F81M gear set question

[Doc]

I've been offered a  3rd gear set from an F81M. A distinct prob I noticed with my F11 was the big gap between 2nd and 3rd. My research shows the F81M and the F11M as well as the F11 all used a 19T/24T combo is this correct?? do the F11M and F11A as well as the F81M share the same ratios? What can I do to fix said gap between gears on the F11? thanks

[JC]

Doc, 

Yes I know the problem too. I have those specs - I will look them up & get back to you.

Gear ratios are lots diff between F81M & F11M. F81M has unusually large jump from 1st-2nd for a racebike (35% as I recall) & close spacing for rest, esp 4-5th.

F11M has much better spacing - almost too close for a bike so torquey.

F11 trailbike had 2 diff sets of ratios - early ones wider. Closer from B model onwards (above #16500 as I recall). Later ones much better. I have wondered long about how to close the gaps since my F11M project has std F11 bottom end. Using the later ratios is much better.

As I recall, early ratio spacings are 35%, 29%, 23% & 17%
Later ones are 34%, 25%, 17%, 17% (not too dissimilar to F81M as I recall)

Will get back to you w actual specs. Nice to hear the F11 is getting some track-time by the sounds. Steve Cox from Nth NSW used to race one too. He likewaise said he was often caught between 2nd & 3rd & I know he had the earlier engine. I put him onto the later ratios, but don't know if he ever followed thro on it. I've since lost touch w him.

[Doc]

Thanks John. I have access to some later F11 bits so I'll look into the swap. I still have the literature you sent me a few years back also and this is great reference I wish I could say it's getting track time but that's yet to come. Very very close but I still haven't been able to get it fired up since fitting the F11M ignition. Might have to go down and take another look now, heaps of fat blue spark, loads of fuel but still not a whisp of blue plume can't be much. Maybe I've the stator bolted in the wrong position or something stoopid cause it was running fine..errr..before I messed with it that is also, you may know, is there a smaller capacity Kawa tank that will bolt on or will I simply have to make something else fit, the stock tank/seat are a terrible shape although it looks quite okay visually

[JC]

Doc,

F81M ratios are 2.45, 1.6, 1.26, 0.95 for gaps of 35%, 21%, 13.5%, 13% ie, wide, good, very narrow, narrow gaps. (Rather strange gaps!) Ratio spread 2.58

F11M ratios 2.23, 1.625, 1.26 1.05, .91;  gaps of 27%, 22%, 17%, 13% ie good gaps/ratios. Ratio spread 2.55

F11A ratios 2.5, 1,625, 1.15, .91, .75;  gaps 35%, 29%, 21%, 17% - wide ratios. Ratio spread 3.33

F11B/c ratios 2.23, 1.47, 1.10, .91, .75;  gaps 34%, 25%, 17%, 17% - in-between ratios. R/spread 2.97

Even if you stick to the F11A ratios you'll find the gap less noticeable if you gear down & use the top 4 ratios, instead of the botom 4. (Start in 2nd gear) With trailbike gearing like F11A I doubt you'd ever get near 5th gear on an MX track. Gear 1-4 ratio spread is 2.75; gear 2-5 spread is 2.17 on F11A. That effectively makes a fairly close ratio 4 sp box which should be quite useable on an MX track. And you still have a very low 1st gear for riding thro the pits (&/or trail riding)

If you did the sanme w the F11B/C box, gear 2-5 spread is only 1.96, which you should also get away with. By comparison, a 4sp maico ratio spread is 1.99. Yr right in the ballpark.

In my experience, 5sp MX close-ratio boxes are actually little/no advantage for most guys on most MX tracks cos most guys start them in 2nd gear anyway & never use 1st on the track. On tracks w a loong straight they are no doubt an advantage. Or if yr name is Joel etc.

I doubt if the F81M gearset would fit an F11 box (tho I haven't checked part nos). The F81M/F9 is a much more massive engine/gearbox & my guess is the gears are wider. Even if it did fit, you'd narrow the 2-3 gap to 22% (from 29%) but open up the 3-4 gap to 28% (from 21%) which would bog the bike even more on that gear-change. (Gaps need to be closer the faster you are travelling)

[Doc]

Thanks a heap John. I'll have a play with the final ratios when I get her running and gear it as a 4 speed as you say. It was just a passing thought as I was offered a nos F81M 3rd gear set for $20 and figured there may be some use for it if the gears would interchange. When I checked the ratios in the parts books it got more confusing. I've ridden the bike before but I've since gone up 10 teeth on the rear and as I said fitted internal rotor ignition. I'd like to build a pipe and fit a bigger carb/airbox and go the 'milder' porting specs from the info you supplied. I've asked this question before but I'll ask again, the bike I have was fitted with 520 chain and sprockets when I got it, I want to go 13T on the front but the only sprocket I can find to match is the original 525 pitch, do you know if there is a sprocket from an earlier or later model in 520 pitch that will fit or will I need to have the 13T 525 machined back?

