[g-manz35]

I was trying some different carbs on my 25cc conversion and I've found one carb that seemed to work well, but one you hit the high RPM it seemed to lean out and then drop in RPM then surge back and then drop off again. I thought the diaphram may have been damaged. Anyone know what could do this. Bad diaphram? Leaking gasket beteen carb and motor? I tried to clean the diaphram and tried it again an now it won't run at all. IF you throw fuel in the carb it will start and die right away.

Help please. George

[Cougar429]

Have never stuck one on the front of a plane, but had lots of fun playing around with getting weed whackers up and running.

Depending on how they are stored, the diaphragms get dried out and brittle if left dry after a season or two of running gas through them. The Walbros I worked with also have a tiny float chamber, so if the needle gets stuck you will have no fuel. This will be opposite the pump diaphragm, just as important and this one can dry out too.

Did you align the pump pulse port in the carb-to-case gasket when you bolted it back on?

[g-manz35]

Originally Posted by Cougar429

Did you align the pump pulse port in the carb-to-case gasket when you bolted it back on?

Yes I did, but the gasket is alittle rough and I was thinking it might be leaking by somewere and not pumping the diaphram properly. I will work on it a bit more tomorrow.

George

[RAMJET]

some carbs have a governor ,when the engine hits a given RPM the carb floods as a way to limit RPM ,most of them have a brass plug in the side of the carb & the passage can be blocked off .Roger

sounds like yours needs a rebuild ,any small engine shop should have the parts.

[grizak]

Sounds like your pulse port may be blocked.

Not sure how the pulse port is configured on your engine (whether it is directly on the carb into the crankcase, or taped off the crankcase and feed through external fuel tubing).

It could be a gasket that isn't aligned properly (blocking the pulse port hole), or it could be debris stuck in pulse port tubing or nipple.


Grizak

[g-manz35]

The pulse port comes right from the head straight to the carb. This carb is from a different engine so the gasket is perhaps leaking out between the carb and block. The hole in the gasket does line up properly. If the diaphram was leaking by would it be visable when looking at it? I pulled the diaphram out and cleaned ity wiht carb cleaner and put it back in then it wouldn't work at all.

George

[HornetFitter]

At the risk of repeating what some of the other guys have said, here is what I would do or advise. Take it for what it's worth...

1. It sounds as though the carb high speed needle was a bit lean (fluctuating RPM at wide open throttle).

2. Carb cleaner is brutal on rubber. The diaphram is fabric with a vulcanized rubber coating. It could have been damaged by the carb cleaner.

3. The diaphram can be inspected for cracking on the circular "ring", which will result in low fuel pressure and a lean condition at wide open throttle.

4. Try to use fuel that contains no ethanol. (it is very hard on rubber).

5. Ensure that the pulse port in the intake adaptor (between the carb and the cylinder) is aligned so that the carb can pump fuel.

6. Take a trip to your local (small) airport and ask if you can purchase some 100LL AvGas. It contains tetraethyl lead which is a lubricant, as well as stabilizers to prevent gum (white or yellow deposits inside the carburetor) and corrosion which will plug the small orfices and air/fuel bleed ports.

You can buy a rebuild kit for about 10 bucks for most Walbro carbs... just Google it for something close to you, or go to a small engine repair shop.

[g-manz35]

Originally Posted by HornetFitter

At the risk of repeating what some of the other guys have said, here is what I would do or advise. Take it for what it's worth...

1. It sounds as though the carb high speed needle was a bit lean (fluctuating RPM at wide open throttle).

2. Carb cleaner is brutal on rubber. The diaphram is fabric with a vulcanized rubber coating. It could have been damaged by the carb cleaner.

3. The diaphram can be inspected for cracking on the circular "ring", which will result in low fuel pressure and a lean condition at wide open throttle.

4. Try to use fuel that contains no ethanol. (it is very hard on rubber).

5. Ensure that the pulse port in the intake adaptor (between the carb and the cylinder) is aligned so that the carb can pump fuel.

6. Take a trip to your local (small) airport and ask if you can purchase some 100LL AvGas. It contains tetraethyl lead which is a lubricant, as well as stabilizers to prevent gum (white or yellow deposits inside the carburetor) and corrosion which will plug the small orfices and air/fuel bleed ports.

You can buy a rebuild kit for about 10 bucks for most Walbro carbs... just Google it for something close to you, or go to a small engine repair shop.

Thanks!!

George

[Luke Planemaker]

I get my repair kits at NAPA, small ones under $10. If the diaphram isn't floppy supple, change it. Check the pump side, stuff accumulates in the screen. Also if stored a long time, the flap valves can stick, preventing fuel movement.

I remove everything, blow out needle holes with compressed air. Clean screens, or put new from kit. Pump side gets gasket with flaps first (so flaps can seal against aluminum and work as check valves) than other one. Main needle chamber gets spacer gasket first, then diaphram.
Some diaphram you have to hook the aluminum pin into fork in main needle lever. Some have screens under welsh plugs, but don't remove unless you have replacements ready (I ruined one on my husky last week)

A vacuum leak on crankcase or pulse port can cause problems, as can too big a carb. How big a carb (venturi) is it?

You probably know most of this already, but maybe it'll help, just by getting you thinking again.