120 hp 2.5 L mercruiser 153 ci carb

tannergallant123 · May 2, 2025

Hi guys I have a Mercruiser 2.5 L 153 ci 120 HP with a carburater I bought off Amazon. It has worked great for me but my spark plugs get pretty black and fouled and then it starts to run crappy after a while. Can someone point out the air/fuel idle screw so I can potentially adjust it to not get so much black soot burnt on my plugs? Also if you could inform me on how to adjust it. Thanks, see below pics

kenny nunez · May 2, 2025

Does the choke open all the way? The idle air adjustment screw should be 1.5 turns out after lightly seated.

tannergallant123 · May 3, 2025

Okay thanks so now that's the gold screw in the 3rd picture right? I'll tighten it all the way then back off 1.5 turns?

tannergallant123 · May 3, 2025

Also my electric choke hasn't been working. It's a 1 wire but don't understand the grounding. Do I need to add a ground wire from the 3 screws on the plastic cover to the base of the carburetor? When I turn the key it instantly begins to heat up

alldodge · May 3, 2025

Plug looks a little long for that motor. It should be MR43T, RV9YC, BR6FS

Dwell 28 to 34*
Timing 8 BTDC

The choke needs keyed 12V power so when key is turned ON it's powered. It is grounded to the carb body so no extra wire needed

Yes turn screw in until it just touches, then back out 1 1/2 turns. Do no tighten bleeder screw down tight, just so it touches

tannergallant123 · May 3, 2025

Perfect thanks so much. I'm using BR6ES right now. Should I switch to BR6FS?

tannergallant123 · May 3, 2025

I believe br6es is up to 1974 and after 1974 it's br6fs. Issue is I don't know if it's a 1972 or 1974 always thought it was a 72 Mercruiser

alldodge · May 3, 2025

The FS is a cooler plug and ES is hotter, hence the reason your plug is longer

My info comes from manual 3 which list for 1978 to 84
Manual 2 for years 1974 to 78 just list MR43T and champion RBL8
Manual 1 for years 1963 to 73 list AC - CR44N and could use CR43N for high speed applications

tannergallant123 · May 3, 2025

Okay perfect ya just cross referenced and it looks like the bR6ES is the correct plug equivalent for 73 and below. Thanks. Maybe it's just the right plug and have to adjust the electric choke and the idle air screw.

alldodge · May 3, 2025

plug cross reference are not exact and never have been, but it's your motor

tannergallant123 · May 3, 2025

Okay I'll look for some c44N online. Thanks for your help

alldodge · May 3, 2025

Your problem right now is not the plugs
If it was my motor I would use MR43T

tannergallant123 · May 3, 2025

Okay thanks. If I run a wire from the positive side of the ignition coil to the electric choke that would work correct? That way when the key is turned it's getting power. Also how should the actual choke be positioned, should I try dead center first? Or do I adjust it so it's 1/8 from closed when cold.

alldodge · May 3, 2025

The coil with points distributor is normally to be 12V when cranking and 9V when running. Manual number 1 does not show a resistor wire but manual 2 does.

Use a voltmeter and measure the + side of coil when running. If it's 12V then you can take it from there, if not then no

If no then follow white wire back to the engine connector plug. Splice into the white wire and run that to the choke

Adjust choke so the opening is 1/8 open when cold

tannergallant123 · May 3, 2025

Ya I upgraded to electronic distributor. I'll meter the coil when the key is turned over and see if I have 12. Thanks a lot for your help.

120 hp 2.5 L mercruiser 153 ci carb

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