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Maico490
08-21-2010, 02:32 PM
Hi everyone. I'm a newbie here and have only posted once on a thread on a primer bulb going dry on a Stihl trimmer. Searching throws up the fact that a lot of people have symptoms which point towards check valve problems. The general reply is "Get a new Carb" which is the case if the check valve is not working on the idle circuit. However the main check valve is fairly easily replaced for $4. This is copied over from a post i made on a chainsaw site:


This has been my first experience with the notorious check valves found on newer carbs. I've been chasing this problem on a FS450 trimmer for a while but luckily it has been between cuts on Water Authority sites. Searching here didn't throw up much but these carbs are used on lots of blowers, trimmers and smaller saws. The carb is a Zama C1Q-S34H which is fixed jet with only what appears to be an air screw (reverse thread) for idle adjustment.Symptoms were hard starting, inconsistant idle which couldn't be adjusted and would run out of fuel with an inch still left in the tank. When operating the primer bulb you could keep pumping without it becoming full and air bubbles were visible in the bulb. At full throttle everything was fine but as soon as you let off it would often stop dead. New lines, primer bulb,fuel filter and a rebuild kit did nothing. The answer was eventually found on Zama's excellent website: http://www.zamacarb.com/tips.html.
Before you do anything to a Zama carb I would recommend reading all the Service Tips. Especially DO NOT BLOW COMPRESSED AIR THROUGH A CARB WITH CHECK VALVES. It seems you also have to be careful with aerosol carb cleaner if directed straight into the valve with an extension tube.

http://www.lawnsite.com/attachment.php?attachmentid=197272&stc=1&d=1282415370

The jet at the bottom of the metering chamber is the main jet which Zama call the "Nozzle Assembly Check Valve". Stihl call it the "Valve Jet".
The hole in the middle, which is normally hidden by the metering lever leads to the idle circuit. To test both circuits use a piece of hose over the jet and blow with your mouth. If clear then try to suck back. If you can the check valve is defective. In general if it is working you can hear a faint click. If you can suck back it is bad news on the idle circuit as it means a new carb but the main jet can be replaced. Zama do a special tool but I managed to use a cheap drill press and one of my extensive collection of broken drill bits. This is clamped in the chuck upside down. If you have adjusting screws remove the H before starting.

http://www.lawnsite.com/attachment.php?attachmentid=197273&stc=1&d=1282414086

It took a fair bit of fiddling to get everthing lined up and more force than I expected to get the nozzle to move. It pushes straight through into the bore of the carb. The chuck is stationary throughout. Here it is out and the new one:

http://www.lawnsite.com/attachment.php?attachmentid=197274&stc=1&d=1282414086

The new nozzle pushed in quite easily with a bit of oil on it to the same level as the old one.
A few minutes of reassembly and it fired right up. Ticks over fine now and is responsive to the LA screw.

Merkava_4
08-21-2010, 06:00 PM
Excellent thread!! Thank you for taking the time to post all this info. :)

BigFish
08-22-2010, 12:52 AM
I agree, great job!!

jtsnipe
08-22-2010, 05:50 PM
Thanks for the info.Being a amateur small engine mechanic it's nice to have one more option in my repertoire.

dboyd351
12-28-2010, 08:28 PM
Excellent step by step post.

jkilov
12-29-2010, 04:19 PM
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.

ricky86
12-29-2010, 06:11 PM
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.

dboyd351
12-29-2010, 06:27 PM
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.

jkilov
12-29-2010, 07:16 PM
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:.

ed2hess
12-29-2010, 10:18 PM
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.

ricky86
12-30-2010, 08:27 AM
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')

jkilov
12-30-2010, 11:27 AM
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.

dboyd351
12-30-2010, 11:57 AM
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

jkilov
12-30-2010, 02:20 PM
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.

ricky86
12-30-2010, 10:59 PM
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.

dboyd351
12-31-2010, 07:11 AM
Oh and Dave, I ride bikes too :drinkup:

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

BigFish
12-31-2010, 12:49 PM
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.

BigFish
12-31-2010, 12:54 PM
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 !

dboyd351
12-31-2010, 02:17 PM
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.

Oli
01-02-2011, 07:12 PM
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

ricky86
01-03-2011, 12:35 AM
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.

BigFish
01-03-2011, 09:16 AM
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.

ricky86
01-03-2011, 02:31 PM
I just love that highlight feature. It really make you look authoritative. Were you or are you a teacher?

BigFish
01-04-2011, 12:40 AM
I just love that highlight feature. It really make you look authoritative. Were you or are you a teacher?

Yeah, me too.

BigFish
01-04-2011, 12:41 AM
I just love that highlight feature. It really make you look authoritative. Were you or are you a teacher?

Ya think???

BigFish
01-04-2011, 12:45 AM
I just love that highlight feature. It really make you look authoritative. Were you or are you a teacher?

Nope, not me!

BigFish
01-04-2011, 12:50 AM
My apologies to Maico490 for unintended hijacking.
As mentioned before, your photos and narrative are great!!!

Just havin' a little fun.

jkilov
01-06-2011, 12:12 PM
Well no. You stated that the main check valve prevents air from entering the carb at idle. That is not true.
Well it is, and all butterfly membrane carbs, regardles if they have bulbs or not, need one.

Your question about 4-stroke float carbs needing check valves is kind of puzzling. Float and membrane carbs are not alike, not even close. Membrane carbs have "wet" idle & main circuits supplying fuel to multiple nozzles and any air will badly screw up metering. Float carbs have only one nozzle, the screw type part that holds the bowl, which mixes fuel with bleed air to form emulsion. This then travels up the emulsion tube or tubes to the throat or idle circuit. Some people mistake the emulsion tube for the main nozzle, which is debatable. So I dont quite get the question? There already is air the emulsion tube, it's coming from the bleed vent to make the carb work properly. If you meant what if it entered the nozzle at the bottom, well it can't cause fuel is always being drawn out, there's no idle detour like on membrane carbs.

Maico490
06-29-2011, 05:11 PM
I haven't looked back at this thread for a while but looks like there have been a few toys thrown out of the pram.
I can confirm that the check valves are 100% needed for correct running. At full throttle there is enough depression across the carb to overcome the main check valve and fuel flows through it. This is the case even if the check valve isn't working. However at idle if it is defective instead of fuel being drawn from the metering chamber air will be drawn back through the main jet and the engine will die.

ed2hess
08-07-2011, 05:10 PM
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 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.

Did they identify the lot numbers that were faulty? My unit is a RB7577A