Excellent thread!! Thank you for taking the time to post all this info.

 

I agree, great job!!

 

Thanks for the info.Being a amateur small engine mechanic it's nice to have one more option in my repertoire.

 

Excellent step by step post.

 

A main nozzle check valve prevents air from entering the carb on idle. When faulty the unit idles poorly and dies on acceleration.

There are two main types: mechanical (like this one) and capillary screen.

Note: only butterfly carbs need check valves, another reason why I prefer barrel carbs.

 

The check valves, idle and main, prevent air from entering the metering chamber when using the primer. Any carb with a primer has check valves. They serve no other purpose.

 

Would it then follow that if you are getting air in the primer bulb when attempting to prime the engine, despite the fuel filter being immersed completely in fuel, that the check valve has gone bad? Any way to know which check valve? I see Maico490 describing putting a piece of hose over the check valve and alternatively blowing and sucking, but I'm not clear on how you get that piece of hose to seal over the check valve.

 

The check valves, idle and main, prevent air from entering the metering chamber when using the primer. Any carb with a primer has check valves. They serve no other purpose.

Partially true, but I'm 99% sure the main nozzle check valve in a butterfly carb is crucial for a stable idle.

Either way a faulty main valve will allow the purge bulb to suck air through the nozzle rather than fuel from the tank.

And yes you can check a mechanical main valve with a piece of hose. Not so with a screen valve, these really can't go wrong other than plugging up which would cause other problems.

Of course you could stop fiddling and get a new carb :laugh:.

 

Partially true, but I'm 99% sure the main nozzle check valve in a butterfly carb is crucial for a stable idle.

Either way a faulty main valve will allow the purge bulb to suck air through the nozzle rather than fuel from the tank.

And yes you can check a mechanical main valve with a piece of hose. Not so with a screen valve, these really can't go wrong other than plugging up which would cause other problems.

Of course you could stop fiddling and get a new carb :laugh:.

Zamba is the only Echo used carb that I can't figure out.....often a tear down and cleaning and new kit doesn't do much of anything. I got three of them in the shop now and debating whether to buy the $11 kits.

 

Partially true, but I'm 99% sure the main nozzle check valve in a butterfly carb is crucial for a stable idle.

Either way a faulty main valve will allow the purge bulb to suck air through the nozzle rather than fuel from the tank.

And yes you can check a mechanical main valve with a piece of hose. Not so with a screen valve, these really can't go wrong other than plugging up which would cause other problems.

Of course you could stop fiddling and get a new carb :laugh:.

Then working off that theory, a float feed carb would need a check valve also, wouldn't it?. There is no fuel being pulled off the main when the throttle plate is closed, so there is no need to shut it off. AND, there are check valves in the primer flange also, which don't affect running but will affect primer operation. I definitely agree with just replacing the carb. Replacing the check valves in the body can be done and the repair will work if in fact that is all that is wrong with that carb.
It's not worth the time in a repair shop environment, and I wouldn't waste my personal time (unless the repair got "personal')

 

Then working off that theory, a float feed carb would need a check valve also, wouldn't it?.

Well no, first you must distinguish between "primer" (enrichment) and "purge", both use bulbs that look alike.

Most membrane carbs (2-stroke) have a purge system, cross-named by 90% of people on this forum as a "primer". A purge system removes air from the carb by flushing/recirculating fuel from the tank, effect is same is you press the bulb 5x or 100x. It does not prime, instead enrichment is achieved by a separate mechanism: closing the choke.

Float carbs (4-stroke) don't need purging, because gravity does the work for you when you open the fuel valve. Enrichment is by choke or on cheaper engines a real primer. This pressurizes the bowl, squirting a small amount of fuel in the intake. Unlike purge, pressing the primer too many times will flood the engine.

 

Well no, first you must distinguish between "primer" (enrichment) and "purge", both use bulbs that look alike.

Most membrane carbs (2-stroke) have a purge system, cross-named by 90% of people on this forum as a "primer". A purge system removes air from the carb by flushing/recirculating fuel from the tank, effect is same is you press the bulb 5x or 100x. It does not prime, instead enrichment is achieved by a separate mechanism: closing the choke.

Float carbs (4-stroke) don't need purging, because gravity does the work for you when you open the fuel valve. Enrichment is by choke or on cheaper engines a real primer. This pressurizes the bowl, squirting a small amount of fuel in the intake. Unlike purge, pressing the primer too many times will flood the engine.

jkilov,
Interesting distinction between purge and primer bulbs, the reason for each, and the consequences of pushing that little bulb too many times. I've wondered about flooding the engine by pressing the bulb too many times. Sounds like on a 2 stroke I don't have to worry about that. Seems like if the tank were positioned above the carb you wouldn't need the purge system on a 2 stroke, either. Having ridden 2 stroke dirt bikes and having had 2 stroke outboards most of my life, none of which had a primer bulb, I always wondered why they had to put primer bulbs and much more complicated, multi-step starting instructions on all these handheld pieces of lawn equipment. On all my other 2 strokes you just choke it until it fires, then alternate between choke off and throttle open and choking it until it fires and runs.
David Boyd

 

Seems like if the tank were positioned above the carb you wouldn't need the purge system on a 2 stroke

Any you wouldn't need a membrane carb to begin with, just a regular float. 2-strokes and membrane carbs go hand in hand, both operate in any position. Oh and Dave, I ride bikes too :drinkup:

Zamba is the only Echo used carb that I can't figure out.....often a tear down and cleaning and new kit doesn't do much of anything. I got three of them in the shop now and debating whether to buy the $11 kits.

