Trials Australia Forums

Strange things happen to the photos. They double up and put the intended photo the end.
When you go to edit parts are missing and you have to reconstruct it. Other parts are showing on the forum but not in edit.
I'm not going to corect order you will have to figure it out

I have a respect for some thing that looks standard but not behave like it.
To the casual eye it looks standard


I've been putting some thought into these and I only have theory
The JC uses a 5 hole emulsion tube (haven't looked at the needle profile).
The C uses two large holes at the top.
The JC has a different end muffler.
The fuel will be finer atomised on the JC. The fuel will be lumpy for the C.
The JC has a 140 main jet the C 240.
The C has a really strange jetting. It seems to run ok on a 180 and 250 just different. Very very large spread. Strangely large. I would be thinking you are out of the range with in 20 either side. Which the JC would probably be.
Conclusion there must be some kind of bigger push with lumper fuel. Possible a longer burn to fully use it up. The finer mist could be a sharper bang


I could change the 24. To a C float bowl / emulsion tube combo and fill the lower end but what would this do to the upper mid to top?
Could taller my own emulsion tube


The answer could be in the needle and air feed to the emulsion tube. I may have enlarged it when I had to drill it out.
The pilot jet was reamed out with a plucked wire from a wire brush. Could be a factor


This tube was miss drilled at the factory as ti meats to the side which the other carbs don't. It was blocked so who knows how rough the flow is


Got my f1 wing


A few needles and jets to choose from should find the spot

Some times my posts / photos are shuffled

Both tanks are starting to. Rust inside




3 step liner and Australian made

I think this will be my last post on engine development.
I have absolutely nailed it. Instant response, pickes up the front wheel in the lower gears with just a wrap of the throttle from anywhere from not much off idle, revs out then just solely fades, idles nicely at really low revs. It's a different motor to what it used to be. Once again it's the tedious combination of a lot of seemingly unimportant changes that work together I did it!

I have documented the process except for the porting so that others could reproduce it. Unfortunately I don't know what some of the parts I used are off but you can get around that as I will explain.

1976 24mm DT 175 carb:
The DT175 needle dose not work till the upper rev range and is boggy down low (to lean)
The TY needle is better because of its finer profile but still a bit off
? Needle (No ideas what it's off or where I picked it up and has no markings?) is perfect the profile is similar to the ty but is comes to the narrowest size about a cm from the end and maintains that shape, where the ty tapers all the way in.

*This means the motor is thirsty really early and is governed by the main jet much earlier than most motors. The main jet now becomes mor critical earlier than normal.

I used a emulsion tube I found at wreckers at the bottom of a really big box of carby parts? It has the same emulsion tube holes as the DT, TY175JC and most modem bikes. Finer atismisation than the to hole at the top TY type. This emulsion tube fits the TY float bowl so I can have the quick change main jet type which is unique to the TY.

The needle clip is second from the bottom which is a richer setting. Middle is ok and bottom is to fat and a touch lethargic. Second from the bottom is perfect

I used a 1.5 cutaway on the slide. This just taylers the mixture to be a bit richer at low throttle openings. At tha 1/4 throttle range the lower cutaway will draw the fuel out more and richen the mix. It alowes you to fine tune for that snap. The DT is 2.5 and 22mm TY is a 3 which makes seance as the 22mm is much smaller and will be drawing harder at around 1/4 a 1/3 throttle.

*I used a 210 main jet

The 240 is to big and feels rich like a normal TY with out that fat rich snap. the 180 is to boggy (lean) don't forget for a TY using the multiple hole emulsion tube 140 is standard.

Interesting thing is the standard DT used a 160 main with the good emulsion tube. The JC use a 140 with the good emulsion tube. The DT has a really soft taper needle where a TY comes to a sharper point for richer up top.

Conclusion:
My motor is really thirsty. I'm using a bigger main jet than a DT and far richer profile needle plus a richer slide. Way way richer than a JC.
The motor needs fuel down low like a TY but much more up top than a DT. TY don't have a expansion chamber so at higher revs the charge is not pushed back in as you get with a expansion chamber tuned to a rev range. This means the motor will lean out above a certain rev range. Normally the charge comes out the transfer ports goes up the back acros the plug and out the exhaust. The expansion chambers job is to be tuned to a rev range where it has a back pulse that pushes the escaping charge back in just before the piston comes up and cuts it off.
The TY muffler can never do that so from a certain rev range it just goes out and is lost. Now I have a better flow and much more charge than any normal TY which gives the motor the ability to rev out more.

The need for a richer mix and profile is the result of flowing more air. More air needs more fuel to keep it at the right ratios for a bigger bang.
When you open the throttle the motor gets a increase in air. If the fuelling profile is not matched to the air flow the explosion will be either to lean (bog) or rich (lartharjic) if its right then you just accelerate.




