chinese carburetor

[Terry Syd]

I used to think that the pump 'pumped' fuel for the carb and that a bigger pump would help to keep it from leaning out at WOT. It was just another misconception about how these carbs work.

Essentially, the metering valve 'floats' off the seat of the metering orifice as fuel demand increases. Once the valve is off the seat, the carb can suck fuel directly from the tank.

 

[Terry Syd]

I thought I'd bring this thread back up, I've worked out some more concepts from that link that XSKIER provided on the Mikuni BN carbs.

If you take a look at their flow diagram you will notice that it implies that the POP provides fuel to 1/4 throttle. That is incorrect, it only affects the fuel flow from the Low speed jet up to (around) 1/4 throttle. The POP is really a RAMP up for the flow of the Low speed jet.

The Low speed jet is a fixed orifice jet. A fixed orifice jet will provide lots of flow initially and then taper off. In order to feed the fuel in slower and ramp-up to full flow, a 'regulator' is used. By using the size of the metering orifice, spring rate and pre-load (POP), a slower feed of the fuel can be obtained.

Here's something else that is important for our saws. Note that the chart shows the Low speed jet providing 1/3 of the total fuel flow of the carb. That figure is only illustrative - IT IS NOT THE ACTUAL FUEL FLOW.

A BN44 carb might have a 120 Low speed jet and a 140 main jet (plus the high speed needle on top of that). Another BN44 carb might have a 120 Low speed jet and a 115 main jet. Clearly, the 30% on the chart is not correct and is only for reference as to where the Low speed jet provides fuel in the fuel curve.

Here's something else I noticed. The BN44 carb with the 120 Low ad 115 High has a much smaller metering orifice and a high POP. In other words, the Low speed jet is providing a lot of fuel and the 'regulator' (metering orifice, spring rate and POP) are SLOWLY feeding in the Low speed jet. You can see the same construction with the BN38 carb.

 

[XSKIER]

Uh ohh. You're going off the deep end now! Look up how many turns out a Seadoo uses on the H screw, and really be confused... I use pop-off pressure tuning to tune throttle response. It has the most effect on a quick off idle stab of the throttle. Once that is clean, then tune the high speed jetting. Good luck!

 

[Terry Syd]

OK, where do the saw carbs fit into all this. I have a Walbro with a .25 Low speed jet. The construction of the low speed circuit isn't like the Mikuni above (or the Zama twin jet). The .25 Low speed jet is the MAXIMUM that will flow with the needle all the way out. Since the needle needs to be turned in to tune the low speed circuit, the actual fuel flow is always LESS than the size of the jet. Essentially, the low speed jet works as a limiter, probably for EPA reasons.

That same carb has a .58 Main jet (plus the high speed needle on top of that). Now, compute the relationship of flows from a .25 Low and a .58 Main jet. (I don't have the actual flow rates through the jets, I'm just using the AREA of the size of the hole). The .25 Low speed jet only provides at the MAXIMUM just 15.6% of the flow of the carb. In actual use, with the Low speed needle turned in and the High speed needle turned out, the percentage of the total flow is far less.

Now I am beginning to see why my modded carbs with the greater Low speed fuel flow are providing so much more torque for the engines.

 

[Terry Syd]

"I use pop-off pressure tuning to tune throttle response. It has the most effect on a quick off idle stab of the throttle."

XSKIER, that's right. The POP is ramping up the fuel flow from idle to 1/4 throttle. If you got the POP too low or too high, the fuel flow wouldn't be following the demand of the engine, you might end up with either a too rich or lean mixture.

 

[Terry Syd]

A few other bits.

On most Walbro type of carbs, the Low speed needle determines the total flow for the low speed circuit. When you need extra Low speed flow to help with the torque, you may need to increase the POP in order to ramp in the low speed fuel flow a bit slower. Since the low speed needle will also increase the fuel flow at idle, you will have to tweak the throttle plate to allow it to flow more air and lean out the idle.

On the Zama twin jet carb, the second jet comes in at 1/4 throttle. This allows you to add more fuel flow down low without upsetting the idle circuit or the ramping of the transition circuit.

