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  1. #1
    bex's Avatar
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    GP1200R backfire

    Seasons greetings again guys.

    I've read enough posts thru the search but just want to confirm a few things with you all.

    As a background on the ski and me, I'm new to 2-Strokes completely and carbs and carb tuning etc.. so some of the advice on adding T-handles when I changed the D-plate went over my head. I purchased a 00' GP1200R that was running but had no top end, motor was shaky/rattling under no load on the trailer and making plenty noise, when in the water, would get to about 4500rpm and die, hard starting wasnt an issue. Long story short, all the pistons were smashed, and ended up taking it to a local guy to do a top end rebuild and had the carbs cleaned.

    Now, I've put 4 hours and 1 1/4 tanks of fuel on the ski since the rebuild. The first full tank was 50:1 premix + oil injection and I kept it under 5K rpm but varied the throttle under load, and let it cool down for about 20mins after a 20min ride until the tank was exhausted. I took it out today on the lake for a quick run and got it up to 7K rpm @ 48mph on the display.

    The three issues that I have noticed with the full tank with 50:1 premix:
    1. Hard starting when the engine is warm...I noticed this after the last 3 fuel bars on the display (fouled plugs?)
    2. Engine was stalling out under load. I could not get the boat out of the hole at any decent rate. I had to drag her out and onto plane, but once at about 4-4.5 she could plane and look decent at 5K
    3. There was a backfire or deep pop coming out of the exhaust once I had it onto plane or while holding the throttle at about 35-40% to get the 4-5K rpm

    Now that I had all the fuel exhausted as per the factory breakin procedure, I did not have a chance to swap out the fuel filter, but I just used straight gas in the tank (no premix) and relied on the oil injection to supply the oil.

    Under this setup, with new plugs, I had no start issues, but still had 2 and 3 from above.

    I had suspected that with the premix + oil injection that I was running too lean. It being the holiday, 4 day weekend here, I wasnt able to get a new fuel filter and assumed that the 100% gas would blow thru the filter no probs and cure my suspected lean condition. However it did not, still bogging out to the point of engine shutdown and also still backfiring.

    I decided not to run the engine again until I got some technical advice on where to look or to proceed next. I am thinking that the carbs are in need of tuning, enrichening the low speed circuit perhaps? The high speed appears to be running fine. I just want to avoid spending another $1K on parts and labour. Overall, as my first GPR, its a great machine and a much better experience than the raider I had before.

    As always, thanks for looking guys.


  2. #2
    Chris03HD's Avatar
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    For your carb setup / questions / jetting / reading, look up OsideBill here.

    Many posts on the correct jetting specs, with detailed instructions and pictures for every step along the way.

  3. #3
    bex's Avatar
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    Thanks will search for all his posts and sift thru what I think I need. Just dont want to run this thing anymore unless this is sorted out.

    Peace

  4. #4
    gaz1's Avatar
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    I would have thought that the premix+oil injection would have made it a rich mix which would foul the plugs and make it run like that.What are the current settings for the high and low speed adjusters and what jets are in the carbs?stock jetting?Maybe ask the guy who cleaned the carbs for you? did he use mikuni rebuild kits?When tuning the carbs,you should start with the low speed adjusters to get your throttle response then tune your highs for max rpm,there is thread on it on how to do it step by step and in sequence,i'll try find it and post it.

  5. #5
    Duke's Avatar
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    Quote Originally Posted by bex View Post
    Thanks will search for all his posts and sift thru what I think I need. Just dont want to run this thing anymore unless this is sorted out.

    Peace
    Osidebill, 2fast4u speak really good 2stroke carb language. Give give either one a PM in case they dont see this thread.

  6. #6
    bex's Avatar
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    Lots of info coming in. I think the guy just cleaned em up for me, he didnt tune them. Money was adding up, but in hindsight should have moved in that direction.

