Primer bulb pulling air but not the usual suspects.

[jarrick]

Got an old Tanaka ECS 3351b (33cc) and the primer/purge bulb gets a lot of air coming in from somewhere.

Replaced all the tubes with new clear ones and the tube going from the carb to the bulb gets a lot of bubbles going through and hardly any petrol.

The saw runs but not as well as it should (though that might be a tuning issue).

I've recently cleaned the carb and put new gaskets and diagrams etc, and all the screws are done up tight.

I even swapped the carb with the one in my spare saw - and that was even worse.

The primer bulb is new and tested ok.

 

Any ideas?

 

[openspaceman]

If the carb you tried was off another working saw and worked well then no, unless the tubes are on the purge bulb the wrong way round.

 

The problems I have had with air being pulled into the purge valve have been worn/leaking throttle enriching pump ( and this cannot be completely deleted, @bmp01 fixed it with a blocking plug that still allowed fuel in the purge circuit ) and the main check valve leaking. In the latter case I think it was someone poking a wire through it or blasting it with compressed air.

[Muddy42]

As above are you sure the primer is connected the right way?  Its short line from (normally the top of the) carb, long line returning to the fuel tank. The fuel line goes from the fuel tank to the bottom of the carb.

 

If the lines are clear, you should see where the bubbles start from?

 

Are you sure the carb was assembled in the correct way? If any of the diaphragms ripped, it could benefit from a carb kit.  You can pressure test a carb, but do it gently to avoid breaking stuff. You don't need a pressure tester for this, if you blow with your mouth on the fuel line you get an idea if the pressure is dropping.

Edited November 11 by Muddy42

[jarrick]

Thanks for the replies - I've been going over it again.

Yes, the bulb is the right way round and the check valve is doing what it should, and I installed the carb kit correctly, without damaging the diaphragm. 

It could well be the other carb has the same issue.

I don't think this carb has a throttle enriching pump.

I put some air down the fuel intake and it held pressure.

I tried sucking on the outlet pipe (that goes to the primer bulb) whilst blocking the fuel inlet pipe and I can hear air coming into the carb from somewhere - But when I also fully screwed in the H and L adjusters, it stopped the air coming in. Is that supposed to happen?

Also, it is easy to blow air into the carb via the outlet to the primer bulb - is that supposed to happen?

Many thanks.

 

[Muddy42]

11 hours ago, jarrick said:

Thanks for the replies - I've been going over it again.

Yes, the bulb is the right way round and the check valve is doing what it should, and I installed the carb kit correctly, without damaging the diaphragm. 

It could well be the other carb has the same issue.

I don't think this carb has a throttle enriching pump.

I put some air down the fuel intake and it held pressure.

I tried sucking on the outlet pipe (that goes to the primer bulb) whilst blocking the fuel inlet pipe and I can hear air coming into the carb from somewhere - But when I also fully screwed in the H and L adjusters, it stopped the air coming in. Is that supposed to happen?

Also, it is easy to blow air into the carb via the outlet to the primer bulb - is that supposed to happen?

Many thanks.

 

 

Air should come through the carb outlet pipe.  If you think about it that's what the purge bulb does, suck fuel from the carb so it gets replaced with fresh fuel.

 

You should not be able to blow back into the carb through the outlet valve.  Are you 100% sure the fuel pump side of the carb is oriented correctly (valves next to the metal of the carb then gasket) and that the metering diaphram is not upside down?  Is the needle valve seating properly?

 

Edited November 13 by Muddy42

[openspaceman]

11 hours ago, jarrick said:

But when I also fully screwed in the H and L adjusters, it stopped the air coming in. Is that supposed to happen?

I don't know the carburetor specifics but generally the idle mixture screw controls all the idle fuel, so if its screwed in nothing can bypass it. The main jet is slightly different in that there is an undersized orifice (part of the check valve) from the diaphragm chamber direct to the venturi, the HI screw just allows additional fuel into this for tuning. The check valve at the top of the main jet should prevent air being sucked into the diaphragm chamber from the venturi.

[openspaceman]

As @Muddy42 says. Perhaps you need to go back to basics. Begin by putting a pump and gauge on the fuel inlet to the carb and see if the inlet valve holds pressure, pump it to 5PSI and the needle should hold if there is no depression in the venturi.

[Mark_Skyland]

If you block the fuel inlet to the carb, just put your thumb over it and create a vacuum on the primer side, put some fuel pipe on it and suck you should not be able to draw air through, if you can you either have a hole in the pump gasket or usually a bad check valve. 

[bmp01]

On 12/11/2025 at 22:37, jarrick said:

when I also fully screwed in the H and L adjusters, it stopped the air coming in. Is that supposed to happen?

Yes that should happen. Now open each screw, one at a time and tell us which one is allowing the air through….

 

On 12/11/2025 at 22:37, jarrick said:

Also, it is easy to blow air into the carb via the outlet to the primer bulb - is that supposed to happen?

I think that’s ok too. You are blowing into the metering chamber only, so long as you have the little valve and it’s gubbins plus the diaphragm in place. That’s the chamber where fuel is supplied to the jets…..

So here you are creating a little bit of pressure on the fuel side of the jets, you are blowing through the main jet and low speed jets in the direction they are supposed to work. The one way valve in the main jet should be open in this direction. With the low speed circuit there is sometimes a little one way valve at the entry to the low speed circuit, which would also be open in this direction.

 

What is not ok is the purge bulb (or you) being able to suck through the main jet one way valve.

 

And it’s not ok, being able to suck through the low speed circuit one way valve if there is one, less of a big deal, but still not good.

 

Worth mentioning, both one way valves are often a little leaky with air, less so with fuel in them. Secondly, if the purge button pops back really slow and fills with air, its likely the fuel filter inside the carb is probably blocked …. bulb can’t suck fuel so it does what it can and sucks air, through the jets. 
 

And finally, leaky fuel line (between carb and bulb) or the bulb itself will allow air into bulb, I think this is pretty obvious. But less obvious maybe is that a leak in this area can easily allow air to be sucked into the carb metering chamber when engine is running…. Food for thought.

 

 

 

Edited 4 hours ago by bmp01