Carb popoff adjustment

[tdi-rick]

I really like this animation for understanding fuel flow in a carby

http://www.dolmar.com/downloads/144/carburetor.exe

OK, I'm a kid that grew up with cartoons

 

[pgg]

Awesome cartoon mate

 

[Terry Syd]

Neat carb

I wonder what carb is being illustrated there. It has removeable jets for the low speed circuit, a removeable jet for an intermediate circuit and adjustable mixture for both idle and high speed.

If that is the carb that Dolmar uses, no wonder they can adapt it to various saw displacements.

 

[tdi-rick]

I'm still trying to figure out changing the fuel curve - if it is possible. As a hypothetical, let's say two different springs allowed the same fuel flow at max revs. One spring is weak and the other is strong, perhaps they have different pre-loads (pop-off), but the effect at max revs is the same.

So what happens as we drop down the RPM range and load the saw? Can one of the springs make the engine more resistant to bogging?

If a saw is tuned to four-stroke out of the cut and then cleans up as it starts cutting, the mixture strength has to have leaned out as the revs dropped. If we set the mixture for the best cutting speed, then that is likely the optimum fuel mixture. When the saw revs are really pulled down in a cut, the carb must lean out even further away from the optimum fuel mixture.

My question, can a change in carb internals help to maintain the optimum fuel flow so that there is less chance of bogging?

Perhaps the spring rate isn't that important, maybe it is the position of the nozzle outlet. It picks up the signal from the venturi which is what creates the demand for the metering chamber.

and you do need to get the delivery nozzle away from the boundary layer, as thin as it would be in the venturi for maximum signal.

I'd love to know what the transition RPM that the HS circuit kicks in on any given saw.

With the Walbro carbies on KT100J and KT100S Yamaha's, most of the tuning is done on the LS circuit.
In fact with the J engine, all the tuning is done on the LS.
There is insufficient depression to draw a reasonable amount of fuel from the HS circuit (IIRC they only rev to around 11-12,000RPM at the end of a straight)
The 'S' revs quite a bit higher, but IIRC the transition from LS to HS is still around that 11,500-12,000RPM mark.

 

[tdi-rick]

Dunno which carby it is, but every time I watch it I love it

 

[nenicu]

Very interesting!

 

[Terry Syd]

Well, it took me a few years after this thread, but I finally got some answers to some of the questions. Here's a thread that may help someone tune one of these carbs. - http://www.arboristsite.com/community/threads/chinese-carburetor.246510/

 

[Martinkart]

Tuning

I'm still not comfortable with my tuning knowledge of diaphram carbs. I'd really like to get my head around some of the finer points. Maybe someone can help me with some of the questions I have.

Assuming no change in the position of the H&L needles -

If the pop-off pressure is set a bit higher, will the carb then act as if a leaner mainjet had been installed in a conventional carb?

If the size of the metering valve seat is enlarged, how does that affect fuel delivery? Is it equivalent to less pop-off pressure?

The same pop-off pressures can be achieved by using a strong spring with less pre-load and a weak spring with more pre-load. How does the spring rate affect the fuel delivery? For example, does the stronger spring slightly lean the mid-range?

Similar question - Does the spring rate create a 'curve' in the fuel delivery rate? If so, could this curve be used to fine tune different portions of the power curve?

Is there an 'optimum' High speed needle gap (say one turn out) that can be obtained by setting the pop-off pressure? If so, why is this necessary/desired?

How important is the taper of the needle? What effects do different tapers have?

The position of the HS nozzle will change the fuel delivery curve (further in will lean out the mid-range). What is the 'working area' (as defined by percentage of venturi bore diameter) of the HS nozzle outlet? How much alteration of the fuel delivery is possible within that working area?

What effect, if any, does a larger outlet diameter HS nozzle have on fuel delivery?

Hello
Its a long time since this was an active thread, but I found it very interesting. And specially this post!!
Have you come to any conclusions here Terry?