 

[k_kindig11]

I've bought several Zama carbs for ms290 and notice the exact same issue. They seem to run great but if you let the saw sit idle for a minute or two, rpm's drop and it sometimes dies. Act the same on several different saws. Just thinking here.... If I didn't want to try to lower the metering arm, do you think I could take a metering arm from a walbro carb and put on it? I've not had this problem with walbros so do you think this would fix the problem without having the trial and error of bending the current metering arm?

 

[Terry Syd]

Why not file a bit off the throttle plate on the side OPPOSITE of the idle port. That will lean out the idle and when the engine is idling the backflow up the intake will be reduced over the idle port (part of it can go past the other side of the throttle plate).

You don't need to take the throttle plate out of the carb to give it a bit of a trim. Just stick something in the carb to hold the throttle plate open. The edge of the plate will stick out of the carb and you can trim it. I use a small fine needle file to trim it.

Make sure you have enough adjustment on your idle screw adjustment before you trim too much off the plate. You will have to turn down the idle as you are effectively opening up the area at idle.

If you go too far, take a soldering iron and run a bead of solder across the area you filed.

 

[rmh3481]

Terry,
Does pump pressure effect the low speed metering?

 

[Terry Syd]

Near as I can tell, no. It would only effect the metering if it blew the metering valve open. In which case, the metering diaphragm would be pushed out and extra fuel would be available, without pulling against the POP, on the next intake cycle. The mixture would tend to get richer if the valve was being blown open.

Which comes back to another thing I had to work on, why do worn pump diaphragms cause rough idling?

It turns out that my first impressions were wrong. The pump doesn't get worn out and decrease in pumping ability - it's ability to pump fuel INCREASES! What happens is that the pump diaphragm gets worn and the distance it can move increases. That means it increases the amount of fuel it can pump. Which can then increase the pressure so that it lifts the metering valve off its seat.

Here's something else, there is a small chamber between the pump diaphragm and the metering valve. I figure this chamber, that has another small diaphragm where the other side is vented to the atmosphere, MAY be the reason that carbs first start showing signs of wear. That small diaphragm appears to act as a pulse chamber to dampen the pressure surge when the metering valve closes. When the diaphragm gets worn out, it can't stretch anymore and absorb the shock.

As an experiment, I'm thinking that I may put one of my extra metering springs under the lid to push on the diaphragm to give it some extra resilience to absorbing the surge. I'll try that when I get a worn diaphragm to work with.

 

[rmh3481]

Thanks Terry. Im working with an older Walbro HDA where a replacement is NLA. This HDA is decent sized with a choke bore of 22.5mm, throttle bore of 20.5mm and Venturi of 17mm. The problem is the inlet needle is seeping and as such will only hold a pop of 5psi. The inlet seat is brass and Ive tried polishing it with a Q-tip soaked in jewelers rouge spun with an electric drill. Usually this practice of polishing the inlet will resurface the seat bringing up the pop and allow the carb to be put back into service. Ive had good luck in the past with the aluminum seats but not so this time with brass. I was able to find a 'full kit' which came with a new inlet spring and related parts. (Generally Walbro doesnt provide the inlet needle spring in their rebuild kits like zama does.) This new spring seems light compared to other springs coming on newer versions of the HDA. Have you experimented with heavier springs on the inlet needle?

 

[Terry Syd]

A POP of 5psi? Wow, you got some problems.

I had a brass seat I tried to drill out (as an experiment) and had similar problems about getting the metering valve to seal. I kept working on it with a round hardwood toothpick and jewelers rouge. The toothpick jammed into the top of the orifice and I could twist it. I think the round toothpick eventually shifted the ragged inside edge out further and smoothed the edges. That carb went on to become my first real successful mod - but only after I got the metering needle to seal.

You've probably already tried a new needle with fresh rubber on the seat, so maybe you might have to try a higher POP to get it to seat.

The information about setting the POP for good acceleration off of idle isn't so critical for the saws. We don't start with a prop in the water or slicks on the track. We hit the throttle without a load on the chain and quickly move past 1/4 throttle. A higher POP will create a slower ramp for the fuel delivery, but after about 1/4 throttle you should be on the low speed jet. You might get a bit of a lag off of idle with the slower ramp, but heck, at least you would be able to use the saw once it got up to speed.

