Carb rebuild, and trouble shooting

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Seandonato73

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So I've been noticing a lot of carb issues as of late. Some from new guys some from not so new guys. Thinks it's about time we have a info page. I'll start with the walbro WT series manual and IPL (illustrated parts manual the ipl doesn't cover our specific models, but it points out all the parts and names for reference. All of the same series carbs are basically the same, with minor changes.) for reference.
I currently have 4 carbs that need rebuilt (for saws) I think 3 of them are WT series. So I'll post up how I rebuild them.
Let me know if you want anything more in depth then the basics.
Cheers.
 

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Ok so some more information, I take credit for none of it.
Carb setting chart:
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Our most popular carbs and venturi specs
Walbro WT-668 15.88mm bore 12.7mm venturi. One of the more common std carbs for rc, under 32cc displacement engines.

Walbro WT-990 15.88mm bore 12.7mm venturi. no choke plate built into carb. "Race version of 668"

Walbro WT-1107 15.88mm bore 13.5mm venturi. Std on Zen 32cc, upgrade for smaller displacement engines. Easy to tune.
Honorable mentions. WT-608, WT-813. not as popular as above.
The 608 is another stock small cc carb. Worthless imo.
The 813 is a 990 with an accelerator pump. Its note worthy that the pump is not replaceable. And if it ? you need a new carb. Theres no real advantage over the 990, unless your tuning to the edge of performance.

Rebuild kits.
K10-WAT. For WT-813 and WT-990
K20-WAT. For WT-608, 668, AND 1107.
K27-WAT. Master WT series rebuild kit. Come with everything you would need for a full on rebuild. Not all components are specific to our carbs, but has all the Welch plugs, ect that you may need.
Thanks to @Top Dead Center for the kit number

Make sure the kits come with a tan or brown fuel pump diaphragm. The blue ones are trash, it's a hard material, doesn't move very well, especially on smaller displacement engines with weaker pulses. Most new genuine kits should come with the tan diaphram. If it don't it's old stock.
Honorable mention,
K3-WYTA spiral diaphram.
Not needed, and it does run about $18.00 currently. But it is impervious to ethanol. I noticed a slight improvement in acceleration installing it in my WT-1107. It comes with a new style gauge for setting the needle level. Pic from DDM sight
20200430_115000.jpg
As far as I know, it should fit any carb mentioned above.

Tools: there are 2 tool kits recommended by walbro. (Sorry tdc, not getting into zama right now) you don't really need the kit but it sure does make life a bit easier.

The old kit is a 500-500. Not a bad kit. doesn't cover newer carbs. Walbro lists it as NLA. it can be had on amazon for around $50.00 (some a bit less)

New updated kit. 500-538 $78.00
Screenshot_20200430-133555_Samsung Internet.jpg

https://shop.walbro.com/collections/products/products/500-538-tool-kit

(This would be my recommendation if your gonna buy a one stop kit. Will work on 95% of all walbro carbs ever produced)
I've rebuilt countless carbs with out a tool kit. When I started doing carbs for friends and neighbors I sucked it up and got the 538 kit. Live is easier with factory tools. That's just a fact.
For those of you not interested in the tool kit. You at minimum need the gauge to set the metering lever.

https://smile.amazon.com/dp/B01HP266JO/ref=cm_sw_r_apa_i_jQWQEbE7JRCTC
if your serious about rebuilding all your carbs it is cheaper to get the 2 pack that has the zama gauge too. (Refer to @Top Dead Center post)
 
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I am, idk. Jury is still out. It should in theory last forever. But when the fuel pump craps and you buy a kit, it has a new diaphragm with it. I did notice better acceleration. But for the price I don't think it's worth it. Under normal conditions your carb will need a kit every 2 to 4 years. Depending on fuel used and how often you run it. So the real question you should ask is, how much is not having to mess with half your carb worth?
 
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For those interested in the spiral diaphram.
This is the 2020 list for WT series carbs.
Pdf. Found on walbro we sight...
 

