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  1. #1

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    GP1200 Help and Advice.

    Hi there everyone.

    New to the forum and jet ski world. And i require some help from the experts.

    I have to say, the forum is very helpful and learned a few things.


    I got hold of a 1997 GP1200 that was a non runner.
    I Managed to get her going and ran quite well on the hose.

    Air/fuel screw has the T handle mod.
    I have removed the automix and went to premix. Running 40:1.



    So the issue is I need to get the settings for the high and low screws.
    I set them to the settings from the manual just to see if it will fire up. And. It did, threw a bit of fuel down each carb and bang it started. After that it starts with the touch of the button without the choke. Even when cold.

    I am not sure about the stock settings as the flame arrestor plate was removed.

    I'm not sure on the jets but it is most likely stock. Also to add, when I removed the plugs for the first time cylinder 1 and 2 were perfect tan brown. But cylinder 3 was black.

    So something tells me it was running well before and with 120psi on each cylinder it should run well again.

    Is there a set specification for the screws with the flame arrestor removed?
    Or is there a guide that I could follow to tune the mixture?

    And also, I have temporarily disconnected the instrument cluster until I am able to repair or replace it.
    Is there anything that I can do to prevent any issues from happening like overheating ?


    Is there any things I need to check on it ? Or any usefull mods I need to do ?

    Many thanks to everyone!
    Last edited by YFZR750; 12-11-2014 at 03:07 PM.


  2. #2

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    Hi there,

    I go the ski in the water today but on the trailor to see how it runs in the water.
    It idled and revved nicely, so after about 5 minuets of running the engine starts to misfire.

    So I begin to troubleshoot and pull off the spark plug wires one by one to see which ones miss.

    So It came out that if I pull cylinder 1 and 2 plug wires off there is no difference. But when I pull cylinder 3 off the engine dies immediately. But if I keep it on a rev and pull off cylinder 3 it misfires very badly.

    Pulled the plugs, 1 and 2 are soaking wet and 3 is dry.

    So I worked quickly to check if there is spark as I wanted to keep everything at the temperature.
    And all 3 have a nice fat spark.

    Swapped in other plugs and it still misfires on 1 and 2.

    It has to be spark related as the plugs are wet.

    Any ideas ?

  3. #3

    Join Date
    Jul 2014
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    Check the oil line going to cylinder 3!!!! Be for you do anything else!!!

  4. #4

    Join Date
    Dec 2014
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    Hi,Do you mean the oil injection hose?I have converted it to premix.... Running 40:1.

  5. #5

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    Okay, found what could be the problem.
    I just saw that cylinder 1 was dripping fuel from the bulls eye... What would cause this ?

    And secondly, anyone know of a good guide on tuning the high and low screws on this engine?
    I would like to do it in the water on the trailer. If its possible to so.

    Kind regards

  6. #6

    Join Date
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    The 65u engine is a very good one. However, it does have its issues. Amoung them, they have a very high pop off pressure in stock form, and they also have a tendency to vibrate a lot. This combination causes the needle and seats to leak fuel through the bulls eyes.... which makes their carbs very difficult to tune. I do not believe that this explains your back fireing. The 65u also have a tendency to crack the internal clear plastic check valves inside the carbs. This may cause backfireing.

    IMHO you should pull the carbs and put some carb. kits in them. In the very least, find out when this was done last. Also, while you are there, inspect your reed valves for any chipped or broken/burned ends on the reeds and replace as needed. Chipped reeds will for sure cause backfireing.

    That engine will run like it's fine on the trailer, but when you put it in the water and add a load on the engine, it will fall on its face.


    www.mikuni.com. Read about SuperBN carbs and how to tune.

  7. #7

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    Okay, I downloaded the manual for the carbs and will be reading through it.

    Also, its not backfiring. Its missfiring, as it is not running on that cylinder. But I found that the dripping carbs were throwing the plugs wet. Which is what causes it.

    So is there a fix for the dripping issue? I have been reading another thread but they never came up with a solution.
    And what puzzels me is that he rebuilt the carbs and it did the same thing again.

  8. #8
    mudslanger's Avatar
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    If your carbs are flowing too much gas through them to the point you are flooding a cylinder out you have a serious carb issue and needs addressed. the first thing is to remove the carbs and totally disassemble them. there are a couple of things that will cause this issue. #1. A bad Oring around the needle SEAT itself. #2 a bad needle and seat. Also if someone has screwed the mixture screws out too far but that doesn't sound like your issue. Like SALTY said new carb kits. also when you rebuild the carbs make sure you pressure test them and adjust to the proper pop off pressure.

  9. #9

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    Two things that I personally have experienced with my 65u both involving high pop off pressure and fuel leaking through the bomb sites....

    Back in 97 the first week I owned my GP, the boat would idle fine but when I tried to ride it (i.e. put a load on the engine) it would not go. A symptom was raw fuel dripping through the bomb site. Turned out that a clear plastic internal check valve had cracked. Yamaha repaired it, and the assumption was that the HIGH factory pop off pressure caused the problem. They fixed it by installing two check valves in the place of 1 to strengthen it.

    After a year of hard riding, it would not run well on the bottom end, but it did run OK once you got the boat to speed. A symptom was raw fuel leaking through the bomb site. The needle and seats had worn due (presumeably) to high pop off pressure and high vibration these engines are known for. Later that summer, when the cylinder was ported, new aftermarket needle and seats were added along with different jetting. The new needle and seats and spring combination LOWERED THE POP OFF PRESSURE. After that, I road that old GP for years without carburation issues.

    Running your boat on the trailer I am afraid will be of little value. I believe that if you were to find some of the post made by OsideBill (God rest his soul) you would do well to read what he has to say about the 65u. More than likely, you will find aftermarket jetting and a LOWER POP OFF PRESSURE (bigger needle and seat/spring combination) that will make your problems go away amoung his post.

    Bite the bullet, dude. Go through those carbs.

  10. #10

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    Okay, I got to the carbs and found that there was a bit of debris in the needle/seat area. Allowing fuel to flow straight through it, and out the bulls eye..

    Started up perfectly, and no more misfire.

    But I have two questions,

    1: Is it possible that the engine can run without the stock flame arrestor in the airbox? I ask because the previous owner probably removed it.

    Or should I fit individual cone air filters (The type used on motorcycles) on it ?
    I would remove the little rubber trumpets and clamp the air filters straight onto where the trumpets fitted and then close it up with the original cover.

    I would like it to run well without changing any jets if possible.


    2: Is there any guide to tuning this engine? I have searched but could not find a guide with clear instructions to do so. I will be able to place the engine under load to do the tuning.

    Many thanks!

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