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    Signs of a bad CDI?

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    • Member since
      July 2005
    • From: Cleveland
    Signs of a bad CDI?
    Posted by polarisboy on Saturday, December 3, 2005 9:15 PM

    My father in law bought a vintage Chaparral SSX last winter.  We had problems with it and chalked it up to bad carburetors.  We found some NOS carbs this past summer and took it out for its first spin today.  It ran great but within 10 minutes it fouls out the plugs.  You put in a new set of plugs and it takes off again and runs great for about 10 minutes till it fouls again.  It has new coils, new wires, new caps we are thinking maybe cdi.  What are some signs of a dying CDI do they go slowly or do they die suddenly?  We are at a loss.  It is getting fuel and the spark is good on new plugs.  10 minutes and it fouls out and plugs are shot.

    Any suggestions would be appreciated I hate for him to spend 140 bucks if its something else.  Jay

    Wish the Hobbies made me money!
    • Member since
      June 2005
    Posted by Anonymous on Sunday, December 4, 2005 9:49 AM
    Jetting?  What are you mixing your gas at?  Or if it's oil injected, is it set rich?  As far as I know, either CDI's work, or they don't, I don't think there is a grey area with them.  Are you gapping your plugs, or just throwing them in?
    • Member since
      July 2005
    Posted by jasper on Sunday, December 4, 2005 10:13 AM
    Yeah, CDI usually works or it doesn't.  Pretty rare one would be intermittent.  Problem might be down at the stator, I've seen some bad coils, connections, and weak flywheel magnets down there affect spark quality on older sleds. Be sure the plugs are the correct heat range also.
    • Member since
      July 2005
    • From: Cleveland
    Posted by polarisboy on Sunday, December 4, 2005 4:16 PM

    Thanks guys,

    I will steer him clear of the CDI.  We narrowed the problem down a little bit from where we started.  It is not on particular cylinder.  It varies back and forth with each set of fresh plugs.  Its basically a race to see which side will foul out first.  The stator was not what I was hoping for but at least it does not mean splitting the motor open.  The sled runs beautifully till the fouling starts happening then it just slowly goes south till a cylinder floods out one side. 

    We were running B7es for a plug and as far as we know the jetting is stock to what the book says.  We may try dropping the jetting down a couple of notches and see what happens.  If that does not work then we will need to pull the stator I guess.  Anybody know the testing procedure for the 74 L/C Chappy's? 

    Also is it possible to knock a two cylinder crank out of phase?  If a sled is out of phase does it not backfire a lot and give problems though?  Believe it or not a bad crank would be an easier fix then the stator. 

    Wish the Hobbies made me money!
    • Member since
      June 2005
    Posted by Anonymous on Sunday, December 4, 2005 7:47 PM
    Does the book say to run the B7? Lot of the older sleds run an 9 or even a 10.
    • Member since
      July 2005
    • From: Cleveland
    Posted by polarisboy on Sunday, December 4, 2005 7:53 PM
    My understanding is that the lower you go the hotter the plug.  The manual we have I believe says 7 and I know my colts run 7's?  Maybe I am wrong on my thinking?  Jay
    Wish the Hobbies made me money!
    • Member since
      July 2005
    • From: Michigan
    Posted by jbshocks on Sunday, December 4, 2005 7:57 PM
    7 is hotter than a 9.  Not to try to push you toward a cdi because they usually fail but years ago 1 guy I ride with had a 77 TXL340 and it started getting about 2 miles per gallon a ran terrible.  it was a cdi.
    Have you checked the engine timing?

    • Member since
      July 2005
    • From: Cleveland
    Posted by polarisboy on Sunday, December 4, 2005 9:00 PM
    No, we haven't gotten into the engine yet.  Its a pain in the butt to get to you have to remove 1/2 the sled literally to get to the motor.  We are going to mess with  the jetting some and if that does not work then we will get into the motor.  Hopefully our manual will help with the timing instructions.  Jay
    Wish the Hobbies made me money!
    • Member since
      June 2005
    Posted by Anonymous on Sunday, December 4, 2005 10:27 PM

    Dump ALL of the fuel out of the tank. Clean the tank and then install fresh fuel. I bet that will clear up the problem. Old fuel even if mixed with fresh can foul plugs quicker than yoiu can say BOO!

    • Member since
      July 2005
    • From: Anchor Bay MI
    Posted by chines on Monday, December 5, 2005 12:41 PM
    Could also be crank seals or  fuel pump diaphram especially after sitting.

