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  1. #1

    Problem with FX140

    Hello, I just registered here after finding this board while searching Google. Great board with tons of useful information!!

    But anyway, I just recently purchased a 2004 Yamaha FX140. The guy I got it from was having problems getting it to start and run and now I'm in the same boat. The engine does turn over.... it has fuel and spark, but it backfires like there is a timing issue. The engine was just rebuilt and it does have good compression. At TDC, the cams appear to be in time. I tried swapping the wires on the coil packs and it actually seems a little better like it wants to start, but it is still backfiring.

    At first, I noticed there was a wire unplugged that turned out to be the engine temperature sensor. The check engine light was also on and it was showing a code 15 (Incorrect engine temperature sensor signal). I ended up pulling the motor out and found that the sensor was never put back into place. So, I ended up buying another sensor and installing it and putting the motor back in hoping this was the problem. Well the check engine light is off now, but it is doing the same thing. I have the engine pulled out again now checking for a bad woodruff key, but everything appears to be good.

    So now I am kind of lost. There is spark, but it just seems like it isn't firing at the correct time. Am I missing something here?? I've searched this board and others numerous times, but I just can't seem to find a solution.

    Thanks in advance for any information that might help me.


  2. #2
    do a compression test to see if all are even and higher than 150 psi. during the rebuild depending on who rebuilt the engine the shims under the buckets may be off causing a valve to be open when it shouldn't be. You can also do another self check to see if all cylinders are firing by pulling one injector clip at a time and seing if the engines rpm or exhaust note changes. Might want to keep a valve for the water close to the flush fitting incase it quits so you can turn it off. pm me if you need any more help. the yamaha 4 strokes are very tempermental with plugs.

  3. #3
    Quote Originally Posted by slociviccoupe View Post
    do a compression test to see if all are even and higher than 150 psi. during the rebuild depending on who rebuilt the engine the shims under the buckets may be off causing a valve to be open when it shouldn't be. You can also do another self check to see if all cylinders are firing by pulling one injector clip at a time and seing if the engines rpm or exhaust note changes. Might want to keep a valve for the water close to the flush fitting incase it quits so you can turn it off. pm me if you need any more help. the yamaha 4 strokes are very tempermental with plugs.


    Thanks for the reply slo! I put a gauge on each cylinder and they all read just under 60psi. I am gonna try and double check it with a different gauge tomorrow. The one I used has been in the garage for a while and I'm not too sure how accurate it is. I wouldn't think it would be that far off though. I also pumped air into one of the cylinders and the engine immediately tried to turn over until a valve opened allowing the pressure to bleed off. I would think that if a valve was hung open, it wouldn't seal enough to allow the engine to turn over? I expected to hear the intake or exhaust port leaking air, but that didn't happen until the air turned the engine over and the camshaft opened up the intake valves. Also, rechecked TDC and the marks on the cam and they seem to be correct. this thing is starting to drive me crazy!


    Thanks again!

  4. #4
    is this backfire on the hose or in the water? and were you able to pull an injector clip one at a time to see if it made any difference. and yeah compression has to be at least 80-100 to even fire in that cylinder and will have a lot of blow by. make sure you have new plugs and can even run plugs for the yamaha r1 motorcycle 2003 model range. check the coil pack on the side of the oil tank and make sure they are plugged in correctly and not backwards causing the firing order to be off. could also be a bad spark plug wire arching out causing a missfire and backfire. heck tps or map sensor could be bad. and lastly a cracked throttle body to cylinder head will cause poor running conditions if it isn't sealed or has a crack in it. Pull the injection rail hold the injectors and crank the ski. make sure an injector isn't stuck open and ake sure all fire independently from eachother and the same spray pattern.

  5. #5
    Quote Originally Posted by slociviccoupe View Post
    is this backfire on the hose or in the water? and were you able to pull an injector clip one at a time to see if it made any difference. and yeah compression has to be at least 80-100 to even fire in that cylinder and will have a lot of blow by. make sure you have new plugs and can even run plugs for the yamaha r1 motorcycle 2003 model range. check the coil pack on the side of the oil tank and make sure they are plugged in correctly and not backwards causing the firing order to be off. could also be a bad spark plug wire arching out causing a missfire and backfire. heck tps or map sensor could be bad. and lastly a cracked throttle body to cylinder head will cause poor running conditions if it isn't sealed or has a crack in it. Pull the injection rail hold the injectors and crank the ski. make sure an injector isn't stuck open and ake sure all fire independently from eachother and the same spray pattern.
    I am trying to start it in the garage without water or anything to it. It never actually starts and runs, it only backfires while attempting to start when the starter is turning the engine over. I will give that a try tomorrow after work and see what happens. Definitely has to be compression related...just not too sure what's causing the low compression now though. Still gonna recheck with another gauge to be certain.

    Thanks again!

  6. #6
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    Quote Originally Posted by enajizer View Post
    I am trying to start it in the garage without water or anything to it. It never actually starts and runs, it only backfires while attempting to start when the starter is turning the engine over. I will give that a try tomorrow after work and see what happens. Definitely has to be compression related...just not too sure what's causing the low compression now though. Still gonna recheck with another gauge to be certain.

    Thanks again!
    I just recently went thru a compression issue that turned out to be a stuck shraeder valve in the compression gauge hose. Take the valve out or replace it with one that has a softer spring. I was getting 60psi on all 4 cylinders of my SHO until I put the softer valve in. Now they all read 125.

  7. #7
    Quote Originally Posted by mikevv View Post
    I just recently went thru a compression issue that turned out to be a stuck shraeder valve in the compression gauge hose. Take the valve out or replace it with one that has a softer spring. I was getting 60psi on all 4 cylinders of my SHO until I put the softer valve in. Now they all read 125.

    Thanks Mike, I didn't get a chance to mess with it today. I'll try and get out there tomorrow and see how it goes. Just out of curiosity, does anybody know if the engine would still start and run without the engine temperature sensor being plugged in?

    Thanks again for the replies!

  8. #8
    Well, just a little update for anybody interested. Motor is out again and I took the flywheel cover back off. I'm pretty sure that somebody installed the wrong rotor onto it. I'm thinking the crankshaft might have been replaced with a used one and this different rotor came with this crank. The one currently installed on it has the number 6D3 and has about 5 raised areas for the pickup coils which I believe is for an engine with individual coil packs. The original rotor for mine should have 2?

  9. #9
    The rotor was definitely the incorrect one...now that the correct one is installed, the engine is up and running!

    Thanks again to those who gave me the helpful suggestions!!

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