the all aussie dribble thread!

[koshari]

speaking of carbs pulled the Keihin PWK35 off the KDX for the 3rd time today and cleaned out all the ports/jets and it was still spluttering and struggling to idle, turned out it was the reed, damn petals had gaps in em the thickness of a credit card, turned em round for now and got some carbon fibre ones on the way.

2 strokes, sometimes you love em and sometimes you loathe em!

 

[David (saltas)]

Ok terry Ill look at some drawings and do some reading and see If I can catch up.

Terry do you remember an electrcal engineer from swinburne tech he was from the sub continent I think.
He made a home made electronic fuel injection kit for his KLX250 as prototype he had the weight down to a kg
Most of it was off the self stuff but it was from all over, boat bits car bits etc.

I also remember HRC for Doohans 500cc gp bike and evinrude buying some injection technology from Ralph Sarich.

I do love EFI for the economy and the torque it has brought to previously carburated engines.

I wont hold my breath to see EFI on a saw but I cant see why Stationary engines cant be done.

 

[David (saltas)]

Terry is this the beasty you are playing with?


Chain Saw Carburetor

 

[Terry Syd]

Yep, doesn't matter if it is a Walbro, Tillitson, Zama - they are all basically the same. If we can establish some tuning principles for one brand, the principles will apply for all the others.

One of the fellows that was using a diaphragm carb on his aircraft mentioned that one of their mods was to use a slightly longer main discharge nozzle. The longer nozzle gave a slightly leaner mixture for when they throttled back for cruising. At lower throttle settings, the speed of the air in the middle of the venturi is slower than the speed of the air next to the edge of the venturi. The slower air flow doesn't have as strong of a depression (signal), so there is less fuel flow. - Using a longer/shorter discharge nozzle is just one carb tuning principle that is available to a tuner.

I'm tuning for a strato and I expect that I may make some changes to the fuel flow that others with a conventional two-stroke may not need. The C3M carb that Rick was referring to with the 'transition jet' would be an ideal carb for a strato and it wouldn't surprise me to see more such carbs used on stratos - and tuning adjustments developed for the transition jet.

However, at this point I would be happy just to conceptualise all the factors that go into the running POP.

 

[David (saltas)]

so the only spring I see is biasing the metering chamber diaphragm and press's on the needle and seat

 

[Terry Syd]

That's the spring - number 3 in the diagram.

 

[Terry Syd]

I think I have a way to test a saw to see if the POP is correct for its powerband. Since there are two 'peaks' to consider (peak horsepower and peak torque), you tune for each of them.

Peak horsepower will be where the saw is doing the most work. Take the saw and put it in a log and tweak the High speed needle to get the best peak horsepower. Then put a load on the saw and pull the revs down to peak torque.

Check the settings at peak torque and see if a change in the High speed needle setting will increase the torque.

If you find the engine needs a bit more fuel at peak torque - then you need to decrease the POP.

If you find the engine needs to be leaned out a bit - then you need to increase the POP.

After changing the POP, you will need to re-test the same way. For example, if you decreased the POP you will likely have to lean out the High speed needle a bit to get the peak horsepower back, then check peak torque again. Rinse and repeat until you get the carb the way you want it.

 

[David (saltas)]

Ok terry I have done some reading

These crabies are crude but clever at the same time.

They are a fuel pump, differential pressure regulator and a the mixing circuit

I am trying to draw a fuel supply y axis throttle position x axis drawing like you would find in Keihin owners manual

I agree that adjusting the pop spring has the effect of richening or leaning the carby it is not the intended was of doing so.

You are adjusting the fuel pressure and will be effecting the sizing of all of the plumbing idle ports and air bleed etc.

I will post a drawing soon and it will be wrong but it will be a start that can be improved upon.

 

[Terry Syd]

The throttle position will be irrelevant for a chainsaw, you pull the trigger and go WOT. However, a work saw may see many different RPMs during a cut, which may change the fuel mixture as the engine speed varies.

I've read that the fuel pressure on the other side of the metering valve is usually 6-7 psi, in other words a relatively constant fuel pressure. What will vary the lifting of the valve will be the action of the diaphragm, which will be influenced by the signal from the venturi and the resistance of the lever.

If you could somehow graph the various changes in fuel flow of a diaphragm carb - I eagerly look forward to seeing that graph (and I'll bet a lot of other fellows would to).

 

[David (saltas)]

contribution of jetting components

some of this is best estimate some of it comes from carby manuals.

at 100% throttle 50% fuel comes from the sum of the low speed circuits and 50% from the high speed circuit.

at idle 80% comes from the low speed circuits and 20% from the v notch.

At 50% throttle 66% comes from the sum of the low speed circuit and 33% from the high speed circuit.

 

[Terry Syd]

Good start on a graph. Just in regards to your colour coding -

You have the transition jet coloured as 'magenta' (bugger if I know what it's called, that's one of those women colours), whereas I think it may be the purple line that you drew.

The idle jet colour of 'lime' appears it could be the 'premix orifice' (the holes just after the idle hole).

As I recall, the relative fuel flows in a chainsaw carb (versus a kart carb) is that the idle circuit provides about 30% of the total flow at WOT and the High speed circuit provides the other 70%.

OK guys, jump in here with some input...

