Carb popoff adjustment

[benmyers]

Has anyone on here messed with adjusting the carb "popoff" pressure? I'm sure someone knows all there is to know about it, but i searched and didn't find anything. I know it's something that I've dealt with on 2-stroke racing kart engines, but haven't heard of anyone messing with it on a chainsaw or any recommendations.

Thanks

Ben

 

[pioneerguy600]

I have just replaced the springs under the metering lever with the proper ones designed for the particular carb I am working on.
Pioneerguy600

 

[benmyers]

I see. I know with karting, popoff pressures can be adjusted quite a bit. Typically the lower they are (to a point for obvious reasons), the more adjustment you have with the high jet. That is, it will take more needle
adjustment to change the air-fuel ratio when the spring is stiff.

 

[hotshot]

Always check

I recommend to check it after every carb rebuild:

First do the usual 7# pressure 'eternal hold' for the needle & seat test, then increase pressure up & check the pop-off (normally 10-14#), and finally let it settle back down to check the needle re-seat pressure.

Please note all testing must be done wet, so fill the pressure tubing up with gas mix or light machine oil (sewing machine oil is cheap).

No telling how many "I can't get it to run right" problems have been solved through leak testing inlet needles & seats. Also have thrown many bodies away, as the needle/seat would still not hold at re-seat with a new needle & metering spring...even with the seat cleaned & re-cut. WT series Walbros seem to be the worst offenders.

You can adjust the pop-off pressures up or down by changing the metering springs out, but a lot of times the new metering diapragm fixes high pop-off problems.

 

[tdi-rick]

Pop off pressure definitely affects carburettor tuning.
Too high and the saw will be hard to tune becoming too lean and be hard to start, too low and it has the opposite effect with the fuel pump pressure overcoming/overpressurising the fuel circuits.

Mentioned here
http://www.arboristsite.com/showthread.php?t=122823&highlight=pop

and here http://www.arboristsite.com/showpost.php?p=1903739&postcount=21

and I know I've talked of it in other threads but can't find them ATM.

 

[Terry Syd]

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?

 

[Terry Syd]

Thread killer?

Not a lot of responses...

Seems like we can put a heap of time into porting and then just hope for the best that the carb will allow it to run. I'm sure a few of you fellows have spent some hours dialing in your carbs and injection systems on bikes and cars - and what, no interest on dialing in the saw carburetion?

OK, anybody want to take a stab at it any of the questions, perhaps get some discussion going on to see if we can figure out what effects various changes make?

I'll start - It seems to me that if the outlet hole of the metering valve was opened up that the metering valve would become more responsive to changes in the float level. That is, that the diaphragm would not have to move as far to flow more fuel.

If a saw had a small carb that tended to lean out at higher speeds, enlarging the metering hole could allow more fuel to flow with less movement of the diaphragm.

Another option to the small carb leaning out may be a lighter spring on the metering lever that would allow more diaphragm movement after the pop-off pressure.

True? False?

 

[tdi-rick]

Terry,
from my experience many years ago I left the pop off adjustments to my engine builder

As is said in one of those links I listed, everyone chops a few winds off the spring to lower the pop off pressure.
Counter intuitive to me as you've just increased the spring rate, but it reduces the pre-load and just plain works.
Apparently there are different spring rates available for Mikuni's but I've never heard or seen in a catalogue a selection of springs for little Walbro or Zama carbies.

I tend to think it's more a blunt instrument approach to tuning.
If your engine is leaning out at high RPM and the HSS is, say, over 1,1/2 turns out, drop the pop off pressure.

This is the basic tuning advice from Tillotson for the carbies we used to use on 100cc karts.

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, and is a tuning factor that you can adjust for the altitude of your racetrack and the type of racing that you do.

 

[AUSSIE1]

In this link down toward the bottom there is a section
"Converting a Walbro carb from Gasaline to methanol"
It is about increasing the flow rate.
If this won't work for increasing straight fuel, can someone more knowledgeable please pitch in?
I suppose there is the risk of the pump being unable to keep up also.

http://tech.flygsw.org/walbro_tuneup.htm

 

[Terry Syd]

Zama

Rick, take a look at this link for Zama carb parts.

http://www.m-and-d.com/zama_carburetor.html

Click on a type of carb, then when the parts diagram pops up, scroll down to the variations of the carb. When you look at each carb you will see different metering springs, throttle butterflys, main nozzles and needles.

