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Engine missing


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I'm having some problems with my 1998 Sportster 5.7 carb with about 580hrs. Our lake is about 6500ft. Boat had been running excellent other than what I describe as carbon loading when Wake surfing or wakeboarding with wedge/ballast. Last week I decided to do some maintenance that I had neglected. I changed the fuel filter, plugs, rotor and distributor cap, and used 2 bottles of TECHRON fuel system cleaner(treats 24 gal). I removed NGK platinum plugs, which after 4-5 years, looked pretty good. I replaced them with Champion plugs that crossreferenced RS12YC, gapped to .045. The engine ran fine at first, then after about 15 min, began to miss and backfire, and wouldn't go above 4000rpm( nml 4600rpm). Went to dock, pulled plugs, none were fouled, but rear plug on each side were dark, others still white, checked all wires, pulled cap and no arcing or obvious problems. After sat for about 45 min, ran well again for about 10 min then problem recurred. Temp stays at 160, good oil pressure. Runs fine without load.

Yesterday I replaced the champions with NGK v-power TR 5, as I couldn't find any BPR6ES's. The plug charts I have found are inconsistant with their cross references. I ran the boat yesterday after adding 20gal premium, and ran much better than with the champions, but was still missing under load. Symptoms were somewhat intermittant. RPM would drop 200-400 at WOT, then improve. The other concern, which was also evident yesterday, is the black film from the exhaust when the engine has been under load. It has been doing this since I bought the boat 7 yrs ago. Very similar to what will wash out of by truck exhaust at the ramp(duramax). I attributed the black slick to running rich from the high altitude.

Any suggestions? I am trying to get a set of AC Delco's today MR43LTS. The boat ran great with the platinum NGK's, but I can't find them referenced and they were inadvertantly thrown away. I also thought it may be time to change the plug wires, but it ran great last week.

Dan

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How big is the fuel tank in your boat? Sounds like you may have added too much Techron, especially if you could fit 20 gallons in after running for a half hour or so after doing the maintenence.

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How big is the fuel tank in your boat? Sounds like you may have added too much Techron, especially if you could fit 20 gallons in after running for a half hour or so after doing the maintenence.

Tank is 32 gallons, Techron states to use 1 bottle for 12 gals, tank had about 28-30 gals with 2 bottles, ran the boat for 3-4 hrs, then added 20 gals yesterday. Do you think the techron could be causing the problem?

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Could be. The boat ran better with the techron laced fuel diluted to about 1/3 the original value (adding 20 gals to a 30 gal tank). Maybe the fuel was getting old??? And the fresh 91 octane helped it out.

Just to be clear, the boat was running well before the maintainence?

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Could be. The boat ran better with the techron laced fuel diluted to about 1/3 the original value (adding 20 gals to a 30 gal tank). Maybe the fuel was getting old??? And the fresh 91 octane helped it out.

Just to be clear, the boat was running well before the maintainence?

Boat was running very well. intermittantly it would drop RPMs when running above 4200, for about 1-2 seconds, but was not audibly missing. Fuel filter had water/debris in it, but now is new. I removed the new fuel filter the day we were having the backfiring, drained it and it was clean. I have been using 30-40gal/week, always premium. At our elevation I run a 10 1/2" prop, WOT is 4600 at 40mph. The "local" boat shop did not have the AC Delco's, and instead recommended to same plugs I put in yesterday.

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Any chance you crossed a plug wire or two?

No, I was careful. It was running excellent for the first 10 min each plug change, then missing/backfiring started. Starts perfect, idles well, revs well without load. Also will still pull 4600rpm. I was barefooting yesterday, so still enough power, just missing. Seems worse under load at slow speed. With the champion plugs, backfiring every3-5 seconds, never backfired with the NGK's. Plugs? Fuel? wires?

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My next step would be to get the factory recommended plugs back in there and see if that helps at all.

I plan to order a set. I wish I had the NGK platinum plug #, Those worked great great for years! I cant find any cross reference for them. I have always been a fan of NGK's. Do you know anything about the electronic ignition module under the distributor cap?

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The Techron might have broke free some gunk that is now in the carb effecting the floats,jets,needles and basically inhibiting proper fuel flow. You are running lean with the backfiring.

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The Techron might have broke free some gunk that is now in the carb effecting the floats,jets,needles and basically inhibiting proper fuel flow. You are running lean with the backfiring.

I was wondering if I was lean or if there was pre ignition from too hot a spark plug. The Champions are 1 heat range hotter than the AC Delco's or NGK's. At my elevation, running lean is something that should not happen. I never had my carbs rejetted. Should I use a carb spray cleaner? Nobody locally has marine plugs. Would it really make that much of a difference to use the factory plugs? Could this all be from the Techron? I'm going to lake tomorrow and will likely add 15-20 gal. Does anybody know about rejetting?

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The Techron might have broke free some gunk that is now in the carb effecting the floats,jets,needles and basically inhibiting proper fuel flow. You are running lean with the backfiring.

I was wondering if I was lean or if there was pre ignition from too hot a spark plug. The Champions are 1 heat range hotter than the AC Delco's or NGK's. At my elevation, running lean is something that should not happen. I never had my carbs rejetted. Should I use a carb spray cleaner? Nobody locally has marine plugs. Would it really make that much of a difference to use the factory plugs? Could this all be from the Techron? I'm going to lake tomorrow and will likely add 15-20 gal. Does anybody know about rejetting?

You can subsitute the MR43LTS plugs with R43LTS if someone carries those. My engine is a '95 but I upgraded my heads to the Vortecs (same as yours). I am currently running R42LTS gapped at .045". My boat runs great. Tops out at 4700 RPM and 42 MPH. I read somewhere (and I can't remember where) that GM or Mercruiser or Indmar changed their recommendation on the heat range of plugs for Vortec engines. It was dropped from 43 to 42 for the AC plugs. So I wouldn't go up on the heat range with those Champs.

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