# Seeking Help with diaphragm carburetors



## bdawg8569 (Jul 6, 2014)

Hello there,
I am new to the forum and was hoping I might be able to pick somebodies brain on secrets to working with Diaphragm carbs. I have been working on small engines for a couple years now and while i'm by no means an expert, i usually do pretty well with anything with a float bowl carb. 

Lately i have been working on a few weed eaters and handheld blowers and I keep coming across the same situation. I have replaced fuel lines, fuel filter, and installed a new diaphragm from a gasket kit and still can't get them to start. I have a couple like this. One is a Ryobi weed eater, and another is a weedeater brand leaf blower. Both units have spark and will fire on starting fluid or gas put in the cylinder. 

Anybody have any good nuggets of information on how to proceed when this type of thing happens? I really love working on small engine stuff but its very discouraging when I can't make any progress on a few items. Thanks for your time.


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## dawgpile (Apr 28, 2008)

When you say you replaced the diaphragm, are you referring to the metering diaphragm or the fuel pump diaphragm? The metering one is the part that has the round ring on it and is on the side of the carb with the inlet needle.

The fuel pump diaphragm is on the other side and is typically either a rubber like material or a mylar film. It basically has two flaps on it that act as check valves.

If you take the time to take it apart, and assuming you bought a gasket kit as opposed to an individual item, it makes sense to go ahead and replace both diaphragms and both gaskets. (Note the gasket goes next to the carb body on the metering side, then diaphragm, then cover, and the pump diaphragm goes next to the carb body, then the gasket, then cover on the other side. It makes a difference!)

A common cause of the problem you describe is the inlet screen in the carb has gotten clogged or gunked over with debris. This screen is on the pump diaphragm side of the carb. If you by chance only replaced the metering diaphragm and didn't open up the pump side of the carb, it's quite possible the screen is clogged. Pull the pump side apart, note the order of the pump diaphragm and gasket, clean the inlet screen and reassemble with new a new pump diaphragm and gasket out of your kit.

Finally, I'd suggest going to the manufacturer's website. There are great resources like service manuals, videos, etc. Most 2 cycle carbs are made by either ZAMA or WALBRO. Check out their sites as they contain a veritable cornicopia of info!

Give that a try and let us know how you make out!

Kelly


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## bdawg8569 (Jul 6, 2014)

Thanks Kelly for the advice, but unfortunately I did replace both diaphragms and both gaskets. I didn't actually replace the screen you are referring to, but i tried to blow it out with compressed air. I have seen lots of videos on doing carb kits on these. But when it still doesn't work i'm not sure what to do.


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## dawgpile (Apr 28, 2008)

You don't have to replace the screen as it usually cleans up nicely by spraying it with carb cleaner.

A couple other items to check...

-Fuel filter stone. Make sure it's not plugged.
-Compression. Most folks on this forum have settled on 90psi or greater for 2 cycle machines. Poor compression due to a worn cylinder leads to hard starting.
-Muffler/Exhaust. Make sure it's not plugged by checking for carbon buildup in the cylinder exhaust port, muffler itself(usually spark arrestor screen), etc.
-Intake/cylinder Leakage. This can be hard to find if you can't get it running in the first place. Best thing to do is make sure your gaskets are good between carb and cylinder and any blocks in between and the carb is secure with even torque on both mounting screws. Make sure the pulse port from the head to the carb is not blocked. Check the fasteners holding the cylinder head and make sure they are all tight.

Finally, if you are going to be doing more than just the occasional repair, I think it's worth investing in an ultrasonic cleaner to clean the carb with. They are in the 20 some dollar range with a 20%off coupon at Harbor Freight. It can be helpful when you run into the unit that hasn't been run in years.

Hope this helps some...

Kelly


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## bdawg8569 (Jul 6, 2014)

Kelly,
Thanks for taking the time to reply. I have seen the ultra sonic cleaning units in a few videos. I usually soak most of the metal pieces in laquer thinner for a couple hours when cleaning carbs. I may see about getting one of the ultra sonic units though cause i do a lot of small engine work. 

I did a compression test on one of the 2 units i was having trouble with. It only measured about 80 PSI, but it starts fine with fuel in the cylinder so i'm still inclined to believe its a fuel delivery issue. 

I have not taken off the exhaust, but again with it firing on fuel in the cylinder or sprayed in the carb, this doesn't seem to be a likely cause. 

I didn't actually change out the fuel filter on either of them, but both seem to be working fine. I blew them out with compressed air, and they are pulling fuel through the carb when priming it. 

At times like this i wish i had a small engine expert that could show me in person what i'm missing. Thanks for all your suggestions!

Brian


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## bob1190 (Apr 18, 2010)

Very possible you have a vacuum or pressure leak. Do you have a Myty-vac to test vacuum and pressure?


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## Sir Thomas (Dec 7, 2013)

Sometimes we look for complicated problems and over look the simple.
The gasket(s), some units have more than one, between the carburator and the cylindar head. Some have more than one hole in it and some only has one. Make sure that you have the holes lined up. If the gasket has a single hole and it's turned wrong, then you don't get the vacuum to operate the metering diaphram. On some machines you have a heat dam, carb adapter or extender which also has holes in them. Make sure they are all clear.
Do you have a template that helps you set the metering valve lever? On some carbs, the edge of the lever is flush with the body. On these the Tet on the metering diaphram is real shallow, like 1/16". Other's like the lever flush with the base of the metering chamber. These have Tet's that are almost 1/8".
Did the kit come with metering check valve? What color is the tip of the old one?


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## geogrubb (Jul 28, 2006)

IMHO, using anything other than spray can carb/brake parts cleaner on the small cube carbs is going to do more damage than repair. Put a piece of aluminum foil in a ultrasonic cleaner, it will eat holes in it and will probably do the same thing to check valves in a carb if left in the cleaner too long. Once a check valve is damaged the carb is junk since most of them can't be replaced, poking wires and blowing with a high pressure are sure ways to destroy a carb. If it can't be cleaned with the pressure from a spray can it is probably beyond repair and you are better off just buying a new one. Have a good one. Geo


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## Sir Thomas (Dec 7, 2013)

geogrubb said:


> IMHO, using anything other than spray can carb/brake parts cleaner on the small cube carbs is going to do more damage than repair. Put a piece of aluminum foil in a ultrasonic cleaner, it will eat holes in it and will probably do the same thing to check valves in a carb if left in the cleaner too long. Once a check valve is damaged the carb is junk since most of them can't be replaced, poking wires and blowing with a high pressure are sure ways to destroy a carb. If it can't be cleaned with the pressure from a spray can it is probably beyond repair and you are better off just buying a new one. Have a good one. Geo


Yeah, most of your run of the mill "Carb Cleaners" are for larger carbs not the cube carbs found in small two cycle engine. I destroyed the first cube carbs by using a carb cleanner and compressed air (type used to clean computer keyboards). If the cleaner doesn't desolve the check valve the compress air will blow them out. Like Georgrubb says most check valves are not replaceable so you can kiss the carb good bye.


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