Paul's Yamaha 3MA track bike

[Paul]

Avgas or pump fuel Paul?

its unleaded 99 octane

[Crank]

Why have you gone off avgas?

Availability?

[Paul]

im sure its all in the text earlier scotty but avgas is going to change soon to be part (if not all) unleaded, it's being phased out. i can get 99 at a few garages, most the supermarkets have it.
i am impressed with it and have had no problems running over a year on the TZ kitted 3xv which is on the road and avgas is impractical.
you get a lot of bang for your buck.
i have even got spare heads and CDI ready to change the TZ over.

[Paul]

I put locator pins for the heads
That'll give me a bit of surety when tightening the spark plugs.

[Louis]

Good work Paul, that will secure the inserts definitely.

[casal-fan]

Yes. Either 1.1 or 1.2mm. Can't remember. I fitted them a long time ago and have never changed them.
I'm listening, what do you rekon?

Hi Paul. Misted this post of yours.
Please, read this as only a consideration (another one), maybe check with your dyno man?
Since the 3MA race manual is available (a big thanks to our friend across the pond), it is for all the see that on all 3MA race models the air correction was reduced.
Another consideration, and I have been searching like crazy without conclusiv results, is that perhaps mikuni jet numbers do not correspond to mm.
The increse in cross area measured in mm is very disproportional - like if 0.5mm has X cross area, one could be inclined to think that 1mm would be 2.X cross area. This is not the case. 1mm cross area is much more then that.
So, if you have a 1mm air jet, and think it is equivalent to a #100 mikuni - not even close.
Important bit being that maybe, perhaps there is nothing wrong with air velocity in the carb, but the ability to supply fuel fast enough when the side is cracked fast because of the large air jet?

I tested a bit this theori with terrible results ... but also one major mistake, I discarted it without an adequated test. I tested with the bike on the stand just by feeling and listening to engine response while it was being reved.
A 3MA behaves very different being reved at the stand, and turning laps at the track.
Just my opinion

[Paul]

Thanks Rui.
I used an air jet at the time that worked best. I wasn't reading manuals or jet size charts, I was using my mini drill bits.
I fully understand the difference between area and diameter and how area increases exponentially as diameter is increased. But like I say I was testing, on the road, what worked best.
It does concern me that the bog feels like too much air and I have wondered if it's the air jet but think it's the venturi in the carb throat.
this single bloody thing (air jet) is so critical and playing with it affects other settings. I curse the Std carbs most of all. I do need to ride it more as I was getting the hang of it and it only bogged about 3 times. I am sure it's related to how quick you open the throttle and I have a shorter twist TZ unit which probably opens too quick.
I am open to thought and suggestions and am waiting to hear how Louis goes with his testing.

[James P]

...perhaps mikuni jet numbers do not correspond to mm...

That is correct Rui, but only for the hex (large or small) main jets, as far as I know . It seems that fuel inlet needle valves, pilot jets, air jets and round (large or small) main jets are all rated according to inside diameter (e.g. 100 = 1.00mm). If anyone knows different, please tell us!

I found a conversion table which may be of interest to convert Mikuni hex main jet "flow rate size" (same as Amal sizes) to actual internal diameter (Keihin, Dell'Orto etc.): http://www.ozebook.com/compendium/t500_files/mikuni.pdf

There is also a table from JetsRUs (http://www.jetsrus.com/FAQs/FAQ_mikuni_vs_dynojet_vs_keihin_sizes.htm), but it doesn't state which types of Mikuni jets it applies to (I would suggest it compares the Keihin main jets to Mikuni round-type main jets).

Regards,
James

EDIT: I have an idea that starter jets type VM17/1002 may also be sized according to flow. This type of jet was used for the power jets in the TM28SS carbs on the 2XT model and others. I have only size 65 of this type - the hole is a fair bit bigger than 0.65mm, but smaller than 0.8mm (this seems to be the reverse of the usual relationship between hex and round main jets!). Again; if anyone else has any insight, please let us know!

[Paul]

After a long time together. SOLD to a chuffed enthusiastic new owner.

[fozzy]

Anyone we know paul

[Paul]

Not a forum member.
A guy who's been off 2 strokes for a while.
It couldn't have gone to a better bloke.

[fozzy]

Not a forum member.
A guy who's been off 2 strokes for a while.
It couldn't have gone to a better bloke.

well it could of...... me 

were hope to ride this weekend paul, if you fancy a fix of castrol