Merc 4.3 T4 hesitating

[ohm529]

I have a '92 4.3 alpha one gen 2 with Thunderbolt 4. I bought the boat in the spring of '19 and used it last summer. The boat struggled to run up from 1500 RPM's and would hesitate and almost stall. If I pegged it to WOT it would sputter, maybe pop, and then run up to speed. I figured it was the accelerator pump, so I rebuilt the carburetor over the winter in addition to new cap, rotor, and plugs. I discovered the exhaust manifolds were cracked so I replaced those and did the risers and flappers at the same time. I ran the boat in the water for the first time yesterday and still have the issue. In fact, it feels a little worse than before and doesn't seem to have enough power once I get it going. At WOT it maxed out under 4K RPM's and the boat felt sluggish.

Any thoughts on what could be causing the issue?

 

[Rick Stephens]

Don't feel bad or that I am denigrating anyone, but still sounds like a carburetor problem. I'd suggest doing it again and making for sure and for certain every port and passageway is clear and clean. Probably get away with reusing the kit you put in it last time. All it take is a little crud to block a passageway or hold a check ball from closing.

Could also be an intake leak.

When it pops, it is running out of fuel. That's where you should be looking.

 

[ohm529]

Could it be a flow restriction before the carb?

Also, I just saw a video stating that if the throttle body is corroded, to replace the whole carb. Mine definitely had corrosion on it.

 

[Rick Stephens]

If it had a flow restriction - I assume you mean airflow - it would be running rich. It is running lean.

 

[ohm529]

Sorry, I meant a fuel flow restriction. Plugged vent, anti-siphon valve, clogged pick up screen...

 

[Rick Stephens]

It runs at WOT once you get it past the lean part, so it isn't running out of fuel. Carburetor is too lean in mid range and maybe, depending on carb, the needles aren't moving when you accelerate.

 

[Bondo]

Sorry, I meant a fuel flow restriction. Plugged vent, anti-siphon valve, clogged pick up screen...

Ayuh,...... Welcome Aboard,..... All those could part of the problem, but I'm with Rick, a rebuilt corroded carb is the more likely source of the problem,......

 

[ohm529]

Thanks for the advice. Since it's lean in the mid-range, is it fair to start looking in the accelerator pump ports?

 

[Rick Stephens]

Thanks for the advice. Since it's lean in the mid-range, is it fair to start looking in the accelerator pump ports?

Don't know what carburetor you have, but I wouldn't be thinking accelerator pump. Those passageways and check balls are large and have very short term effect on acceleration. You can bypass them entirely in testing just by slowly opening throttle. Accelerator pump is easily checked just by disconnecting throttle cable, pulling flash arrestor off and looking at the inside when opening the throttle plates.

Midrange may have its own ports, or may be main jets with needles controlled by vacuum and springs. And some of that is determined also by what is meant by midrange. What throttle positions have limited power because of leanness.

 

[ohm529]

It's Mercarb 2 barrel. Hesitation starts happening around 35-40% throttle. If I push from 25% directly to WOT, it sputters and then goes. Maybe that's the main jets vs. the power valve at the bottom of the bowl?

 

[Scott06]

It's Mercarb 2 barrel. Hesitation starts happening around 35-40% throttle. If I push from 25% directly to WOT, it sputters and then goes. Maybe that's the main jets vs. the power valve at the bottom of the bowl?

You need to look at them all only way to fix is to completely tear down, soak in carb cleaner then run thin wire through the ports.

 

[Rick Stephens]

Agree with Scott. Anal clean isn’t too much to ask for.