Tales from the Emporium

[NordieBoy]

I've got the front grip and lounge chair

[NordieBoy]

And once I replace the scored brake banjo I'll have some front brakes as well

[warewolf]

I've got the front grip and lounge chair

I've got the front grip, and the lounge chair with vibro-massage!!

[NordieBoy]

I've got the front grip, and the lounge chair with vibro-massage!!

And some intense reading lights

[NordieBoy]

Somehow I just did 345km to reserve (20km/L).
I normally get 300-310 max.
I'll see how this next tank goes.
Either the cut carb spring/header weld grind really helped out or I forgot to reset the trip meter at some stage.

Well I got 350km on 17.9L (19.55Km/L) this tank and that was including my 70-100kph full throttle roll-on tests.
The header weld was done 3 days before the end of the last tank but the carb spring was cut the day before the last tank.

Looks like I'm getting more pickup from the header weld and spring being done and the spring is giving me 4km/L better fuel consumption!

I have never gone better than 16-16.5km/L previously.

[Transalper]

Well I got 350km on 17.9L (19.55Km/L) this tank....the spring is giving me 4km/L better fuel consumption!

I have never gone better than 16-16.5km/L previously.

How does that work then?
What does the spring do with in the works?

[NordieBoy]

How does that work then?
What does the spring do with in the works?

One of the guys on thumpertalk with an air/fuel meter on his DR thought he'd try shortening the stock spring to the same length as one of the aftermarket ones (from 111mm to 101mm) as they were only $4us to replace.
He was very surprised how effective it was in increasing the bottom end and had no noticable downside.
He also finally did the header weld grind and was surprised how much of an effect that had as to to how hard the bike wanted to go, right through the rev range without requireing any jetting/idle changes.

The chopped spring decreases the preload on the carb slide so it moves off the bottom quicker but as the spring is slightly stiffer it is a bit slower to open through the middle/top of the slide slider thingy.

I'm thinking my increased fuel economy must be that the increase in bottom end gets the bike up to the revs quicker and as the slide isn't opening as fully/quickly it's not sucking through as much fuel - Wonder if the guy in the States has found increased mileage?

The header grind is getting the gasses out more quickly, allowing the pipe to work properly - restriction to flow is all in the muffler now so it can do what it's designed to do - so that shouldn't use any more fuel unless you open the other end even more to let more air in and have to richen it up more.

The weird bit for me is that I'm finally getting up to the mileage the US guys get normally and I couldn't get near that with the bike completely stock.

Ryan's stock pipe looks like it can flow 60% more gas than our little NZ spec ones going by the size of the exit hole so I'd expect that the jetting I used on my first dyno run (removed snorkle, 1mm shim under needle) would do the business on his as well.

Too much thinking, my brain hurts...

[Crisis management]

Nordie, I've been getting 19 to 19.5km/l from my bike fairly consistantly. I run a cycleworks exhaust and 15/44 gearing with no other mods to the fueling that I know of. I think Merv gets a similar mileage (from previous posts somewhere...) so it is not too hard to achieve.
Your mods to the exhaust header weld should increase the efficiency of the engine but the slide spring shortening should not have that effect.
The CV (constant vacuum) carb relies on the vacuum created in the venturi to lift the needle slide to allow fuel to flow thru the main needle jet. The higher the vacuum, the higher the slide lifts and the greater the fuel flow. There is no mechanical connection between the throttle twist grip and the carb slide, the twist grip operates the butterfly controling the vacuum created to operate the slide.
The common mods to the slide are:
1/ Drilling a second bleed hole thru the slide from the venturi side to the top of the slide (vacuum chamber) allowing a greater vacuum to be drawn on top of the slide opening it faster. This gives faster throttle response, wheelies etc.
2/ Shortening the slide spring which again reduces the resistance of the slide to upward (opening) movement. Again this gives faster throttle response. As he spring provides the balancing force (at constant throttle) it determines the open position of the slide at any engine load, shortening the spring should make the engine run slightly richer as the needle jet will be more open at any engine load now.....
Neither of these mods should increase your fuel mileage just give more throttle response and bottom end / mid range power.


Hope the brain doesn't hurt too much now.......

[NordieBoy]

Nordie, I've been getting 19 to 19.5km/l from my bike fairly consistantly. I run a cycleworks exhaust and 15/44 gearing with no other mods to the fueling that I know of. I think Merv gets a similar mileage (from previous posts somewhere...) so it is not too hard to achieve.

