wigglesworth
Booned
Did you change the fuel line? On that series of saws, they will split right at the grommet where they enter the fuel tank, and be hard to see if you dont pull it out some. Just a thought....
Metering lever ?
- Thread starter Brian VT
- Start date
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Rustynutz
ArboristSite Operative
How to test Diaphragm Carburettor without disassembly
QUICK TEST
Prove metering system works
Put 10psi on fuel inlet and see it hold there, (will creep down slowly) put tube on diaphragm vent hole and blow, watch inlet pressure drop
If it holds and drops that’s working
Test Main jet and Check Valve
Screw low Needle in until snug fit (Do Not Over tighten you will damage seat) then remove High Needle, get red nylon tube from WD40 spray can or similar and place in jet hole (takes some fiddling to get a good seal) when you think you are in the jet blow then suck you should feel the valve opening and closing as you do this (easy to blow harder to suck) if your not getting a response try sucking and blowing a little faster, if still not noticeable attach your WD40 and squirt some in (look in chamber and you should see it come out of the main jet, then try the suck blow test again (remember to clean the tube or you will get a mouthful)
Test Low Jets
Remove L needle put tube in and spray with WD40, look in chamber you should see it coming out of both holes.
NB
Using WD40 like this is OK for short periods (Don’t leave it in there for to long) I usually flush through with petrol after the test, A Quick blow on the supply tube will prove fuel can get there.
Put both needles back in till snug then back off 1 and ¼ turns (most working Carb’s will start at this setting)
General Notes
Main jet is in the centre of the chamber. Low Jets are each side of the throttle butterfly (Engine end). The H high needle (shorter one) is always closest to the air filter end and the L low needle (Longer one)is always closest to the engine. I made my pressure tester with a bicycle pump that had a gauge or you could use a car tyre pump and make an adaptor with an old valve stem and some fuel line (just remember not to go over 10psi or you might damage the diaphragm Zama) Walbro can go to pop off.
If it passes all these tests it should start and run now???
QUICK TEST
Prove metering system works
Put 10psi on fuel inlet and see it hold there, (will creep down slowly) put tube on diaphragm vent hole and blow, watch inlet pressure drop
If it holds and drops that’s working
Test Main jet and Check Valve
Screw low Needle in until snug fit (Do Not Over tighten you will damage seat) then remove High Needle, get red nylon tube from WD40 spray can or similar and place in jet hole (takes some fiddling to get a good seal) when you think you are in the jet blow then suck you should feel the valve opening and closing as you do this (easy to blow harder to suck) if your not getting a response try sucking and blowing a little faster, if still not noticeable attach your WD40 and squirt some in (look in chamber and you should see it come out of the main jet, then try the suck blow test again (remember to clean the tube or you will get a mouthful)
Test Low Jets
Remove L needle put tube in and spray with WD40, look in chamber you should see it coming out of both holes.
NB
Using WD40 like this is OK for short periods (Don’t leave it in there for to long) I usually flush through with petrol after the test, A Quick blow on the supply tube will prove fuel can get there.
Put both needles back in till snug then back off 1 and ¼ turns (most working Carb’s will start at this setting)
General Notes
Main jet is in the centre of the chamber. Low Jets are each side of the throttle butterfly (Engine end). The H high needle (shorter one) is always closest to the air filter end and the L low needle (Longer one)is always closest to the engine. I made my pressure tester with a bicycle pump that had a gauge or you could use a car tyre pump and make an adaptor with an old valve stem and some fuel line (just remember not to go over 10psi or you might damage the diaphragm Zama) Walbro can go to pop off.
If it passes all these tests it should start and run now???
Yeah, I did replace the fuel line. It was split right where you said.
roncoinc
Official Stihl Disser
Rusty, I went through your checks. Mity Vac is handy. 
Seems to check out fine.
Can anyone confirm that the H and L screws are the same (Walbro WT) ?
They came in the kit and weren't labeled and appear to be the same and fit in either hole.
Next step is to mess with the metering lever.
Seems to check out fine.
Can anyone confirm that the H and L screws are the same (Walbro WT) ?
They came in the kit and weren't labeled and appear to be the same and fit in either hole.
Next step is to mess with the metering lever.
I just did that. The grease didn't budge.
No impulse line to replace. What next ?
roncoinc
Official Stihl Disser
Rusty, I went through your checks. Mity Vac is handy.
Seems to check out fine.
Can anyone confirm that the H and L screws are the same (Walbro WT) ?
They came in the kit and weren't labeled and appear to be the same and fit in either hole.
Next step is to mess with the metering lever.
screws not the same.
longer one is low.
Thanks. I just realized I was thinking about the screws in the carb. kit. For the Homelite 360 I'm also working on. 
Anyway, I see the impulse spout seal in the IPL. I guess I found my problem.
I'll go dig the seal out of the old cylinder. I assume I'll be up and running soon.
Thanks for bearing with me, guys.
Anyway, I see the impulse spout seal in the IPL. I guess I found my problem.
