reverse of pop off leak test for 2 cycle carb.

[38racing]

I've never done this test before. I have a walbro wt-685 right now that I can't seem to get running right. I see that the correct tool from my supplier is pricey at $85 CDN. Watched someone make one on youtube. I had a 90 psi gauge and the schrader valve so made one but bought cheap pump from amazon. Even with fuel line connecting hose plugged with a nail punch in the end the set up does not hold pressure so that's a bust. I wondered if applying vacuum to the fuel inlet would test anything. I have a lisle vacuum pump and I now realize it's numbers are not psi but in hg. No idea what the conversion is. If I apply vaccum to the fuel inlet should it hold vaccuum?

[bgsengine]

Yes, should hold vacuum. I might suggest a mightyvac brake bleeder vacuum/pressure pump - get one with a gauge - 90 PSI gauge would be far too inaccurate for such small values as you'd be working with on a 2-cycle carb - (usually less than 10 PSI) No conversion to vacuum, really but if you can pull 14 inches of mercury (which is what Hg refers to) then you know the needle is seating (that or something's plugged solid) - Pop off test only tests for whether the metering needle is STUCK , it otherwise proves next to nothing (other than the strength of the metering lever spring) - You want to be able to pressurize to about 7 PSI (not much more) and have it hold pressure for 1 minute , if it does, needle and seat are OK. I don't see how you'd manage to read 7 PSI out of a 90 PSI Gauge though... (much less anything involving a schrader valve?)

But if you get the mightyvac tester then you can do a much more complete fuel system test (I have posted the procedure in these forums at least once, if not twice or more, I believe) for a more complete fuel system diagnosis..

[38racing]

Yes, should hold vacuum. I might suggest a mightyvac brake bleeder vacuum/pressure pump - get one with a gauge - 90 PSI gauge would be far too inaccurate for such small values as you'd be working with on a 2-cycle carb - (usually less than 10 PSI) No conversion to vacuum, really but if you can pull 14 inches of mercury (which is what Hg refers to) then you know the needle is seating (that or something's plugged solid) - Pop off test only tests for whether the metering needle is STUCK , it otherwise proves next to nothing (other than the strength of the metering lever spring) - You want to be able to pressurize to about 7 PSI (not much more) and have it hold pressure for 1 minute , if it does, needle and seat are OK. I don't see how you'd manage to read 7 PSI out of a 90 PSI Gauge though... (much less anything involving a schrader valve?)

But if you get the mightyvac tester then you can do a much more complete fuel system test (I have posted the procedure in these forums at least once, if not twice or more, I believe) for a more complete fuel system diagnosis..

Thanks. I have the lisle 75000 vacuum pump. Gauge goes to 30 in hg which is apparently about 14 psi. Carb does not hold. Just steps its way back to zero.

[KE4AVB]

38, What you in the line of a tester is the Mityvac MV8500. This tool is highly for the pressure/vac test of the crankcases of two cycles too. What you currently got is basically a brake line bleeding tool.

As said the fuel pump side should be able to hold 5-10 psi steady for 10+ minutes (30 min is better). If leaking under this pressure either the needle/seat or pump side gasket is leaking. This is where water submersion helps as you can see where the air is leaking. And yes I have seen the needle seats to leak around the outside edge. Testing under vacuum is really a waste of time in my opinion as it would only test the gasket for leakage as the vacuum would just pull in the needle valve tighter.

You will not be able to test the metering diaphragm side due carburetor design. It will always leak down on both pressure and vacuum tests. On pressure most of it will go out the main nozzle and on Vacuum, the vacuum will bleed off through the mixture screws.

[38racing]

since it still leaked even with vacuum pulling needle there's an issue. Needle is new so I'm going try the qtip/valve compound on seat. Maybe get a carb on its way from china.

[KE4AVB]

since it still leaked even with vacuum pulling needle there's an issue. Needle is new so I'm going try the qtip/valve compound on seat. Maybe get a carb on its way from china.

That I suggest a water bath using pressure as I had a few seats not to leak at the needle to seat point but leak around the seat and the carburetor body. No amount of grinding or counter sinking will fix that.

[38racing]

since it's a trimmer carb the q-tip is too big lol.

[KE4AVB]

That's why I brought a six flute 61 degree counter sink bit. I have only use those bits a handful of times. The last two times that I had cube seat problems it was the mating of the seat to the carburetor body and no matter how much seat was polished it will not fix that problem.