Chainsaws

Richard,

There are a number of possibilities, as Eugene suggested double check that the carburetor is assembled correctly, and that there are no gasket leaks, sometimes a little WD40 on the carburetor gaskets will help seal the gaskets better when re-assembling.

Make sure you have no airleaks from the induction manifold between the engine and carburetor, If air leaks around the carburetor sufficient air won't be pulled thru the venturi of the carburetor in order to pick up the fuel.

Check that the throttle linkage hasn't changed and that both the choke plate and throttle plate are in the correct position for the following three positions, choke starting (choke completely closed & throttle open just a little), Idle (choke wide open & throttle open 1/4 -1/3 open) and finally Wide Open throttle (choke fully open & throttle fully open). 

You already stated that your getting a vacuum pulse from the engine to the carburetor, so you should be pumping fuel, however if you carefully take apart the carburetor after a few starting pulls you should see a little puddle of fuel in fuel pump reservoir inside the carburetor, If you don't  see any fuel inside the carburetor, then you may have a problem with getting fuel to the fuel pump diaphram (ie plugged fuel line or fuel filter) or a problem with the fuel diaphram.

Venting on the fuel tank has also been known to restrict the fuel pump from working, so make sure your fuel tank vent is working.

I hope one of the above suggestions help, Oh and a last case scenario since you stated you re-built the engine, did you replace the engine seals ? and check the Cylinder gasket ?, and are you sure the engine is both pressure and vacuum tight, Again any air leaking into the engine will reduce the amount of air that needs to get pulled through the Carburetors venturi, and reduce the carburetors ability to function properly. So you have to have an air tight engine all the way up and thru the induction manifold to the carburetor. The only place that you want air to be pulled into the engine is through the caburetor.

Karla

PS. I am Not a Husqvarna Representative or Sales Representative, just an experimental chainsaw person

Eugene,

Thanks for the Info, and link.

Richard,

After looking at the IPL drawing,  I do know that getting the induction manifolds back into alignment with the carburetor are problematic with this design. Also, if they are not fiting up properly, and even the smallest air leak will result in the carburetor not working. So its real important to carefully inspect the manifold and gaskets from the engine cylinder to the carburetor. As shown in Page A, Parts # 537 20 43-01, 537 20 47-01,  537 20 49-01, and 537 20 45-01 of the referenced link sent by Eugene.

I am now more suspicious of these parts having an air leak, then I am of the carburetor not working correctly, I have assembled similar air intake manifold designs. I don't have any good tricks to there assembly, other than Inspecting the parts to make sure there is no damage to the molded rubber parts that creates the seals, and being real careful and take your time assembling, that they seem to feel like they are fiting together correctly.

I hope this helps,

Karla

Richard,

Yes - Positively, and based on what you have already tried and worked on the carburetor, and my own similar experiences. A small amount of air leak which allows air to be pulled into the engine without going thru the venturi of the carburetor will cause this problem.

An engine air test may be appropriate to also include the induction intake manifold, I would block the exhaust, I use a piece of bicycle inner tube cut into about a 1 1/2" x 3" strip. and sandwich it between the cylinder port and muffler. Next, You will need to block the air intake of the carburetor which may take a little creativity based on your design, I have sandwiched a piece of rubber between the carburetor and the intake manifold, but on yours it may be better If you can leave everything in place so you don't have to un-bolt the carburetor, and block the intake side of the carburetor, Its not nuch pressure needed only about 5-6 PSI, so maybe like a rubber ballon stretched over the intake side or some rubber bands with a small plastic poly bag. You will need a fairly accurate low range pressure/vacuum gauge and a source of low pressure air (5 - 6 psi like a bicycle pump) Do not use an Air compressor. Some Home Auto stores have a reasonably priced gauge, and also may have a 14mm or spark plug adapter fiting that will let you put air into the spark plug or if you have the separate impulse line you can keep the spark plug inplace and use the impulse line to get air into the engine. Then pump the engine up with about 5-6 psi, No more, make sure all your air connections are tight, I use the plastic barbed fitings and some air or fuel line, also available at most Auto stores fairly reasonable. Leave the pressure on the gauge, and make sure it maintains the pressure for 20-30 seconds, If the air bleeds off fairly quickly then you have a leak. Oh, you can also rotate the piston to the bottom of the cylinder, if you are putting the air in thru the spark plug hole.

I would definitely try the engine and induction manifold air test and check for leaks before buying a new carburetor, unless you have a local dealer that will sell you a carburetor, and be able to return it, If it doesn't solve the problem. If I had access to a second carburetor, and you have the same symptoms with the new carburetor then by process of elimination or substitution, you will also know that you probably have an air leak.

