Mercury Inline 6 max rpm at 4000

[blfink]

I have a 1985 115 inline 6. Had problems with shutting off at low idle, especially when shifting between forward and reverses (ran good otherwise). Felt like when it was only running on 5 cylinders. Looking at the #6 plug you could see water. Turns out I had a bad exhaust plate gasket. Had gaskets replaced last month and got the boat out for the first time yesterday. Ran great for about 5 minutes, got it up to 5400 rpm on the first try, but that was it. After that it would start cutting out at about 4200 rpm and wouldn't go past. All seems ok up to 4000-4200 rpm (hole shot and all). As time went on it sometimes would fall down on the hole shot too. Had new gas, changed plugs, checked fuel screen before carbs (no filter that I'm aware of). Seems like a fuel problem to me. Guess I'll take it back but I don't understand why it ran better with wet plugs. Any suggestions...is this a common problem with the inline 6?
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[jimmbo]

Re: Mercury Inline 6 max rpm at 4000

sounds like a bad trigger. check all the connections on the switchboxes and coils. also look for frayed wires that could be shorting .

 

[Chris1956]

Re: Mercury Inline 6 max rpm at 4000

Have someone pump the primer bulb and see if that fixes the high speed problem. If so, rebuild the fuel pump and clean the fuel screens.

If that motor has back drag side bowl carbs, there is a stainless screen under the brass fitting that allows fuel into each carb.

 

[blfink]

Re: Mercury Inline 6 max rpm at 4000

I hope I found the problem! Took the time today to do a good visual. I found a screw hanging in the air with four ground wires (with terminals) loosely dangling on it. The four wires were the top 3 coils and the rectifier grounds. The screw at one time was fastened to the top corner of the top switch box. I'm thinking this was attached for the first 5 minutes when it ran good . I need to take it out to test this weekend. Does it make sense that this would cause it to run ok up to 4000 rpm then miss and not go past?

 

[blfink]

Re: Mercury Inline 6 max rpm at 4000

Had it out today and had the rpms up over 5100. Last time, with 250 lbs less in the boat I did 5400, so 5100 seems reasonable to me (right?). Had a little trouble idling when its been running a while, and every here and there I had to back off on the hole shot because it would stumble but would be ok the next time. I think I will go through a link and sync.