Theroritical carb testing question ??

[Ax-man]

I have been pondering this question for a long time and now that we here in the upper part of Illinois are in a deep freeze till the weekend I am going to throw this little brain teaser out onto the table. I haven't tried this but was wondering if it would work. Just want to see what someone else might say.

The question is . Can you test a carb for damaged H or L damaged needles or seats using an air pump like a Mighty Vac ????

Let's use a simple carb as an example that you have no history about where it has been or what might have been done to it, no priming system and no limiter caps to prevent you from turning the H an L screws all the way in till they lightly seat themselves.

Lets assume the carb checks out OK and hold pressure using the M/V on the pump side gaskets and the inlet needle doesn't leak.

Now to check for possible bad H or L needle or seat damage we remove metering cover and gaskets and the inlet needle and then reinstall the metering gaskets and cover without the inlet needle . Then we turn the H and L screws in till lightly seated or closed completely. Now we pump air into the carb . I am assuming air can't get past the metering diaphragm gaskets.

Now the answer to my question is this. If both H and L needle and seats are good they should hold steady pressure and not leak down . Correct or am I missing something ????? that would cause the carb to leak down and lose pressure by escaping somewhere else in the carb or hold pressure regard less of the condition of the needle and seats of H an L screws in the closed position.

I hope I am explaining this right and I am not way out in left field with my thinking by not actually doing this test on a sacrificial carb before posting it here. Just fishing for answers if any.

 

[Ozhoo]

Theoretically yes it would seal. But those seats aren't machined to that tolerance.

 

[Ax-man]

Good answer . I didn't think of that but it makes sense.

 

[hotshot]

Theoretically yes it would seal. But those seats aren't machined to that tolerance.

Cabin fever is setting in for the OP!

Besides that, they are never fully closed in operation or else the fuel couldn’t get through & thus the saw would not run.

The only time you’d want the L fully closed is when you test the check valve on the main nozzle.

 

[Chainsaw Jim]

This is something that should be done by visual inspection and feel.

 

[Ax-man]

Hot shot ,you must be having the same weather we are having right now.

Could you elaborate just a little more on your last sentence. Are you referring to closing the L screw , H screw turned out 1 turn, pumping up the carb and then tripping the metering diaphragm with a tag wire or rubber coated paper clip through the vent hole releasing the air pressure and watching the gauge on the M/V to see if it falls off quickly indicating that the H circuit is unrestricted. If that is the case I am familiar with doing that and doing that in reverse to see if L circuit is restricted.

If your talking about something else I'd like to hear it.

Chainsaw Jim, I hear you about the feel of turning the screws in and out but trying to peer down to look at those seats with a mag light doesn't always tell the story or at least for me it doesn't. I'm just tying to see if I can fine tune my carb diagnostic skills.

 

[Chainsaw Jim]

You can't always peer down the hole. I back light the passage from the nozzle and which lights it up really well from the inside. Punching the nozzle out helps in most cases.

 

[Ax-man]

Thanks , I never tried that but will definitely be trying it on the next carb I have apart.