Again, I hope this helps, I have had several chainsaws that I have had trouble starting or running due to an Induction Air Intake problem. My very first one was a 1987 Homelite Model 330, of course a much simpler air intake system just a one piece rubber boot clamped between the engine and the carburetor. A small slit in the boot or if not sealed properly and the engine would not start or run properly, also you would swear it was the carburetor.

Also air leaks are one of those things that take some persistence, and perserverance.

Karla

Richard,

The vent should be like a one way check valve, It will allow air into the fuel tank, but won't allow liquid out of the vent. So theoretically if you turned the chainsaw on its side or upside down while your using liquid fuel will not come out the vent, however air has to be able to get into your tank or your fuel pump in the carburetor would be pulling against a vacuum and that will not work.

Some individuals have tried loosening the fuel cap, but be careful doing this since obviously fuel could come out. But If the engine runs with a loose fuel cap then it is probably a venting problem, and most of those vents there is nothing to clean or fix, its just a replace item.

I have not looked at your specific Model 570 xp vent, but I will take a look a little latter today, I don't think there is anything extra special about your vent, but I will double check for you.

Hopefully my explaination will help you troubleshoot

Karla

Richard,

I just looked at your IPL drawing, and on page "I" of the link Eugene sent, you will see part # 537359501, this is the actual vent, they are usually white plastic in color, yours has a piece of tubing to it, that must have been an after thought (probably an attempt to keeping dirt or sawdust from getting stuck in the vent), because other models use the same type of vent without the tubing.

Anyhow the broader end of the vent is stuck into a hole which goes into the fuel tank, and the narrower end goes to that tubing. Atmospheric air as allowed to go into the tank thru the narrow end and come out the broader end inside the tank. Air should only pass one way, "Do Not Use an Air Compressor to try and blow air into this Vent", I know individuals that tried that and blew up there fuel tanks into pieces, really!.

A small soda straw and small puff should easily pass air into the tank with the cap off the tank. However if you had fuel in the tank and turned it towards the side where the vent is located, you should not see fuel come out the vent, hence its like a one way check valve.

I hope this helps.

Karla

Richard,

I understand about fuel getting into the carburetor, so from your description it sounds like your fuel pump is working, But Just a word of caution, I would dis-agree with Robert about using steel wire to clean out cast aluminum ports, these are very precisely drilled to delivery the fuel to the engine. Since steel is much harder than aluminum you will almost certainly ruin the carburetor. I would recommend nylon solid fishing line, it will certainly push thru some dirt and gasoline deposits, and will not destroy the aluminum casting of the carburetor, I use 17lb test stren plastic fishing line, anything around that test weight will fit, however 20Lb test line may start to get to big in diameter. 

In answer to your question there is a total of four (4) ports, three for the low side (L) circuit, and one for the main or (H) circuit.

The three on the low side are located on the low side of the carburetor (The side of the carburetor that would be closest to the engine cylinder) where the throttle plate opens and closes, you will have to hold the throttle plate open to see them, and if you notice a notch in the brass throttle plate, that is the side of the carburetors throat that you want to look at. I wish I had a microsopic lens for my camera and I could take a picture for you, they are really tiny holes and are suppose to be small, they create a spray of liquid fuel which is carried by the air going into the engine. Oh, if you remove the "L" threaded_screw  on the carburetor and with a straw type spray can of carb cleaner, squirt it into the "L" screw_hole and you should see carb cleaner come out the three tiny holes.

The main jet or high side is located in the center of the carburetor and is midway between, the choke plate and the throttle plate, and looks like a 3/16" brass fiting, the jet is not that big but the fiting is because it contains a check valve inside of it, when you pull on the trigger to get maxium RPM's, sufficient air is pulled thru the venturi during wide open throttle (WOT), to pull fuel from the main jet. If you open up the carburetor on the metering lever side of the carburetor, you will see one fairly large hole toward the center of the carburetor, you can spray carb cleaner straight into that hole, and you should see it spray out the main jet, you will have to hold the choke plate open. Also wear safety glasses when spraying out these jets, carb cleaner has a way of getting into your eyes. Oh, If you want to test the "check valve" on the main jet, remove the single_screw cover or the fuel diaphram side of the carburetor, and put the spray can straw into the center_screw hole for the cover, and it should come very close to where the main jet is inside the venturi, and give it a spray and nothing should go thru in that direction proving the check valve is intact. But, If your having a starting problem, then its on the "L" side, and not the "H" side. If your having a running problem or WOT problem then its on the "H" side of the carburetor.

Again, I hope this helps, and is not too wordy or confusing.

Karla

Yes karla I took everything apart and checked each piece as I reassembled checking if the parts rubber gasket ( pre-maid on the parts) to make sure every thing would seal. Thanks for the carb info.

Richard