How to test vacuum on 394xp and WJ 39-1 carb

IanB

Member
Sep 26, 2013
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Yeah, AS is down, so i am looking for some help here. My carb just won't draw in gas from the tank. It doesn't feel like a carb issue as I I cleaned it out (photo's below) and put some diaphragm rebuild kits in. So I am off to do the vacuum and pressure testing. I get the procedure, but I don't have a Impulse Line from the tank on my carb, it is just a very very small hole that I can't hook the mityvac up too.

How do you hook up the mityvac when you don't have an Impulse Line like most saws?

photo 1 after carb clean.jpgphoto 2 after carb clean.jpgphoto 3 after carb clean.jpgphoto 5 after carb clean.jpg
 

thomas1

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Oct 22, 2012
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Yeah, AS is down, so i am looking for some help here. My carb just won't draw in gas from the tank. It doesn't feel like a carb issue as I I cleaned it out (photo's below) and put some diaphragm rebuild kits in. So I am off to do the vacuum and pressure testing. I get the procedure, but I don't have a Impulse Line from the tank on my carb, it is just a very very small hole that I can't hook the mityvac up too.

How do you hook up the mityvac when you don't have an Impulse Line like most saws?

View attachment 1139View attachment 1140View attachment 1141View attachment 1142
Can you modify a spark plug and test through the cylinder?
 

IanB

Member
Sep 26, 2013
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Yeah, I am going to have to try the spark-plug approach

I think modifying the spark plug-approach is what I am going to need to try. Don't have a compression tester right now, so it's about all I got available to me. Glad I can at least USE the spark-plug to test vacuum and pressure, that is a good thing.

I'll report back.
 

sefh3

Well-Known Member
Mar 7, 2012
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What is your issue? Your carb looks clean. If it's a starting problem more than likely not seals on the crankcase. Usually when seals go bad it makes the saw scream at WOT. If your having a starting issue, check your fuel line. If it's cracked it will cause a no start issue.
 

Eccentric

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Jan 13, 2012
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Yeah, AS is down, so i am looking for some help here. My carb just won't draw in gas from the tank. It doesn't feel like a carb issue as I I cleaned it out (photo's below) and put some diaphragm rebuild kits in. So I am off to do the vacuum and pressure testing. I get the procedure, but I don't have a Impulse Line from the tank on my carb, it is just a very very small hole that I can't hook the mityvac up too.

How do you hook up the mityvac when you don't have an Impulse Line like most saws?

View attachment 1139View attachment 1140View attachment 1141View attachment 1142
The impulse line (or passage in the case of your 394XP) doesn't go from the tank to the carb.....................it goes from the crankcase to the carb (or in your case from the crankcase through a passage in the cylinder base to the intake manifold, and then through a passage in the intake manifold to the carb).

CAREFULLY examine your intake manifold, especially around the impulse passage. The 394XP saws have a reputation for cracking intake manifolds.
 
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IanB

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Sep 26, 2013
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I have to say

The intake manifold looks surprisingly good. No cracks at all. Could have been replaced recently? I am a recent owner of the saw.

Tomorrow will be the day for the vacuum test. I think the Mityvac came with a number of tools that will allow me to test vacuum with the carb off, and spark-plug in. So, I'll let you all know in the morning how things test out.

I am also going to see if I can use the vacuum on the mityvac to draw-in fuel from the carb by putting a bit of vacuum on the impulse inlet, maybe that can tell me if my carb will draw in fuel when the saw creates the same vacuum on that impuls inlet?

Tests will show tomorrow when I get a minute on the bench and will report back.
Ian
 

Eccentric

Walkin' Boss
Jan 13, 2012
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The impulse signal from the crankcase to the carb is like alternating electrical current. The + and - pressure signal makes the carb's pump diaphragm move back and forth, pulling fuel through the inlet check valve, then pushing it through the outlet check valve to the metering chamber. Jist putting vacuum to the carb won't make it pump.

I suppose if you could get a good enough seal with one of the tapered mitivac tips in the impulse port of the carb, you could put the hose (attached to that tip) in yer mouth and suck/blow (no comments from the peanut gallery please) to provide your own impulse signal to drive the carb's pump section. Would probably work so long as you don't get tired first. Not something I've tried....
 

IanB

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Sep 26, 2013
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So...

All sucking and blowing aside. I have put some time into vacuum testing this morning. Still need to get a complete test on the case with gaskets and all, but I have found one thing.

The carb is supposed to hold vacuum (maybe up to 10 on my gauge) and it does not, it holds zero pressure. Unless the small hole at the top of this photo is closed (I put my finger over it) then it holds vacuum just fine. Otherwise it just draws air right through that hole, and doesn't hold vacuum.