    Self proclaimed President, Southeastern Muskamoot Bay Drag Racing Association. 5 members, and a few more that I'm considering.no dues. God bless the second amendment and thank god for the fifth.

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    • Member since
      July 2005
    • From: Cleveland
    Posted by polarisboy on Monday, December 5, 2005 1:24 PM
    BNO carbs have pumps on each carb and the carbs were brand new this past summer.  Crank seals would lend to a lean condition not a plug fouling problem.  Jay
    Wish the Hobbies made me money!
    • Member since
      July 2005
    Posted by jasper on Monday, December 5, 2005 5:46 PM
    Just for ha ha's, have you checked the sparks intensity when this problem starts to happen. See if the spark becomes really weak or intermittent.
    • Member since
      July 2005
    • From: Cleveland
    Posted by polarisboy on Monday, December 5, 2005 9:03 PM
    Once the plugs foul the spark is shot.  Put in the new set of plugs and you get a nice blue spark.  If you put the fouled plugs in another sled they still won't spark either.  Jay 
    Wish the Hobbies made me money!
    • Member since
      July 2005
    Posted by jasper on Tuesday, December 6, 2005 12:28 PM
    I guess what I meant is using an inline spark tester while running the sled on a stand if its possible to see if your losing spark quality at the time the plugs begin to foul.
    • Member since
      July 2005
    • From: Cleveland
    Posted by polarisboy on Tuesday, December 6, 2005 1:08 PM
    hmm, very interesting idea jasper.  Would have never thought of that one.  Will it still read a good spark though if the plug is fouled?  Wouldn't the plug fouling cause your ground to go creating a low spark?  I am willing to try it if you think it will work!  The other thing I thought of last night was to run new plugs till they fouled and then to remove the plug and put put it in the cap of a "good" sled.  Then compare the spark between the two?  Also, somebody mentioned on the VSCA site that bad rings can cause this issue?  Wouldn't I see a drop in compression if the rings were bad?  Jay
    Wish the Hobbies made me money!
    • Member since
      October 2005
    • From: Big.D
    Posted by frosty the snowman on Tuesday, December 6, 2005 4:41 PM
     chines wrote:
    Could also be crank seals or  fuel pump diaphram especially after sitting.
    cranks seals would cause a lean condition. chines may be right about the fuel diaphram. i had an xlt  that did the same thing rebuilt the fuel pump and it ran fine.
    • Member since
      July 2005
    Posted by jasper on Tuesday, December 6, 2005 4:53 PM

    If compression reads about the same on a cold engine as when warm, I doubt its the rings. you can pour a little oil down the plug hole to see if you get a drastic increase in compression opposed to what you had. That would indicate worn rings. Most 2 strokes are also way harder to start when rings are shot. As far as the spark goes, I would think  you should see some evidence in time of a good blue spark changing to a weaker pink color or cutting out completly just before the plugs foul if it were electrical related. 

    • Member since
      September 2005
    • From: West Monroe, NY
    Posted by Foxs on Wednesday, December 7, 2005 11:11 AM

    I had an old 86 indy 400 for my son that did that exact same thing you are discribing. After hours of fixing the wrong things, it turned out to be the CDI box. So I guess you can't rule that out yet. It would foul one or both plugs (or so we thought), change plugs and by time we did that the spark was back. Very frustrating, but it sure feels good when ya figure it out.

    www.foxsdiscount.com

    • Member since
      June 2005
    Posted by Anonymous on Wednesday, December 7, 2005 4:45 PM
    had a cdi go poo on an old tx...was fine when the temp outside was minus 20...got warm and just ran like ***...let it cool..ran better..ran best when it was super cold outside..new cdi fixed it perfect
    • Member since
      July 2005
    Posted by jasper on Wednesday, December 7, 2005 6:00 PM
    Gezz, going way back I once had the same problem on an old 89 modified Indy 500, turned out the CDI unit was prone to heat after running hard for a while which caused a loss in performance so we mounted it up on top of the water pump cover to solve it. Its rare, but yeah I wouldn't rule it out either.
    • Member since
      July 2019
    Posted by Mofo on Friday, July 26, 2019 9:07 AM
    I have a jet ski that keeps dying after 5-10 minutes, no spark after it dies, after a week of researching, and diagnosing, I may have narrowed it down to the stator, they day when it gets hot it sends a sign to the cdi to kill the spark(not enough volts) like you said, but my plugs arent getting fouled, they just get oily(2 stroke) from lack of spark after it dies. Ive ordered a stator, and an oil plug off kit, im going to premix to rule out any lean or rich ratios and install the new stator, hopefully this works.

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