rough idle on 72 18hp

[popsmazz1]

my motor runs good at higher speeds but at half throttle and idle it seems to run rough, it"s had the carb rebuilt this spring, is this normal for this motor? I also replaced the coils, has strong spark. the carb only has low speed adj. I have it set were it idles the best it will but it vibrates quite a bit at idle. I think it should be smoother but am not sure. by the way will a carb from a 1957 18hp work on this?

 

[kfa4303]

Re: rough idle on 72 18hp

Hi pop. cool old motor you've got there. It should definitely NOT have a rought idle. On the contrary, it should idle down nice and low. While you can cetainly reclean/rebuild the carb, I suspect you may actually only be running on one cylinder. To be sure, use a pair of insulated pliers and disconnect the spark plug wire from each plug, one at a time. The motor should be able to run on one cylinder, if it dies altogether when you remove the lead, then you know you that was the one cylinder it was running on. Thankfully, your model still uses the much loved OMC universal magneto, which is very cheap and easy to fix. I'm not sure about the carb compatability, but I've posted the links to each motor's parts catalog so you can compare them. Here are some great links that can walk you through a tune up from to bottom. You can get any routine parts you may need right here at iboats.

Cold start the 1958 Johnson 18 H.P. - YouTube (this is want you're looking for, but make sure it is a submerged a couple more inches in the test tank than shown in the vid)

Maintaining Johnson E & FD Serie
1972 Evinrude 18202R 18 hp Outboard Motor Parts ('72)
1957 Johnson FDE-11 18 hp Outboard Motor Parts ('58)

 

[popsmazz1]

Re: rough idle on 72 18hp

has all new ignition in it, I believe it"s running on both, this was a 25hp. orig. and it was replaced with an 18hp. powerhead, it may have the wrong carb on it, would that cause problems? how can I find out? thanks.

 

[AlTn]

Re: rough idle on 72 18hp

what are the compression numbers?

 

[popsmazz1]

Re: rough idle on 72 18hp

I looked up the part # and found the 25 carb# is 0384530 and the 18hp. carb is 0384529, are these # on the carb? or how can I find out what I have?

 

[popsmazz1]

Re: rough idle on 72 18hp

the casting # on the carb. is 315082-d2, does that mean anything?

 

[popsmazz1]

Re: rough idle on 72 18hp

funny thing, I was having a hard time pumping up pressure with the squeeze ball, so I put on my new one and now it seems to run much smoother. maybe it was not getting enough gas or sucking some air? any thoughts?

 

[kfa4303]

Re: rough idle on 72 18hp

Hello again Pops. It's not uncommon for the primer bulbs to go bad over time, especially with modern ethanol fuels. Keeping the bulb in a vertical position with the arrow on the bulb pointing up, can help the check valves inside of the bulb function properly. As long as the bulb starts off soft, then gets and stays firm when you squeeze it prior to start up both the bulb and fuel pump should be ok. Once the motor is running the primer bulb will become soft again. If the motor should start to fail, but picks up when/if you manually squeeze the bulb, then that probably means the fuel pump is bad.