Mercruiser 4.3 running badly need help diagnosing problem

[bmcclean]

Hi All
I have a 1992 Mercruiser 4.3 V6 Thunderbolt IV ignition 2BBL Mercarb. Location: South Pacific
Boat: Bayliner 2052 Cuddy
Ok so before I start pulling the engine apart I thought I see what the experts have to say.

After the winter I put the boat back in the water it hadn't been run for 6 months.
I notice at full throttle I was only get 2800rpm and moving very slow. It all sounded good but felt like it was running on 4 cylinders. I also noticed it didn?t want to idle and would pop and back fire through the carby when shifting between forward and reverse.
I have fresh fuel, replaced the Fuel/Water separator and no water in old one.
Fuel pump is working well and carby appears to be working well, choke open and good flow of fuel from jets.
I have replaced the spark plugs, distributer cap, rotor and leads.
When I pulled the plugs I think I found a clue.
4 of the plugs are dry and light brown/black and look good, but when I pulled cylinder 4 & 6 the plugs where clean and wet with what looked like oil, it may have been water mixed with oil but was tick and brown like engine oil.

After this a did a compression test and all cylinders came up good around 150-170psi although both 4 & 6 came up at 185psi which makes me think once again oil.
So this is where I am at now, my guess is either Head Gasket, Intake gasket or exhaust gasket. I know that cylinder 4 & 6 have side by side exhaust ports but the intake ports are separated.
So as I said I would like to hear from the experts before I pull off the head.
Thanks

 

[Leedanger]

Re: Mercruiser 4.3 running badly need help diagnosing problem

I wouldn't pull the head off yet personally. Whenever I've had a backfire on that motor it's almost always been a fuel issue or occasionally timing. You certainly have adequate compression so if it were me, the first thing I'd do is rebuild the carb. If I remember right that has the 4bbl weber and definitely will respond well to a rebuilt or vice versa. If you hook up a vacuum gauge to the fuel line and the readings aren't right you may also want to replace the anti syphon valve or at least be certain the check ball is not stuck.

 

[MCL]

Re: Mercruiser 4.3 running badly need help diagnosing problem

Try running it on muffs,if its burning oil it will show as blue smoke at the exhaust.Also try touching the manifold closest to the exhaust ports,that will tell you if 4&6 are firing,if they dont they should be cooler than the rest of the cyl.

Also check that you have connected the sparkplug wires correctly.

 

[dubs283]

Re: Mercruiser 4.3 running badly need help diagnosing problem

Also check that you have connected the sparkplug wires correctly.

1-6-5-4-3-2 if memory serves

verify timing is correct and is advancing properly under load

verify rpm drop when disconnecting plug leads one at a time at the cap at about 1500 rpm

check fuel pump pressure at idle and higher rpms preferably under load especially if mechanical

verify fuel system feed lines, anti-siphon valve, pickup, filters, tank vent, etc are all clear and functioning properly

then go through this list here: http://forums.iboats.com/showthread.php?t=295070

once you have exhausted all these possible causes, then you can pull the head

 

[bmcclean]

Re: Mercruiser 4.3 running badly need help diagnosing problem

Ok just to close this tread, the problem turned out to be the timing was way off practically running at TDC. of course it took a while to find the problem as I have never touched the timing and not sure hwo ti could be so far out.

Maybe Thunderbolt IV have a habit of losing the timing.

Regards