"COKING" explanations???

[JIS1974]

Hey All. I just bought an '84 Evinrude 90hp VRO<br />E90 MLCRD<br />The engine ran at medium throttle but would cut out and die at WOT (or anything over about 3000 rpm). There was no warning horns and the motor wasn't exceptionally hot. The engine would crank over and start back up quickly after stalling out. ...it will not idle now in gear and it sounds as if a cylinder isn't hitting. <br />After inspection:<br />Carbs were seemingly spotless inside. (I rebuilt them anyway.) Compression check: 90, 80, 75, 55<br />Clear tube for VRO was crumbled and cracked with age.<br />I'm considering a rebuild, but somebody mentioned that the rings might be "coked." What, exactly, does that mean? Is it likely to happen to all four cylinders at the same time? Is there a way to confirm this condition? ...any easy fixes? If there is something I can do before tearing into the powerhead, please let me know!!<br />Thanks Everybody!!<br />KS

 

[12Footer]

Re: "COKING" explanations???

Coking is the buildup of carbon in the engine's comustion chamber, and exaust.<br />It is like a goey black soot, that gets under the ring lands, and wears your engine out prematurely. It sticks to everything after the ignition-point of the fuel ,because it is the byproduct of the mixed gas/oil.<br />It is very important to routinely use some form of decarbon cleaner in the fuel,or as a spray treatment.<br />It comes in spray types, which can be administered underway if you wanted to,or at the driveway. It is applied directly into the carb, or the engine maker's brand sometimes comes with a fitting, so you can attach it to the "FOGGER" fitting.<br />Other types are added to the fuel.<br />Don't ovcerlook this. And you can never decarbonise or 'de-coke' an engine too much.<br />It may be causing your problem by restricting the exaust ports,or by sticking your rings.<br />Do a compression test. Treat the engine using the spray type (follow the instructions on the can). Then, retest compression.<br />Personally, I don't think this is your problem.<br />Although decarb every 80hour of normal operation, and sooner, if you idle frequently.<br /><br />It sounds more like a coil or switchbox failure.<br />Try shooting premixed gas into each carb,and see if performance returns briefly (watch out for backfiring while doing this). If it does improve, you have a fuel problem with that carb.If no changes, or it looses performance durring spraying fuel mix, it is an ingntion problem....<br />Now, if so, get some very well-insulated plastic tongs, and pull one plug wire at a time. If you find one that when pulled, does NOT stall or otherwise have a negative effect,that is the dead cylinder. Check that plug for fouling, and suspect it's coil,trigger, or the switchbox.

 

[Dhadley]

Re: "COKING" explanations???

Sounds like there may be a broken ring on the cylinder with 55 lbs. That could be the worst case result of coking. Look inside that cylinder thru the plug hole (or remove head) and let us know what you see. It usually shows up on one side first (worse) than the other.<br /><br />Good luck!