1978 15HP Johnson won't run right!

Hey guys I am new to the forum and I need help. I recently acquired a 15hp Johnson 78 model. Model # 15R78B

This motors lower unit was rebuilt 2 years ago and is in great condition. I had to fix the pull start, install a new impeller and then I tried to get it to run. the motor would run but it had to be given alot of gas while pull starting to get it to fire up. Then after I got the motor running it wouldn't idle low. I would try to run it at start speed or shift speed and it would die. I figured the low idle jet needed to be cleaned. A carb kit was in store. I took apart the carburator and cleaned out all of the nooks and crannies with carb cleaner and an air hose. (got instructions from another thread) I made sure everything was nice and shiny and then I installed the new kit. Put everything back together and it fired up the 2nd pull. Idled alot lower than before but still wasn't perfect and would die if you turned it down to low. I played around with the low speed jet tightening it in and then back out 1 1/2 turns like factory recomends. Now the dang thing won't hardly start. I can't sometimes get it to fire while giving it some gas and then as soon as it starts running it acts like it floods out. What is doing this?

Re: 1978 15HP Johnson won't run right!

1-1/2 turns is the factory set point, but you can adjust the needle to where you can get a smooth idle. Once you make a 1/8 turn in or out, wait for 20 seconds for the motor to adjust to the setting. You will be able to fine tune it this way.

One other thought, your butterfly needs to be all the way closed to set the idle right. You may have to adjust the linkage.

Re: 1978 15HP Johnson won't run right!

Cofe said:

1-1/2 turns is the factory set point, but you can adjust the needle to where you can get a smooth idle. Once you make a 1/8 turn in or out, wait for 20 seconds for the motor to adjust to the setting. You will be able to fine tune it this way.

One other thought, your butterfly needs to be all the way closed to set the idle right. You may have to adjust the linkage.

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butteryfly? and any idea on the fact that it starts and then immediatly drowns out? i figured it had something to do with my float but everything looked great when I put it all back together. I held it vertical and my float set parrallel to the bowl. However if it wasn't parallel how would you change the floats position? Also I have a little play in my tiller handle? would that make a difference?

Re: 1978 15HP Johnson won't run right!

is the roller from your carb linkage centered on the timing cam mark just as the butterfly is beginning to open?...is the roller at the highest part of the timing cam when the butterfly is at wot, and is the butterfly perfectly horizontal at wot?

Re: 1978 15HP Johnson won't run right!

AlTn said:

is the roller from your carb linkage centered on the timing cam mark just as the butterfly is beginning to open?...is the roller at the highest part of the timing cam when the butterfly is at wot, and is the butterfly perfectly horizontal at wot?

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Ok guys the mark on my timing plate does not touch the roller. The roller doesnt come in contact with the plate til about 1/4-1/2" past the mark and the carb is not opened quite all the way when the plate is at full throttle. How do i adjust this so it is correct? This could explain my idle problem

Re: 1978 15HP Johnson won't run right!

loosen the 2 screws that hold the timing cam and adjust it so that mark lines up with roller when the throttle handle is in the start position...adjust also that at wot the timing cam is at its highest point and the throttle plate ( butterfly ) is horizontal...I adjusted a 1978 9.9 so the timing mark is slightly beyond centered on the roller...timing advanced...for start...seems to start easier this way..fwiw...this may not be all that's going on, but it's a starting point

Re: 1978 15HP Johnson won't run right!

Ok I understand how to do this but will it make the timing plate hit the roller at the arrow on the timing plate? seems like it would just change the tiller handle? I may be dead wrong but I am just curious. Think that will fix it? also the motor will start when I have the throttle turned up and then it dies off like its out of gas. Recon its the fuel pump going out?

Re: 1978 15HP Johnson won't run right!

green_head_getter said:

Ok I understand how to do this but will it make the timing plate hit the roller at the arrow on the timing plate? seems like it would just change the tiller handle? I may be dead wrong but I am just curious. Think that will fix it? also the motor will start when I have the throttle turned up and then it dies off like its out of gas. Recon its the fuel pump going out?

Click to expand...

Follow in this order!
The roller must rest in the arrow area when the tiller throttle is all the way down. If it doesn't, correct the tiller idle all the way down. Then adjust the timing with the arrow. Then adjust the butterfly so it all the way closed.

