What limits RPM on clone engine?

itsid

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No load RPMS are 6040 right now and 5980 with a load. That's pretty good I think.
pretty good? no .... sub 100 is awesome!
but since we now know it's the coil limiting it's unfortunately not yet telling how close you are to the perfect gear ratio ;)
luckily that'll be solved once the new coil arrived :D

..coilgap can affect the strength of the spark
the magnetic field strength drops by the distance squared,
so if you are having a weak spark a small change can already make a big difference..
Since it doesn't appear you are having any issues due to a weak spark,
I'd say 0.4 is likely alright for your setup
(also heavily depends on the quality and field strength of the magnets in the flywheel of course)

Since a hotter spark also means a hotter coil I'd not change the coil gap w/o a good reason
like your coils seem to have a short lifespan and you are going through a lot of coils for unknown reason,
then a bigger gap might help you to keep it cooler,
well or having a weak spark at high rpms... then a smaller gap might be the answer
(I'd go for a slightly hotter plug first if the coil gap is within specs)

'sid
 

thedude306

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pretty good? no .... sub 100 is awesome!
but since we now know it's the coil limiting it's unfortunately not yet telling how close you are to the perfect gear ratio ;)
luckily that'll be solved once the new coil arrived :D


'sid
I need to edit this. I'ts more like 300 rpm. Which I still think is pretty good.

My "no load" was with the back of the snowmobile raised in the air. Spinning the clutch/chain/track. My load testing was with my boy on the sled doing a test run and looking at the recall for max RPMs. BUT, I was doing some clutch testing and removed the chain and set the snowmobile back down. With no clutch and not spinning any track I was getting 6250 RPM. So I got out the timing light again and it's flickering at 6200 but not at 6000. Hooked every thing back up and raised the sled and I am back down to the light flickering at 6020rpm. ???

Weak coil under load? I don't really get that. Or maybe that's just the inconsistency of a cheap coil? Or maybe heat?

Oh and I learned a lot about clutching my Max torque SS clutch. Had silver, green and black springs with light and heavy shoes and one set of heavy shoes cut in half. Found my best combo with a black spring with one coil cut off using three heavy shoes and three light shoes. Heavy shoes over the holes on the backing plate (it made a difference) Clutch clatter at 2600 and full engagement at 2800 rpm.
 

itsid

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My "no load" was with the back of the snowmobile raised in the air. Spinning the clutch/chain/track.
that's the correct way here.

With no clutch and not spinning any track I was getting 6250 RPM.
this is not... a free revving engine (no clutch no drivetrain) gives absolutel no information in terms of "useable max rpm"
since it can turn it's shaft with the help of nothing but the flywheel even if the actual combustion now delivers close to ZERO torque.

incosistency in the coil is indeed odd..
it shouldn't bother much about if the engine spins anything or not...
heat might be an issue of course... but again... where would that come from during the test.. that tiny amount of load (clutch/chain/track)
couldn't cause much additional heat really.
even if we're including the 200 additional rpms on the fanblades to blow the heat off.

My idea here would be resonating frequencies ....
engine vibration vs. the vibration caused by engine and the moving track
maybe the coil (or just the spark plug boot) has a tiny hairline crack that opens just enough at a certain frequency
to cause a weak spark.
tough to tell did you notice any resonance happening w and w/o the the clutch that also shifted slightly up?

'sid
 

thedude306

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Im getting shocked holding the coil wire/spark plug wire at high RPMs. Definitely an issue. Picking up coil and new driver for the track tomorrow and will sort it out. Also need new clutch spring for the max Torque SS Engagement is back down to around 2200. Sled won't move if you are in any snow. Worked great at 2600. More updates at the end of the weak
 

thedude306

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well it hit 6980rpm no load (just track and clutch in the air) with the new coil and no goofy stuff on the timing light. Hope it pulls into that 6500 on the snow. Also set the valves. It got darn cold here (-34C last night) so I pulled the motor again and I am making a parts run for a new driveshaft for the new driver that showed up. It uses a 1.25" hex shaft and I only had 1" Should have that sorted out on monday and then need to machine it to fit. Clutch and chain should be here friday from NR Racing.

Wife also made me a cool spreadsheet will all the gear ratio combos I have and formulas for RPM and speed.



new 5 tooth driver!



old driver and shaft

 

thedude306

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Everything is back in the sled and I am happy! It looks like after half a dozen WOT runs that we are going to be in that 6400 RPM range for max rpm. I am happy with that.

Now to start playing with gearing. I think I can get more out of the engine as I am definitely losing RPM due to load. Our test run was with 21/40 1.9 @ 38mph and 6440RPM but I am going to up that to 18/40 2.22 and even go as far as 18/45 2.5 We just want to be over 30 mph...

thanks for all the feedback.
 
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