Mercruiser V6 won't idle and heaps of blowby out of rockers cover

[Squizz]

Hello,
I was wondering if someone can help me out with an unusual problem.
I have a 1986 4.3 V6 Alpha one 205hp. It has a 4 barrel rochester with thunderbolt ignition. It was rebuilt about 5 years ago.
It idles well (purs like a kitten)with no issues at home in the drive on the muffs. However on the boat ramp in the water and on the water it will not idle or idles rough with lots of black smoke coming from rocker covers via the blowby valve hole, it will cut the engine due to fumes if engine cowl is closed, it does not use or burn any oil.
At 3200 it easily planes at 20 knots and runs well, same at 4000 rpm. There are no noises, tappet or knocking, the motor sounds good. What I have done so far:
- Compression test 140/150 psi per cylinder, no difference wet (oil).
- New plugs, leads, Cap and rotor.
- checked flapper valve (perfect)
- cleaned spark arrester

 

[Bondo]

Re: Mercruiser V6 won't idle and heaps of blowby out of rockers cover

However on the boat ramp in the water and on the water it will not idle or idles rough with lots of black smoke coming from rocker covers via the blowby valve hole, it will cut the engine due to fumes if engine cowl is closed, it does not use or burn any oil.

Ayuh,... Sounds like a serious Carb problem to me, so far...

 

[mkast]

Re: Mercruiser V6 won't idle and heaps of blowby out of rockers cover

idles rough with lots of black smoke coming from rocker covers via the blowby valve hole

Choke/carburetor problem.

 

[Squizz]

Re: Mercruiser V6 won't idle and heaps of blowby out of rockers cover

Choke/carburetor problem.

I know the choke is good.
If its carb, why all the smoke through the tappet/rocker covers?

 

[Squizz]

Re: Mercruiser V6 won't idle and heaps of blowby out of rockers cover

Ayuh,... Sounds like a serious Carb problem to me, so far...

but it runs fine out of water.

 

[mkast]

Re: Mercruiser V6 won't idle and heaps of blowby out of rockers cover

I know the choke is good.
If its carb, why all the smoke through the tappet/rocker covers?

Is the choke good and adjusted?

 

[Squizz]

Re: Mercruiser V6 won't idle and heaps of blowby out of rockers cover

yes watched choke with spark arrester off, it will slowly open and stay open when heated up. Symptoms are the same whether hot or cold.
Why is black smoke coming out of breather instead of exhaust?

 

[Bondo]

Re: Mercruiser V6 won't idle and heaps of blowby out of rockers cover

Why is black smoke coming out of breather instead of exhaust?

Donno, but Black smoke is unburnt fuel...

 

[mkast]

Re: Mercruiser V6 won't idle and heaps of blowby out of rockers cover

yes watched choke with spark arrester off, it will slowly open and stay open when heated up. Symptoms are the same whether hot or cold.
Why is black smoke coming out of breather instead of exhaust?

Blow-by, past the piston rings.
Because the choke plate is closed when the engine is cold, and opens as the engine heats up does mean the choke is working.
But, it does not mean the choke is adjusted as per specs.
If there is "Black" smoke at operating temperature, the carburetor is the first place to look.

 

[Squizz]

Re: Mercruiser V6 won't idle and heaps of blowby out of rockers cover

The carb is fairly old, I'm fairly good with Holleys etc... is there a good replacement carby, as it will be hard to get a Rochester of the same vintage?

 

[mkast]

Re: Mercruiser V6 won't idle and heaps of blowby out of rockers cover

You might try rebuilding the carburetor first, a rebuild kit (good quality) is less expensive.

 

[Squizz]

Re: Mercruiser V6 won't idle and heaps of blowby out of rockers cover

You might try rebuilding the carburetor first, a rebuild kit (good quality) is less expensive.

What about Float level, on the boat ramp and in the water the angle is higher than when in the yard? Its flat in the yard and on the plane?

 

[Squizz]

Re: Mercruiser V6 won't idle and heaps of blowby out of rockers cover

Update so far:

Fitted new carby - no change
Choke works fine
3 different sets of plugs - no change
removed exhaust flappers/shutters - no change
adjusted ign timing retarded and adv - no changes
new leads, dizzy cap and rotor - no changes
Very good spark
compression cold and dry - 175/180 psi

I'm leaning towards valve guides or cracked valve spring (no noise from valve train though!!)
Noises from motor at all??

4.3 V6 Mercruiser
Alpa one Gen 1
4BBL Rochester
Thunderbolt IV

 

[Squizz]

Re: Mercruiser V6 won't idle and heaps of blowby out of rockers cover

Ended up being valve guides...

The reason it was worst when in the water was the back pressure in the exhaust was higher due to the exhaust gases having to push past the water. Therefore the exhaust guides were the least path of resistance and allowed exhaust gas to push up throught the guide and out through the rocker cover.

Both guide and valve were absolutely stuffed. The rings were fine.

Goes fantastic with new valves and guides...

Update so far:

Fitted new carby - no change
Choke works fine
3 different sets of plugs - no change
removed exhaust flappers/shutters - no change
adjusted ign timing retarded and adv - no changes
new leads, dizzy cap and rotor - no changes
Very good spark
compression cold and dry - 175/180 psi

I'm leaning towards valve guides or cracked valve spring (no noise from valve train though!!)
Noises from motor at all??

4.3 V6 Mercruiser
Alpa one Gen 1
4BBL Rochester
Thunderbolt IV