Once again to the casual observer it looks standard


Ps
I forgot to mention I have also worked on the piston. Basically the hole in the back of the piston matches the inlet port whith and lower, possibly higher I dont remember what I did but it would have looked right for that barrel. There is a balancing act between that and the bottom of the piston skirt dropping off. you also have to be carefull how you leave the filed surfice otherwise you could encorage a crack. The time between a crack and failour is instant as I have never seena bent one. I set mine at should be good for the life of the piston. Also did other mods for better. Lubricating the skirt and rings.
By enlarging the hole in the back of the piston you can maintain the flow energy you create. When the piston goes up it creates a vacuumed in the crank area as that void is being filled you create a certain momentum. All I am trying to do is not hinder the airs desire to fill a void. This again equates to more fuel/air for a bigger bang.

Mods summary:

Exhaust burnt out.
You can do this with oxi but I chuck them in a big burn off pile until they glow red and then some. Clean them up after and repaint. This guarantees you have a clear exhaust for flow.

Porting the barrel.
Exhaust
The exhaust port has ridges on the sides as the studs run past that area. Flatten it out as thin as you dare to go. 1mm would be enough. The top of the exhaust port needs cleaning up as there is a ridge there. Remove it and square it up a bit. You can polish the port if you like.
Transfers
Clean them up top and bottom remove dags and match the metal to the alloy. Check for small chamfers on top of port and none on sides. At the base of the transfers sharpen the divider and around smoothen. Not polished. Never leave scratches going in the direction of flow. If you have to leave scratches always make them diagonal.
Inlet
Clean up the same way but don’t remove any material. In my opinion the area is too wide for the read block and could do with packing. When you clean up go for a burnished look and not polished. Polished drags on the flow.

Piston
The back of the piston has a big influence on how much you can pack into the crank area. Match the back of the piston hole to the port area. The less piston you have the better but you have to leave some as it stops the piston from rocking which limits you win what you can do. Chamfer all the bottom edges of the piston. The front and back should be done already just match the sides. This forces oil between the piston and boar and not cut/scrape it off. If you have a DT piston where the top ring is not at the very top edge, match the exhaust port profile to the piston and shape (hard to explain but it makes a big difference)

Reeds
Do as I did. The small boyesen reeds are good for opening early and staying open a fraction longer as they don't have the desire to snap closed like the thick base ones do. Good for low revs. The single long read is thin but beeping one price it is a bit tougher

Reed block
You will have to use Boyson ones or some other one piece. Thinning the centre bar out as much as I did would mean the standard reeds would fall in. Only thin from the sides and not the depth as you need every bit of strength you can get when it’s that thin. Leave about 1mm at the ends to seal everything else can be removed and shaped.

Mounting block
Replace with the type I have used. I really don’t understand why they use that one to the side. There is no reason for it that I can see and really blocks/messes up flow.

24mm DT 175 carb
Clean it up and check every passage, jet, replace o rings, jet the best you can like I did.

Inlet track
Just copy

If you want me to do it for you it would cost you $500 plus postage, parts and up to 3 months. Wouldn’t do it on a old crank or piston.

The fact that I could Easley jet this carbie means a 24 is to small


I'm going to put a 26 hear


The maths says this inlet system is to small. I'm going to have to make my own

I have a plan
Maths, heat, filtering and volume.
Lets see what happens?


What a sweet motor. Lowdown with follow through. Don't forget the eara as good as my mates 250.
The frame and suspension are predictable. The slower I go the better it gets. Unfortunately we are on a mission to find the east cost track.
We are pushing through following through every track, trail, tyre print, orentering markers and bent stick. The north edge looks promising. We turned back. I was ready to turn around instead we found dirt roads back along the north boarder to about halfway.
The south edge has as yet impassable ravines. East is still a mystery


The last bike I enjoyed working on like this one was a RS Honda.


Running repairs since last trip
- to rich
- quieten the end muffler
- bend the gear selector arm as its not selecting 1st or 2nd well. Could be due to having to grind it out to remove it. My welding is not that good.
- paint strip the right crankcase cover while its off
- new fork seals
- new float bowl gasket
- reset the float level
- rejet slightly leaner across the board. When it gets hot its on the over rich side. Rich seems to be good for this bike but when it's hot it's a bit to much so I have to compensate heat build up. What I really need is more fins or water cooling.
- tighten brake leaver bracket
- re position the speedo a bit to take the pressure off the cable
- reposition handle bars
- change plug, oil, oil chain
- monitor the perishing splits in the tyres

Got about 1/4 of the way but a forced keep heading north
My mixture issue was sorted with just moving the needle one clip leaner to the centre position. This way I keep the rich cooling mixture

I got this at the wreckers for $10 bucks, I found it in the back left hand corner under a pile of junk carbs where I'm sure I have looked before.
I think this carb has been weighting for the best offer and I'm it.
Two days ago I ordered a 1998 YZ80 carb that looks like this one. It's really hard to tell the difference some times in the Mikuni VM family. I think this one is a YZ80.
1


Meet three of the VM family. There is a big variety over the years. This is one series in boar size but there are many variants. Ty and other 22mm,DT and other 24mm and YZ and other 26mm.
2


The 26 on the right doesn't come with a auto lube nozzle so it's likely it's a ZY but then again if you had good auto loob directly into the bearings and such.
3


