backhoelover
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Carburetor Main Nozzle Replacement
- Thread starter SteveSr
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- main nozzle walbro zama
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That's not an easy answer. There are a plethera of variables to consider.
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I would assume that you have you put a "known good" , and same model carburetor on the saw in question with good results, which would rule out any mechanical issues?
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Did you confirm the carb in question that is on the saw is really the model that's supposed to be on the saw?
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Some saws had both walbro or zama as oem carbs through their production years. Some Zama carbs have accelerator pumps, which when bad, can cause a bad stumble or bog upon acceleration.
This can be caused by not only leaking air past the plunger o-ring through the throttle shaft bore, but the absence of a pump shot of fuel, which would further lean out the mixture of fuel for the transition from idle to WOT.
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What saw are we talking about?
Chris-PA
Where the Wild Things Are
I found the Tillotson diagram (attached), and it doesn't show any check valve nor is there any discussion of it. I took apart an old Walbro HDC which is similar in size, and it has a screen held in by a snap ring, although there is no welch plug over it. On the Walbro there is definitely a check valve behind the screen, and it is called out as such on the diagram.
The only way I can see the Tillotson HS10 design functioning is if the carb is relative small for the engine, such that the main venturi is pulling a bit of a vacuum even at idle - not enough to pull much fuel, but enough to keep air from feeding back down the H jet. Then the fine mesh screen might help seal it off if soaked with fuel due to surface tension? I dunno, I'd much rather have a check valve.
EDIT: I found this in the Walbro Service Manual: "The capillary screen seals rely on the capillary action of gasoline against a fine wire screen to keep air from coming back though them at idle"
I think this is pretty much what I described above, and I don't think it would seal against a large pressure differential.
Interesting design. Less moving parts.
I agree with you in that I'd rather have a check valve.
It must have not been a very good design, as it was superceeded.
As the carb wears, (throttle shafts, etc), maybe the combined air leaks create an overly lean condition that can't be corrected, or compensated for through tuning ?
I agree with you in that I'd rather have a check valve.
It must have not been a very good design, as it was superceeded.
As the carb wears, (throttle shafts, etc), maybe the combined air leaks create an overly lean condition that can't be corrected, or compensated for through tuning ?
Chris-PA
Where the Wild Things Are
Maybe lower oil ratios change the viscosity or surface tension?
SteveSr
Addicted to ArboristSite
The saws in question are an 021 with a Walbro WT-503 and an MS210 with a Zama. Both were fixed by changing out the carburetor.
SteveSr
Addicted to ArboristSite
Both of these carbs were fixed "H" jet - no adjustment. They would run correctly one minute but not the next. The 021 had already had the carb kitted which made no difference. The only thing left was a wacky nozzle check valve.
backhoelover
Free service manuals and service tool info
I had a leaky intake boot one time would pass press and vac test with proper tools. Saw wouldn't throttle up installed new carb etc.
didn't throttle up right until I changed the intake was weird stihl 021
didn't throttle up right until I changed the intake was weird stihl 021
Helvehammer
New Member
Hello Fellas - Helvehammer here. I have signed up just to comment on this main discharge check valve thread. IMO this is the one issue that separates the guys who just HAVE TO finally triumph over these little engines from the "wannabe-s" who want to APPEAR as if they know what they are doing !
I just had to run my mouth on the subject of "no check valve" carbs. Tillotson "H" and "HL" carbs as well as the much later "HS" had no check valves.
I noticed the only ones of those things that ever actually worked had the carb sitting level, with discharge at the bottom of the venturi - and super duper soft metering springs in relation to the size of the diaphragm that the spring worked against. I guess if you take gravity into account then they start to act like sort of a float bowl carb where the fuel "wants" to stay at a certain level. That level-effect resists air being pulled back in the main discharge hole because the fuel under it doesn't want to get out of the way for it to pass by. The super soft metering spring means the suction on the idle discharge needed to get enough fuel is very low suction indeed. Hey it was all we had in the old days if you didn't count Brown brand carburetors (and nobody DID !).
Okay - thread is a year old but man did the two-cycle world rejoice when Walbro carbs came out in the early 70s ! It took me a decade more to realize that Walbros had main discharge check valves and they were the cat's meow.
Lastly - guys - always always always make sure you got mechnical integrity (rings, crankcase seals, reed and base gaskets are tight) and ignition integrity
(spin it with an air drill, sparkplug out and a 3/16" spark gap - for 10, 15 seconds, not skipping any) and that you can continuously run it with a steady stream of fuel out of a tiny hole in the lid of a squeeze bottle. Get the right size tiny hole and you can make it idle, accelerate and cut under full power (if you got a helper who moves the trigger s l o w l y) with the fuel system completely dry ! If you got a machine that runs great, and burns it's fuel cleanly and eagerly - then and only then can you start to figure out why the #@*%!!! stupid fuel system does not want to meter out fuel the way you need it to.
I don't want to imply I am the total master of carbs - I still do not understand how the h*ll fuel metering even works on a piston port engine !
But I will say that on diaphragm carbs on 4 strokes and reed valve two strokes I can even slightly modify carbs to get 'em to work "just right" - and that is "priceless" ! ! !
I just had to run my mouth on the subject of "no check valve" carbs. Tillotson "H" and "HL" carbs as well as the much later "HS" had no check valves.