Wonder what would happen on 14/44?
No jetting mods?
Should be running very lean then?
Even stock I was only getting around 300-305km to the tank.

Your mods to the exhaust header weld should increase the efficiency of the engine but the slide spring shortening should not have that effect.

I'm just trying to eliminate anything from the equation.
It could be something as simple as the carb is sealed better this time when I put it back together...

The common mods to the slide are:
1/ Drilling a second bleed hole.

Done a long time ago - improved the pickup noticeably.

2/ Shortening the slide spring which again reduces the resistance of the slide to upward (opening) movement. Again this gives faster throttle response. As he spring provides the balancing force (at constant throttle) it determines the open position of the slide at any engine load, shortening the spring should make the engine run slightly richer as the needle jet will be more open at any engine load now.....
Neither of these mods should increase your fuel mileage just give more throttle response and bottom end / mid range power.

It reduces the preload on the slide but the spring itself is stiffer when shortened.
I'm tempted to buy a stock spring and put it in and see if there is a difference in km/L.



I know I could just ride the damn thing but I'm nosey

[Crisis management]

It reduces the preload on the slide but the spring itself is stiffer when shortened.

If thats the same spring cut shorter then it is not stiffer, a spring has a compression of "X kgs per mm" as a result of its steel composition / heat treatment and winding rate. Cutting a spring just makes it shorter, it still needs the same a mount of load per mm of compression. You will have reduced the preload on the slide which will alter the neutral position of the slide (sitting higher) and running slightly richer.

As far as my jetting is concerned, I haven't looked in the carb yet so don't know what the jetting is. However, cycleworks recomend their exhaust with stock jetting and the plug looks about right to me. All I was suggesting was, that the 19km/l is readily achievable, rather than a sales pitch that is unachievable.

[NordieBoy]

a spring has a compression of "X kgs per mm" as a result of its steel composition / heat treatment and winding rate

Yep, "X kgs per mm" at spring length Y.

Compress a spring with 10 winds 5mm and each wind deflects 0.5mm.
If that spring had 20 winds each wind would only have to deflect 0.25mm to get the 5mm total movement so would only need half the pressure.

That's why the boy racers who cut their springs end up with very hard rides - if they're not riding on the shock bumpers.

It was weird seeing all these Yanks getting 20km/L and me with a stock bike getting 16km/L.

Now I'm curious as to whay my bike was getting such low mileage before but I'm not putting everything back to stock to find out

[Crisis management]

Compress a spring with 10 winds 5mm and each wind deflects 0.5mm.
If that spring had 20 winds each wind would only have to deflect 0.25mm to get the 5mm total movement so would only need half the pressure.

This is getting down to nitpicking.......wheres the lice comb!

Unfortunately you are incorrect here.
For example, a 100mm long spring that compresses 10mm per 100 grams will require 500 grams to be compressed to a final height of 50mm. If you cut 10% or 10mm off the spring it is now 90mm long, but still compresses at 10mm per 100 grams: to achieve a compressed height of 50mm tho it will only have to compress 40mm requiring 400 grams of load.
So, if you shorten the spring it will require less load to compress to the same height (preload in the CV spring case).

I'm not even going to go near cut springs in cars.........I've tried explaining that to my son (gangly, pimply, youth standing beside a lop sided mazda with a hacksaw in his hand.....you get the picture?)

I've read the Thumper article about the spring mod and it certainly seemed to make a difference, oh for an air/fuel mixture gauge like that!

As far as mileage goes, my readings are for a cross section of gravel roads and city riding, there seemed to be no difference, I always hit reserve at 180km's and thats about 9 to 9.5 litres.

[cooneyr]

Yep, "X kgs per mm" at spring length Y.

Compress a spring with 10 winds 5mm and each wind deflects 0.5mm.
If that spring had 20 winds each wind would only have to deflect 0.25mm to get the 5mm total movement so would only need half the pressure.....

I'm a dumb engineer and I still dont get this one.

If we take a spring of 10 coils with a spring rate of X (in kgs/mm) and compare to a spring of 20 coils with a spring rate of X then coils has no difference as the spring rate is still the same (assuming they are both the same length). It is all about the material the spring is made from. Obviously with the second spring the material will have to be different because if it were the same for the 10 and 20 coil spring then the 20 coil would have a higher spring rate. Why then by removing coils does the spring rate change? I think the following is possibly a better way of looking at it?