I only found one difference on the plumbing and its a tough one
The DT175 has the air jet in the lower option and jetted emulsion tube. The ZY? Has jetted emulsion tube but uses the top air jet option like the TY. The TY has no emulsion tube jetting and used the top option?
4


I wish I knew what they were thinking
5


That's a 2.5mm drill that fits in the channel that feeds the emulsion tube. Variation for TY175.
6


I estimate it to be about .4mm it has a jet in the tube.
7


I guess these are 0.4mm as well. Interesting that the DT and YZ? Have a jet
This is a emulsion tube for a DT and TYJC the TY comp has a a posing two hole system documented previously.
8


But it dose come with this and I'm exited.
If we are going to be so good then I'm going to have to touch up the air passages
9


I'm going to build this one from the ground up. I'm thinking that if I make this one and jet it and get a 10% improvement using the stock air cleaner then I may just park it there and call it good
I think I will coat this one in silicon only
10

10


I need bigger gearing I started with a 13T front jumper to a 15T and now need a 16T possibly 17
Anything less than a 15 is just a waist of time.


Started hear and only wanted to round the edges off


Ended up hear as one thing leads to another. I am looking to have as much flow as possible with partial throttle openings as I need that low down snap.


No that you can tell but this is a 26mm same boar as a 250.

The first run up and down the drive was disappointing no mid range, had to screw the air jet right out to make it idle. Really dull motor
Second run I changed the main to a 140 as I am now running the restricted air entry to the emulsion tube with the six hole type. Better
Next run I changed the slide to a 2.5 as a posed to the 1.5 for the 24mm carb. Worse

Took it out and drilled the emulsion air entry tube out to 2.5 like the TY like to run. I have another carb coming if I stuff this up. Put all the settings back to what I was running on the 24mm even changed the pilot jet over.
Fixed
I don't know what it is but TYs seem to have there own way of running they have a particular sound and feel that is achieved through a having a well aerated fuel delivery. Which is not used in DT or YZs.
My drive way up the side of the house from the road is not particular long but if I do the same thing every time and wrap the throttle on in the same way it picks the front wheel up from low revs in second gear. I turn at the bottom of the drive an roll it on fast before I'm straightened. I can only turn so fast and giving it to her before you are straight is always the same. I may be going up in front sprocket sizes but so is the power

I have to draw inspiration from that and I'm struggling


I don't know it could be a lot of work for little gain.

At the moment I would say. My mods
With a 22mm you get to 1/2 throttle and it shuts down
With a 24mm it gets what I would say is a very balanced carbie ration.
The 26 is a bit flat low down but the problem may be me and over accelerating. Top end needs tuning and the mid range is unmapped.


The velocity stack I want to use is 2mm from the exhaust and touching the frame at the tip
I have to make the UNI filter fit in there,, flow, heat shield and direct to suck from the top

I can't afford for it to let me down. I'm taking it places that I won't get out. Best is bury it for a week and get out.
I have my concerns



Biggest I can go in that style
Biggest that style


26mm
Drilled 2.5mm Main air jet
1.5 cutterway
Fine taper last 1cmm needle
Clip 5 richest
360 main
DT175 pilot
Two screws out on the pilot air jet
Bell mouth modification
26 specs



I like how the cables sit. Big bends
Good design

Fully modified intake system.
I have my concerns:

The reeds are experimental. The top read should have a reed backing plate I'm with out

The reed cage is on the edge

I think YZ intake manifold

I don't know the carbareta model DT style main ar jet, YZ 80 size, TY250 bellmouth and size, later model features with no auto lube

TY float bowl and jetting system and attachments

DT175 float system

I think YZ rubber manifold between the carb and air box. Very nice design for high flow

Plastic manifold between rubber manifold and air cleaner. This piece has its problems. For starters it made out of a morning fresh detergent bottle. It has to mate with two rubber ends with no solid base to press against. It's warped in vinyl tape and herald together with 4 clamps. It has design reliability issues.

I prefer the wire, cloth air filters but this old one fit

The support is questionable

16t front sprocket

It picks the revs up a bit to quick for that heavy flywheel. There is a concerning difference between picking up the revs under load and coming back down again with the clutch in.

But it all works. Now I can pick it up from low and running it out for a few thousand revs in second.


I can't really fault that


The air intake manifold is 25% over minimum size.


With a bit of luck it shouldn't fail. When I'm out bush I won't be able to do much if it dose.
Should hold



I had a plan and this is where I ended up
So far

A few cable tyes from the unifilter clamp is all that holds it in place. I took it around the house to heat it all up ant test. After that I loosened the carby clamps to see it wanted to move but it was happy to sit there so I'm guessing it will last.


One of my requirements was maths and this certainly has over capacity


Another requirement was heat. That exhaust is a killer for performance with all that hot air it will generate.


My ideas is to shield it and that fibreglass material the they lag exhaust with springs to mind. I have some but it's unpractical to use so I make this up of three sheets of fibreglass matting left over from some long forgotten project.


It should take the sting out of the heat and it seals off from the frame back to the mudguard and seat base