March 10, 2014 at 12:22 pm #23559
I have tried looking at old threads but haven’t found anything that addresses my problem exactly:
I recently bought a used 2009 rotax max, which seems to run well, but at about 11,000 RPM or so the Mychron usually drops down to around 5,000 or 0. So, it doesn’t allow me to figure out my max RPM and it also messes with laptimes (the mychron thinks it’s shutting off).
This is with the RPM lead correctly placed, and with a new battery in the Mychron. Furthermore, I had the exact same setup with my previous rotax and it had no problem reading.
My local shop thinks that this is usually a sign of a weak spark coming from the coil. So, I’m trying to figure out what I need to try to change in the electrical of the kart to get a stronger spark. I measured the voltage of the battery after a full charge prior to going on the kart and it was slightly over 13v, and it starts the kart without any problems.
I tried to go through all of the connections and they all seem fine, including the leads on the bottom of the coil. I will check them again.
What should I try first? I was thinking of trying a new On/Off Switch, or a new lead from the coil to the plug. Or could it be the loom or the coil? I will also check all of the grounds (the coil has a ground, and the wiring loom ground is connected as well).
Any help would be appreciated if anyone has had a simlar problem. Thank you!
March 10, 2014 at 2:56 pm #23570Bill BoxellParticipant
Check the Maximum RPM setting in the gauge.
March 10, 2014 at 3:30 pm #23573
First, Bill has a good idea. Won’t argue with your shop’s logic but it should have given you a place to start. I’ve had this happen to me. After downloading the data from a MyChron4 at the end of the day, noticed that I had one more session stored than the number of times on the track. Found that the gauge thought it saw zero RPM’s during a 25 minute race. The only change made for the next race was to wrap the RPM sensor tighter around the plug wire. Ran the rest of the season without a problem.
You could try to tighten the lead around the plug wire and you could add an extra wrap of the lead around the plug wire. There is the possibitity that the RPM wire is damaged or theere is problem with the guage. Call AIM. Their service is excellent.
Don’t think it could be the on/off switch unless your engine is cutting out. From your description your engine is running and just the gauge is giving a false RPM reading. The spark plug wire is part of the coil. Now you’re talking bucks.
LAD Specialties customer / tony kart / rotax / kt100
March 10, 2014 at 6:10 pm #23582
March 11, 2014 at 2:53 pm #23637
What I was referring to is page 10 of the user manual: http://www.ogracing.com/images/tech/MyChron4-user-manual.pdf
Looking at it again, it just says that it is “sufficient,” but there’s not a reason not to wrap the lead around. I will try that, and it seems likely that it will pick up the spark doing that. I was mostly concerned as to why the spark would be weaker than normal/ my last motor. When I first ran this new one, it was cutting out a little bit, so I cleaned up the leads on the bottom of the coil and bent them slightly so they would get a better connection to the harness plug, which improved how it ran. I thought that this might be related. But, if it seems to be running good I will just forget about it.
March 11, 2014 at 4:19 pm #23639Jim WhiteParticipant
Check and clean all the ground wires too.
March 11, 2014 at 5:35 pm #23644
IMO, in this one instance, the manual is incorrect when it comes to a Rotax. Even two wraps of the rpm lead around the spark wire may not be sufficient. I have three. The rpm lead is only measuring flux changes as the coil discharges. Don’t quote me on this, but the Rotax coil wire may have better insulation, hence a weaker signal to detect. If your engine is stumbling at high rpm’s, you might have a marginal coil, but doesn’t seem to be the case.
There must be other Rotax guys out there running a MyChron on the Forum. It would be nice to here from you on how you have the sensor attached.
LAD Specialties customer / tony kart / rotax / kt100
March 13, 2014 at 3:12 am #23716Chad LandersParticipant
I’ve got Mychron 3 plus, the one with the box. I’ve also had a standard Mychron with out the box on a Rotax. I have my rpm wire wrapped around the plug wire 3 or 4 times and then zip tied. Never had a problem reading RPM unless the settings on the Mychron accidentally got changed.
Try wrapping you RPM wire around the plug wire first and see if that solves the Mychron problem. If if doesn’t then I’d trade Mychrons with someone. If that doesn’t work then it would more than likely be the Rotax.
I had a weird electrical issue with my Rotax. Sometimes it would start and then others nothing. It would sometimes die taking off on the grid. Ended up being the ground wire on the coil relocation bracket. I recently removed it and had the ground wire on the opposite side of the coil bracket changed it to the other side and worked perfect.
Also check the crank sensor plug on the bottom of the backside of the motor. I had one that came loose and it would cut out badly at higher RPM’s.
March 10, 2014 at 4:30 pm #23574
Thanks guys. Unfortunately it’s not the max RPM setting, I checked that. Sometimes it will read 13,xxx, just not consisently, and if I look at the min RPM for each lap, it’s typically 0.
I don’t think it’s the Mychron as I had it on the previous engine with no problems, and just swapped the engines out. However I will switch with a friend’s mychron and lead and see if that helps.
The mychron lead is not wrapped around the spark plug wire– it is just attached using the plastic clip – I believe that is how the instructions said to attach it for a rotax, and that’s how it worked on my old one. I’ll try wrapping it around a few times to see if that helps, unless there’s a reason not to do that.
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