    Everything is stock in the stock positions I am assuming, its a used machine 10 years old, so who knows whats happened to her along the way. I know the Cat is gone, I changed it myself, there is a riva free flow exhaust kit in the back also, other than that, I am clueless and will assume stock parts.

    The information is massive on this board, hard to assess everything that comes in on searches, I guess thats why noobs like me throw up the same posts over and over, seeking guidance basically.

    My thought are also that the premix at 50:1 + injection would foul the plugs resulting in the conditions that I posted, but it happened the same way after the full tank Yamaha suggests for the break in period and after break in when I am running straight gas in the tank and no premix. The moral of the story is I know to sit the boat down before the bills add up. I want to change out the fuel filter, as the filter on it was the one it came with, history unknown. I doubt that will solve the problem but I am willing to give it a shot and see whats up.

  7. #7

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    Quote Originally Posted by bex View Post
    Seasons greetings again guys.

    I've read enough posts thru the search but just want to confirm a few things with you all.

    As a background on the ski and me, I'm new to 2-Strokes completely and carbs and carb tuning etc.. so some of the advice on adding T-handles when I changed the D-plate went over my head.

    Now, I've put 4 hours and 1 1/4 tanks of fuel on the ski since the rebuild. The first full tank was 50:1 premix + oil injection and I kept it under 5K rpm but varied the throttle under load, and let it cool down for about 20mins after a 20min ride until the tank was exhausted.

    The three issues that I have noticed with the full tank with 50:1 premix:
    1. Hard starting when the engine is warm...I noticed this after the last 3 fuel bars on the display (fouled plugs?)
    2. Engine was stalling out under load. I could not get the boat out of the hole at any decent rate. I had to drag her out and onto plane, but once at about 4-4.5 she could plane and look decent at 5K
    3. There was a backfire or deep pop coming out of the exhaust once I had it onto plane or while holding the throttle at about 35-40% to get the 4-5K rpm

    Now that I had all the fuel exhausted as per the factory breakin procedure, I did not have a chance to swap out the fuel filter, but I just used straight gas in the tank (no premix) and relied on the oil injection to supply the oil.

    I had suspected that with the premix + oil injection that I was running too lean. It being the holiday, 4 day weekend here, I wasnt able to get a new fuel filter and assumed that the 100% gas would blow thru the filter no probs and cure my suspected lean condition. However it did not, still bogging out to the point of engine shutdown and also still backfiring.

    I decided not to run the engine again until I got some technical advice on where to look or to proceed next. I am thinking that the carbs are in need of tuning, enrichening the low speed circuit perhaps? The high speed appears to be running fine. I just want to avoid spending another $1K on parts and labour. Overall, as my first GPR, its a great machine and a much better experience than the raider I had before.

    As always, thanks for looking guys.

    It is always a wise thing to ask for advice whenever you feel that you are in over your head.
    I will comment on a few of the things that you posted to help get you on track.

    Forgive me because the way you posted it implied that you confused the handles with the installation of the "D" plate. The point that people were trying to make is that once the pipe is removed, it is it is a simple thing to install "T" handles to your carbs.

    The "T" handles that you read in post refer to the high and low adjustment screws you have for your pilot jet (lows) and main (high) jets on your carburetors. It is good to have the "T" handles because the GPR is so hard to work on. It is easy to grab these handles and change the amount of fuel you want in the low or high speed circut. The stock adjustment screws are actually set at the factory and covered up, so you cannot even see them if you wanted to. You have to dig the anti tamper caps off to adjust your fuel flow.

    You are right in thinking that the added oil could potentially lean your engine out. Think about it this way: if your carbs are adjusted perfectly with the right amount of fuel and oil, then if you add oil into the fuel, then you are not getting enough fuel because oil is taking its place. As for the stock fuel filter, I believe it has internals made of a plastic mesh. I do not see any reason that you should change it unless there is lots of funk build up (trash) in there. Assumeing that your carbs are adjusted properly, once you get all of that oil out of your fuel tank, your plug fouling should go away.

