Merc 5.7 bogged down under load

[ukstu]

Hi all, I hope you may be able to offer some advice please.
Engine is a Thunderbolt 5.7.
Just lunched the boat for the start of the year. Took for a quick run and it’s really bogged down, no power. It rev’s freely in neutral and ram up on the muffs no problem.
If i coached it up through the revs it would eventually get running and Rev up nicely but as soon as it comes off the power it is bogged down again.
Last season it was running perfectly with loads of power.
nothing changed over the winter other than a service and run up.
could anyone offer any advice please?
mom assuming a fuel problem of some sort, either not enough or maybe choke stuck open?
It was a new engine last year and I’m not sure how the new choke that’s fitted works, but wondered if it may have anything to do with it.. picture attached..
Thanks in advance

 

[nola mike]

Start with fuel. Assuming you didn't change your fuel filter. Do that, and check the contents for crud and water. Did it get properly winterized? Check the oil level, make sure it isn't too HIGH. Don't rev it in neutral, or on muffs. Proves that a V8 will spin when there is no load applied to it. Choke stuck open would cause a starting issue, choke stuck closed would cause a running issue. Pull your flame arrestor and look at the choke plate when the engine is warmed up.

 

[Lou C]

That looks like an electric choke conversion for a 4bbl Quadrajet. I'd make sure the choke plate closes with the engine cold and completely opens with the engine warm, electric chokes usually open faster than the older style manifold heated chokes. These usually work well but if there isn't a constant 12v source to the choke heater they will not open up at all.
If they fitted a new engine, it is possible (can't tell from the pic) that they used a newer Vortec engine, these do not have the exhaust crossover in the intake manifold for the old style manifold heated choke, so that might be why you see the electric conversion there. Those conversions are actually made for the old style intake manifold so I'm not sure how they mounted it there, the Vortec intake does not have the little mount on it to bolt the bracket on the choke to....
other causes:
rusted/seized anti-siphon valve on the fuel tank
any restriction in the fuel system can cause this..

 

[ukstu]

Thanks for all the replies.
yes it’s a vortec engine, and the choke has been mounted on a makeshift bracket.
carb is 4bbl quadrajet.
Fuel filter was new last week.
Oil good, correct level and clean.
I’ve not yet got my head around this new choke that is fitted, could you please point me in the direction of what and how to check if it is in fact operating as it should?
thanks again!

 

[LundAngler1650]

Similar issues with a neighbors boat a couple years ago, once I removed the mouse nest and food, rebuilt the carb, changed the fuel filter, and spark plugs it ran perfectly. Rodents are a royal pain in the....

 

[ukstu]

I think I’m heading towards a carb issue to be honest. Think I might take it off, strip it, clean and rebuild. Not been done for a few years and may have some old fuel clogged up from sitting foe the winter. Going to go back down in the morning and re check all the basics first.
only thing I do not know about is if the choke is operating correctly or how to check it.

 

[Bondo]

I think I’m heading towards a carb issue to be honest. Think I might take it off, strip it, clean and rebuild. Not been done for a few years and may have some old fuel clogged up from sitting foe the winter. Going to go back down in the morning and re check all the basics first.
only thing I do not know about is if the choke is operating correctly or how to check it.

Ayuh,..... On a cold motor, actuate the throttle to Wot, once, which sets the choke,....
With the throttle at 1/4 to 1/3rd, turn the key to start,....
When it lights, pull back the throttle to, say 1,000 rpms, 'n allow it to warm up,.....

Once the choke is "Set", power from the ignition circuit heats the pull off coil, 'n the choke opens, motor runnin', or not,....
Of the 2 wires on it, one is from the ignition circuit, 'n the other is ground,....

 

[kulle]

i had similar issues with a 4.3 merc. under load would bog/stall. muffs/neutral ran fine. went to perform transom repair and found half of a water shutter valve( piece that sits in the exhaust risers to prevent water from getting in engine) lodged in the exhaust.

 

[ukstu]

Solved ( where’s that embarrassed emoji..?)
HT leads for cylinder 1 & 3 round wrong way.
where I had fogged the cylinders I had obviously mixed them up putting back on.
Thanks as always for info and advice.

 

[LundAngler1650]

Just glad to hear that it was a simple fix.
Happens more often than you think.
Had a customer come to me last year, said he changed his own plugs, truck ran like crap since.
I pulled out the new plugs, checked the gap on all 8, reinstalled, double checked the resistance in each wire with my ohm meter, all was good. Rotor and cap were new, looked fine as well. I double checked the firing order just incase he had it wrong, and sure enough he had the plug wires in the wrong order. That beautifully restored 1978 Chevy Big 10 left purring like a kitten in no time. Love those old trucks, so easy to work on, plug swaps couldn't be easier, or most other things.