Mercruiser 3.7 choke question

[Ugarph1982]

I've got an '88 Mercruiser 3.7 with an Alpha 1 gen 1 and a MerCarb 2bbl. I know the motors have their problems, but this one is in geat shape. I've done cam sleeves, alternator conv., pertronix with new coil, and a reseal of the upper unit. Boat runs great, for the most part.

My issue is with the choke, and maybe it's just a misunderstanding. I've adjusted the choke where it just closes the plate, and then gave it another less than quarter inch to provide some pressure. When I turn on the ignition, i have 12 volts at the choke, measured when grounding on all 3 choke retaining screws and the red meter wire on the purple terminal.

The problem is that, even after being on for 90 seconds without the engine running, the choke plate does not loosen and allow me to even push it open without it springing back shut. The engine starts fine and forces the choke open with vacuum, but never really opens all the way. The engine is running a bit rich and I believe this is part of the reason why. I'm going to do a float level and drop check to rule that out also. However, I believe the choke is the main culprit. The boat idles fine and accepts power nicely on the water, but I believe the choke is holding it back. Initial timing is dead nuts 4 BTDC.

Should the choke be completely loose and stay open on its own after simply applying 12 volts for a couple of minutes? Mine does NOT do this, and remains spring loaded, even after engine has run for a few minutes. Is there another way to test the choke? Does it sound like a bad unit? In lieu of buying a $100 choke, other suggestions for choke? Anyone had luck with a cable controlled contraption?

Thanks in advance!

 

[Scott Danforth]

make sure its adjusted properly. it could simply be set wrong

you can measure the electrical resistance of the choke coil. if you get zero resistance, no amount of electric current will open it.

regarding your idea of a cable - fix it right.

 

[Bondo]

I've adjusted the choke where it just closes the plate, and then gave it another less than quarter inch to provide some pressure.

Ayuh,.... Welcome Aboard,.... Loosen the choke adjustment,....

Insteada pressure against closin' it,... Leave an 1/8" gap, from fully closed,....

The motor vacuum will suck it shut enough to work,....

The throttle linkage sets the choke with a full Wot motion,...
Then the electric coil pulls it off,....

 

[Ugarph1982]

Thanks for the responses!

I loosened the choke a bit and the motor is no longer running rich. It is idling too fast, but I'm working on that with cable and screw adjustments.

Funny thing is that the choke still doesn't move. Pulled it out and put it on the bench with 12 volts. 0 ohms and 12 volts is what I get on both sides. Choke is dead. Looks like advance auto sells a replacement for about $50. I will pick it up tomorrow and keep yall posted!

 

[Ugarph1982]

OK, so I went out and bought a new choke this morning. Fits perfectly. As soon as I fired her up, the choke began opening immediately. Money well spent.

​Until the choke opened up. That's when the adventure began. So, once the choke hit nearly the full-open position after about a minute and a half, it proceeded to stall out. Well crap, too lean. If I blipped the throttle to recover it, it would backfire violently out of the carb. Quite a spectacle with the flame arrestor off. Basically, it would not idle below 2500 RPMs. I had seen this before when I had a bad spark wire and the number 1 cylinder was not firing reliably. Something tells me that the dead choke was masking an issue for the previous owner. When I changed plugs out before this choke fiasco, they were all pretty black, indicating a rich condition.

​So, I took the air horn off of the carb to take a look, and I was pleased with what I saw (I did not take the base from the manifold):
--Float level and drop were close to spec
​--The power piston was in place and movable
​--I took both jets and the power valve out to find that the jets were clear and the power valve moved easily and had clear passage
​--The venturi passages were clear
​--Airhorn gasket in good shape
​--The fuel in the bowl was clear and free of debris
--The accelerator pump plunger looks great and gives 2 healthy quirts when you blip the throttle

Also, I have replaced the fuel pump filter, checked the carb inlet screen, and verified that the anti-siphon valve out of the tank was clear ... all of which looked good. Both initial and advanced timing are to spec (4 BTDC and ~30 BTDC)

​Before sitting down to type this message, I cranked it up after reassembly. The results were the same. It runs fine until the choke gets almost all of the way open. This time, after a restart attempt and a couple of throttle pumps, the thing threw an EPIC FIREBALL out of the carb before it quit. I'm a pilot and I've seen carbureted airplanes do this when they are cold and too much fuel is thrown in (when it happens, keep cranking!!!) ... Couldn't do this in the boat -- I hopped my butt out of the boat so fast I must have set some type of record.

​So I seek more advice ... What else can I check? The carb looks good so far as I can tell. It runs too lean, back fires out of the carb, acts like it's starved for fuel, and won't idle below 2500. Vacuum leak in intake manifold, blown head gasket cross-firing between cyl 2 and 3 (please no!!), something else in the carb, or am I just missing something being a novice working on a 29-year-old tired engine?

 

[Ugarph1982]

In case anyone comes across this thread, I thought I would post the resolution to my issue. It turned out to be a gummed up carburetor.

If you are using the mercarb, make sure you take the carburetor completely apart if you are going to perform a rebuild. These two barrel carburetors have more small passages that you could possibly imagine. The two that were clogged on mine were the slit openings directly next to the butterfly valves on the throttlebody.

If you are receiving a popping through the carburetor on acceleration and at higher engine speeds possibly accompanied by the engine shutting down due to fuel starvation, check to make sure you have pressure in the line and fuel in the bowl. If you have both, more than likely the carburetor is gummed up in some way.

 

[Bondo]

In case anyone comes across this thread, I thought I would post the resolution to my issue.

Ayuh,..... Thank you for the update,.... So few folks get 'round to it,......