2011 F70LA slight miss above 3300 RPM.

[cprodave]

I am helping my buddy with his 2011 70hp 4-Stroke F70LA (6CJ) that has a slight miss running above 3300 RPM or so. It runs fine at idle and has good holeshot, only problem occurs cruising along at high RPM. A couple weeks ago he had a problem where the engine would only putt-putt back home at low speed (I am unsure if the engine cut out and restarted, but I do know that there was no Alarm (overheat etc) when the problem arose. The engine would not run at higher RPM's. After the putt-putt home he changed the fuel/water separator which fixed the putt-putt problem . The contents of the old separator were disposed of without viewing in a glass jar etc to confirm (or not) the presence of water in the fuel. I then installed new like-for-like NGK LKR7E spark plugs (with as-received Gap 0.035inch) and took a test run. Engine is still missing slightly at high RPM. I removed the separator and dumped the full contents into a clean glass jar--absolutely no water was observed. My buddy runs only non-ethanol gas from a marina that sells high volume and has a reputation for good fuel.

On the old spark plugs I noticed the lowest-position plug was slightly browner/oily compared to other 3 higher-position plugs. Also looking at the spark plug holes in the cylinder head only the lowest position has a brown runny residue.

So we are looking for input on: 1) is it high risk to run this engine ( has only around 330 hours) with this slight high RPM miss? 2) is it normal for lowest position cylinder/head to have some brownish residue? 3) if not normal, are there any other actions we should take before taking it to a local Yamaha mechanic? 4) I am unsure if the order/placement of spark plug wires is correct. I was careful replacing spark plugs only one at a time, but wonder if somebody previously got wires wrong (my understanding is that on most engines wrong order/placement will destroy the engine).

Thanks for any help, I will try to post photos soon.

I will update in a few minutes with photos if possible.

 

[cprodave]

[No message]

 

[cprodave]

Sorry can't post photo of lowest position spark plug hole vs. other holes--size limit problem...

 

[cprodave]

Could it be a thermostat that is opening too fully? Somebody told me if there is too much water flow through the cooling paths one cylinder (the lowest position one--furthest from the thermostat/inflow) doesn't get up to optimum temperature, therefore some fuel is only partially burned, leaving a residue while that cylinder occasionally "misses". Make sense?

 

[Texasmark]

Millions of engines ran just fine without stats over the years and those with them, located at the top of the cylinder bank.....to ensure that the entire crankcase fills with cooling water......work just fine. Stats can't open too much as there are mechanical limits besides lots/most higher hp engines have bypass ports, larger than the stat port that are forced open at higher rpms for additional cooling. Plugs fail at hight rpms. Get a full set of new ones. If the engine does a lot of trolling/slow running, especially at colder ambient temperatures.....like trolling for Walleye in the winter....if northerners do that, get some WOT time on the engine to blow the crud out. Using an engine cleaner, like Sea Foam (my favorite) in the fuel helps a lot in keeping the cylinders and plugs squeaky clean.