dave de

My Feedback: (12)

I read a great thread about checking pop off pressure. Has anyone tried this?? after buying that part, it looked to me like they used a spout from a ca container & attached a fuel line to it. what should the pressure be for a rcgf 45cc is 10 -12 psi correct or 10-11 psi

Tired Old Man

My Feedback: (1)

Never tried that before. Nice to see some new stuff in here...........

10-12 works, but imo is not optimum. If the engine runs fine don't fix it. You aren't going to gain rpm with pop off pressure.

w8ye

My Feedback: (16)

If you keep on reading, you will find that a higher pop off pressure leaves the carb less susceptible to outside forces like weather changes or the difference between running on the ground and in flight. There are many variables and compromises involved here. You can read in the Walbro manuals about pop off pressure and then check the pressure on your DLE engine and be surprised.

This is getting in to some deep theory.

Sort of like trouble shooting your radio system? On your radio - in a way is not important if you have 6.8 volts or 4.8 volts unless you take into consideration the discharge characteristics of your battery installation.

Truckracer

My Feedback: (19)

I played around with pop off pressure a couple years back when so many were talking about it here in these pages but couldn't see any real performance or handling differences after making changes. Other factors in an airframe / cowl can affect an engine far more than pop off pressure so I really don't worry about it any more. Unless pop off pressure is excessively low for some reason, I can't imagine there being a problem.

Tired Old Man

My Feedback: (1)

Pop off has more affect on mid ranfe tuning than anything else for our purposes. It's a ton of trial and error using a lot of different length springs. For most, if the pop off pressure falls anywhere between 12 an 25 psi it's fine.

ggraham500

My Feedback: (4)

OK, I'll bite. Can someone explain the basics of "pop off pressure" please? Thanks.

dave de

My Feedback: (12)

I,m no expert by any means, but from what I read its the point when the needle lifts to let fuel into the chamber. Its measured with a auto cooling pressure pump. Alright guys, I hope your not laughing too loud, but isnt that the basics.......lol I,m learning something new just about everyday on this site.......

jedijody

My Feedback: (1)

That's exactly right, but just as important as the pop off pressure is the pressure that the inlet needle again re-seats. There is a very wide acceptable range for the pop off pressure point, generally anything from 15-30 PSI works fine, it should re-seat and hold by about 10 PSI. If it's less than 8 the fuel pump could over ride the inlet needle and cause impossible to adjust rich running problems.

w8ye

My Feedback: (16)

On a Walbro type carb, there is a pump and a regulator. They are both diaphragm operated.

<u>The pump</u>
diaphragm is operated back and forth by positive and negative pressures created inside the crankcase of the two stroke engine. By utilizing two rubber flapper type check valves that are sometimes part of the edge of the diaphragm, this pumps fuel from the tank to the carb.

<u>The regulator</u>
also has a diaphragm that opens a needle &amp; seat type valve. This diaphragm is actuated by the negative pressure created in the venturi of the carb. The two or three little holes of the idle fuel passages in the venturi of carb and also the high speed fuel passage is where this negative pressure signal comes from. These passages are uncovered by the carb butterfly valve as you open the carb to make the engine run faster. There is a lever under this diaphragm that reverses the direction so when the diaphragm is sucked down, the action raises the needle off the seat and opens the valve from the pump. There is a spring under the diaphragm that counteracts the vacuum created in the venturi. On the atmospheric side of this diaphragm there is a little vent hole on the outside of the carb that you can see without taking the carb apart. This regulator works as a demand regulator. If your throttle is closed and/or the engine is not running, there is no vacuum signal to the regulator diaphragm and the needle and seat remains closed. The more you open the carb and the faster the engine runs, the more open the fuel valve.

In diagnosing problems with your carb, there are times that you want to check the operation of this fuel regulator. Does it shut off? Does it open?
It is difficult to recreate the vacuum back up through the little passages of the carb throat/venturi to make the fuel regulator work.

<u>The simple diagnostic solution</u>
You can apply air pressure to the carb fuel inlet nipple. This pressure will go right past the pump because the two directional check valves of the pump will let pressure go right on past them in this direction. But the needle and seat type fuel valve of the regulator will resist being opened because of the spring underneath the regulator diaphragm. If there is enough pressure against the needle, it will open the needle anyway. This is the 12-30 pounds <u>pop-off pressure</u> you keep hearing about. On the other side of this valve, it only takes a very slight vacuum on the diaphragm to open the needle but on the fuel pump side of the valve, it takes a lot of pop-off pressure to open because of the difference in surface area when comparing the size of the diaphragm and the size of the end of the needle in the valve. <u>This pop-off pressure test makes for a simple non invasive test of the regulator valve's action.</u>

For our trouble shooting purposes, we can use the pop-off pressure check to determine that the fuel regulator valve does work within a range of normalcy.

It is beyond the scope of this explanation to go into the inter-reactions of changing spring pressures to have different pop-off pressures and the corresponding effects on engine performance.

