Topics

1

Tech Talk / Mikuni needle, part 2, taper start.

« on: September 03, 2024, 01:35:55 pm »

Regarding Mikuni 6-series needles, I have always wondered, " how much of the needle straight section is still available after the idle position of the slide" so I measured it. Mikuni quote a distance to the start of the taper for all their needles, but, how does that relate to throttle application?

These measurements were taken on my VM38 with right side idle adjuster (my left side idle adjuster measures differently, turns out it's an OEM Kawa slide.) I guess there is a bit of variation between the various models of VM carbs, especially when it comes to Chinese carbs, my Chinese carbs measured way different.

I took the measurements with the slide at 1.00mm from totally bottomed-out, my bike had the idle running sweet at .9mm but I've had some bikes that had the slide raised about 2.0mm at idle, although I think the idle mixture too rich and necessitated the extra air from a higher slide?

2

Tech Talk / Mikuni 6 series needle comparison graph

« on: August 31, 2024, 11:17:03 pm »

Hey Chaps, I have, in a semi accurate process, created a couple of Mikuni 6 series needle graphs, I only used the details of the needles that I carry, only because it would have been too busy a graph to have more info. I find these graphs easier for a quick comparison between needles.

I am not familiar with creating online graphs using Excel, Microsoft or Google-sheets, so I drew the graphs in Excel then joined the dots, I am sure there would be someone among us that could create a professional looking job? Ian Williams Tuning had an even rougher looking chart on his website, I figured I could make something a little more legible. It was difficult to squeeze in needle diameters of .01mm increments (but I did it) in a graph, I also did one in .02mm increments but it wasn't any clearer.

I hope some find it helpful, regards, John.



My quoted 6FL14 specs do not match the MikuniOz, or any other Mikuni seller's website, as after measuring, their dimensions are wrong, I did find a website listing that does quote, what looks to be correct dimensions, this website even has all the specs for OEM fitment needles!

3

Tech Talk / Twostroke VM jetting, just want to think out loud.

« on: August 25, 2024, 10:00:50 pm »

I am amazed, 50 years after owning and tuning, aircooled twostrokes, with VM Mikuni's, and thinking back then, that I knew what I was doing ...... ha! I think I may just get all this sorted before twostrokes are banned ;-)

After 40 years of building racing fourstroke fours, I am back playing with aircooled twostrokes again, the thinking, and methodology, of a 65 year old is so much different from when I was 15.

I'm enjoying juggling the various VM fuel circuits, in particular the needle straight section v needle jet size, many considerations that had never entered my head before, I feel sorry for my young naive self, I guess also all the young kids tackling life with minimal life experience.

I wonder if I will learn as much in the next 50 years? :-)

4

Wanted / WANTED: Yam fork bottoming cone.

« on: August 15, 2024, 03:19:52 pm »

Hey Guy's, I was wondering if anyone might have 1 or 2 Yam DT2 fork bottoming cones, that sit on the bottom of the damping rods, I think most early Yam twostrokes, DT, RT, YZA, MXA might run the same cones, or actually any bottoming cones with a 13mm ID? Could they be posted to Central Victoria? Thanks in advance.

5

Tech Talk / Yam DT1 front brake cam improvement?

« on: August 11, 2024, 05:27:39 pm »

Hey Guy's, does anyone know of a front drum brake cam that will work in place of the stock cam on a DT1 (and just about any Yam dirtbike from '68 to '76), the actuating pad between the brake shoes has a very gentle action, causing a spongy lever action. I guess I could weld a bead of weld across my stocker but I'd rather find something that will bolt-in from another model if possible.

6

Tech Talk / NGK B10E & B10ES

« on: March 29, 2024, 04:22:46 pm »

Hi Chaps, any know the difference between B10E and the B10ES ? They look the same.

7

General Discussion / Just checking in .....

« on: January 11, 2024, 10:15:44 am »

2 weeks no posts, just keeping things active :-) Come on Chaps, just talk anything VMX, I need my fix ;-)

8

Tech Talk / CR250M inlet port height different, why?

« on: September 08, 2023, 05:32:25 pm »

So here is another useless rambling post from me :-)

I've known for many years of the odd inlet port of the Honda CR250M, but I only just now wonder why? I guess I look at employing such trickery on another bike. With the top of the two inlet port windows at different heights, below is a link to the cylinder liner. I guess at a couple of the following reasons ....

* Added crankcase turbulence?

* Flow from each port directed in a different direction?

* With one port starting to close before the other, could it give a mild supercharging effect to the later closing window?

