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Messages - Momus

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Tech Talk / Re: Fork leg straightening
« on: Yesterday at 07:45:14 PM »
A straight edge and eyeball is all that is required to check the stanchion.

Tech Talk / Re: Fork leg straightening
« on: January 15, 2018, 11:28:49 PM »
Look for the rub witness and hone the leg with a brake cylinder hone if only slight. Other method would involve sliding the leg over the straight stanchion and heating to relieve it.

Tech Talk / Re: Measurements for side-bleed expansion chamber?
« on: January 13, 2018, 02:25:40 AM »
On the practical side old steel bicycle frames are a good source of relatively thin wall tube for this sort of work.

Tech Talk / Re: Fork leg straightening
« on: January 13, 2018, 02:23:04 AM »
Ian at Detroit Flame Hardening in Moorabbin straightens steel axles after heat treating; he may do your stanchion.

Tech Talk / Re: Measurements for side-bleed expansion chamber?
« on: December 22, 2017, 09:05:28 PM »
I cant see that there could be much (any?) difference between a “side bleed” and an “internal stinger” that starts where the side bleed would exit. (You could even bend the stinger so the entry was near the edge of the chamber, where the entry to the side bleed would be) Externally, the second option looks standard, just an apparently short stinger (some hidden in the chamber)
Yes, there shouldn't be much difference, but I suppose it depends on whether one wants the silencer to stay in the same place. If so, the "internal stinger" arrangement provides an extra length of tube which is nearly straight (and hence as short as possible), while for the "side bleed" arrangement, the extra length of tube cannot be anywhere near straight (so must be longer)...although I wonder whether this admittedly small difference in extra length would have any detrimental effect :-\ .

I should point out so far that I am satisfied that both "internal stinger" and "side bleed" designs should provide significant noise reduction with little or no effect on performance. The only question which remains is the necessity (or not) of increasing the diameter of the stinger to compensate for the increased length (in order that the cylinder does not retain any significant extra heat).

A suggestion has been made to me for converting an existing "normal" pipe design (with 25.4mm ID stinger, about 380mm long) to the "internal stinger" arrangement, whereby the internal portion of the stinger would be made from 28.6mm ID tube (about 250mm long), but the external portion would still be 25.4mm ID and retain its usual length. As I lack expertise in this area, I must ask the following questions:
a) Would the proposed increase in length require an increase in stinger diameter to prevent excessive heat retention in the cylinder?
b) If so, will the suggested stepped design achieve that objective?
c) If an increase in stinger diameter is required, is it better to make the stinger of uniform ID throughout, rather than the suggested stepped design?

AG Bell offers some handy advice about stinger lengths and IDs in his book (120-280mm long and 26-28mm ID for a 250cc cylinder), but these measurements must only apply to "normal" stinger arrangements because he makes no mention at all of any alternative stinger design. A brief look through one of my other two-stroke tuning books (JC Dixon) revealed some info on both "internal stinger" and "side bleed" arrangements, but annoyingly no advice is given about length or diameter (other than to effectively state that diameter is very important and length is not so important).
Jennings' recommendation for stingers states diameter to be 58-62% of lead-in (header?) pipe diameter and length to be twelve times whatever the stinger diameter is. In my case, the ID of the stub manifold at the cylinder is 44mm, giving a stinger diameter of 25.5-27.3mm and a (nominal) length range of 300-330mm. Of course, Jennings' experiments with internal stingers were carried out while keeping the stinger length constant - only its position was varied.

This comment over on the Penton board may give some leads...

"...Ollie McKagen built some wild pipes. He also designed and fabbed some wild and crazy suspension concepts. He was the smartest/most "out there" guy I ever met...I saw him weld pipes with the "internal stinger". He was extra diligent to ensure the backflow went into the center of the pipe to provide bottom end grunt. He wrote some articles for Dirt Bike or Dirt Rider. He patented several items too. There is some info on him on the Eurospares website...
Thanks for the reference. I have had a look at the Eurospares web-site, but it only deals with Ollie's suspension arrangements (all the bikes in the photos are four-strokes). A few brief Google searches did not reveal any results relating to his internal-stinger pipes, but there were a few more results relating to his suspension arrangements - most particularly his "funny front ends".

Unfortunately I ran out of expertise on this topic a long time ago :( . Also I have to go and lie head hurts :-[ . Further contributions from anyone knowledgeable, experienced and/or interested are welcome!

Thanks & regards,
No one knows the answers James. Make your chamber with a flange, gland or spigot at the end of the baffle cone and test variations. If you do this thoroughly you will probably be the world expert on this sort of empirical testing on your model engine.

Yamaha / Re: YZ125K Rebuild
« on: November 23, 2017, 08:47:22 PM »
I'll have a look tonight Lozza.

Stator: 1442    54/A/18
Coil:    105458
Rotor:  940 - with a 3/4" bore from the B50 crank.

