Home Forums TaG Rotax Max FR Max shuts down and wont restart.

This topic contains 3 replies, has 2 voices, and was last updated by  Jim Giesen 2 years, 2 months ago.

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  • #68053

    Jim Giesen

    Hi guys. I have a FR Senior that I rebuild after a rod failure and its ran 6 races (dirt cart oval track) with out any problems.  Just for the hell of it I removed the AKA exhaust restrictor and went up to 170 from 160 on the main just to see how she would go against the KT100’s.  It ran like crap, pull like a steam train away from the pack into the 1st corner on song then starts to go off song some times it will have a little back fire but feather the throttle down the straight and she would come back on song as she hit the corner then pull away again.. removed the carby and stripped it down no faults found, checked the pump flow was good, (fresh rebuild), plug was black changed the main to a 165 same problems. Engine temp was stable around 92deg C, same fuel and oil used before 98 RON castrol TT oil. Found the exhaust had come loose from the ball due to broken spring..  caned that night..  Next meeting same problem went fatter and leaner on the jets no difference..  End of season..  I did a check on the top end no problems but fitted a new ring.    On the test track she would now pull but then randomly  die or if you feathered the throttle she would pick up and come on song. When she would die you would wait for 4-5 mins and she would start 1st kick and run like there is no problem.. she would shut down hot or cold. When she is dead she will crank but not fire. Checked all connections and checked battery voltage at coil , over 12 V  then she would start again.

    Replace the battery, coil assy, exhaust system, went through the loom with a fine tooth comb, new main switch. rechecked the carby and found another sunk float (3.6) Thought maybe It was a problem. Gave the carby a “hot” ultra sonic bath and rechecked the reed valves. Went to go racing last night and found fuel in the pump pulse hose, replaced the pump, cleared out the excess fuel in the case and she ran and idled (not done that for a long time) Son went out on the track did 3 warm up laps starting at ROF then screamed passed the field by the 1st corner (all other karts are KT100s). got to the next straight and she shut down….. refused to restart.  The only way I can get it to fire is to hand choke the carby. will fire lean out then die… rechecked the pump, OK, drained the bowl and was full…  Sounded like I had low crankcase compression…. Called it a night and ran the Yammies for the night.

    Today she fired straight up.. ran for about 10 mins off idle then started to stumble then die.. hand choking was the only way to to make her fire.. I sprayed carby cleaner down the throat as she was dying made no difference she would just shut down..  Hand choke and she would fire again. Checked the plug (new) and it was wet…. have taken the engine off, sealed the crankcase off and done a vac and pressure check. no problems.  When she shuts down and you hand choke over the carby you can feel a strong crankcase suction.

    Ive run out of ideas, I work on 2 strokes for a living but this one has kicked me in the guts..

    Any one have something that i may have over looked no matter how stupid it may seems….    The TAG yammy is looking good to replace this FR.


  • #68128

    Marshall Martin

    Check the plug on the crank sensor.  Very close to an issue I had one time.  Looks plugged in but was just short of full plug in.  Push the plug until you hear it click.



    Marshall Martin

  • #68130

    Jim Giesen

    Thanks Marshall. The plug was locked in. I did remove the sensor tonight, there was very fine metal fillings on the magnet…  scary. The sensor has strange marking like a crack. Have 2 spares will refit and try again.

  • #68566

    Jim Giesen

    Fixed!!!!   Faulty crank sensor. Replace sensor and ran on the track for 1/2 hour without any problems. The sensor tests fine, put it on a scope to check wave forms, OK . heated it up and was getting a variable wave form.

    never going to get stuck on that fault again…. :)

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