I noticed the only ones of those things that ever actually worked had the carb sitting level, with discharge at the bottom of the venturi - and super duper soft metering springs in relation to the size of the diaphragm that the spring worked against. I guess if you take gravity into account then they start to act like sort of a float bowl carb where the fuel "wants" to stay at a certain level. That level-effect resists air being pulled back in the main discharge hole because the fuel under it doesn't want to get out of the way for it to pass by. The super soft metering spring means the suction on the idle discharge needed to get enough fuel is very low suction indeed. Hey it was all we had in the old days if you didn't count Brown brand carburetors (and nobody DID !).
Okay - thread is a year old but man did the two-cycle world rejoice when Walbro carbs came out in the early 70s ! It took me a decade more to realize that Walbros had main discharge check valves and they were the cat's meow.
Lastly - guys - always always always make sure you got mechnical integrity (rings, crankcase seals, reed and base gaskets are tight) and ignition integrity
(spin it with an air drill, sparkplug out and a 3/16" spark gap - for 10, 15 seconds, not skipping any) and that you can continuously run it with a steady stream of fuel out of a tiny hole in the lid of a squeeze bottle. Get the right size tiny hole and you can make it idle, accelerate and cut under full power (if you got a helper who moves the trigger s l o w l y) with the fuel system completely dry ! If you got a machine that runs great, and burns it's fuel cleanly and eagerly - then and only then can you start to figure out why the #@*%!!! stupid fuel system does not want to meter out fuel the way you need it to.
I don't want to imply I am the total master of carbs - I still do not understand how the h*ll fuel metering even works on a piston port engine !
But I will say that on diaphragm carbs on 4 strokes and reed valve two strokes I can even slightly modify carbs to get 'em to work "just right" - and that is "priceless" ! ! !
Welcome to AS Helvehammer.
SteveSr
Addicted to ArboristSite
Back at it again... Apparently I haven't learned my lesson yet! Ran into a Walbro Wt-227 that had a check valve problem and decided to change out both the check valve and main nozzle since both were the same age (old) and been exposed to the same environment. With this particular carb the check valve feeds both low and high speed circuits. Its main purpose is to keep the blasted primer system from sucking air back through the low speed circuit thus preventing it from working properly.
The check valve came right out after drilling a drywall screw access hole in it.
The main nozzle ended up being an S.O.B.! I couldn't get it to budge in either direction using a drift or improvised slide hammer. I even went so far as to putting a drift into the drill press chuck. Although I applied enough force to deflect the drill press table the nozzle refused to budge.
At this point having nothing to loose I decided to improvise a nozzle puller as shown in the photos below.
NOZZLE REMOVAL
1. Cover any small orifices to prevent entry of drill chips.
2. Drill out the main nozzle all the way through with a suitable 3mm, #8-32, or 4mm drill.
3. Tap the nozzle with the correct tap.
NOTE: Due to the fuel channel in the nozzle it MUST be drilled and tapped ALL the way through.
Here is the #8-32 nozzle puller installed and ready to go:
The puller sleeve is a 1/4" I.D. bronze sleeve found at a local hardware store.
Here is the puller with the extracted nozzle:
The puller is just a #8-32 bolt, nut, and suitable washer.
Here are some photos of the nozzle showing the reason to drill and tap it all the way through:
Once I had all of the pieces ready and assembled the nozzle pulled right out without even a whimper! Much more efficient than the slide hammer approach.
NOZZLE INSTALLATION
1. Tap the new nozzle into position at the correct depth using a suitable drift.
2. DO NOT try to seal around it with nail polish or other sealant. The sealant won't hold up to the ethanol in the gas, will come off, and clog up other passages in the carb.
The check valve came right out after drilling a drywall screw access hole in it.
The main nozzle ended up being an S.O.B.! I couldn't get it to budge in either direction using a drift or improvised slide hammer. I even went so far as to putting a drift into the drill press chuck. Although I applied enough force to deflect the drill press table the nozzle refused to budge.
At this point having nothing to loose I decided to improvise a nozzle puller as shown in the photos below.
NOZZLE REMOVAL
1. Cover any small orifices to prevent entry of drill chips.
2. Drill out the main nozzle all the way through with a suitable 3mm, #8-32, or 4mm drill.
3. Tap the nozzle with the correct tap.
NOTE: Due to the fuel channel in the nozzle it MUST be drilled and tapped ALL the way through.
Here is the #8-32 nozzle puller installed and ready to go:
The puller sleeve is a 1/4" I.D. bronze sleeve found at a local hardware store.
Here is the puller with the extracted nozzle:
The puller is just a #8-32 bolt, nut, and suitable washer.
Here are some photos of the nozzle showing the reason to drill and tap it all the way through:
Once I had all of the pieces ready and assembled the nozzle pulled right out without even a whimper! Much more efficient than the slide hammer approach.
NOZZLE INSTALLATION
1. Tap the new nozzle into position at the correct depth using a suitable drift.
2. DO NOT try to seal around it with nail polish or other sealant. The sealant won't hold up to the ethanol in the gas, will come off, and clog up other passages in the carb.
Helvehammer
New Member
Very very good work. Guess you showed that sucker who is boss! I have said many times over the years (silently so nobody thinks I lost all my marbles)
"Machine - by the Grace of God I am a living being and YOU ARE NOT. If I just do not give up you can't win. You can waste a bunch of my time - but I will figure out the problem and fix you." Aaaand - several times it took another identical unit that worked perfectly and swapping parts until there was no doubt about what was bad. But I got it and the NEXT TIME I knew what to look for. Poulan and Homelite both got informed of many weird problems by phone "back in the day" and I have a small collection of letters from their engineering departments thanking me for it.
