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Carb rebuilding how to ?

junkman

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One of my faults is sometimes i can fix a carb ,sometimes i can not ,is there more to it than a kit and a can of carb cleaner ? sometimes they work after cleaned out ,sometimes they do not ,like my string trimmers .what am i missing besides blowing out the passages on something that will not start ? Anyone care to do a in detail how to on this subject ?
 

trooney

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Yeah I'd be interested in it too. I think you've got to understand how a carb works to get a thorough knowledge of the workings of them. I know the basics but as far as fuel flow, vacuum and such not so much. How can you tell a check valve went bad, and can it be fixed?
 

Terry Syd

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How can you tell a check valve went bad, and can it be fixed?

When the saw is at an idle with the throttle plate closed, there isn't much air flow across the discharge nozzle (check valve) to create a low pressure on the discharge nozzle. That's why the check valve is there, it prevents air from being pulled back into the 'wet side' of the carb.

Inside the carb, the fuel is being sucked out of the 'wet side' of the carb. It is easier to suck in air through the discharge nozzle than fuel through the metering system. Eventually, you run out of fuel and the engine quits idling.

If you have a saw that will idle for a little bit, then have the revs increase a bit (leaning out) just before it dies, that could be a symptom of a bad check valve. If you have to use the choke to get it restarted (even when the saw is already warm), that's another sign it is a bad check valve.

Unfortunately, I don't know of a way to fix the bad check valve other than putting in a new discharge nozzle. However, they're not that expensive and easy to replace.
 

angelo c

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When the saw is at an idle with the throttle plate closed, there isn't much air flow across the discharge nozzle (check valve) to create a low pressure on the discharge nozzle. That's why the check valve is there, it prevents air from being pulled back into the 'wet side' of the carb.

Inside the carb, the fuel is being sucked out of the 'wet side' of the carb. It is easier to suck in air through the discharge nozzle than fuel through the metering system. Eventually, you run out of fuel and the engine quits idling.

If you have a saw that will idle for a little bit, then have the revs increase a bit (leaning out) just before it dies, that could be a symptom of a bad check valve. If you have to use the choke to get it restarted (even when the saw is already warm), that's another sign it is a bad check valve.

Unfortunately, I don't know of a way to fix the bad check valve other than putting in a new discharge nozzle. However, they're not that expensive and easy to replace.

Terry,
good to see ya here.
Maybe a explanation of a 'pop-off' test might help some guys too.
those were some great posts you had on that other site.

guys, if'n y'all don't know Terry, he's VERY good in a carb.
 

Terry Syd

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Oh Gawd, don't say stuff like he's VERY good in a carb. I mentioned on the other site that it took me three months to find a leaking welch plug - that I had installed!

I suppose you mean 'pop-off' as in trying to find out if your metering valve is leaking. Perhaps someone with pump and gauge set-up could show some pictures (pictures, they're worth a thousand words).
 

Duane(Pa)

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I want to learn some stuff here too. I don't KNOW this, but I think you can do plenty of harm with compressed air. Blowing stuff into oblivion I guess. Any truth to this?
 

Lee H

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Having a pop off pump and gauge is a good tool to have when
rebuilding carbs. Things to pay attention are gaskets and diaphragms.
Make sure they are exactly like to originals. Hope the carb still has the
originals as i have had my fair share of previously rebuilt carbs.
Diaphragm orientation is also a spot that gets over looked. They have
to be assembled in certain way. I very rarely do the welch plugs but
some really dirty carbs like ones that haven't been run in 30 plus years
may need to have them removed.
Keep in mind that not all carbs are salvageable especially when it comes
the very small ones on trimmers, blowers and other small equipment.
I did one on a trimmer and had it apart like 4 times. Still ran like crap and
wouldn't pull fuel right. I finally took my shop air host blow it through
the high needle hole with carb mounted and assembled. Worked after that
one quick shot of air. These small carbs have tiny passages that sometimes
can't be unclogged.


Lee
 

angelo c

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I want to learn some stuff here too. I don't KNOW this, but I think you can do plenty of harm with compressed air. Blowing stuff into oblivion I guess. Any truth to this?

Gotta be regulated or with a mity vac with a gauge. 10 psi max.
 

skidooguy

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I have used cutting torch tip cleaners on a few occasions to clear out jets and other small carb passages. Don't know if that's a very good practice or not but it works I guess.
 

