1970 Mercruiser 120 wont idle when warm

Royalknight

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Feb 20, 2018
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New to this forum and Looking for some advice, I have a 1970 caravelle with a merCruiser 120. last few years it has slowly started to have issues with idling when warmed up. So this is what I have done: installed electronic ignition rebuilt the carb, tuned and tuned again. I rebuilt the carb again and took it out this weekend. worked great at first but after about 45 minutes of starting and stopping (we were fishing) it was back to the same problems. It would idle fine sometimes and not at all others. when it would idle it would die when I shift into gear. I am very mechanical and have been through the MerCruiser/sterndrive book checking adjustment rigging on everything and I am not finding any joy, I have got to be missing something and was hoping to get a new perspective you experts. I love this old boat and just want to keep it alive and well. Any help would be terrific.
 

Bondo

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Ayuh,.... Welcome Aboard,...... Sounds like a Carb problem,.....

To rule out the ignition,... When it stalls, check for power at the coil's tiny (+) terminal,....
 

Royalknight

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The chock is opening, works well. I will check the to ensure the ignition is not the issue. thx
 

alldodge

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Might be running out of fuel. The things which can cause this issue
Anti-siphon valve
Fuel pump
Restriction (line, filter, fitting)

When was the last time the fuel filter was changed?
When trying to start do you see any fuel dripping down the carb?

Get a vacuum fuel pump pressure gauge and place it with a Tee between filter and carb
 

Scott Danforth

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which brand of electronic ignition did you install. if you installed a pertronix, try bolting in a set of points and a new condenser.
 

Royalknight

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Ok, I ran it a long time before it started acting up (runs great until it fully warms up, not overheating) never lost power on the coil. it acts like it is running out of gas. will stay running if I keep it above 800 RPM (most of the time).

The boat had this problem before I put the "hot spark" electronic ignition kit (with new coil) on it last year.

I changed the fuel filter last month. No fuel dripping around the carb, I can see fuel being siphoned inside the carb while running and the accelerator pump works fine. My next step will be to check the fuel pump pressure. maybe at low RPM the pressure is too low?

haven't given up yet, thanks or all the advice.
 

mickyryan

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i would bet cam is lobe is worn and at lower rpm you are barely pumping , try a electronic fuel pump see if it stops.
 

kenny nunez

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Have you checked the voltage at the + side of the coil when the problem shows up ? It is possible the resistor circuit is weak. There should be a minimum of 7-8 volts at the + side of the coil at all times. Try with a jumper wire going from the + battery to the + side of the coil when this symptom starts to happen. This will tell you if the circuit is the problem. Does the rotor shaft have a lot of side to side movement, and have a worn out distributor, have you replaced the rotor ? These are some things worth checking also.
 

fishrdan

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My next step will be to check the fuel pump pressure. maybe at low RPM the pressure is too low?

Too much fuel pressure can cause that issue also, fuel pushing past the carb's needle and seat, overfilling carb, spilling into carb.

Check all the fuel hoses/lines to make sure they are connected tight, and nothing is cracked. A loose/cracked fuel hose can suck air, but not leak fuel.
 
Last edited:

QBhoy

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Presumably the compression is all good ? No water ingress from exhaust system etc ? A weeping exhausts or riser allowing water ingress will see a low idle. Just a thoight. If not a compression test will rule out anything serious.
 

Royalknight

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Ok, update on what I have been doing:
  • Compression check good-all around 110.
  • Fuel pressure 4-5 psi at all RPM.
  • re-rigged the ignition cut out switch, gave me issues last time I took it out.
After checking everything I went back to the carb. pulled it disassembled the top, cleaned and inspected the passageways, reset the float. got it back together yesterday, set the timing, mixtures and Idle, it never died or struggled to stay idling.

Next test is to take it out and see what it will do on the water, crossing my fingers.

I will post the test results when I get back. thanks for all the advice so far!
 

fishrdan

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got it back together yesterday, set the timing, mixtures and Idle, it never died or struggled to stay idling.

Once you have the boat in the water, get the engine warmed up (take it for a 5 minute spin), bring it back to idle in gear, THEN adjust the idle speed and idle mixture screws again. You may have to adjust them a couple times to get smooth idle, mixture/speed/mixture/speed/mixture/etc. If the engine starts stumbling/dying while adjusting, blip the throttle to clear it up, then go back to adjusting.
 
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