Idle and acceleration issue

robinsbd

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Dec 7, 2013
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A little bit of background: I was having an issue with my engine not being able to idle last year. I did a complete carb rebuild last year and I also did an initial tune up (plugs, wires, cap, rotor, ignition coil). After the carb rebuild, I had the boat out on the water last fall (warm 60 degree days during the first week of November). When I did the carb tuning, I was able to get it to idle pretty decent. I took it for a long run and didn't have any issues bringing it back to idle and docking was great.

This year, first time out, I ran into a similar issue. I could run up the river no problem at speed, but as soon as I want to idle, it just stumbles and dies. While in forward gear, I was only able to get the RPMs down to about 1200 steady. Anything below that, and it would stumble and die. My first thought was that the fuel is getting old (a little over 2 seasons old w/ one treatment of stabil). I know this ethanol gas goes bad fast, so I pumped all the gas out and filled a fresh tank of gas and I also added seafoam in the correct proportion with maybe 5 or 6 gallons untreated so the mix is a little weak on the seafoam, which is fine. I just figure it helps stabilize the gas a bit and maybe cleans a little bit as it passes through the carb.

The engine still wouldn't idle. So, I went through the carb tuning procedure from the Merc manual the same as I did last year. I have the Thunderbolt V in base timing mode and set the idle mix and speed screws to their default settings. I was finally able to get it to idle steady after backing out the idle mix needle more than 2 turns. That seemed pretty normal and I think that was close to the same as last year. It's idling in neutral at about 700 RPM according to the dash tach. Seems a little rough. I backed the idle mix screw out more and more and the idle kept going up. I was able to back it out at least 5 turns and she still runs. That's how I got the idle all the way to 700. I never touched the speed screw and left it at the default setting (2 turns in).

Now, when I try to rev the engine slowly, it hesitates and stumbles and acts like it wants to die. If I punch the throttle, she revs way up to 3K no problem and then I let it back down to idle. It stays idling, but as soon as I try to accelerate gradually, it just stumbles and stumbles. Also, I got it to back fire once through the carb. I turned the idle mix screw back in to somewhere around 3 turns out, but the acceleration issue is still the same. When I push the accelerator forward, it just hesitates, starts to hiss a little louder and then stumbles and it will back fire if I do that too much. I also tried the same thing in normal mode (not base timing mode), and it still acts the same. Again, if I punch the throttle, it will stumble some and then accelerate way high, but it's definitely not working right.

Somethings going on here, but I just don't know where to look next. I did all my testing in the water tied up to a dock. I don't want to leave the dock in this condition, so I'm back on the trailer again.

Thanks for your help!
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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pull the carb apart. your old broke-down fuel gummed up your idle circuit and accelerator pump circuit
 

alldodge

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Agree, It's a carb issue and check the accelerator pump adjustment, bore and check balls

Roch 2G.JPG
 

robinsbd

Petty Officer 2nd Class
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Dec 7, 2013
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138
I'm also thinking carb. It ran great last November right after I rebuilt the carb. World of difference.

I was thinking since i just rebuilt it that it would be good for a while. However, I did leave it sit all winter in storage with the old gas in the carb. So, this makes sense.

I'm going to get in there and clean it again.

Now, do you guys have any recommendation on how to store the carb over winter? I was thinking maybe a fuel shutoff somewhere would allow me to let the engine run the carb dry. Unfortunately, the mercarb does not have a float bowl drain. What do you guys think?
 

Rick Stephens

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Aug 13, 2013
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When you clean up the carb, pay particular attention to float level.

If you leave it dry over the winter it will shrink the gaskets. Just put stabil in it, and if you can, ethanol free fuel. Mostly keeping the water and crud out with good filters. Do you have a canister type fuel filter in line? I had to add one to my boat, didn't have one from the factory.
 

robinsbd

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Dec 7, 2013
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The only filters I have are the water sep canister and the fine mesh filter in the carb inlet nut.
 

alldodge

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My Rinker carb has never been touched and I don't use anything in the gas. Using E0 now but for many years it always had E10 in it. I think there was some crud that was not removed between filter and the carb, or left in the carb. From what I'm seeing in your post, your idle circuits are ok for the most part, its the acceleration part that needs works.
 

robinsbd

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Dec 7, 2013
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I have a question regarding tuning the carb with a vacuum gauge. The Mercarb has no vacuum ports. I have a pre-vortec intake manifold. How do I check vacuum? Is there a port on the intake manifold?
 

alldodge

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There are no ports on the marine intake, so using a vacuum gauge is not doable
 

robinsbd

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Dec 7, 2013
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In that case, then I'm stuck with RPMs. Do you have any recommendation for a good shop tach? Last year, I tried connecting a Bosch digital meter to the negative side of the ignition coil and I got no readings there. I have the Thunderbolt V ignition. It would be really nice to get accurate RPM readings right there at the engine. I used the dash tach last year and it was difficult to see it from the engine bay and also not as accurate. Thanks for all the quick responses!
 

alldodge

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I have one but forget which one, not a home right now. Look up hand held auto tachometer. Many are laser so they are very accurate and not to expensive.
 

robinsbd

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Dec 7, 2013
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I was looking around my intake manifold and I think I found what might be a place for a vacuum guage. Can you confirm if this is what I'm seeing in the following picture?

 

robinsbd

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Dec 7, 2013
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Just wanted to report that all is well now. It was the carb. Darn Ethanol!! I took my carb apart, gave it good cleaning, and reassembled with new parts. The boat is running great again. I had it out on the 4th and acceleration was very smooth.
 
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