By all reports the CR250M was the fastest 250 in its time, the inlet port being the only thing that seems to stand out from the 250's, makes me wonder, hmmm.

https://www.ebay.com/itm/184383481671?hash=item2aee1ca747%3Ag%3AEcMAAOSwfs9fJG2q&fits=Year%3A1973%7CModel%3ACR250%7CMake%3AHonda

Feel free to offer your own crazy ideas :-)

9

General Discussion / Anyone know US buyers that can post parts to Australia?

« on: August 30, 2023, 10:55:04 am »

Hey Guy's, I want to purchase some parts from the US, the company that sells them does not ship to Australia, is there a company, or rider, that I could pay to purchase the required parts and post to me in Australia?

10

System problems? / Is security certificate going to be updated?

« on: August 08, 2023, 02:04:48 pm »

See above :-)

11

Tech Talk / RM125S Piston rings without notch for pin, eh?!

« on: July 15, 2023, 09:06:13 pm »

I have some NOS pistons & rings to suit '75 Suz RM125S, the rings (the .60mm thick ones) do not have a notch where they end at the pins, with the standard size rings fitted into a new 56mm bore, there is a 1.4mm end-gap. I think that when fitted to the piston, and slipped into the cylinder, a minimum of 1.2mm gap is required for the pin thickness, so the 1.4 would allow them to not jam hard against the pin when hot.

What are your thoughts of running such a wide end-gap? I was thinking of running .25 over rings, and grinding an angle at the ring end, to minimise the end-gap, but I worry that a .6mm think ring does not leave much metal to handle the heat?

Has this matter ever arisen for RM125S owners before?

12

Tech Talk / 1980 Hon CR125 head stud spacing same as Suz TS125... YES!

« on: June 02, 2023, 04:20:43 pm »

I was wondering if anyone has used a 1980 Honda CR125 copper head gasket, 12251-466-000, on a Suz TS125? Could anyone advise me of the distance from stud centre to stud centre? TIA

EDIT; Yes it is the same :-)

13

Tech Talk / Mikuni hex main jet thread !!!

« on: May 24, 2023, 08:06:41 pm »

Hello Chaps, I was looking for a thread-tap to suit Mikuni hex main-jets, according to the Mikunioz website the thread is 5.3mm diameter by .9mm pitch, what tha! After a bit of searching I am not having any luck finding a tap, anyone got any ideas? Me not having a clue about anything imperial, would there be an imperial measurement that would come close?

14

Tech Talk / Where can buy jet reamers 2.0mm to 3.0mm?

« on: April 30, 2023, 04:51:11 pm »

Anyone know where I can buy tapered jet reamers, maybe a set of 5 or 6 that range between from 2.00mm to 3.00mm (yes to open needle-jets :-) ), style as per the ones in this link:  https://www.misterworker.com/en/kukko/nozzle-reamers-set-12-pcs/59063.html?gmc_currency=8&SubmitCountry=AU&gclid=Cj0KCQjwgLOiBhC7ARIsAIeetVAJy33u6HZdY3VViFi2whChlZCXy1f5LJhZY4fv_kYDTA_Fm-fYAbcaAnJ7EALw_wcB  But up to 3.00mm, or maybe 2.80mm

15

Tech Talk / Needle jet still works with a closed throttle !

« on: April 28, 2023, 06:33:02 pm »

I thought I'd run something past you guys (what again!), trying to get my TS125/VM32 jetting fine tuned, I think (?) umm how do I put this into type .... ok I'll start somewhere.....

In neutral, rev the bike up, chop the throttle, rpm's hang up (probably around 1500 rpms?) like too lean (which it is), after aprox 20 seconds the idle comes down to aprox 800 rpm (nice idle), then finally conks-out.

I think, as the rpm is quite highish, with a closed throttle, just after closing the throttle, the engine is drawing fuel from both the pilot/airscrew AND the needle-jet, once the idle rpm can creep down, fuel stops being supplied by the needle-jet and then the pilot/airscrew has sole control.

So, in all my years I have never had a bike with this kind of issue, probably because I am basically starting from scratch, rather than someone else (the manufacturer) doing the hard work. I never considered that the straight section of the needle/needle-jet can be drawing fuel with a closed throttle! I guess only while the rpm is high enough to have air speed to do so.

While confusing myself, I had the pilot jet too rich and the needle-jet to lean, bike revs its tits off above normal idle rpm (lean), conks-out at normal idle rpm range (too rich) ..... all rpm affected, with a closed throttle !

Ok I'll work on my theory tomorrow, hopefully I am correct :-)