Zamba sounds like house dance or voodoo. Anyway, Zama (owned by Stihl) had several faulty batches of carbs. Something with aluminum materials developing hairline cracks making the carbs leak internally, these can't be fixed. The correct way to diagnose it is to do a carb leak down test. Since 3 new carbs cost some money I would try to !properly! clean them.

 

Well no, first you must distinguish between "primer" (enrichment) and "purge", both use bulbs that look alike.

Most membrane carbs (2-stroke) have a purge system, cross-named by 90% of people on this forum as a "primer". A purge system removes air from the carb by flushing/recirculating fuel from the tank, effect is same is you press the bulb 5x or 100x. It does not prime, instead enrichment is achieved by a separate mechanism: closing the choke.

Float carbs (4-stroke) don't need purging, because gravity does the work for you when you open the fuel valve. Enrichment is by choke or on cheaper engines a real primer. This pressurizes the bowl, squirting a small amount of fuel in the intake. Unlike purge, pressing the primer too many times will flood the engine.

Well no. You stated that the main check valve prevents air from entering the carb at idle. That is not true. The check valves are for primer operation. So your explaination about primer types and their respective operation really has nothing to do it.

 

Oh and Dave, I ride bikes too :drinkup:

A lot of dirt bikers on this thread. It was started by Maico490.:drinkup:

 

Would it then follow that if you are getting air in the primer bulb when attempting to prime the engine, despite the fuel filter being immersed completely in fuel, that the check valve has gone bad? Any way to know which check valve?. I see Maico490 describing putting a piece of hose over the check valve and alternatively blowing and sucking, but I'm not clear on how you get that piece of hose to seal over the check valve

Just take a piece of tygon tubing ( fuel line hose ) and cut the end square with a razor blade. Then hold/push to seal the cut end against the flat of the check valve and suck/blow. I use the Stihl vac/press. tool but not really necessary.

 

Well no. You stated that the main check valve prevents air from entering the carb at idle. That is not true. The check valves are for primer operation. So your explaination about primer types and their respective operation really has nothing to do it.

Your wrong, bud !
jkilov is right on the money with his accurate and helpful description! Some people need to pay more attention !

 

Just take a piece of tygon tubing ( fuel line hose ) and cut the end square with a razor blade. Then hold/push to seal the cut end against the flat of the check valve and suck/blow. I use the Stihl vac/press. tool but not really necessary.

Thnx BigFish.

 

Great thread and very informative, but I got a little confused as to whether a bad check valve would cause a flooding condition if the purge bulb was pushed excessively. I have a Husqvarna 326 LS trimmer with a C1Q carb. Pushing the purge bulb more than what is required to just fill the bulb causes the engine to flood. Engine starts and runs fine with just 1-2 pushes on the bulb. More than that and the engine floods. Is the check-valve bad? Thanks, oli

 

The check valves, IN the carb, are suppose to close when the bulb is used. If they were not working (stuck open), the primer wouldn't work very well, if at all. They prevent air from being sucked into the metering chamber, through the main and idle circuit, during the priming process. If air is sucked in, the purge process can't work. If they are stuck closed, no fuel can enter the engine, so it can't flood. You carb might have a slightly rich setting on the inlet needle, among other things. But if it starts, do you care?
There seems to be some dispute if they serve any purpose during running, which is OK. It's sort of a theory vs actual.

 

The check valves, IN the carb, are suppose to close when the bulb is used. If they were not working (stuck open), the primer wouldn't work very well, if at all. They prevent air from being sucked into the metering chamber, through the main and idle circuit, during the priming process. If air is sucked in, the purge process can't work. If they are stuck closed, no fuel can enter the engine, so it can't flood. You carb might have a slightly rich setting on the inlet needle, among other things. But if it starts, do you care?
There seems to be some dispute if they serve any purpose during running, which is OK. It's sort of a theory vs actual.

No disputing the facts. Go here http://www.zamacarb.com/pdfs/TechGuide_2007.pdf and READ paragraph 5 under IDLE OPERATION. Should make it clearer for ya.

Oli, it sounds like the check valve in the primer bulb base is bad/weak, which is pressurizing the metering chamber and squirting gas out the main nozzle.

 

I just love that highlight feature. It really make you look authoritative. Were you or are you a teacher?

 

I just love that highlight feature. It really make you look authoritative. Were you or are you a teacher?

Yeah, me too.

 

I just love that highlight feature. It really make you look authoritative. Were you or are you a teacher?

Ya think???

 

I just love that highlight feature. It really make you look authoritative. Were you or are you a teacher?

Nope, not me!