SixGun

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My dentist gave me a box of root canal files, they are super strong and taper down to a fine point. I use them on carbs all the time .They are spiral cut files so they grab all the grime and pull it out.
I have had many carbs that just didnt work anymore . sometimes its better to get a new one than spending hours cleaning, mounting, testing and doing it again
 

CR888

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Another thing about check valves is they either work or don't. Its not that common for them to fail but an easy way to check them is pop out the welsh plug above main dischare nozzel and get at few inches of fuel line put it over and you should be able to blow through air and if you suck (haha if u suc!) it should be much harder to bring air through, it won't stop ALL air but restrict most of it. If you don't want to remove welsh, you can do same test by removing H needle and putting a piece of fuel line in their and doing the same thing.
 

CR888

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Qtips soaked in fuel/solvent are good for gently cleaning the inlet seat after you remove the inlet needle. I used to be a 'I hate friggen faulty carbs' person and would replace them in short order if some carb cleaner and air did't magically solve the unknown problem. Nowdays I like them and enjoy the challenge. We always fear what we understand least. Properly reading the Walbro/Zamma tech books (about 30 pages with big pics) is a very good start, but hands on is essential too.
 
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czar800

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Qtips soaked in fuel/solvent are good for gently cleaning the inlet seat after you remove the inlet needle. I used to be a 'I hate figgen faulty carbs' person and would replace them in short order if some carb cleaner and air did't magically solve the unknown problem. Nowdays I like them and enjoy the challenge. We always fear what we understand least. Properly reading the Walbro/Zamma tech books (about 30 pages with big pics) is a very good start, but hands on is essential too.

Well said, you have a link to the books?
I'm not a carb expert, but when I rebuild one I just slow down and be a little more careful.
 

jacob j.

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Sometimes- you can just take the carb body if it's from a saw that's been sitting a long time and just soak it in clean straight gas for a few hours and that will loosen all the gunk in the small passageways. I've resurrected a few carbs that way.
 

CR888

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Well said, you have a link to the books?
I'm not a carb expert, but when I rebuild one I just slow down and be a little more careful.
I am not good at computer stuff, but I have them both in pdf form. They have been posted in threads on 'the other' site but google 'Walbro tech guide' and 'Zamma cube carb service' and you will find them. You don't 'need' the pro tools to do carb servicing, just like you don't need factory tools to service saws but......the proper tools found in the Walbro P/N 500-500 kit really enable you to do a far more professional job. So a good start would be that kit (50-$80), maybe the box of various metering springs ($30-$60) and a carb pressure tester Walbro p/n 57-11 (I have a mityvac 8500 press/vac tester but the walbro being much smaller is easier to use) to test pop-off and air leaks. A ultra sonic is on my list when I find a good deal on a decent one. Micro drill set and pin vise. Cans of carb cleaner. But another thing is instead of chucking no good carbs in the bin start keeping them or stripping them down so you have a a parts bank, that gives you options and allows cost effective repairs. There's more stuff too but those things are the basics that will get used the most. Practice and patience are also required, l got sick and tired of letting carbs beat me down, now I appreciate the role they play.
 

czar800

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I am not good at computer stuff, but I have them both in pdf form. They have been posted in threads on 'the other' site but google 'Walbro tech guide' and 'Zamma cube carb service' and you will find them. You don't 'need' the pro tools to do carb servicing, just like you don't need factory tools to service saws but......the proper tools found in the Walbro P/N 500-500 kit really enable you to do a far more professional job. So a good start would be that kit (50-$80), maybe the box of various metering springs ($30-$60) and a carb pressure tester Walbro p/n 57-11 (I have a mityvac 8500 press/vac tester but the walbro being much smaller is easier to use) to test pop-off and air leaks. A ultra sonic is on my list when I find a good deal on a decent one. Micro drill set and pin vise. Cans of carb cleaner. But another thing is instead of chucking no good carbs in the bin start keeping them or stripping them down so you have a a parts bank, that gives you options and allows cost effective repairs. There's more stuff too but those things are the basics that will get used the most. Practice and patience are also required, l got sick and tired of letting carbs beat me down, now I appreciate the role they play.

Thanks I'll take a look over there.
 
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