Johnson 120 drill exhaust relief

[aw1575]

I have a 1985 120 Johnson that hesitates (bogs) when going from idle to full throttle in a skiing take-off situation. The motor has a 20" shaft and the transom sits fairly low in the water at idle. It has been suggested that these factors prevent the exhaust pressure being relieved properly until the transom lifts when underway. Makes sense .... I understand there is a procedure for relocating or drilling the exhaust relief holes which may assist this. Does anyone have any information or diagrams re this? Thanks.

 

[Dhadley]

Re: Johnson 120 drill exhaust relief

That could help but lets start at the begining. What is the top rpm? Has it always done this or just started doing it?<br /><br />Make sure the carbs are clean - I would start there. We may have to rejet for todays fuel. Lets clean the carbs and go from there.<br /><br />Good luck!

 

[4link1955]

Re: Johnson 120 drill exhaust relief

Dhadley,<br />I'm having the exact same problem! If I trim the motor up it's not as bad. My carbs are clean as a whistle. I'm going to decarb tomorrow. Speaking of decarb, If I spray the decarb in the motor and shut it off, let it sit overnight and run it down the river the next day, will it hurt anything for it to sit over night? I know the can says to let sit for 15 minutes or longer. Also I bought some additive to pour in the tank that I'm going to pour in there after I run the motor. Any tips for me before I go about doing this? <br /><br />Also I have another topic on this forum I want you to take a look at if you can.<br /><br />Thanks:<br />Jason

 

[aw1575]

Re: Johnson 120 drill exhaust relief

Hi there. Thanks for the prompt replies. This has been a bit of an ongoing saga so a lot of work has been done already. The carbs have been stripped and cleaned by two different authorised OMC service agents and all gaskets replaced. The idle jets have been replaced also. Top RPM is 5700 with 17" prop. Power head has been removed, bearings checked and rings replaced. Compressions have been checked and all are good. Fuel hoses have been pressure tested and replaced where necessary. Fuel flows have been checked (don't know how) and pronounced OK. We have tried different size and pitch props. Unfortunately none of the above has changed the personality of the motor. I spoke to OMC head office here yesterday and they advised re their experience with excessive back pressure on short shaft motors. It seems to stack up as moving weight forward in the boat or trimming it up slightly for takeoff (running on the second pin) certainly helps it clear its throat. The motor seems to be mounted correctly in relation to the bottom of the hull but I haven't yet tried lifting it a notch as I feel it may start to cavitate. Thanks again for your help. Alan.

 

[Dhadley]

Re: Johnson 120 drill exhaust relief

Raising the motor is the next step. Dont worry about cavitation right now while were trying to solve a backpressure issue. A prop change may be in your future anyway.<br /><br />Keep us posted!

 

[BAINOR]

Re: Johnson 120 drill exhaust relief

Might get another opinion or two on this but saw this old thread on prop mods. Kinda describes what is happening to you

 

[stockyjr]

Re: Johnson 120 drill exhaust relief

Yes you can drill holes I have done this on 2 boats it helps to get the revs up on take off.<br /><br />Check the address below<br /><br /> http://marinemechanic.com/site/page76.html

 

[aw1575]

Re: Johnson 120 drill exhaust relief

Thanks for your replies. I am familiar with the technique stockjr mentions. However I was not referring to drilling the prop, rather drilling the exhaust relief ports where they exit out the back of the leg above the water line just below the motor cowling. I will try raising the motor first but if anyone has info on the exhaust port modification, it would be interesting. Thanks.

 

[seahorse5]

Re: Johnson 120 drill exhaust relief

Drilling out the exhaust relief holes, which was needed on the '85 V6's, was only for better idling when the motor was mounted very low to the water, such as with a bracket mount.<br /><br />If you are bogging on acceleration, the carbs are too lean or a pickup tube restricted, float setting low, or cracked high speed nozzles. Also make sure the relationship between the spark advance and throttle opening is correct. Techs call it sync and link. Also make sure the thermostats are warming the engine up to around 140 degrees F.

 

[Hooty]

Re: Johnson 120 drill exhaust relief

I think Seahorse has it. Bob's Machine Shop makes, what they call, an "Exhaust Housing" but it mounts just above the cav plate and it's more for taking some of the exhaust off of over-the-hub props. They sound good too. Here's what it looks like.<br /><br />

<br /><br />c/6<br /><br />Hooty

 

[Jamar]

Re: Johnson 120 drill exhaust relief

Looks sweet hooty, do they sell those for v-4's?

 

[aw1575]

Re: Johnson 120 drill exhaust relief

Hi there. After a bit of a delay I can now report on progress. The boat shop lifted the motor 1 hole but felt it was higher than they would prefer. After a run today it is slightly better but no real improvement. The same "personality" exists and caviation is much worse in turns. I notice that at rest, the exhaust relief holes are about 1" below the water line - i.e. under water! Is this likely to be an issue in terms of get up and go? Perhaps I am fighting a loosing battle here with this short shaft motor on this hull making it transom heavy? Thanks.

 

[sparkroost]

Re: Johnson 120 drill exhaust relief

is it possible that your pickup timing is out of whack? If your timing is too low when you crack the throttle plates open it will bog off idle.

 

[aw1575]

Re: Johnson 120 drill exhaust relief

Hi sparkroost. The timing / sync & link has been checked by several different dealers and all pronounced OK so I can only assume this is not an issue. Thanks for the thought though.