Iflyglow

My Feedback: (79)

Pop off pressure was very critical in Cart racing with Tilotson carbs if you wanted to be competitive in the class that I ran in.

jedijody

My Feedback: (1)

Adjusting the pop off pressure is just like adjusting a fuel pressure regulator, for some applications a setting within a very narrow range can be very advantageous. In order to fine tune the setting one must first have a thorough understanding of how the carburetor works (good job Jim ), then also know how those changes will relate to the specific application, operating conditions, and loads the engine is used in. For the vast majority, including the Walbro factory, a very wide range is acceptable. Most of the engines these carbs are intended for are used in passive load conditions, ergo, centrifugal clutches, air or water impellers/propellers, when the load is more solidly connected to the end of the crankshaft this area of tuning can be a little more important. </p>

captinjohn

If you are looking to fixa mid-range that is too rich...try cutting the moon on the throttle plate just a tad bigger. A lot more easy than messing with the regulator spring. There are photos ect on how to do it in one of these threads. Capt,n

jack19345

My Feedback: (1)

There is fuel on the diaphram at all times, not vaccum! Of course the fuel is on the inside of the diaphram.

pe reivers

To all purposes, I thought this horse was dead! Most modern walbro crabs have pop-off in excess of 30 lbs, and equally raised seat pressures. For our purpose this might be just as well, because it helps reduce plane attitude mixture variations caused by the weight of the fuel and membrane. This influence is less at high pop-off / reseat values.

w8ye

My Feedback: (16)

The fuel between the diaphragm and the needle is under a vacuum at times created by the engine itself

captinjohn

Lets think this out. Maybe the fuel is pumped into the metering side of carb by pulses from the crankcase which causes the pump to work because of one way fuel valves.Then as the fuel if "drawn" (a vacuum) into the venturi &amp; into engine, the atmospheric pressure thenshoves on the metering diaphram to open inlet valve and let more fuel into metering chamber from pumping chamber....something like that!!!!!!!!!!<hr />Edit...added (a vacuum) "by drawn" some spelling corrections.....I did say "maybe &amp; something like that.
I thought it would be fun to see the replies. Well it was not a copy &amp; paste anyway...just fun!

jedijody

My Feedback: (1)

Wrong, read Jim's explanation of how the carb works in post #9, it's quite accurate. The fuel pump is only capable of 5-7 PSI, it can take near 30 PSI to push the needle off it's seat. Without vacuum from the enginefuel would never get to the metering side of the carb, let alone into the engine.

av8tor1977

My Feedback: (6)

Actually I think Captain John said "metering side of the carb" when he meant to say "pump side of the carb", but other than that his description is accurate and the same as W8YE's. (Just written differently.)

AV8TOR

captinjohn

Thanks AV8TOR for the reply. I was just goofing around trying to word something close, &amp; not a copy and paste. I am just not great with words! Ha. Every body should know that by now.

Here is a little link to Vacuum types. I did not know about this. I did know there are various ways to create a vacuum. The one that comes to mind right now is the lips on a nice healthy woman! Like a good Kiss!<hr />Never mind...here is the link http://en.wikipedia.org/wiki/Venturi_effect

jedijody

My Feedback: (1)

I wasn't picking on John, he's a friend of mine and I know he knows how a carburetor works . I'm just a very detail minded person and I feel what is written in these pages for those that ask should be as accurate as possible. Though the wording leaves some to be desired for true accuracy it is as he states, "something like that".

captinjohn

Below is what seems to be a clear description of my post. I am sure there are even better. It is a cut copy &amp; past. Capt,n <hr />If the engine must be operated in any orientation (for example a <font color="#0645ad">chain saw</font>), a float chamber cannot work. Instead, a diaphragm chamber is used. A flexible diaphragm forms one side of the fuel chamber and is arranged so that as fuel is drawn out into the engine the diaphragm is forced inward by ambient air pressure. The diaphragm is connected to the <font color="#0645ad">needle valve</font> and as it moves inward it opens the needle valve to admit more fuel, thus replenishing the fuel as it is consumed. As fuel is replenished the diaphragm moves out due to fuel pressure and a small spring, closing the needle valve. A balanced state is reached which creates a steady fuel reservoir level, which remains constant in any orientation

w8ye

My Feedback: (16)

Having run go carts back in the late 50's early 60's and aware of the Tilotson carbs, I noticed B &amp; S came out with a diaphragm pumper carb and I was all excited for the lawn mowers. But I soon found that the diaphragm pump on the B&amp;S only pumped fuel into an overflowing bowl. The carb fuel feed was manifold suction from the constant level bowl. A complete position one system.

av8tor1977

My Feedback: (6)

No worries Jody, I didn't think you were picking on him. It's just that I read his post several times and thought "Except for mis-stating "metering side" for "pump side", he's basically got it right. So I thought I would mention it to clarify both his post, and the carb theory for everyone.

AV8TOR

captinjohn

<hr />
<hr />Jody, is Indeed a friend. He has the right thought....be as accurate as possible! I do the same,but it is hard for me to convey precise workings into words. If ever I get a chance to spend some time flying with Jody &amp; talking engines....it would be a dream come true. Remember.....one has to have things to look forward to...even if sometimes it is impossible. Best Regards Capt,n

pe reivers

OK then John av8tor and Jody. What do you think of this one:
Fuel supply to the jets is a delicate balance between jet fuel draw and fuel demand replenishment by the air pressure on the membrane balancing with the fulcrum lever spring and fuel pressure from the pulse pump.
With the carb in downdraught position, the weight of fuel AND membrane is added to the outside airpressure, SEEMINGLY like higher air pressure and thus a rich mixture. With the carb in updraught position, this weight influence tends to lean the mixture. Under normal circumstances maybe not very much, but in high G manoeuvres this may become quite noticeable and even a nuisance.
Any ideas? Fixes?