the dwell on the points spot on will automatically mean the timing is correct - is that true? i can't get my head around the relation between the two..

cheers guys

adam

 

[Blk-n-Blu]

Re: Finding TDC on number 1....advice please..merc 120

With the plugs out you might try tightening the alternator belt and use it to turn the motor.

 

[Bt Doctur]

Re: Finding TDC on number 1....advice please..merc 120

Can you acquire another outdrive inputshaft, make a T handel tool from 1 inch pipe and weld it to the back of the shaft. Remove the drive and insert it to turn the motor slowly.
Dwell has nothing to do with the timing settings.Dwell is the amount of time in degrees that the points remain closed to charge the coil.Dwell and the point gap are closely related ,smaller point gap ,longer there closed.higher dwell, slightly advanced timing.
wider gap, shorter there closed,more retarded timing, lower dwell reading.You could assume the dwell reading is the points gap setting, meaning you could install a set of points by eye , adjust the dwell by opening or closing the points tiil you read eg: 35 deg. and then go back and measure the gap opening .Next time it needs points just gap them and that setting.

 

[libellav15]

Re: Finding TDC on number 1....advice please..merc 120

Cheers guys.

I am constantly looking on flea-bay for a shaft to make an alignment tool.

i will look extra hard now thats a fab idea Doc.

and cheers for the dwell pointers (pardon the pun) thats exactly what i thought too.

Re tightening the belt - good idea - don't want to stress the alt bearing though - she's quite an old lady.

The timing is becoming a bit of a bug-bear really. It seems like i can get it set up, go for half an hour potter about (literally - 4 mph max on the rivers here) and then suddenly it starts missng, hesitating etc.

Guess it could just be the plugs fouling due to continuous low speed - practically idling at 4mph as you can imagine.

Thought it might be the mechanical advance in the dizzy playing up when its hot.

Don't want to move on to checking the fuel system out until i'm sure ignition is perfect - don't want to chase my tail.

Starting to think it might be dirty fuel getting around after half hours agitation. The boat was sitting for 3 years before i got it. Seems obvious sitting here as i type this - lol.

 

[rodbolt]

Re: Finding TDC on number 1....advice please..merc 120

if that balancer ring slipped, then you may get to post about a broken crank snout. best I remember the 2.5 had a solid hub and the 3.0 had a balancer.

 

[libellav15]

Re: Finding TDC on number 1....advice please..merc 120

if that balancer ring slipped, then you may get to post about a broken crank snout. best I remember the 2.5 had a solid hub and the 3.0 had a balancer.

Broken snout is out of the question.

the timing mark is constant and fixed steady under the timing light - just in the wrong place.

and on the 120 the pulley IS the balancer.


cheers for the supportive (not) and short snotty reply though