Take a look at a carb of a certain size venturi and look at the options. There's gotta be a reason for the variations in the internals.

I contacted Zama previously and their metering spring numbers are all over the place. Essentially, each has a different pop-off pressure for the carb, however the overall length, spring pitch and wire diameter are different for each numbered spring. I felt like I should purchase their entire selection of springs for my carb and just see what effect they had. After playing around with various springs I might have a better idea what to do if I wanted to adjust the metering.

I just found the reply I got back from Zama about spring pop-off pressures -

"The metering springs are as follows 0019014 30gm / 0019002 16gm / 0019009 19 gm / 0019012 25 gm / 0019013 35 gm."

That gives you some idea about the scope of the various pop-off pressures. Of course, there are some with the same pop-off pressure, but with different spring rates. So, what under what conditions do you use a different spring rate, but keep the same pop-off pressure?

 

[Terry Syd]

Strato carb

Here's another idea I had. There was some recent discussion concerning ported 441s. It seems the engines would rev out until they began to four-stroke, then start leaning out again and the revs would climb further. That appears to be a carburetion problem that is affected by the strato function.

What appears to be happening is that the fixed orifice of the HS needle will reach a point where it starts to be restricted to flow. On a conventional two-stroke the HS needle is set to four-stroke at the top of the cutting speed. However, the strato has a second port, the strato port, that continues to suck in straight air. As the carb on the intake port begins to max out with the fuel delivery, the strato continues to keep on drawing in more air - the result is a leaning out condition as the revs rise.

A solution to the problem could be a bigger carb so that the intake velocity won't be as high, thus flattening out the overall fuel delivery.

Another solution could be to position the HS nozzle outlet further into the venturi to lean out the mid-range of the carb.

Another solution could be to slightly enlarge the existing carb's venturi and leave the HS nozzle in the stock position, thus ending up with a slightly larger carb and a nozzle positioned further into the venturi.

But what about the metering spring? What type of metering spring would be most likely to correct the ported 441s high speed lean out?

 

[tdi-rick]

Cheers Terry.
I have about 4 different Zama tech guides and catalogues, but not those ones.
Makes mixing and matching easier for my C3M carby.

 

[Terry Syd]

Metering springs

Take a look at the price of the metering springs - 75 cents.

If you went through their entire catalog and ordered one of each number 0019002-00190?? - it wouldn't be that great of an expense, but you could have a selection of metering springs to try an optimise the powerband of your saw.

I expect that most people are just hopeful that the saw fires up after they mod it. Whereas they could be trying a few more mods to the carb to maximise the low-end torque and the cutting speed.

 

[tdi-rick]

[snip]

But what about the metering spring? What type of metering spring would be most likely to correct the ported 441s high speed lean out?

To me the most obvious 'fix' is to lower the pop off pressure incrementally and see if that works.
That's the 'old style' solution to that problem on a non-strato two stroke.

 

[Terry Syd]

Modded strato

Yeah, that may work with a retune of the LS circuit. I wonder if the guys tried it?

The problem I see with the 441 example is that the carb has been set up with internals for a given RPM range. When it is ported and the flow through the wider intake port, the longer intake duration and the greater demand (velocity of air) of the higher RPM, ends up exceeding the original operating parameters.

If the carb was on a conventional two-stroke the four-stroking could be set at the top end of the cutting speed. However, the strato function just keeps on functioning and it then begins to lean out.

I would try the slightly oversize venturi and the nozzle in the stock position. Give it a go and see how much change it made. If it was close, then perhaps another .5mm off the venturi. If that wasn't enough I'd try a weaker spring with the same pop-off pressure (considering that the low-speed circuit is working fine at that pop-off). Or, maybe make the metering valve inlet a bit bigger.

These are ideas off the top of my head without actually testing the mods, so I could be way off.

 

[pgg]

The diaphragm pops the fuel valve open from the impulse, the valve always opens no matter what the spring rate, chainsaws don't operate at 1/4 1/2 3/4 throttle, it's full gas or nothing, at idle the fuel keeps dribbling through from the slower pulses, varying pop off pressures make no difference on a chainsaw

 

[tdi-rick]

The diaphragm pops the fuel valve open from the impulse, the valve always opens no matter what the spring rate, chainsaws don't operate at 1/4 1/2 3/4 throttle, it's full gas or nothing, at idle the fuel keeps dribbling through from the slower pulses, varying pop off pressures make no difference on a chainsaw

The pop off pressure determines the fuel pressure in the control chamber, ie. the fuel pressure the jets actually see.

