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  1. #1

    96 SL780 will not accellerate

    Hey all,

    I have a 1996 SL780 that I have been working on for while now. I have cleaned the carbs, replaced the fuel, put new spark plugs in, and made sure fuel is getting to the carbs. I just took it out to the lake and I still have problems. It starts right up and will idle fine in the water. When I give it just a little throttle (maybe 1/16th or 1/32nd) it will pick up speed but only until about 6 mph. If I give it any more throttle it bogs and dies. It doesn't matter how slowly I apply throttle once I pass maybe 1/16th throttle it dies. If I choke it while plotting along it will die. This leads me to believe I have a severely rich condition, is my conclusion correct?

    I have not messed with the mixture screws but I assume previous owners did all sorts of stupid stuff (there is a lot of evidence to support that statement). I think my next move is adjusting the mixture to lean it out but I wanted to see what you guys think. Unfortunately I don't have easy access to a lake so it is a guess and check scenario for me. If you had one thing that you would try before taking it back to the lake what would it be? Any input would be great.

    Thanks,

    Charlie


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    If you have not yet internally cleaned the carburetors, that would be the first thing to do. And replace ALL the fuel lines.

    Click the link below, then click on Fuji engine

  3. #3
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Welcome to the Hulk.

    What's your compression?

    If you haven't touched the adj screws, you haven't properly cleaned the carbs.

    Remove your carbs and give them a THOROUGH cleaning. (if not a rebuild due to age alone)

    Set your pop off pressure to spec

    Set your adj screws to spec for your model year

    Next remove the spark plug boots and cut the wires back about 1/4", then rescrew the boots back on and zip tie them in place.

  4. #4
    Thanks for the input. Just did a compression test and here are the results. From front to back the numbers are 116 psi, 112 psi, and 118 psi. This was done on a cold engine with a pretty well (not fully) charged battery. The manual says anything below 120 psi is low but they are well within 10% of each other. I took a look at the spark plugs while I had them out and the back and middle look like the mixture, when it does run, is OK. The front plug looks brand new. So part of my problem is that the front cylinder is not firing. I will check for spark and then go from there. I'll report back what I find.

    Thanks again for the input.

  5. #5
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Check for spark. If that's good, check for fuel saturation. (rich)

    you may also want to look for water intrusion. It will literally "steam clean" the internals if you have a water leak into the combustion chamber.

    Try removing your spark plug boots, cutting about 1/4" off the plug wires to expose clean wire, and rescrew the boots back on. See if that helps with no spark.

  6. #6
    Have spark in the cylinder that isn't firing. It appears the same as the spark at the other plugs. I put the high and low speed settings back to factory. My only dilemma was whether my carbs are the "model with first transition hole below throttle plate at idle" or the "model with first transition hole above throttle plate at idle" Any quick way to differentiate between the two. Out of water should I be able to fire this engine up on one cylinder? If so that will make verifying each cylinder is running easy. If not, how can I make sure each cylinder is operating (in my garage). Also, since I am a newb, is the PTO cylinder in the rear and the MAG at the front, or the other way around?

    I am at around 5500 feet altitude. I know that usually that makes carbed engines run rich so you usually would lean things out to increase performance. I am not concerned about that right now, but assuming I get this thing running, any guesstiment as to how much I will want to change from the stock settings?

    Thanks.

  7. #7
    Quick update.

    Was able to get the engine to fire using only one cylinder for each cylinder. So I think I am at least running on 3 cylinders now. I took your advice and cut the spark plug wires down and reinstalled the boots (with zip ties of course). The fresh copper was nice to see. The high and low speed settings are stock now (which should be safely rich at this altitude). I will take it out tomorrow evening and throw it on the lake. Fingers crossed it will perform better than last time.

  8. #8
    Another Update.

    Took it to the lake to test out. The first time I fired it up I was able to get it to rev (on the trailer, in the water) to about 3600 rpm. After that I was stuck at 2600 rpm. My front cylinder (MAG?) is definitely not firing. I messed with the mixture, rotated spark plugs, etc. The front cylinder would not fire consistently. The cylinder felt like it was getting warm and it was firing intermittently but not consistent enough to run right. I will probably pull the carbs again and check them out, but I am wondering if it really is a spark issue. I know I get spark but maybe not a consistent spark. Any way to test this? Any suggestions on where to look? Perhaps this is electrical? Any input would be appreciated.

    Thanks.

  9. #9
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Did you try cutting the plug wires back?

    Get yourself an inline spark tester to see if it is a no spark condition.

  10. #10
    Cut the wires back and just did a test with an inline tester. Looks like I do indeed have good spark at that cylinder. So, good compression, good spark... guess that means fuel. I have another sl 780 sitting on the trailer right next to this one that I think I am going to pull the carbs off of. I will clean those up and throw them on this sled. If I have the same problem then I am back to square one. If I can get the MAG cylinder running then I know it is the MAG carb on carb set #1.

    I'll let you know how it goes. Thanks for the input.

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