Mercruiser 120 - timing/dwell/carb conundrum please...

[libellav15]

Hi guys

Info = 1972 Mercruiser 120 GM in Line 4 - same year Pre Alpha leg. Rochester 2bbl carb.

The story so far guys - hope someone can help please.

I bought the boat 4 weeks ago.

Used on the river - runs quite nicely - but tended to miss and surge a bit - so i suspected ignition service due and set the dwell and timing.

So i changed the plugs, leads, points, rotor, dizzy cap etc.

And on i went to do the timing. Set the dwell no probs. Then the fun begins

Turned out that with timing light the timing mark on the balancer was about 2 inches to the left of the static mark scale on the block.

If i turned the dizzy to line the marks up at 6 btdc the engine stalled before i got anywhere near.

So i put it back to where it was and the engine ran/idled/started etc sweet as a nut.

But it still bogged about - at random - and the fact that the balancer may have slipped was playing on my mind.

So i have been down to the boat today - to add a correct mark to the balancer.

So i pulled the plugs, turned the engine by hand until No. 1 pot was at TDC (compression) - and low and behold the timing marks where totally spot on. Good news that the balancer hadn't slipped - but the question then arises - why won't she run with the timing correct. 6 BTDC at 650RPM in gear.

I have gone to fuel issues -

so my questions (finally you all say )

1. I have found some water in the fuel - pumped some from 'after filter' and let it settle in a glass - definitely h2o contamination. Could water in the fuel explain why the boat won't idle at the correct timing - but will idle smooth as silk if the timing is majorly advanced?

2. Having stripped the carb down it was fairly varnishy and in a bit of a mess - but looks to have been rebuilt in the last couple of seasons - all seals/gaskets etc are good. Could dirty internals explain why the boat won't idle at the correct timing - but will idle smooth as silk if the timing is majorly advanced?

3. Finally, the 2 idle mixture screws have very little effect - apart from making it run very rich if i screw them right out - literally OUT of the carb body. Could blocked carb airways explain this?

Please help guys - i'm usually ok with engines - but i'm stumped with this one.

Spark - Yes
Fuel - Yes (but not the best)
Compression - Excellent all 4
Ignition - timing and dwell set up with guages.

But can't get her to idle

And what do you all soak your carbs in to clean them? Over here all i can buy is an aerosol of quick blast and quick dry stuff - I want to give it a good 48 hour soak to loosen any crud - and then blow out with the aerosol cleaner - and then compressed air.

cheers fellas - any and all suggestions welcomed please guys - i know from experience that the obvious and most easiest things are usually the ones missed by the tech working on the engine - can't see the wood for the trees and all that

Adam

 

[Fishermark]

Re: Mercruiser 120 - timing/dwell/carb conundrum please...

Could water in the fuel explain why the boat won't idle at the correct timing - ....



2. Having stripped the carb down it was fairly varnishy and in a bit of a mess .....Could dirty internals explain why the boat won't idle at the correct timing -



Fuel - Yes (but not the best)

Well, I can't really explain why it runs well so far out of time, but you need to start eliminating the obvious problems first.

Try running on a different fuel tank with fresh fuel - a portable outboard tank works well. That eliminates the poor fuel part of the equation.

If it still runs poorly, then I would move on to the carb. It is possible you have some water and debris in the bowl - and even some clogged passages. Some carbs just cannot be rebuilt. It may be time for a new one.

 

[libellav15]

Re: Mercruiser 120 - timing/dwell/carb conundrum please...

thanks so much buddy

i am thinking the same

just had another thought - if the springs in the centrifugal weights on the centrifugal mechanical advance had broken, and the weights were loose - presumably it would start advancing itself straight away

can't get back to strip the dizzy until tomorrow - but what do you reckon?

as far as i know the mech advance shouldn't do anything until after 600rpm - do you agree?

which would mean because the carb is so dirty its been set to idle high to prevent stalling - and hence the mech advance is kicking in.

i'll look tomorrow - and post an update

any other thoughts chaps?

cheers

adam

 

[libellav15]

Re: Mercruiser 120 - timing/dwell/carb conundrum please...

Any thoughts guys?

don s bt doc - where are you guys

 

[Bt Doctur]

Re: Mercruiser 120 - timing/dwell/carb conundrum please...

If #1 was TDC compression, IF timing marks are accurate, then you used a different cylinder with the timing light? What is the point gap now? Are you positive you used #1 plug wire to check the timing, did you use a "dial-back" timing light.??????????????????????????
Sometimes corrosion and other junk can be removed/loosened by soaking it in plain old Coca-Cola.The carbolic acid it contains will loosen the junk,cleanup with hot water.immerse it in a pot and gently simmer. DON`T LAUGH, IT REALLY DOES WORK

 

[libellav15]

Re: Mercruiser 120 - timing/dwell/carb conundrum please...

cheers doc - as usual

re the coke as a cleaner - fab idea - when you say simmer do you mean put it in a pan and put it on the cooker? over here simmer means boil gently..

sadly it is deffo on #1 lead, with #1 at tdc (comp stroke), and rotor pointing at #1.

no dial back adjustment on the sun, straight forward light-only job.

Update -

distributor removed and stripped - mech adv springs both connected - but corrosion was making movement stiff - and the mech adv weights weren't quite sitting 'home' - approx 2mm from fully returned when static

doesn't sound alot - but i bet it would represent quite a bit of advance

and fuel seperator was full of crud and water - so that won't have been helping - do you just buy a new screw on filter for the seperator - or replace the whole lot - or even better can the old one be cleaned out and re-used?

cheers doc

adam