Then proceed to adjust the fuel mixture.

The float can be adjusted in the inverted position with the float perpendicular to the body. If it isn't then bend the float arms to make it so.


Yes your fuel pump could be bad, but you first have to do a proper link and sync as outlined above. Hope this helps you. Cofe

A good OMC manual describes the proper link and sync procedure. Look on ebay for one for your year of motor.

Re: 1978 15HP Johnson won't run right!

Cofe said:

Follow in this order!
The roller must rest in the arrow area when the tiller throttle is all the way down. If it doesn't, correct the tiller idle all the way down. Then adjust the timing with the arrow. Then adjust the butterfly so it all the way closed.

Then proceed to adjust the fuel mixture.

The float can be adjusted in the inverted position with the float perpendicular to the body. If it isn't then bend the float arms to make it so.


Yes your fuel pump could be bad, but you first have to do a proper link and sync as outlined above. Hope this helps you. Cofe

A good OMC manual describes the proper link and sync procedure. Look on ebay for one for your year of motor.

Click to expand...

Hey Cofe thanks so much. My tiller handle turned all the way down shouldn't ever kill my motor right? thats what a kill switch is for. So I turn it all the way clockwise and then set my timing plate so the arrow is on my roller correct? I think that may put my "start" and "shift" at full throttle. But I'm not sure on that one

Re: 1978 15HP Johnson won't run right!

Tiller handle should return to the idle position when you let go of it. If it don't you need to find out why........ is it binding because the rest of the linkage is adjusted too far up? Lack of lubrication?....... You need to discern what you're linkage is doing wrong or right. Once again it is hard to describe how you should adjust your linkage over a type written forum. You really need a OMC manual for your motor..

Re: 1978 15HP Johnson won't run right!

Cofe said:

Tiller handle should return to the idle position when you let go of it. If it don't you need to find out why........ is it binding because the rest of the linkage is adjusted too far up? Lack of lubrication?....... You need to discern what you're linkage is doing wrong or right. Once again it is hard to describe how you should adjust your linkage over a type written forum. You really need a OMC manual for your motor..

Click to expand...

you make a good statement, if im running full throttle and I let off of my tiller handle I don't think it goes back down to idle

Re: 1978 15HP Johnson won't run right!

what could cause my motor to drown out as soon as it fires up?

Re: 1978 15HP Johnson won't run right!

bump! need info, im planning on working with it in about an hour

Re: 1978 15HP Johnson won't run right!

green_head_getter said:

what could cause my motor to drown out as soon as it fires up?

Click to expand...

When you did the kit, did you replace the float the same as the old one (orientation correct?) , is the clip installed on the needle and is the float height parallel to the carb bowl?

Re: 1978 15HP Johnson won't run right!

Yes i made sure the float is good, i disassembled the carb again and cleaned everything once more and reassembled. No air can be blown through when the carb is upside down. However what all goes where the low speed valve screws in? "rich and lean" i unscrewed that all the way and removed the red rubber seal and cleaned everything then replaced the red rubber seal with the new one an reinstalled the needle. Is there anything else to that other than the 2 pieces? I reassmbled everything and started my motor. It is still hard to start but it will actually stay running as long as i keep the rpms well obove idle. When i start to slowly lower the rpms it will reach a point where it sputters and almost dies. sometimes giving it more gas brings it back however sometimes it kills it off. Is it a spark issue or a fuel issue?

Re: 1978 15HP Johnson won't run right!

did you remove the core plug < resembles a small freeze plug> on top rear of the carb when you rebuilt it..if so.. did you clean the 3 tiny holes found in the "well" there?...these openings and your idle mixture needle control the idle circuit..they also provide some additional fuel when the choke is fully closed on start up

Re: 1978 15HP Johnson won't run right!

AlTn said:

did you remove the core plug < resembles a small freeze plug> on top rear of the carb when you rebuilt it..if so.. did you clean the 3 tiny holes found in the "well" there?...these openings and your idle mixture needle control the idle circuit..they also provide some additional fuel when the choke is fully closed on start up

Click to expand...

yes I did pull that and clean all of the holes in the well. I just can't imagine what it could be. I guess im going to have to take it to the shop and say fix it!