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?

[Raceman]

Did you look for water on the plugs again? If somebody tore into the exhaust chest on an inline 6 and didn't know what they were doin' they might have it leakin' worse now than before.

Have you run a compression test?

[Mark75H]

I agree on the exhaust divider possibility. Distant second is weak ignition box.

Also could be a fuel tank vent issue or fuel line partial pinch

[blfink]

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?

[Bartman39]

Grounds are an often overlooked issue that can cause problems that just dont make sense so you may have found it...

But be sure as stated earlier to check for any water in a cylinder... Normaly it would be in #5 or #6 because of the water sitting in the exhaust divider plate near the bottom and pitting out the plate... It can even come through the outside plate as well, had the happen before...

[blfink]

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. Saw posting of highly recommended step by step procedure for the inline 6 link and sync last year but can't find them now. And by the way, plugs seem ok, couldn't find traces of water anymore.

[Mark75H]

Getting down to the 5,000 mark when it used to do better is a big warning to to the compression test and confirm there is nothing major wrong inside. The peak number is not as important as having them close. Hopefully 20 or less variation thru all 6.