No one usually has to touch the spring on a stock saw as the manufacturer (both saw and carburettor) have determined the best compromise and installed it, but, if you modify the saw all the air and therefore fuel parameters have changed and there's fair odds the pop off pressure may have to be altered too.
Sometimes even a stock saw needs the pop off adjusted http://www.arboristsite.com/showthread.php?t=122823&highlight=pop

Greater velocity through the venturi = more depression = more fuel flow through the main jet if there's enough fuel in the control chamber.
Stock saws have low BMEP's and therefore their carbies are pretty tolerant of changes, just look at the 64cc and 79cc Dolmars, they use identical carbies, but, if you really start to increase efficiency pretty soon that stock carby won't be able to deliver and will do odd things like momentary lean outs at WOT/full revs.

 

[pgg]

I've always thought the spring is just there to close the valve when the throttle is released, regardless if the saw is upside down or on it's side or whatever. Unlike float controlled carbs which control the valve by fuel level in the chamber as the carb always sits level. On a saw, tweaking the lever height seems to have much more effect on rich/lean symptoms than the spring pressure, I've lost those springs a couple of times and just shoved any old spring I could find back in there and the saw still ran exactly the same. Just the tiniest height adjustment always makes a noticeable difference though. On a fixed jet carb, (non-adjustable hi circuit)the lever height makes a big difference to whether the saw screams or 4-strokes at full throttle

 

[tdi-rick]

The lever height would determine how high the needle lifts, and to a much smaller extent change the spring pre-load.

here's some pop off reading

Pop-off is the measurement of the point where atmosphereic pressure overcomes the fuel draw vaccum of the carb. The entire regulator chamber is nothing more than a big check valve. It is the step between the fuel pump, which is providing more pressure than the carb needs to deliver, and the jets, which meter a fixed amount, within a certain pressure range. The regulator chamber is in place to maintain that pressure range.

The whole regulator chamber gets filled with fuel. The fuel is coming from the pump, sits under the diaphram, and is flowing through the jets. If it was just an open flow, with no needle and seat, the jets would be supplied with too much pressure from the pump. As the fuel is sucked out the jets, the fuel supply in thereis drawn out, and the diaphram goes down with it. As it gets to it's low point, it contacts the lever arm, and lifts the needle from the seat. That allows more fuel into the chamber, pushing the diaphram back up, re-seating the needle and shutting off the flow from the pumps, so the jets aren't over-pressured. This happens many, many times every minute as fuel is consumed.

If the pop-off point is set too low, there isn't enough resistance on the needle to stop the fuel pump flow, and the jets are over-pressurized, giving you a rich condition you will never tune out. The same goes for leaking needles.

If the pop-off is too high, there isn't enough fuel pressure to overcome the vaccuum and spring pressure, and you will have hard starts, as the engine is not spinngin fast enought o produce enough pulse, to make the pump supply enough pressure to flow the fuel into the regulator chamber.

It is adjusted by changing the size of the N&S, and the spring tension. There are 4 general strengths of springs - the less the spring is rated the lower the pop-off pressure will need to be to unseat the needle. For example, a 65 gram spring may give you a pressure of 15 psi, where a 115 gram spring may give you 50 (not actual numbers).

You test the pressure with a pop-off pressure gauge. With the regulator chamber open, wet the N&S with WD-40 or gas. With the pump body removed, attatch the gauge to the fuel inlet fitting on the body. Pump the gauge until it 'pops' noting the pressure gauge. Repeat this 3 times to get a nominal reading. Installing a heavier spring will raise the pressure. To adjust in small increments lower, cut coils 1/2 coil maximum and re-test.

NOTE: You cannot accurately test with the pump body and fuel filter still attatched to the carb - you MUST disassemble it and test directly into the fuel inlet orifice, where the fuel filter sets. Otherwise you are testing through the pump checkvalves, throwing your readings off, too high. If you have an external fuel pump, this does not apply.

I'll try and find exactly where I pinched that from and provide a link, it was from a jet ski forum.

 

[Terry Syd]

Tuning

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.