 

[Terry Syd]

Yeah, most of the information that was available about the diaphragm carbs was BS. You can't imagine the BS I had to wade through to finally figure these things out. I had developed my own theory (which was bloody close) based upon empirical evidence - then I finally got access to Mikuni's document which confirmed and filled in the gaps of my knowledge.

The Chinese carburettor thread was where I first started dumping what I had learned and then later the 7900 carburettor thread where Rich decided to jump into the deep end of the pool and build a few carbs using the concepts. If it wasn't for him buying the drills and giving it a go, this whole carb tuning issue would have probably died a long time ago.

It blows me away that MANY of the posters on this forum are familiar with tuning carbs/injection systems on cars, trucks and bikes, but when it comes to chainsaws they freeze - Crikey, it is the same concept - just do it!

 

[Martinkart]

I'm a novice in the carburator busines - but I have some questions that I hope you can answe for me
Popoff pressure - is only the opening pressure important or is also the closing pressure important?
If so - the difference in opening and closing pressure (ex opening at 12 and closing at 8 0 = 4) is that an important factor?

 

[Chris-PA]

The function of the metering diaphragm/lever/spring assembly is to regulate a fixed pressure drop from atmospheric. The drop is determined by the spring force. Because the pressure in the chamber is held below atmospheric it will not spill, and the carb is then all-position.

When the air velocity through the venturi creates a pressure drop at the fuel outlet that is below that of the metering chamber, then fuel flows through the fuel jet and into the air stream.

If you raise the lever height, you reduce the effective spring pressure and reduce the drop from atmospheric - so the pressure in the metering chamber becomes relatively higher, and the mixture will be richer.

The intent of the orifice at the inlet needle is not to meter fuel - that is the job of the fuel jets. It should be sized so that it can flow at least as much as needed under any circumstances as it is not supposed to be a metering restriction.

In general one wants the various subsystems to operate independently. So you would want the pressure regulator to hold a fixed pressure drop under all circumstances, which would mean a spring that has a more constant pressure with length changes. Then you would handle dynamic conditions like acceleration with a pump providing an extra fuel shot, or by fuel outlets positioned to pick up the edge of the throttle plat at partially open positions (although this affects both part throttle and acceleration). You may be able to modify the mixture under dynamic conditions with a non-linear spring, or a weak spring set closer to atmospheric, but then the various metering circuits become more interdependent.

There are many ways to approach setting up a carb and handling the various functions, both steady-state and dynamic.

 

[Terry Syd]

POP is some of that BS that I had to wade through. If you read the 7900 carb thread you will notice that I never mentioned 'POP' to Rich - that's because it is irrelevant.

What lifts the needle is the diaphragm, not pressure. When the fuel level drops in the metering chamber, the diaphragm moves down and lifts the needle.

Here's something to consider, say there are two carbs with identical metering springs. The only difference is the size of the metering orifice under the needle. One orifice is small and the other orifice has an area that is twice the area of the small one. If you measure the POP for each carb, the one with the small orifice will have twice the POP as the larger one. That is because the pressure works on the size of the area of the orifice and the one with the smaller area requires more pressure to blow off the needle.

However, in the real world of how it actually works, both carbs will LIFT the needle at the SAME TIME. When the fuel gets sucked out of the metering chamber, the diaphragm moves down. Now, how fast a carb re-fills the metering chamber is a process controlled by the size of the metering orifice, the pre-load on the spring, the spring rate and the available pressure.

Chris is still thinking of the carb function as some sort of pressure regulator. He could quickly test his theory if he was to remove the pump diaphragm from the carb and start the engine. The engine will run without the pump diaphragm, however it won't idle, he'll have to keep the revs up to keep the engine from stalling.

I covered a lot of this carb theory in the Chinese carburettor thread. That thread also contains a link to Mikuni's manual on tuning their diaphragm carbs.

 

[Mastermind]

Great post Terry.

