Decarbonizing 1999 Johnson 175 Carb.

[Scott]

What up? Ran hard today with Part I and could not bust 4600 rpm. 1000 below standard. Ran like a top but not even close to top end. Bad/fouled plugs? Going to try new ones to see.

What size wheel are ya spinnin'?

[Bigshrimpin]

If you were spinning 5600 before and you're only turning 4600 now (no changes except for the decarb) then you're firing on 5 of your 6 cylinders.

Pull all the plugs and inspect the white ceramic part . . . all of them should be paperbag brown in color. One will be white. That's the one that isn't firing.

Replace that one spark plug and/or make sure the coil and plug caps are on tight. Take her for another spin.

Oh yeah . . . You probably have to gap your plugs on the OMC motor . . . so make sure that your in spec with the gaps.


See the carbon on the ends and the paperbag brown color on the tip. That's good . . . The one that comes out looking brand spanking new (from a nice petroleum high pressure clean). That's the culprit. Fiddle with the wires and the plug cap for that plug and you'll be back up at 5600.

[Michael Vezzosi]

PeterB, I have not yet checked the compression.
Scott, OMC SS 14 1/2" X 19". Since 1999. Perfect match.
BigS, I think you are on to something. That sounds closest. Remember I used original plugs that were used during the decarb, because I didn't THINK that they were that bad. Boat ran flawlessly all day until I wanted to run WOT for a minute or so. After sitting on a sand bar for the last hour, we decided to make 5 minute run back to landing. Upon arrival, overheating alarm. Did not run over 3500 revs. to ramp, and was not overtrimmed. Like 12-15 hours on waterpump. Hope to pull plugs today. Thanks all. Part II.

[Finster]

Mike, just a thought. Why don't you check your linkage and control cables? Maybe your not getting full throw out of them. Check your timing aswell.