Small 70's Mercury keeps cutting off.

Just wondering if any of you have had any experience with the following issue on one of the 70's series Mercuries (or any motor for that matter). I have a '70's 45 (4.5hp) and just got the waterpump in yesterday. Went to start her up today, (first time since I've owned it) and she started right up. Idled pretty well, but had a bit of a misfire, nothing to write home about. Anyways, I warmed her up and had her running pretty well, put her in gear and did very well at open throttle. Had TONS of power for a motor this size, felt like it would BLOW my Evinrude 4hp away. Anyways I cut the motor off to go and get my video camera, because one of my buddies is interested in buying it, and came back outside.

Motor only wanted to start when I put the choke on. Ran with the choke on(choppy as you would expect) at idle speed, and up higher in the RPM band. The second I flip the choke off, the revs pick up, and slowly the engine dies out. Starts EVERY time with choke on, dies out EVERY time I take choke off. Won't idle or go high speed with choke off. The motor was JUST RUNNING FINE!!

So what are you guys thinking, ignition or fuel supply?

Re: Small 70's Mercury keeps cutting off.

Anyone got any advice. I noticed that the tell tale feed out on top of the powerhead was leaking. Any chance that this could be caused from a wet ignition system?

Re: Small 70's Mercury keeps cutting off.

As a merc newbie (take that for what it's worth) here are my thoughts...
Runs with choke on is proly a fuel problem.
Check that the vent is open on fuel tank
Check fuel filter
Check fuel pump, see if she keeps running if you pump the fuel bulb
Next is pull, clean and adjust the carb

Re: Small 70's Mercury keeps cutting off.

Yeah trust me man, I wish it were that simple. Im going to end up pulling the flywheel this weekend if I can't get any more insight. I just pulled the carb filter cover and checked the float needle and it looks fine. Guess I'm going to pull the high speed jet, I may have gotten some crap in it. My entire fuel system on my boat is brand new so I know it's not that.

Re: Small 70's Mercury keeps cutting off.

Don't think the problem lies within the ignition system and I think you'll be wasting your time and effort. By the sounds of what you have described I would almost definately say CARB/fuel issue somewhere if it only wants to run with choke on. That's were I would start. Pull the carb off and give a damn good clean and blast all pasages out with air or carb cleaner. Your air/fuel screw make sure you wind it in until lightly seated and count how many turns out it was before you remove it. It could be your mixture is a bit on the lean side, hence why it wants more fuel......
Good luck.

Re: Small 70's Mercury keeps cutting off.

Yeah the only reason that it is puzzling to me is that the motor was running FINE 20 minutes prior....

Re: Small 70's Mercury keeps cutting off.

What mercuk posted. It only takes alittle crud in the carb to mess things up

Re: Small 70's Mercury keeps cutting off.

I had the same thing happen to me, turned out to be a piece of crud in the jet. After I got it out the motor started up and ran just fine.

Re: Small 70's Mercury keeps cutting off.

Only running with choke at idle is classic dirty low speed jet symptoms, so I'll ditto everyone else. Since it's only a single cylinder and it's running at higher speeds, it's very doubtful it's ignition and I would NOT fool with it until I recleaned the carb. Since it ran fine for 20 minutes, I would also suspect your fuel lines are breaking down on the insides (common due to age and ethanol) and unless you change them, you may have constant relapses of clogged carby.
GOod luck,
JBJ

Re: Small 70's Mercury keeps cutting off.

jbjennings said:

Only running with choke at idle is classic dirty low speed jet symptoms, so I'll ditto everyone else. Since it's only a single cylinder and it's running at higher speeds, it's very doubtful it's ignition and I would NOT fool with it until I recleaned the carb. Since it ran fine for 20 minutes, I would also suspect your fuel lines are breaking down on the insides (common due to age and ethanol) and unless you change them, you may have constant relapses of clogged carby.
GOod luck,
JBJ

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Thanks for the reply, but it actually is NOT running at higher rpms. Ill just have to take her apart and clean her up I guess. Just havent seen these symptoms with a carb before....