The vacuum load is the same at a set throttle opening as the vacuum side of the carb doesn't change pressure (for pressure read load over area i.e. load over the diaphragm area which is constant). The vacuum load and the spring load are fighting each other else the slide would jump to wide open throttle at any throttle position.

Before the cut, the spring might have been applying a preload of 0.3kgs (arbitary number) to the slide. Say the vacuum builds up a load of 1kg then to counter we need to build up and additional 0.7kgs of load with the spring to counter the vacuum. If we cut the spring so it is applying 0.1kgs of preload then we need to build up 0.9kg of load with the spring to counter the vacuum.

Say the spring rate is 0.1kg/mm then in the first instance we need 7mm of movement to counter the 1kg of vacuum. In the second instance we need 9mm of movement to counter the vacuum.

Following from this what it also means is that with a cut spring the slide will move sooner as the preload is less. The lower preload equates to a lower vacuum required, which inturn equates to a smaller throttle opening to achieve the same amount of slide movement (say 1mm).

Two points in summary.
1 - the carb will run richer with a shorter spring (I know this is not what you are experiencing Nordie)
2 - the slide will move of the idle position with a lower vacuum i.e. a smaller throttle opening.

In conclusion - cutting the spring is another way of adjusting the richness and responsiveness of the carb but I would suggest it not be done in conjunction with a needle shim unless other mods such as exhaust and airbox to make sure that the bike doesn't end up rich. Drilling the slide and cutting the spring would give the best throttle response. Drilling the slide would seem a much better way to improve responsiveness and shimming the needle would seem a much better way to control richness (as easier to control than moding the spring).

Maybe with both a cut spring and a drilled slide you are getting such good throttle response nordie that you are not opening the throttle as much during acceleration (where you use most fuel round town). Maybe if you do a long distance run at constant speed you might find you use more fuel than previous for a similar run but around town you are using less compared to previous round town runs. Buggered if I know.

Cheers R

P.S. I know my use of load is not quite correct but it will do for this discussion.

[NordieBoy]

If we take a spring of 10 coils with a spring rate of X (in kgs/mm) and compare to a spring of 20 coils with a spring rate of X then coils has no difference as the spring rate is still the same (assuming they are both the same length). It is all about the material the spring is made from. Obviously with the second spring the material will have to be different because if it were the same for the 10 and 20 coil spring then the 20 coil would have a higher spring rate. Why then by removing coils does the spring rate change? I think the following is possibly a better way of looking at it?

But....
If you take that 20 coil spring and cut it in half shouldn't it require double the pressure to get the same amount of compression as the individual coils would have to move twice as far?

Squeezing the leftover 3.5 coils of my carb spring 5mm takes a lot more effort than squeezing the full length one 5mm.

I'll just have to ask someone who knows everything.
Where's a teenager when you need one?


The thing with carbs seems to be that what you think will happen may bear no resemblence to what actually happens.
As mx_rob has found and I'm happy for him to find these things out first

Apart from maybe going to a different pipe - stock or otherwise - I think my carb tinkering is over

Both tanks included at least 80km of 110kph constant running.
I'll find out more this weekend when this tank is due to empty

Signed, Confused of Nelson.

[warewolf]

If you cut 10% or 10mm off the spring it is now 90mm long, but still compresses at 10mm per 100 grams

This is the bit I'm challenging: don't think this is correct at all.

If you uncoil the spring into a straight line, it forms a lever. (After all, a coil spring is simply a more compact form of a straight spring.) If you shorten the length of any given lever, you reduce it's mechanical advantage. We perceive this as "stiffer".

Try it with a ruler on the edge of your desk or bench; 10cm hanging over the edge is a lot stiffer than 20cm. The stiffness of the material doesn't change, but the effect of the leverage changes the effect at the free end quite a bit.

Nordie's point about riceboys is readily-visible evidence of this.

Crisis, I think you are confusing shortening the spring to increasing the preload (which shortens it's captive length). Increasing the preload frequently involves the incorrect premise that the spring rate has now changed, since the initial travel (the bit that is felt at a standstill) is "stiffer"; that's where you are right, under those conditions the rate doesn't change.