    As you said, you do have an older ski, and you may have an electronics issue. But in combination, poor hole shot and backfireing really are indicative of fuel delivery or carb problems. Carbs are finiky, they have to be right for your ski to behave the way that you expect. There are sooooo many things that can go bad with a carb. On a GPR, you have to multiply those problems by 3. One thing that you can do to convince you that it is a carb problem on your bottom end bog is to choke the carbs a tad (5-25%). Choking a carb helps to create a bit more vacume which helps to draw more fuel into the low speed circut. If your hole shot improves, you have a carb problem. Backfireing is NOT good on a two stroke. You could have an internal (plastic) valve that is creased and not functioning.

    There are many good carb guys here on the forum and they will give you great advice and sound tips...Oside Bill being only 1 of them. The problem is that there are so many things that can go wrong, and there is so much work involved. I understand that you had someone clean them, but I really do not know what that means. Did he go all the way through them, or just hose the external bodies with carb cleaner?

    I would strongly suggest that you do what you are thinking and defer to someone locally who is good with carbs. Ask them to go all the way through the carbs to make sure they are right. Take it one step futher and ask him to show you how they work. You see it done once or twice, and you know exactly what you are looking for.

    Hope this helps

  8. #8
    bex's Avatar
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    Appreciate the input Salty, trying to isolate issues one at a time here without breaking the bank at the same time. I wont be able to put the ski in the water until Thursday to get a good check of everything that was mentioned.

    Sadly, just purchasing the ski without a lot of history makes the troubleshooting a bit harder as new gremlins pop up. Had to spend most of my spare time today dealing with another issue, there was a mild clanging from the rear end that got noticeably worse, impeller blade was basically shot, ground down to like a honeycomb appearance on the blade tips and the housing was swollen or warped causing the two surfaces to meet. Replaced both of those parts today. Got a very good to excellent condition 68N00 impeller to replace the 66V impeller and a new housing.

    I'll try to ride the ski for a short time on Thursday with the seat off to make sure the PVs are moving and in the mean time, I'll take a look at the stator just in case its had a run in with water and is causing a weak spark. Trying to go by process of elimination to narrow it down to the root cause.

    I couldnt figure out what to do with the T-handles when I changed out the D-plate thats why I was puzzled. I guess I didnt search hard enough, but I will be looking to add them very soon in the new year.

    Thanks for all the great advice, I'm creeping my way along here and its been a big assist to have some great advice from you guys. I'm confident that I can track down the root problem before I have to go back to work in January.

  9. #9
    bex's Avatar
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    Progress...

    Decided to look at the fuel system today, starting at the filter and going to the tank before removing the manifold to get at the carbs.

    Found the filter very dirty, filled with sediment and debris. This moved me to drain the fuel tank and give it a clean out. Flapper valve at the end of the fuel post has broken off, its not in the tank so I guess that was removed prior to me owning it.

    Getting good spark when grounding out all three plugs on the engine and dry running for about 4-6 seconds, enough time to examine all three plugs individually.

    Decided to just change out the filter for a new one at this stage and fill the fuel tank to about 25% before taking anything off. I have time to run it tomorrow and see if the gremlins have went away. If they remain, and she still bogs out under load at low speed, I'll wait to take down the carbs until I have the parts for the t-handle mod, just dont think it makes sense to go any further without the handles.

  10. #10
    Moderator beerdart's Avatar
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    Did the shop that did the carbs rebuild them or just clean them? There are small filter screens in the carbs that can get plugged. With the dirty main filter you found I would check the carb screens. Did the shop inspect the reed's when the top end was rebuilt? Has the machine ever suffer a cat-converter failure? Do you power valves cycle when the start button is pushed and released without starting? Are you running the stock air intake system?

    Oh and welcome to the Hulk..

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