# Tillotson Carb



## RKDOC (Jun 20, 2006)

I rebuilt a Tillotson HU40 Carb, Completly cleaned and installed new gaskets and diaphrams. Now I have two problems with it.

Problem 1: after rebuild i put it in a jar of gas and pressure tested it. Poped off at 16 lbs. reseated at 10 lbs. After reseat however it would leak a bubble out the main jet every 3 or 4 seconds. Should it seat completely with no leaks? or is an occasional bubble ok?

Problem 2: When I put the carb back on the saw and connect the guage to the fuel inlet, then pull the starter rope the guage shows no vacum, indicationg the fuel pump is being actuated.Yet when I take the carb apart the carb is full of gas. Can it be full of gas and not be pumping? Engine will start on carb cleaner, but won't keep running.

Any help on these problems will be greatly appreciated.


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## 30yearTech (Sep 14, 2006)

Some carburetors may not seat completely till about 8 psi. 

Did you pull the rope with the choke on? If not it's not likely that you will draw enough vacuum at cranking speeds to open the inlet needle on the carburetor.

Hope this helps...


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## RKDOC (Jun 20, 2006)

Thanks 30 year. That will help.
With the carburetor test guage, connected to the fuel inlet, shouldn't it show some vacume drop when you pull the rope , to indicate pulse at the fuel pump?

Thanks fot the replys.


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## 30yearTech (Sep 14, 2006)

No, fuel pump pressure alone will not overcome the inlet spring pressure holding the needle and seat closed (or at least is should not). 

If you have the choke closed then vacuum drawing fuel out of the metering chamber will pull on the metering diaphragm and this will open the needle and you would see a drop on your pressure guage.


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## RKDOC (Jun 20, 2006)

Thanks 30 Year.

According to the walbro service manual.(I am working on a Tillotson,but assume principle to be the same) Connect the carb pressure tester to the fuel inlet of the carb. Pressurize and see if the needle pops off. When reseated. pull the rope and you should see a drop in vacum pressure indicating the fuel pump is working.

When I pull the rope I get no pressure drop. Does this indicate a problem with the fuel pump? or that ther is no impulse vacum being created by the engine?

Thanks.


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## RKDOC (Jun 20, 2006)

In addition to my previous post. I cannot have the choke assembly attached when I have my pressure guage attached to the carb. However when I put the air filter, which has the choke built into it, back on the engine the saw will not start. a quick shot of carb cleaner will start and run until cleaner is burnt off, then nothing.

When I pull the carb apart to check it, there is gas in both the pump and metering chambers. However the gas is not getting into the engine. The main jet is clean.

Thanks for any reply's.


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## 30yearTech (Sep 14, 2006)

RKDOC said:


> Thanks 30 Year.
> 
> According to the walbro service manual.(I am working on a Tillotson,but assume principle to be the same) Connect the carb pressure tester to the fuel inlet of the carb. Pressurize and see if the needle pops off. When reseated. pull the rope and you should see a drop in vacum pressure indicating the fuel pump is working.
> 
> ...


I have done the test your talking about countless times, your not looking at vacuum pressure, the carburetor has positive pressure from the pump and your checking to see if the engine will draw fuel and open the inlet needle. If you cannot use the choke for the carburetor then use your finger to block the air inlet and pull the rope and see if the needle valve will open. It's possible you may have an obstruction in the high speed adjustment loop or the high speed check valve may be stuck and not allowing any flow into the carburetor venturi.

You can check the impluse from the engine with the carburetor off, by placing a drop or two of oil into the impulse hole in the insulator, then pull the rope. It should blow the oil back out of the impulse hole, if not then look for an obstruction there.


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## RKDOC (Jun 20, 2006)

Thanks 30 Year you were exactly right. I put my hand in front of the carb and it will start every time. The engine runs great once it starts.

This carb is on a Stihl 028AV Chainsaw. The choke on it is the strangest I have seen. It is built into the air filter, and it does not seem to work right or I am missing something. When the kill switch is in the off position a lever pushes against the choke lever and holds it open. When in the run position it just lets the choke free float. This does not hold the choke against the carb and make a seal against the carb body. I am guessing that the intake to the carb is supposed to pull the choke flap against the carb body? if so then what happens when the engine is running? Has anyone seen this type of setup? Am I missing some parts or not following what is happening to choke the engine? Any knowledge about this saw would be greatly appreciated.

Thanks in advance.


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## 30yearTech (Sep 14, 2006)

It's been awhile since I have worked on an 028, but I believe there is supposed to be a small spring that holds the choke open, and then when you put the choke lever into the choke position it closes the choke plate and holds it down.


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