I've Always Lurked on here and picked up lots really helpful info regarding tuning & speed over the years and thought i'd pose a question myself.


has anyone previously fitted a 930 concentric with any joy, i remember picking up on that size from a thread over the years which i cant seem to trace back to now. and i bought a carb at the time which ive had sitting on a shelf ever since after some more recent mods i thought it maybe time to give it a go.

abit of background on the motor. it is a stock bored m21, currently ported and polished to suit the 26mm concentric on it, stock sized stainless valves which ive under cut for slightly more flow, ally head which i have welded and re shaped along the lines of a Harley K model, and lastly which i fitted in the week a pair of goldie cams (42/46) which have really picked up the pace for the old thumper. it pulls and rev's really good up and just past around 3/4 throttle but seem's to hit abit of a wall which has had me think abit more carb may help. it has always ran out of steam pretty quickly gearing wise so a taller motor sprocket is on the cards too. at the minute it tops off around 70/75 but like most things speed related you always want to try for abit more.

attached is a couple photos of the bike in question, it's a total bitsa but works to my eyes. built along the lines of a 1940s american flattrack/street bike. it was a M33 originally but after a trip to normandy a couple years back the old tired motor bit the bullet and i kind of took a backwards step with the sidevalve but glad i did as it suits the bike alot more, ive never been to precious about it (don't worry there's no NOS parts to be found here) using it as a daily bike whenever i get chance, the odd bit of a green laning & last year abit of sand racing on Sword beach of all places, it helds its own against a fair few early Harleys which was a pleasant surprise.

anyway enough waffling.

any help and idea's are appreciated.

Thanks
H

email (option): harleydp@hotmail.co.uk

Harley, There may possibly be a lot of purists who will disapprove of what you have done but I think it looks great. Sorry I cant help on the tuning as I am a Matchless rider. Cheers JT

Can't help either but it looks good. I like the Harley risers on the forks.

Hi Harley, I love the look of your bike. If the head is ported at 26mm, as far as I know there's no advantage putting a 30mm carb on. I would lean the plug and do a "plug chop" at full throttle and see what the colour is to find out if it's running lean. If it is you can go up a size on the main jet. If it's the correct colour or black, it doesn't need any more fuel. That's where I'd start.

email (option): horror@blueyonder.co.uk

That should say clean the plug, it wouldn't let me edit it.

email (option): horror@blueyonder.co.uk

Thanks guys, Luckily no WD parts were harmed in the making of this. The forks are actually Indian copies. Despite there bad press touch wood after all the abuse I’ve given them they’ve held up pretty good over the last 7 years. If I was piecing together a stocker then more than likely I’d have come across problems with them.

H

And the Guru who would've known in a flash doesn't frequent this forum anymore.
🙁

email (option): Fuzzyempire@hotmail.com

That's interesting that you're using the Indian made forks with success, Especially as the very subject came up earlier today, and even more especially as you are using your bike at above normal speeds and terrain.

I guess there are good and bad in everything......But how do you tell them apart:thinking_face: Ron

email (option): ronpier@talk21.com

If you search the forum just using the word "Concentric" you will find this subject has been discussed several times.
At different times I have run a 932, 930 & 626 concentric on my WM20

The 932 was ripped off a B50 , in haste as the 276 decided to become a fountain & I did not have time right then to sort it out.
With the 932 on the bike flew.
It accelerated up to 60 mph then just wound on to 85 mph.
Idle was unreliable and it drank so much fuel it made a rocket III look frugal.
Eventually I got jack of using 3 times as much fuel as I should, and having to hold the throttle on at traffic lights os I dregged up a 930.
Wearing that and tuned as best as I could the bike would run out to around 75 mph and got about the same fuel economy as an A65F on firebird gearing , This worked well as the person I rode most with rode an A65F on original off road gearing.
Again it gobbled up fuel . tended to over rev & empty the oil tank all over the bike via the breather.

Finally I forked out for a new 626, dropped the gearing back to standard and decided not to go on any runs that had highway or expressway riding in it
On the 626 it is back down to 65 mph but now idles reliably and no longer overheats drastically on long WFO stretches of road.
It will go up hills without going back to 2nd and is a much happier bike.
Apart from some work on the inlet ( done prior to me owning the bike,) I had done nothing other than to fit GS touring cams with the inlet retarded one tooth.
The instruction I got from a period racer was to retard it 1/2 tooth but I am yet to send the old buggered cams up to Ivan for building up & regrinding.
Space it tight because you have to add around 1& 1/2 to 2 & 1/2 inches of spacer to clear the magneto
That leaves bugger all space for an air filter so it runs firebird / spitfire ) pill box filter which has to be assembled on the carb as there is not enough space to screw it onto the carb. OTOH there is also not enough space for it to fall off either.

Can not remember what the jetting I used was.
The 932 was as per B50 T , Can't remember what the final jetting on the 930 was but it might be in the archives and the 262 was jetted as per B40

email (option): bsansw1@tpg.com.au

Thanks for all the suggestions and good words. After a chat with Ian I think we’ve come to the conclusion I’m at the limit of my messing around with the motor with out delving deeper into it and doing some more mods to the rotating assembly. So I think the plan for now will be a gearing change and keep using the hell out of it. That is until I get bored and rip apart again.

H

Wait till the winter and do it then...The God of motorcycle tuning says you must..:japanese_goblin: ..Ian

email (option): ian@wright52.plus.com