Top-side of carb.jpg

So, After putting some tape over that hole in the carb shown above. I tested the 'simulated impulse demand' suggested, with the mityvac hooked up to the fuel-intake fitting. When performing the same functions the saw does when cranking over, (requires very little air in & out to move the diaphragm in the carb) The carb would put enough action on the fuel inlet to just bump the needle over on the mityvac, showing that there was some vacuum action happening now on the fuel intake side of things now that the tape was installed.

After covering that top hole with the electrical tape, and blocking off the air-intake with my hand for testing. I turned the saw over a few times with the pull cord and fuel came up like crazy from the tank. Obviously something created enough suction/vacuum to draw the fuel up. The saw starts for a moment, and then dies. Starts then dies. I am going to wait for some of that excess fuel to evaporate etc, and then try to start/stop it again, and adjust my idle valve.
 
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srcarr52

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Feb 11, 2013
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All sucking and blowing aside. I have put some time into vacuum testing this morning. Still need to get a complete test on the case with gaskets and all, but I have found one thing.

The carb is supposed to hold vacuum (maybe up to 10 on my gauge) and it does not, it holds zero pressure. Unless the small hole at the top of this photo is closed (I put my finger over it) then it holds vacuum just fine. Otherwise it just draws air right through that hole, and doesn't hold vacuum.

View attachment 1202

So, After putting some tape over that hole in the carb shown above. I tested the 'simulated impulse demand' suggested, with the mityvac hooked up to the fuel-intake fitting. When performing the same functions the saw does when cranking over, (requires very little air in & out to move the diaphragm in the carb) The carb would put enough action on the fuel inlet to just bump the needle over on the mityvac, showing that there was some vacuum action happening now on the fuel intake side of things now that the tape was installed.

After covering that top hole with the electrical tape, and blocking off the air-intake with my hand for testing. I turned the saw over a few times with the pull cord and fuel came up like crazy from the tank. Obviously something created enough suction/vacuum to draw the fuel up. The saw starts for a moment, and then dies. Starts then dies. I am going to wait for some of that excess fuel to evaporate etc, and then try to start/stop it again, and adjust my idle valve.
If you have to plug the vent on the top of the carb then either you have the diaphragm in wrong or it has a hole in it.
 
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srcarr52

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IanB

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Sep 26, 2013
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Spent some more time

And I very much think I have a leak somewhere not related to the carb. Cut gaskets to cover up the inlet and exhuast, and although the Mityvac came with a rubber fitting for the top of the cylinder (sort of fits) I can't hold a lick of vacuum from the smark-plug hole. I will get my spark-plug modifier tool out and see if I can get a proper vacuum reading 'before' any leak detection work with the pressure side from my compressor.

Things don't look good at the moment. and may need to strip the saw down much further to diagnose problems with leaking. Will report back, and thank you for all the help. REALLY appreciated. Need my 394xp to come back to life! Good news is that the piston is still in ok shape, and I am going to keep running it.
 
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jacob j.

When I let my hair down, my friends call me Dick C
Jan 9, 2012
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If you have to plug the vent on the top of the carb then either you have the diaphragm in wrong or it has a hole in it.

This is likely his problem. The newer WJ kits come with two fuel pump diaphragms. One has two round holes in it and the other doesn't. 9 times out of 10 when a WJ carb is leaking or isn't pumping gas properly and the owner has "fitted a new kit" this is the problem.
 
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IanB

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Sep 26, 2013
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This is likely his problem. The newer WJ kits come with two fuel pump diaphragms. One has two round holes in it and the other doesn't. 9 times out of 10 when a WJ carb is leaking or isn't pumping gas properly and the owner has "fitted a new kit" this is the problem.
Here is what I have installed, and it's orientation in the install. (really very simple)

sequence of carb 1.jpgsequence of carb 2.jpg

sequence of carb 3.jpgsequence of carb 4.jpgsequence of carb 5.jpg
 
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jacob j.

When I let my hair down, my friends call me Dick C
Jan 9, 2012
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Here is what I have installed, and it's orientation in the install. (really very simple)
You have the wrong fuel pump diaphragm installed. The hole shown here: sequence of carb 1a.jpg

Isn't supposed to be there. That is why your carb is leaking and is not pumping fuel correctly.
 
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srcarr52

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We've found your problem. There should have been two brown diaphragms in your carb kit. One with the hole circled below and one without. You should have used the one without. Rright now you are venting your fuel inlet out the top of the carb so you are just sucking air. That vent hole is supposed to be a fuel inlet damper to damp out the pressure from the fuel line inertia.

WrongFlapper_Diaphragm.jpg
 
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