TM 400 Full Circle Crank ? ?

[pokey]

Read someplace last night that the lobed crank were used on the TM400 so it would rev its arse off . and it did .

[jimg1au]

flywheel weight make a lot of difference to motors.heaver flywheel less revs more lowdown power.my mates son rides eduros and he was very good.they used to change rotor flywheel weights to make the yz250 4t behave different for diferent races.old english trials bikes had heavery flywheels to their mx and road race bikes with the same cc.my nephew has a 1000 bmw that revs it tits off as the flywheels have been lightened.i once pulled a second gear wheelie on it some 15 years ago.he still has it but it is stored away with all the other bikes he has ever owned.
just my thoughts on flywheels 2t or 4t same result for light and heavy
cheers
jim

[Doc]

yup, loads of variables and not forgetting there were various weight aftermarket flywheel weights made in the US for the TM series back in the day that simply bolted straight on top of the inner rotor. I do believe however the main (engine) problem the TM400 suffered was the light switch advance curve of the ignition that seemd to apply itself at random and was totally unpredictable.

[Lozza]

The crank shape has little to do with the dynamic of what's happening at the wheel(correct Maico31). This full crank will have a greater mass than the bob weight crank, it does 2 things simply lessens the rotational speed variation of the crank in 1 cycle,ie effectively lengthens power stroke and port durations.The higher inertia rating 'carries' more rpm through corners. GP 125's fitted with them(IF you can afford one) dataloggers show the rider on the power 3-5m earlier every corner,and why VHM charge 2000Euros for one. Make more top end HP (from the lengthening of durations)as well, a TZ125 fitted with one went from 47 to over 50HP. Some tungsten slugs in a bob weight crank will achieve the same effect. The greater volume in the  in the bob weight crank gives more revs, less volume more bottom end.This is the reason 'lightened' cranks never work very well at all.

 

[suzuki27]

Good to have your input back Doc. I like the description of the a " light switch advance curve of the ignition that seemed to apply itself at random". That pretty well describes the 'Cyclone". I was lucky enough to have never ridden one. Some of the riders of these bikes must have went in search of smoother power after falling victim to a random act of violence from the mighty TM. Do you know if JP Morgan made changes to the ignition timing Doc ? I am going in search of parts diagrams for the TS 400's now!

[Doc]

ta Suzuki27, I can't remember JP Morgans bikes details as it was a little while ago and I was looking for an answer on the crank and where it came from but I'll go back through the emails and see if other mods were mentioned. There is a largish article on this bike somewhere from memory but that fails me too damn it The Cheney Firko mentioned earlier 'appears' to run a TS185 (same item as TS400) flywheel, TS185 stator and PE175 CDI. Have not ridden the bike but from latest reports it fairly honks will go hunt up that email

[Doc]

no other info sorry..here's the reply but again, all I asked about was the crank

Doc,
   I have two donor motors in my attic with full circle cranks. One is numbered TM400-188##, possibly had a full circle crank installed by previous owner, and TS4003-25759. Not knowing the years and modsels, this is all the info I can pass on. The later pork-chop TM400 cranks weigh the same as the full circle cranks. Good Luck, JP Morgen.

Not sure I could sleep at night knowing the was a TS400 motor in the attic above me..best left in the basement I'd reckon!

[Doc]

seek and ye shall find..scroll down the page and the workings of JP ignition is revealed

http://100megsfree3.com/ahrmanw/Machines.htm

the '76/'77 TS400 heads are apparently the way to go

[suzuki27]

Thanks for that Doc. It is interesting that the "Pork Chop" and "Full Circle"  cranks are the same weight. I wonder if there has been any Poltergeist activity from the attic that houses the TS/TM 400 engines?  I will get on and read the info you rounded up.  Firko might have the makings of a good article for VMX with his project TM400. For a bike that is widely known and feared in stock form, I think the transformation to the smoother, better handling B&S framed version, would make good viewing. Cheers.