I like my saws to rev like a light switch, so I can be a bit fussy, but it really isn't necessary for most cutting. My 372BB explodes off of idle, but I'll never use it for limbing so it is just a personal preference.

 

[XSKIER]

I like my saws to rev like a light switch, so I can be a bit fussy, but it really isn't necessary for most cutting.

Me too! I just had to "tweak" (Tim Taylor) my MS 441 R C-M with a little higher pop-off. It has an accelerator pump too, and was just too blubbery for me.

Keep up the good work!

 

[Terry Syd]

"I've bought several Zama carbs for ms290"

I didn't know there were Zama carbs for the MS290. I looked on the internet and all I could find were Chinese copies of Walbros that had been branded as Zamas. You could tell it was a Wabro copy as the discharge nozzle came in at an angle from the side. The Zama has the discharge nozzle on the very bottom (or top, depending on how it is mounted).

I ended up taking the hardware off of a 044 carb and putting it on a Zama twin jet from a 365.

I'll try and upload a picture of the inside of a Zama twin jet carb.

 

[Terry Syd]

Well, let's see how this goes.

The two jets I refer to sit either side of the welch plug. The one on the left controls flow to idle and transition circuit. It is essentially the 'Low speed jet' that Mikuni refers to. The one on the right side of the welch plug is a separate jet that comes in after 1/4 throttle. I refer to it as a 'Torque jet' as that is what it does for the engine.

 

[Terry Syd]

If you think the inside of the carb looks butchered up, it is. I was making an air bleed for the High speed circuit. Here is what it looked like after I added to end of a discharge nozzle (with the valve) to the High speed inlet.

Without the valve the carb would suck air back into the metering chamber at idle.

The other picture is the hole for the air bleed that I drilled.

 

[Terry Syd]

OK, I'm still learning how to post pictures, somehow the picture of the air bleed hole didn't work.

The air bleed system on a saw carb really isn't needed. If I had a kart, I would definitely put it in and work out the fuel/air ratio and perhaps the position of the discharge nozzle in the venturi. I would also be able to monitor the EGT and cylinder head temps as I rode the kart.

When the air bleed circuit is enabled, the High speed circuit can be made to give a steady or slightly leaning condition as the revs rise. In other words, instead of the High speed circuit becoming too rich and preventing any further rise in RPM, the carb revs like a motocrosser. You definitely want a limited coil if you are going to play around with the air bleed.

 

[Terry Syd]

You may have notice in the picture of the inside of the metering chamber that the back of the discharge nozzle has been notched to clear the movement of the metering arm. That's because I recessed the overly long discharge nozzle back to the edge of the venturi.

This was one of the Chinese copies of a Chinese Zama. The Zamas have shorter nozzles which just protrude from the edge of the venturi. By having the nozzle opening close to the venturi, the High speed circuit will come in earlier. That's because the airflow along the side of the venturi is faster because it has a longer distance to travel (like on an airplane airfoil). As the volume of air through the venturi increases, the low pressure gradients in the middle of the venturi will change. You can tune when the High speed circuit becomes more effective by moving the discharge nozzle in or out from the center. Since I was looking for a flatter fuel curve, I moved the Chinese copy nozzle out to the edge. It came out so far in the metering chamber that I had to seal around it with GB Weld.

Here's a before picture -

 

[Terry Syd]

And here is an 'after'

 

[Terry Syd]

There's one more circuit that I don't have pictures for and I am just beginning to play around with. There is a 'hidden' potential air bleed for the idle/transition circuit just behind the discharge nozzle. If I put it in it will lean out the idle and transition circuit, which may mean I can use the standard Low speed needle instead of a slower taper needle.

The extra air bleed gets cut off at about 3/4 throttle and allows full flow through the Low speed circuit.

I just took the 'torque jet' on a carb out to .45mm. It feels like it may be a bit rich as the throttle plate opens it up. If the extra air bleed circuit is installed I expect that it won't be so rich at that point. However, once the throttle gets to 3/4 throttle everything comes in.

This extra fuel flow may give the bigger engines like my 372BB or a Makita 79/85cc engine some extra flow for really good low end torque. It's a work in progress....