Mercury 115 / 80 Powerhead swap question??

[bonttw]

I have acquired a 1979 80HP inline 4, and a 1978 115HP inline 6. The 115 is shot with a beautifull mid section, lower unit, etc... The 80HP runs great but has rough shape mid section and lower unit. I have swapped powerheads as everything seemed identical. Problem I'm running into is that motor starts great, idles great, but when you put it into reverse or forward she cuts right out. You can idle good in forward at about 4 or 5mph, but try to give it anymore and she just dies right out. I have been reading for hours trying to figure this out. Could it be that the exhaust restrictions on the 115HP Mid-section is not allowing the 80HP to get going. Heard this possibility from a retired local mechanic, and hoping someone here would know if this is/could be my issue?
Thank you so much in advance for your response. Any and all input would be greatly appreciated.

 

[Chris1956]

Re: Mercury 115 / 80 Powerhead swap question??

Bon, This is posted in the wrong forum.

However, I would not think you have an exhaust issue. More likely the carbs are set too lean. Richen them up a bit and see fi acceleration improves. Start by opening the top carb idle mix screw 1/8 turn and retest acceleration. Repeat with bottom carb.

 

[bonttw]

Re: Mercury 115 / 80 Powerhead swap question??

Oh wow. Thank you Chris. I appreciate your opinion and I will try it. New here and not sure how I landed this on this Forum. I will attempt to move it.

 

[Handyman1]

Re: Mercury 115 / 80 Powerhead swap question??

Hi check the gear ratio the 115 hp was stronger so it was able to push the boat no problem the 80 hp and bigger gear ratio it could be too much for it try a smaller prop

 

[Chris1956]

Re: Mercury 115 / 80 Powerhead swap question??

If you are using the gearcase from the 115, the gear ratio could be different from the 80. You may (may) therefore be overpropped by one size. if the 80HP motor is marginal for the hull, with the original prop, using the gearcase from the 115HP and the original prop might (might) be just to much to get on plane. if you can try a 2" smaller pitch prop, you will find out if that is the issue.

You might check the 80HP motor gearcase to see if the ratio is different. If it is it will be 2.3::1, versus a 2::1 ratio on the 115HP gearcase.

 

[bonttw]

Re: Mercury 115 / 80 Powerhead swap question??

Thank you both very much. I will be looking into this gear issue very soon. Thank you for the ideas. never thought of that. I will let you know how it turns out.