And FWIW...ALL the carbs used on our toy airplane engines were originally made/calibrated for some other engine, to the specs from the manufacturer...If ANY ultralight mfr would get a carb made SPECIFICALLY for their engines, there would be NO NEED to screw around with the carb settings..Mr Luis Salas is the MAN at Walbro in Cass City...Maybe a person that knows more about Walbros should get in touch with him...Might get an education from the MASTER of Walbro carbs...AT THE FACTORY....
Spelling and grammar OK, teacher ?
The WB 25 was made for a Sachs Dolmar model 166 CHAINSAW..118 cc..Not exactly a match made in Heaven...
Some of the other WBs were made for GoKarts, not some unltralight engines...That it works so well is a tribute to Walbro...
I shoulda stayed banned...

MVVS uses the WB27 in their 116 and 152 engines. I order to get the midrange right, they linked the idle needle to throttle valve rotation such, that the idle needle is opened further as the throttle is advanced from idle to midrange.
I have played with pop off pressures on this carb as well, but end results were no better than those obtained with needle adjustments. So I restored the original 30 lbs/in2 pressure.
Because I had too much power with that engine on my 1/3rd scale PA18, I fitted a smaller carb. This provides nearly as much low end power for pulling a 27x12" three blade prop in low(er) noise operation. This carb had a pop off in excess of 30 lbs and has excellent mixture from idle to full rpm in a dive. I do not know the exact pressure. My pressure gauge only goes to 32 lbs max.

Just like what Mr Luis Salas at Walbro told me....Ya think the engineers who make the carbs just might know what they're talking about ???
Playing around with the pressure is just a band aid for the real problem, the manufacturer of the engine doesn't want to spend the money to get a carb calibrated for each engine..

Originally Posted by rcign

I shoulda stayed banned...

Nah...we'd miss you too much Ralph.

Originally Posted by rcign

snip
Playing around with the pressure is just a band aid for the real problem, the manufacturer of the engine doesn't want to spend the money to get a carb calibrated for each engine..

Any Idea what that cost would be Ralph? An educated guess would do. Looking at the conjet system, it is not cheap as well, and the cost is there for every engine that is produced.
I think it is only affordable for mass produced engines like Stihl, Honda, Zenoah etc. If you look up the carb numbers on the Walbro site, there is the list of all the manufacturers that went down that road.

I have been playing with aux jetting inside the carb as well and managed to get rid of the conjet system, but my engine did not run nearly as well, so I stopped the effort and went for the smaller carb.
Now, if Luis had looked over my shoulder and added some quiet whispers, I might have gotten somewhere.

Must be a large cost, no one has done it..

Ralph,

You open to a phone discussion on what you found with catastrophic failure with one engine? I need to do some comparisons.

As for the carbs, the saw and yard engine manufacturers have a great interest in delivering a product to market that is extremely user friendly in the rpm bands used by their equipment. R/C rpm bands and loads are not even close to the same as the equipment engine manufacturers. They use things like belt and gear drives, or clutches to extract and use the most effective torque from their products. We use a simple and light flywheel called a propeller and seek high RPM instead of torque. That by itself blows propeller and engine efficiency out the window. Unfortunately the vast majority of gas engine users will never understand that.

Pe,

I see you understand why I prefer a minimum of 25 pounds pop off in my engines. It makes tuning a lot easier, especially in mid range rpm bands.

I have seen large variations across carbs, but the lowest was about 20 lbs. So it seems Walbro loves higher pressures as well. At least I do, because the needle seats more reliable for better idle.
I often wonder what happens at about 7,000 - 10,000 rpm. Will the needle lift with every single cycle and allow just the right amount of fuel into the chamber, or will the needle be partly open all the time. In the first case, a high spring pressure would help seating the valve at elevated rpm figures due to higher natural resonant frequency.
The carb will draw fuel for about 160° of the cycle. The remaining 200° will be used to replenish the fuel that the jets took from the chamber, seat the valve, and wait for the jets to deliver fuel again.

That's just plain weird....you've been looking into my head haven't you, LOL. It seems our mind's eyes are very similar, I'm glad I'm not the only one that sees things like that in my head. At least they aren't talking to me yet...in my head that is.

What's wrong with a minds eye talking to oneself, I do it all the time, don't tell anyone

Originally Posted by Jedijody

That's just plain weird....you've been looking into my head haven't you, LOL. It seems our mind's eyes are very similar, I'm glad I'm not the only one that sees things like that in my head. At least they aren't talking to me yet...in my head that is.

Ah well, mechanical things have a mind of their own. You might as well talk to them, then maybe one day they talk back to you . As it is now, you have to analize the symptoms and come to a conclusion.
I would love to have the facilities to place a sensitive pressure sensor in the fuel plenum chamber, hook up some electronics, and watch what happens. I am sure Walbro has done just that.

I had read somewhere (a Walbro carb manual) about pop off pressure. It mentioned that they were not too worried about when it popped off, but when it would reseat. Since the fuel pump pressure was around 5 to 7 psi then wanted it to seat at 10 psi or more. I have two brand new Walbro WJ71 carbs. I checked both to see what the pop off pressure and reseat pressure was at. Both popped off at around 20 to 24 psi and reseat at 12 psi. I will be putting them on my DL100 (changing the EMAS carbs) to see what differences they have. I know one EMAS carb high speed needle will not supply enough fuel to make the rpm at WOT drop in very much (maybe 50 rpm). The other motor will drop the rpm at WOT down 200 plus rpm (both metering levers were about the same).

Here is the info from the Walbro manual.

Very small differences in machining the angles of the needles and seats...And the size of the hole the tapered end of the needle fits into...Doesn't take much....Metering levers are not made to be adjusted, just set at the factory level and left alone...

But the pop off pressure and reseat can be adjusted by replacing the spring with a spring in the kit or by lengthing the current spring slightly, or am I wrong on this? Agreed the metering levers should be set to factory specs.