[cyclegod]

Here's a nice pipe Chris...

http://cgi.ebay.com/ebaymotors/NOS-TORQUE-ENG-EXPANSION-CHAMBER-VINTAGE_W0QQcmdZViewItemQQcategoryZ35596QQihZ011QQitemZ320292152595QQrdZ1QQsspagenameZWDVW

[Doc]

aye, thanks CG! if that pipe were local or specifically F11 I'd pounce but being as it's not exactly the pipe I want (F8/F9 350) and knowing it's going to cost $$ to get it here makes it a dicey proposition that may not even fuction correctly. I have all the dimensions of the cones I need but I need a sheety or someone to roll them up and tack them so I can piece it together on the bike for a snug fit. What I really need is a small guillo and a rolling machine (easy to make) and I could make my own cones without the crease lines like I had in previous attempts where I rolled them by hand..hmmm..I wonder  

seek and ye shall find!
 http://cgi.ebay.com.au/Sheet-Metal-Roller-2-5mm-Sheetmetal_W0QQitemZ290256472317QQihZ019QQcategoryZ35625QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

although it's near double the price of the same thing on offer from the US It only has to handle 0.80 thick cold rolled sheet and be adjustable both sides. With a bit (a lot) of trial and error, presto!..we have cones!! need to hunt up some more info 

[JC]

Doc,

I don't know. I was going to ask you if you found a 520 sprocket that fit. If I get a chance I try whatever I've got in the shed.

[Doc]

eh, well I do have 520 but no idea how, someone mentioned there was an aftermarket 520 kit you could buy. I'd reckon between the KLX/KDX and KX there'd have to be one that interchanges to get a front but the rear could be a prob

[firko]

I'm the current top bidder on that pipe. Just what I need for the F81M Hindall.

[JC]

Yes it is too, Mark.
Just what you need. Hope you get it

[firko]

Bt coincidence, the seller of the pipe is Bob Neilson, the bloke I bought the B&S from. I didn't realise it until he emailed me to tell me he hoped I win it.
 BTW John to answer your email question, all of my stuff including the B&S and Jonesys stuff is still in Long Beach, ties up by a container backlog. If I win the Kawa pipe it might just make it to Longo to catch up with the rest of the shipment.

[Doc]

  I was going to take a stab on the bell with that pipe Firko but I won't now. I emailed the seller and postage was very very reasonable as seen in the online reply. I have my doubts it would work on a 250 so best leave sleeping dogs lie..go to it big fella! I'm seriously hunting up a roller, been talking to a few sheety metaly type mates and got the good oil for my needs PS that F81M gear set is still on ebay too Mark..now reduced to $7.95 each if interested?

[Doc]

oh and I know this is a looooooooooooooooooooong shot but I'll ask anyway since Ihave been searching for many moons with no joy..my holy grail is an F11M clutch cover, dead or alive I'd really really like one!

[JC]

Doc,

Somewhere you asked about fitting another (smaller) tank. I don't know if they'd go staright on but the 71-72 F6/F7 tank, G5 & G4 tanks are all the same shape but smaller/shorter. 73 & later F6/F7 tank was different shape (& quite ugly in my view)

If you want a KX shaped tank, the KS/KE 125 & KE175 of mid 70's is almost same shape as KX (& considerably smaller than F11)

Wrt rear wheel swap, I've been thinking of using a 130dia brake/wheel on mine, mainly cos they're a fair bit lighter & have ample strength & braking power. In fact, thats what the japs used on their 250s in later 70s.  I'm not convinved you'd lose much if you lost the floating brake. If the point the rod connects to the pedal is over (or very close to) the geometry of the system is such that you effectively have a floating brake. As I recall, the std F11 setup is very close to the pivot. The works bikes didn't have the foating brake, & w'out it you have more room to tuck in a down pipe. If you wanted to keep the floating brake, the Yam 150dia hubs are very similar & many of them have floating brake setup. Most people wouldn't notice it wasn't original.

Yes, I'd love an F11M clutch cover too, as well as ign cover & sprocket cover & head etc