I'll go dig the seal out of the old cylinder. I assume I'll be up and running soon.
Thanks for bearing with me, guys.
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BUT Brian how did it run at all then if it's had no impulse all along ?
If it were me I would have had it fall off while I was working on the carb and I didn't notice......but it's odd to me it ran at all without it.
John H.
?If it were me I would have had it fall off while I was working on the carb and I didn't notice......but it's odd to me it ran at all without it.
John H.
I dunno, maybe the choke/throttle lock somehow brings some fuel up ???BUT Brian how did it run at all then if it's had no impulse all along
If it were me I would have had it fall off while I was working on the carb and I didn't notice......but it's odd to me it ran at all without it.
John H.
I figured I lost the seal on my shop, like you said, but it was sitting in the old cylinder. DOH !
I'll get back to it in a minute. I needed to shake my head over a beer.
I gotta stop working on more than one saw at a time. I'm too scatter-brained for that. LOL
BUT Brian how did it run at all then if it's had no impulse all along
If it were me I would have had it fall off while I was working on the carb and I didn't notice......but it's odd to me it ran at all without it.
John H.
Some saws will start and run,although not very well without the impulse working. The tank will pressurize on saws that have the one way air vent that only allows air to enter the tank when fuel is drawn out. These type tanks will pressurize from fuel expansion, just shaking/moving the saw will cause the fuel to expand resulting in the fuel being fed to the carb under a little pressure. The saw will start and run a little but will not operate for long without the impulse pulling a steady flow of fuel up to the carb.
Pioneerguy600
Welp, I got the spout seal in there and figured that was the source of all my problems and I was laughing at what a dope I can be.
I tried the grease trick over the impulse hole before I put the carb. back on.
No sign of impulse. I figured it might have some when it was running, so I put it together anyway.
1 1/4 turns out on both screws. It runs the same. It gives a good burp with the choke on, it almost starts. It lights right up and revs at the first pull with the choke off/throttle lock on. I click the trigger to release the throttle lock and it idles not too bad but dies with any throttle applied.
The new fuel line is colored so I can't see if fuel is being drawn.
Time for more beer. I'll just sit here drinking until a new idea comes up. LOL
I tried the grease trick over the impulse hole before I put the carb. back on.
No sign of impulse. I figured it might have some when it was running, so I put it together anyway.
1 1/4 turns out on both screws. It runs the same. It gives a good burp with the choke on, it almost starts. It lights right up and revs at the first pull with the choke off/throttle lock on. I click the trigger to release the throttle lock and it idles not too bad but dies with any throttle applied.
The new fuel line is colored so I can't see if fuel is being drawn.
Time for more beer. I'll just sit here drinking until a new idea comes up. LOL
Try blowing a little comp air through the impulse port or run a thin wire through it.
Pioneerguy600
Pioneerguy600
It's a brand new jug. I guess maybe something could be in there ? I'm pretty clean when working on these things. I guess I'll have to try to see in there with a light or pull the damned jug off. 
Is there a way I can test the "impulse circuit"(?) by putting the piston in a certain position and blow air into the plug hole, or something like that ?
Is there a way I can test the "impulse circuit"(?) by putting the piston in a certain position and blow air into the plug hole, or something like that ?
It's a brand new jug. I guess maybe something could be in there ? I'm pretty clean when working on these things. I guess I'll have to try to see in there with a light or pull the damned jug off.
Is there a way I can test the "impulse circuit"(?) by putting the piston in a certain position and blow air into the plug hole, or something like that ?
You can just try pulling vacuum through that port with a vac gun. Of course it should nopt hold vac, if it does its blocked. Blowing comp air through it will clear most blockages without removing the jug.
Pioneerguy600
I'll try sucking it with the Mighty Vac. Not sure about blowing into it. I'm just afraid that, if there is something there, I'm blowing it into the crankcase ? I'm picturing a casting chunk that wasn't ground off ? What the heck else could be blocking it ? I don't really understand the impulse thing so straighten me out if I'm just being paranoid.
Again, thanks for sticking with me on this.
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I'll try sucking it with the Mighty Vac. Not sure about blowing into it. I'm just afraid that, if there is something there, I'm blowing it into the crankcase ? I'm picturing a casting chunk that wasn't ground off ? What the heck else could be blocking it ? I'd prefer to blow it out, if that's somehow possible. I don't really understand the impulse thing so straighten me out if I'm just being paranoid.
Again, thanks for sticking with me on this.
I can`t say exactly where the port for the impulse runs on a 55, if it runs through the base of the cylinder down through the base to the crankcase then either the base gasket could block the passage if installed in reverse or if no gasket was used then the sealer can get into the port blocking it. The impulse works from the movement of the piston up and down in the bore, on the way up it develops a low vacuum and on the way down a low + pressure, transferred to the carb pump diaphragm via the impulse port or line depending on saw model.
I doubt it is a piece of casting, did you verify that the 55 cylinder base has a port that lines up with the 51 crankcase deck?
Pioneerguy600
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