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  1. #1

    gp800 stumbles on first start

    I took my 98 gp800 out a month ago and it started stumbling when first started then it would clear up. I rebuilt the carbs and it still stumbles when first started and after being shut off for a little bit it will stumble when first started. If i ride it for about 30 seconds to a minute it clears up and rides fine. It feels like a misfire then goes away. It is all stock and the carbs are at stock settings. Its almost like its loading up when turned off, yet idles fine when running. It could also feel like its losing fuel to carb when shut down. I mix 32:1 .

  2. #2
    07vette's Avatar
    Join Date
    Jun 2007
    Corpus Christi, TX
    Try running 40:1 , with some good oil. Keep a eye on your plugs to see whats going on.

  3. #3
    I am running amzoil dominator i will have to check the plugs. Do they sound fouled. I am not too familiar with 2 strokes. What does a fouled plug look like exactly. I thought the power valve motors need 32:1

  4. #4
    Banned User
    Join Date
    Aug 2011
    an oil fouled plug will be wet and black. white is lean. grey is a happy medium 40:1 is will just fine. your getting too much fuel/oil imho. put new plugs in and try 40:1 see what that does. a plug can be fouled in many ways, wt or dry, depoits, etc. pull plug, snap on boot, and hold against frame to see if it has good spark. do it at night and dont hold wire or plug with bare hands.

  5. #5
    If that dont work...and you are sure you have good reeds/diaphrams.Could be slow screws are out of want to achieve 1 turn out on each but they can run between 1 to 2 turns out unless you have over size jets..

  6. #6
    I just rebuilt the carbs so the diaphrams are new and the reeds looked good when they were out. I will pull the plugs and check.

  7. #7
    I checked the plugs they didnt look fouled to me, I could be wrong though. Does anyone have a pic of a fouled plug? If it was the plugs why would it only do it when first restarted?

  8. #8 want be able to see a foul plug it can happen to a brand new one...two kinds of fouls.One is plug just wont fire.. easy check just pull all plug wires away from ground check one at a time engine should run on one plug while patting the gas..motor should run..on each one...second is a miss firing plug or wire grounding to block..start in dark for wire check..check plug for burnt lines on white part of plug for miss..Plugs should have black oil burned oil on it to be a good running plug..If its bright gray and ashy your running to lean and at risk of piston shattering..Other option could be cat. or water box loading up.Or leaking Vac or fuel line...Plugs are prob fine since you say it clears up and runs fine.Normally slow screw or pilot jet is your cause of bad take off...

  9. #9
    Its the weirdest thing it is either like it is flooding out or losing prime when it sits not running for a minute or two. Once it starts it clears up after about 30 secnds and then runs,idles,accelerates fine. It started doing this before I rebuilt the carbs, so I figured it was carbs. I rebuilt them and found a cracked chevk valve in the pump section. I figured that was the problem, yet it still has the same symptoms. I have checked the fuel lines and they seem ok from the pickup all the way to the carb. Could it be the reeds? The slow screws are set to stock specs.

  10. #10

    Join Date
    Jan 2006
    Texas City, Texas
    Did you test your needle and seats when you did the rebuild by doing the pop off pressure test? Yamaha's traditionally have high pop off pressures, and when the N&S get old, they will leak before they pop off as the pressure is building . When this happens, you leak fuel in the carbs before it needs it.

    In other words, leaking N&S can prematurely flood the lower circut with excess fuel.

    When you do the test, no bubbles should form as you are pumping up the carb to induce the N&S to pop off. If bubbles form to any significant amount before your N&S pops off, you should change them.

    If you really believe that you do not have enough fuel at the bottom end, then lower the pop off pressure so that you can start to tap into the high speed fuel circut a little bit earlier.

    Go to 40:1 unless you race that thing full time.... the Amzoil will give you all the protection you need at 40:1

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