Yamaha / Re: YZ125K Rebuild
« on: November 23, 2017, 02:52:36 AM »
I'm going through something similar having been given a PVL analogue ignition supposedly to run a 500cc 4 stroke single, that when correctly i'd, was an 1800 windings 2 stroke unit for a kart.
PVL's site says, at least in regard to the analog systems " the approval of an ignition coil/stator unit combination must be registered by PVL. We urgently discourage random combinations with a view to obtaining a different ignition curve, as we do not know if the electronic components are compatible..."

I've fitted a couple of smaller PWK's and found same deal with slow running and pilot passages. Also found that the seal of the jet block and the passage alignment might need a blueprint. These were OKO branded units.
Those will be the Chinese rip off Okos, saw a thread on how to pick them. Real Okos have Taiwanese flag on box is first clue, next is jets actually have markings
Have been running a real Oko for years on my Sherco & it's as good if not better than a Keihin PWK
Yes the OKO carbies with provenance are good and the cheapies easy enough to get right. I am using the sub 30 mm size which are very compact and ideal for smaller 4 stroke fitups. Benchmark for these though is the Keihin FACE.

I've fitted a couple of smaller PWK's and found same deal with slow running and pilot passages. Also found that the seal of the jet block and the passage alignment might need a blueprint. These were OKO branded units.

General Discussion / Re: BSA B50, Cheney, CCM Pre 75 Reference.
« on: November 10, 2017, 05:13:03 AM »
If you are planning on building your own frame Marcus, make sure you show us with pictures ..... lots of them.

Have you looked at Faber Frames ? Local-ish to me and they are very very nice. Not CCM or Cheney, but still good.

Thanks for that. I spoke to Howard and Miles about their frames a few weeks ago and agree they are well made and with the right provenance. Not wanting to be critical of any of the blokes, but build and delivery time from the UK is an issue. I have the bender tools and other gear so will give DIY go.   Pictures will  start with a shots of a few lengths of steel tube, a headstock tube on my fixture and a set of cases.

I have picked up another B50 engine for core parts so will be able to keep the existing matching numbers bike I have intact. Despite the rear frame stays having been butchered it is in better knick than I thought.

General Discussion / Re: BSA B50, Cheney, CCM Pre 75 Reference.
« on: November 07, 2017, 12:48:34 AM »
It's worth asking Mark, just in case he has one in stock. If you are going to fire an email off, do it today as they are both off for a couple of weeks sun catching .....

Grouty, received the following from PES:

"With regards to the 73 frame, our are slightly different to Grouty’s due to strength issues. C500 is the part number and costs £2500.00 UKP and a picture can be found on our website.

With regards to frames, we do not take orders or deposits because some people get upset if dates are missed, but if someone expresses an interest we will make it as soon as we can and if they still want it, it is theirs."

I have some '73 Cheney dimensions now so will have a look at that.

General Discussion / Re: BSA B50, Cheney, CCM Pre 75 Reference.
« on: November 03, 2017, 01:50:26 AM »
Thanks Grouty. Interesting information about the frame work.

I have a swag of PES and NEB engine bits for my build. I didn't make a direct enquiry of Mark but I gather from his postings on the B50 page  he is not that keen on making frames. Given the time line and that I fabricate chrome moly tube at work I might have a go at a replica frame over Christmas. Tidying up the oil system appeals and so does the extra space gained for induction using the short spine.

I've been wondering if anyone has converted a B50 or B44 etc to wet sump to do away with a lot of the external plumbing?

Not sure what happened to my photo, thought I had it sorted for free hosting.

Cheers Marcus

General Discussion / Re: BSA B50, Cheney, CCM Pre 75 Reference.
« on: November 02, 2017, 07:01:40 PM »
Ok, I've been wrestling with Photobucket and seem to have their slightly changed arrangements pinned. Thanks for your photos and offer Grouty. Nice bike you have built and very much along the lines of what I have in mind.

My B50 shown came with the lay down shock modification to the frame which has been powder coated.

I've spoken to CCM/B50 replica frame makers Joe Maxwell and Simon Cheney about their frames and had a look at a couple of other lesser known and IC efforts.

Joe Maxwell informs me that his CCM 1974.5 bikes have 6.5 to 7 inches of rear travel, so not kosher for Pre75.  A Cheney, or replica,  appeals; low, light and simple and with the winning history and no fuss design approach.

General Discussion / Re: BSA B50, Cheney, CCM Pre 75 Reference.
« on: October 26, 2017, 05:14:13 AM »
Thanks Alison. That question and subsequent trip to the archives just cost me about 110 quid.  I have finally invested in a copy of Bill Lawless' 'Rolling Thunder' CCM history courtesy of Ebay.

Long story short is I've been collecting B50  bits for a few months now and am deciding whether to build up a CCM replica, a Cheney or other special, or stick with a done up B50.

General Discussion / BSA B50, Cheney, CCM Pre 75 Reference.
« on: October 25, 2017, 05:03:29 PM »
Can anyone point me towards a good photo and text reference for the various Pre 75 BSA B50MX, Cheney, CCM and other specials from the era?

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