 

[tdi-rick]

Sorry guys, F1 is on, been watching Le Tour the last few nights (been following Cadel's career since 1990/2000) so a bit, umm, tired and pre-occupied

BTW, found the stray current problem.

Rummaging through some of Dear old Dad's test gear I found a panel DC 20v scale meter set up with leads for testing which worked out as an ideal test instrument.

Had about 0.2v through the coolant when checking all sources from the coolant to the rad, but noticed a spike to 0.5v when engine cranking.

Quickly found the battery earth terminal loose/dirty

All fixed now, no current at all ATM through the radiator.

 

[David (saltas)]

Rick Good find on the stray current.
Many have tried and failed hence the earth strap they gave you. :confused2:

 

[David (saltas)]

The walbro manual I was reading was quiet specific in the order the three sets of hole work in ie the whole closest to the piston first and then the second closest then the third.

transition supplied fuel first mostly due to the v notch to start the saw then when it fired it ran on the transition and idle.
On opening the throttle the premix started to supply fuel.

I tried so hard not to keep getting throttle position and rpm mixed up

I was thinking and drinking after I posted and I may have miss interpreted the manual I will post a second graph it should be able to prove or disprove which if these is closer to the truth.

 

[David (saltas)]

more likely contribution of jetting components

The manual clearly stated 80% fuel was supplied from the low speed circuits at idle.
It did not say where the other 20% was coming from.
This graph has it coming from the high speed circuit.
In this graph the v notch is only important while you are pulling on the starter rope to make the transition jet suck hard.
When the saw fires the Idle circuit starts to supply more of the fuel

I know the butterfly is never at zero.
It was too hard to draw so I will ask for forgiveness rather than tried for permision up front.

 

[David (saltas)]

So what you ask

Well I have herd people comment that a saw that had been tuned properly.
IE idle speed was good, max rpm was good, little 4 stroke but cleaned up in the cut.

The comment was that the saw was sluggish to spin up from idle to wot.
Or conversely that Brand X was better than brand Y because Brand X saw had a snappy throttle response.

Opening up the premix orifice by a couple percent might be the cure for the sluggish one

 

[Terry Syd]

Maybe my definition of a 'transistion jet' is different from yours. The jet I'm thinking of has an opening just below the edge of the venturi and is fed from the High speed circuit. It helps transition from the low speed circuit to the high speed circuit and is primarily affected by the low pressure of the throttle valve as it moves past it. On your graph the closest line colour would be the purple line.

I also think the magenta line would be the idle orifice and the lime line would be the premix orifices that reside just in front of the idle orifice. The fact that their outputs blend together as the throttle valve continues to open supports their origin from the same source - the Low speed needle.

The graph does a good job of showing how the various orifices contribute their fuel in sequence through the movement of the throttle valve, rather like a graph of a slide carb and how each jet makes its contribution. When the various curves are blended together you end up with the actual fuel flow of the carb. If you have a carburetion problem at a certain point in throttle movement - you have a good idea where to look to create a fix.

Nice work.

 

[Terry Syd]

I agree with your comments about sluggish carburetion. There are a few things that will affect an engine's ability to rev, flywheel inertia, piston mass, but the biggie is carburetion.

I have put significant amounts of mass on flywheels and you could hardly tell the difference in the ability of the engine to rev without a load (however, traction in rough terrain was altogether different). I think it came down to the fact that the flywheel was a store of energy and would carry the engine through the passive phases of crankcase and cylinder compression much easier.

I often read on this forum about how a certain saw has an ability to quickly rev - like it is some secret porting trick or something. All it is is superior carburetion. If you want a saw to rev better, clean up the carburetion.

 

[Terry Syd]

Instead of opening up the premix orifice a couple of percent, there is another fix.

I used to write tech articles for a motorcycle magazine and one of the articles I wrote was called 'That's a Different Problem'. It was an article about how sometimes people will confuse one problem for another.

What precipitated the article was when I was out riding and a came upon a fellow that had broken off both his front brake lever and the clutch lever. Luckily, the spare lever I had with me would fit his bike. However, when I checked his clutch lever mount it was tightened down so hard that it would not move if the lever took a hit. I told him that he needed to loosen the mounting screw so that the lever mount could move in an impact. He told me he tried that, but that the screw would keep loosening up and the mount would get loose.

I looked at him and said "That's a different problem".

That was the problem of 'loose screws', you fix that problem with loctite or star washers or lock washers or wired screws. He looked at me and the light came on "oh, yeah".

End prelude to carb fix.

I read of guys on the forum that can get good throttle response if they open up the Low speed needle a bit, however it makes the idle too rich and the saw will load up. Guess what - that's a different problem.

You can fix that problem by taking a small rattail file and filing a small divet in the throttle plate OPPOSITE the side where the idle hole is. That will allow more air to pass the throttle plate at idle, which leans out the idle. You then have to open up the Low speed needle to get the mixture right. Of course, the idle will now be a bit too high, so you turn down the idle screw. Now the engine idles perfectly.

The result is that the idle has a proper fuel mixture that won't load up the engine. When you open the throttle, the butterfly will move across the premix orifices and will pull in extra fuel since you have opened up the Low speed needle. Now you have both a snappy response and a perfect idle.

 

[MCW]

For crying out loud guys keep this carby talk to private messages. If any Americans read this they'll think that us Aussies are smart or sumfin...