 

[Chris-PA]

Chris is still thinking of the carb function as some sort of pressure regulator. He could quickly test his theory if he was to remove the pump diaphragm from the carb and start the engine. The engine will run without the pump diaphragm, however it won't idle, he'll have to keep the revs up to keep the engine from stalling.

You are confusing a few things here Terry - it is not the carb that is a pressure regulator, it is the diaphragm, spring and needle that form a pressure regulator. They hold the pressure of the fuel in the chamber under the diaphragm at a fixed pressure under atmospheric pressure - that is how these carbs are all-position, because the fuel is at a pressure under atmospheric it never leaks out while running.

It is not regulating the pressure of the fuel pump, and the pump is not necessary once the main venturi is drawing a significant amount of fuel. Once the air velocity through the venturi creates a pressure at the fuel outlet that is less than the metering chamber, then fuel flows through the fuel jet and out the fuel outlet into the air stream.

Further, for a venturi/fixed jet to work you must have a controlled pressure pushing on the fuel, otherwise the fuel flow is not just a function of air velocity through the venturi. In a float carb that is atmospheric pressure pushing on the fuel in the bowl and that remains fairly constant, but here it must be less than atmospheric ambient pressure or fuel would leak out - but it must still be relatively constant.

At idle there is not much air velocity or flow though the venturi formed by the edge of the throttle plate, and it must create a pressure drop that is not only below atmospheric like in a float carb, but below the offset below atmospheric created by the diaphragm/spring/needle. If it has trouble getting down to that pressure it needs some help supplying fuel volume, which is where the pump comes in. So your proposed test does not prove anything about whether the diaphragm/spring/needle is a pressure regulator as you've misunderstood what pressure it is regulating.

 

[TEZZA]

Is this helpful?

If you find that the engine will not chop off clean or is slow to come back from high rpm's when you lift the throttle, this is a sign that the pop-off pressure in the carb is too high. The cure for this is to lower the pop-off pressure. If you experience loading-up problems, this is a sign that the pop-off pressure may be too low. The cure for this is to raise the pop-off pressure. As a general rule of thumb, the pop-off pressure will be from 8 to 12 lbs.

Ref: http://www.iwt.com.au/tillotson.htm

 

[Tyler Keniston]

Old thread I know, but it seemed unfinished.
The last few posts has my mind in a twist.

slow to come back from high rpm's when you lift the throttle, this is a sign that the pop-off pressure in the carb is too high.

My best guess as to why this is is because the diaphragm will be lower due to the higher pop-off (creating pressure closer to atmospheric in metering chamber) leading to excess fuel being fed after throttle closing. Not sure if that makes sense, seems like that would load it up? Either way, based on Terry's and Chris's words it seems accurate to me that the pop-off isn't precisely what would affect this in reality as much as the spring pre-load, spring rate, and diaphragm.

I'm not sure I see the difference between what Terry and Chris are saying (seems real similar to me), but in regards to the pump providing needed pressure:

At idle there is not much air velocity or flow though the venturi formed by the edge of the throttle plate, and it must create a pressure drop that is not only below atmospheric like in a float carb, but below the offset below atmospheric created by the diaphragm/spring/needle. If it has trouble getting down to that pressure it needs some help supplying fuel volume, which is where the pump comes in.

I assume that the pressure produced by the pump (in chainsaw carbs at least) does not exceed pop-off pressure (bypassing the lever action via diaphragm). In other words, the pressure from the pump would (should) never on its own pop the needle off its seat? If this is correct, than how would the pump provide any additional pressure to the metering side beyond what is permitted by the diaphragm/ spring / needle?

 

[Terry Syd]

Here's a thread where we used the concepts to mod a carburettor for a Dolmar 7900 - http://www.arboristsite.com/community/threads/dolmar-7900-carb-upgrade.229528/page-2#post-4863935

The Chinese carburettor thread (linked above, post #27) also has a lot of information, especially the Mikuni manual.