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Thread: dead cylinder??

  1. #1
    Rasta Mon Condoms We Be Jammin!!!!! TxVirageTx's Avatar
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    dead cylinder??

    98slth.stock lost the mag cylinder to a clogged carb screen,was able to hone and replace with new stock piston and rings.completely cleaned the carbs and replaced all the fuel lines.ran o'kay for about an hour then started acting up.like its on the limiter barely revs to 3500.pulled the carbs,everything checks o'kay.no ruptured diapraghms,no clogged screens,return orifice clear,seperator clean.removed the breather.the front plug gets hot to the touch the pto gets warm.i can run it with the seat off as well and hold the throttle wide open and completely choke off the pto carb with my hand,when i pop my hand off it stops missing and accelerates hard for a bout 5 seconds to about 5400 rpm and no higher and starts missing again.i can repeat the hand over the carb thing time and again and it will accelerate hard when i pop my hand off.the pto carb is the one with the fuel pump on it.compression is 148 mag and 143 pto.possible one coil is out or weak??or tank vents??just can't find anything with the carbs


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    If you swap the coil primary or spark plug wires, does the problem move to the other cylinder?

    I am pretty sure the Ignition Update Kit for the twin sparks both cylinders together every time, but I don't know if the original 1998 twin ignition also did so.

  3. #3
    Rasta Mon Condoms We Be Jammin!!!!! TxVirageTx's Avatar
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    coil

    i thought the same thing,i'll move the box around and switch coil wires.if the problem moves to the mag cylinder i should be able to get away with just a coil replacement.would be just a weak spark and not a trigger issue.i wondered if by capping the carb top that it makes the fuel pump drive harder(pto carb has the pump)builds enough pressure to run and when i pop my hand off it runs on that built up pressure till its gone,or is it that when i pop my hand off that cylinder goes way lean and can fire until the fuel flow catches up inside the throat the mixture gets correct again and can't fire

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    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by TxPro1200 View Post
    ...capping the carb top that it makes the fuel pump drive harder (pto carb has the pump) builds enough pressure to run and when i pop my hand off it runs on that built up pressure till its gone,

    or is it that when i pop my hand off that cylinder goes way lean and can fire until the fuel flow catches up inside the throat the mixture gets correct again and can't fire
    The weak fuel pump theory sounds plausible

    When you have capped the PTO carb while the engine is running, the vacuum created would be pulling raw fuel into the manifold and crankcase, where it could accumulate (since air flow is very low). When you release your hand and let the air flow again, it now has lots of fuel and air, and away you go!

    Either way, if the problem isn't in the spark, then it sounds like the PTO carb needs attention. Could you temporarily swap the two carbs?
    Or insert a fuel primer bulb into the fuel feed line, and hand squeeze it to artificially boost the fuel pressure into the carbs?

    I would think the carb could recover from excess lean in way under 5 seconds, so that doesn't sound as likely (in my opinion ).

  5. #5
    Rasta Mon Condoms We Be Jammin!!!!! TxVirageTx's Avatar
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    ???

    anybody else??

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    SPEED KILLS, BUT YOU GET THERE QUICKER Keddano's Avatar
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    I'm thinking coil,But what about a reed valve stuck? is it just missing or have a little backfiring? A couple of the in-line Spark checkers work good,put one on each cyl, take to the water and load on the trailer,you'll see if the spark faults under load then.

  7. #7
    SPEED KILLS, BUT YOU GET THERE QUICKER Keddano's Avatar
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    Also,check your fuel pressure on the return side.

  8. #8
    Rasta Mon Condoms We Be Jammin!!!!! TxVirageTx's Avatar
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    spark

    i guess a automotive style spark checker from autozone or such will fill the bill??the more i think about it the more i keep coming back to the spark,seems like the mag had a sharp blue across the gap,the pto had a fat orange

  9. #9
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by K447 View Post
    If you swap the coil primary or spark plug wires, does the problem move to the other cylinder?

    I am pretty sure the Ignition Update Kit for the twin sparks both cylinders together every time, but I don't know if the original 1998 twin ignition also did so.
    Did you try this?

  10. #10
    Rasta Mon Condoms We Be Jammin!!!!! TxVirageTx's Avatar
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    swap

    didn't swap them,can you link me the wiring diagram for the 98 slth??should be able to tell if the original coils fire independently.don't really want to have to listen to a huge backfire if i don't have to

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