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Ok for the meat and potatoes portion.
First things first (im going to assume you know how to take the carb off)

Give the outside a bit of a cleaning before disassembly. I just use a brush and some carb cleaner. Not getting real particular at this point. Get most the crud off, and carry on.
There are 2 sides of the carb. The fuel pump side
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And the metering side
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Let's start with taking it apart
Remove the screw in the middle of the pump side. Pop the cover off. You'll find a gasket, and the pump valve/ diaphragm membrane.
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Set them aside for now. (So you can make sure you have the right stuff from your kit)
Next pop out the screen. This is kinda like a last chance filter of sorts.
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Just stick a pick in the center of it and pop it out. This can be trashed. The kits mentioned above only come with one screen.
Next flip the carb around and take the 4 screws holding the primer bulb on, out.
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Remove the primer bulb, plastic spacer, metering diaphragm, and gasket.
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9 times out of 10 the gasket and diaphragm will be stuck together. Just set them aside for now.
Now remove the needle, lever, and spring. Do not loose the spring! Contrary to popular belief the engine manufacturers spec what spring to use in the carb. The kits may not come with the right spring. They are generic kits for a particular set of carbs. Reuse you original spring.
Theres a wee little screw to take out, that holds the whole assembly in.
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Now I don't always do this part, and truth be told it doesn't really need done unless the inside of the carb is really crapped up.
And that is to remove the Welch plug covering the idle passages.
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Basically you need to poke or drill a small hole in the center of it, and pry it out.
DO NOT DRILL TOO FAR! I'm talking an 1/8" bit or smaller. Just drill far enough till you can get a pick in the center to pry it out. If you drill too far, DDM sells new carbs.
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I mean small hole, and I normally don't even try to drill through it. Just need to get a pick in there. Not held in by much
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And here's the idle holes.
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Now take your mixture screws out. Turn them in till they lightly bottom out while counting how many turns it takes to bottom out. Write this down so you can reset your carb to where you had it adjusted.
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Now this is where my process may differ a bit from yours.
I do tons of carbs a year for small engines. So a long time ago I scored a small ultrasonic cleaner (heated) at a flee market.
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It's old as sin. Works perfect though. Toss the carb and all associated parts in it. Fill it with hot water and some simple green, or whatever you like. (Nothing with strong acid in it. It will etch the aluminum) and go grab a beer, or three...
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If you don't have an ultrasonic cleaner. You get to scrub. Carb cleaner works well. Make sure you get it perfectly clean. I will not say I'm a fan of jet cleaner rods. Too much of a chance of messing a calibrated hole up. If you cant squirt some carb cleaner through it to satisfaction, its junk imo. Remember this is a $40.00 part. Not worth it not being right and messing up a $400.00 +/- engine.
 
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Now the metering side.
Grab the welch plug and pit it in its hole.
Take the 5/16" punch and give it a tap to seat it. Flat ish is what your going for. (I'm not using any tools out of my walbro kit, save one. So you can all see that common tools get the job done, the tool for the welch plug is way nicer then the punch)
15888836957416472787624395068989.jpg
I smacked this one a tad too hard, but it will be ok.
Now grab the needle, lever, shaft, and your old spring.
Set the spring in place
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Put the shaft through the lever, and place the needle on the fork side of the lever
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Stick it back where it belongs and secure with the screw removed earlier. This is sometimes easier said then done.
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Next make sure the lever is positioned properly. No way around this you need the walbro tool. Use the side of the tool appropriate for your carb series. (WT in our case) you want to adjust the metering lever just till it touches the gauge.
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This is without a doubt the most important part of assembling a carb. Too high and it will flood the engine at idle, too low and it wont open to let enough fuel in at full throttle. So take your time and make it right.
Next up gasket and diaphragm.
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Make sure the machined part is pointed down towards the lever. This is what allows fuel into the metering side of the carb, to feed the jets.
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Clear as mud?
Now just bolt the plastic spacer and primer bulb back on. Good chance to check the condition of the check valve and primer bulb. Replace as necessary. (I haven't replaced check valves in years. The material was updated, and takes alchohol rather well.)
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This primer is ok for another use.
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Reinstall your mixture adjusting needles, to whatever turns they were set at, or refer above for stock settings
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Cant mess this up and they are different size threads for low and high. Low is the smaller high is bigger.
Congratulations you just did a carb rebuild. Go bolt it on, and see how it works.
No Spiral Diaphragm Sean?
Nope, literally tore this one apart because the carbs on my saws don't have primers. If you notice I was going back between 2 carbs lol. This is an old 668 I had laying around I used for most of the pics. I have a few more carbs to do. One is getting the spiral, just waiting for it to show up. I'll post up pics of that procedure too, if you like.
Edit: anything else anyone wants to see with carbs?
 
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My man... thanks ?

I use to have a bunch of old stock rebuild kits ,some of them were for some huge carbs ,unlike what I
normally see ,ive never seen them before ,they look like the smaller carbs that are being used today ,
but they are bigger in size!

Do you know what they might be for ,maybe a old style larger chainsaw?
 
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