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Rpm Tach Reads Double??

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racerx351

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Discussion starter · #1 ·
I rcvd the second fly wheel that was cut ... installed and the tach still reading double .

example. at 1500 rpm it reads 3000rpms .. crack the throttle and you are at 12k ( i wish it was able to rev that fast)



the original fly wheel was cut and re-welded by one machine shop. I was thinking that the magnets were damaged during the welding of the pick up.

The 2nd fly wheel was cut correctly not touching the pick up on the fly wheel.



here is the catch......... i have a stock fly wheel on the other st90 , I use that one and the tach is back to normal??????????????????



what to do next????????????





Tried moving the pick on the motor further away from the fly wheel ..bike will not start .

tried the new fly wheel and the bike is very hard to start it feels like the motor has to catch it self and than starts ... like if the new fly wheel is spun ... like a bad harmonic balancer in a small block chevy .. i hope i am making sense!!

thanks
 
#3 ·
Obviously the Tach reads somehow off the flywheel RPM and the circumference of the flywheel plays a role in this equation.



Let's look at this from a different angle. You are going to need to understand why it's reading double and need to figure out the wiring to see what wire is used as the tach pickup.



You can start by looking at the back of the tach and figuring out what the wires are for. You'll have lighting wires, power, ground and tach pickup. If you find what wire is the tach signal wire, it probably will be the same color at the stator assembly/ coil, etc.



I'll need to see some pictures of a stock flywheel and the modified flywheel to see how they are different.



Isn't modifying stuff fun?
 
Discussion starter · #4 ·
z88r,

it sucks if you ask me !!! lol but, I firgue that a new fresh cut correctly fly wheel will give me the tach back.............NOT!!! i wonder if inside the tach there is adjustments??

here are some pics... the weird part is that the bike is very hard to start with the second fly wheel....



Image




Image




Image
 
#7 ·
racerx,



I am not sure what that small pic is.



You have to compare the trigger size of the cut one and the stock one. Is the cut one smaller?



I had to recalibrate my tach because the trigger on the 125 flywheel is a long rectangle, like the stock ZB. The trigger on my 110 was a small round trigger, the size of an eraser head on a pencil. Maybe that is causing your starting issue and the higher RPM issue?
 
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Discussion starter · #8 ·
the small pic is a pic a fly wheel that has been cut ... the gpx fly wheel has that pencil earser tip looking like pick up...or small trigger .. i did try a longer pick up(trigger) same result... I think it maybe due to the amount of weight that it was taken off the flywheel that may be causing the rpm double readings!! i try another fly wheel stock form tach is back to perfect.. use the old cdi same ..double reading.. at this time i will try anything. next step will be taken the rpm tach apart and see about an adjustment
 
#10 ·
Here's a crazy idea. Put a stock flywheel back on. Since you aren't competitively racing and it's obviously driving you insane, go back to stock. It's not a huge gain in performance anyway.



Most electrical tachs run off the coil leads and not the stator assembly, so this makes no sense to me. All of these bikes have a waste spark system so there must be something in the flywheel that prevents the coil from sparking twice like the common waste spark setups.



You need more detailed pictures of the stock flywheel to show how it's changed.



Perplexing...
 
Discussion starter · #11 ·
maybe you are right!!.. but I would love to really know what is going on .. !! i will get some pics up so you guys can see how the fly wheel is cut etc. I am not sure if the weight of the flywheel will change the reading ???
 
#12 ·
It's not the weight of the flywheel. RPM is measured through the coil. All electronic tachs use the coil for a reading. However, with your motor they spark on the compression and exhaust strokes, and for some reason, the flywheel either suppresses one of the sparks and you machined something off that was needed. Take off the flywheel again and take some detailed pictures of the stator assembly as well. There has to be a logical explanation for all of this.
 
Discussion starter · #14 ·
i took a look at the 2 machined fly wheels... and the stock one not machined.



the 1st was was cut and the tip was machined in error by the shop they reweld the tip back on the flyw wheel and the bike started..no issues just the tach. I was under the impression that the re-weld of the tip damaged the magnets making the tach read double. i installed the stock i have tach was fine... so i had another fly wheel cut by a diff machine shop in florida and mailed to me .. the bike was very hard to start and when it did the tach did the same. the 2nd fly wheel that was cut was cut perfect.

if you were to look at the stock one it ionly has a tip (pencil eraser tip) and there is nothing else like other metal welded nor any other markings.



I need to get some pics up so you guys can see what's up!! I know it works of the neg side of the coil .. its just weird....
 
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racerx351
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