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Full throttle starting required

Old 10-14-2020, 11:56 AM
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Default Full throttle starting required

I have a 1996 Johnson 15hp outboard that seems to require a full throttle to start (using manual pull starting). How best can I address this issue, as it seems unusual in comparison to most outboards, and the motor body torques violently to the left when it starts due to the fully open throttle.
Old 10-14-2020, 01:10 PM
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Originally Posted by Billjersey View Post
I have a 1996 Johnson 15hp outboard that seems to require a full throttle to start (using manual pull starting). How best can I address this issue, as it seems unusual in comparison to most outboards, and the motor body torques violently to the left when it starts due to the fully open throttle.
definitely don't do that again, fix it before you try starting it
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Old 10-14-2020, 01:20 PM
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Having to crank a motor with the throttle wide open to get it started is usually indicative of a flooding situation. Too much gasoline to air for the mixture to combust. Throttle being opened allows more air to flow. When the additional air flow results in the mixture being right for combustion, the motor will start. And as you note, you have to be quick to retard the throttle to prevent an over speed condition.

I would focus on cleaning the carburetor. Making sure that all adjustments are within specifications.

In the good ole days of carburetors in cars and trucks it was quite common to flood the motor. Procedure was to press the gas pedal to the floor and crank away. That procedure will still work today with fuel injection models although they are much less prone to flooding.
Old 10-14-2020, 02:38 PM
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Thank you, Alloyboy -- I will definitely look into that.
Old 10-14-2020, 06:14 PM
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Do you disconnect the fuel line at the engine when not using the boat , and open the tank vent or cap before re-attaching , so a heat -pressurized fuel tank can't potentially force fuel past a carb float valve ?

When you pump the primer bulb , does it reach a point where you can't squeeze it anymore,.... or does it always have some give as if the carb float valve was not closing properly when the bowl is full or it's height possibly adjusted incorrectly , or the float is "gas-logged" ?

Have you tried starting with no, or only much less, bulb squeezing? Plugs wet before or after priming ?

When it starts , will it then idle OK/normally at idle setting in N or in gear , or is higher throttle position required to keep it running ? (Clean carb idle /low speed circuit.)

Have you messed with the Low Speed adjustment knob, if yours has that like my old '77 ?

Does RPM noticeably drop as it should when either 1 plug wire is removed?

Old 10-14-2020, 08:14 PM
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Sounds like several worthwhile checks I can try. Thank you, TTaxi!
Old 10-14-2020, 08:15 PM
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Good advice, thank you mystery!
Old 10-14-2020, 09:02 PM
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I have owned several outboards and all of them started easiest with just a small amount of throttle. Are you using the choke?
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Old 10-14-2020, 09:09 PM
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Will not start at all without choking, RussH. Several pulls always required with choke on -- choke off -- choke back on -- let it sit awhile -- etc.
Thanks for any thoughts on this.
Old 10-14-2020, 10:23 PM
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Just a thought, but I believe the carbs on those up into the mid-90s were all pretty similar, and there may be a low-speed circuit at the top of the throttle body which is prone to plugging up.

I'm dipping pretty far into my memory here but I think there's a needle valve directly above the intake which could be unscrewed to change the low speed mixture. If you pull the carb, you should see a little hole maybe .020" on the top of the main passage, aft of the main butterfly. If you remove the needle from the forward end and hose it out with carb cleaner, and carefully poke a fine wire through the little hole at the aft end, you should be able to clean it out.

I recommend confirming my memories with YouTube or something, because I have never had one that new apart, I just recall that they were all very similar carbs for a very long time, well into the 90s. I THINK a 96 would still be basically the same carb.

If so they're simple carbs, a low speed circuit that used to be adjusted with a long brass screw, and a fixed high speed circuit. All you can really do with them is clean out two passages: the one that feeds the main jet on the bottom, and the one that feeds the low speed circuit on the top.

I'm not at all an expert on carbs or anything but it seems plausible that needing to crank the throttle wide open to start it could be indicative of a clogged low speed circuit.
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Old 10-14-2020, 10:32 PM
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Make sure the choke flap is closing all the way when the knob is pulled. May need to take the silencer box off to see it.
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Old 10-15-2020, 01:52 AM
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Originally Posted by Corndog38 View Post
Make sure the choke flap is closing all the way when the knob is pulled. May need to take the silencer box off to see it.
Just curious, how would that benefit if he has to open the throttle all the way to presumably add air to start now?
Old 10-15-2020, 04:41 AM
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"Just a thought, but I believe the carbs on those up into the mid-90s were all pretty similar, and there may be a low-speed circuit at the top of the throttle body which is prone to plugging up."

CR- this is my recollection an well. Remove the carbs and clean all the passages and new gaskets.

My .02
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Old 10-15-2020, 06:45 AM
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In addition to the comments above, make sure the ignition mechanical advance works by visual inspection. Also, a leaky float needle valve is suspect. The intake gets flooded before the first pull. Does it work well once started?
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Old 10-15-2020, 06:59 AM
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Originally Posted by TTaxi View Post
Just curious, how would that benefit if he has to open the throttle all the way to presumably add air to start now?
If the choke is not closing all the way, opening the throttle will put more vacuum against the choke, pulling in more gas to get a start.

Not saying it is that likely, but it is easy and free to check.

To the OP- once started how does it run? Idle ok and power up ok?
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Old 10-15-2020, 07:21 AM
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i had a lot of manual choke motors over the years. I find each of them required their own system and once you figure it out they tend to fire off easily but if the condition of the carb is uncertain you will probably want to either clean it or have it professionally cleaned. Checking for the obvious stuff also- condition of plugs, choke engagement, etc
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Old 10-15-2020, 05:50 PM
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Thanks for the reply Corndog.
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Old 10-15-2020, 05:56 PM
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clean the carb. I had a 25hp johnson and at one point needed lots of throttle to start and didn;t like to idle. A thorough disassembly and cleaning of the carb solved the problem.
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Old 10-15-2020, 05:59 PM
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Thanks to you and all others for your input -- sounds like we have a consensus that the carburetor is probably the culprit, but there are other possible things I can check as well. Everyone's help appreciated!

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