need small engine diagnostic... (1 Viewer)

[e9999]

OK, save me!


situation:

- weed wacker
- fuel line breaks at the carb
- while trying to pull it out of the tank some more it breaks again. Filter falls off.
- I pull it all the way out until it's flush with the tank wall inside so I can reach the carb.
- Decide to run it without filter to finish urgent job. After cleaning tank and draining it to remove most of the crud if any, can't see anything much.
- Works fine for a bit. When the level of fuel gets low it starts to die down. Usually can restart it after letting it stand. I think I see bubbles in the line usually when it dies. Thinking just sucking air, but not obvious cuz still some fuel in .
- start to get progressively more difficult to run.
- put more fuel in, should be plenty to cover the line stub in the tank.
- still gets more and more difficult to run.
- finally have to run it with the choke 1/2 on (per label, but it's really with the throttle plate 9/10 closed) and full throttle trigger on. Without the throttle trigger in all the way it does not run, just dies
- does not run at all without choke 1/2 on. Choke full no run either.
- lacks power.

so what does needing to run at 1/2 choke on all the time mean, with little power?

feels like not enough gas getting there

is it possible that not having the filter on messes up the fuel delivery rate and plays tricks with the carb?

Don't tell me there is now crud in the carb or I damaged the carb, please, I don't want to hear that!
(but if there is crud, can I just blow into the carb fuel intake with compressed air or is that a bad idea?)



sigh....


TIA

 

[Mace]

Most likely there are more cracks in the fuel line.

Cheap weedeater with the really fine hose?

 

[fsusteve]

you have crud in the carb. it takes a minute to pull it off and clean it out with a paper clip and carb cleaner, fix the lines while it's off.

 

[e9999]

Most likely there are more cracks in the fuel line.

Cheap weedeater with the really fine hose?

don't think there are cracks. No leak visible.

pretty good Craftsman Bushwacker, the bigger version of a weedeater. Yes fine hose, about 0.140" OD.

 

[e9999]

you have crud in the carb. it takes a minute to pull it off and clean it out with a paper clip and carb cleaner, fix the lines while it's off.

eh! I said I didn't want to hear about crud in the carb!!

but, yes, unfortunately it does seem like crud... Dang, I thought I could get away with no filter for 1/2 hour....

can these carbs be taken apart easily? diaphragm type I think, whatever that means...

 

[D'Animal]

My second guess would be a crack in the fuel line.


My first guess is the cap is not venting. Next time it starts acting up, loosen the fuel cap and see if it straightens itself out.


My third guess is that varnish/crud is now sucked up into the carburetor. There is a secondary filter or screen inside the carburetor. It is a fine screen that will stop water but let fuel through. It is on the side of the carburetor that generaly has one screw holding the plate on, that is assuming it is a Walbro Carburetor.

Does it have a primer bulb? If so, is the primer bulb empty of fuel when it starts to run poorly?

 

[e9999]

My second guess would be a crack in the fuel line.


My first guess is the cap is not venting. Next time it starts acting up, loosen the fuel cap and see if it straightens itself out.


My third guess is that varnish/crud is now sucked up into the carburetor. There is a secondary filter or screen inside the carburetor. It is a fine screen that will stop water but let fuel through. It is on the side of the carburetor that generaly has one screw holding the plate on, that is assuming it is a Walbro Carburetor.

Does it have a primer bulb? If so, is the primer bulb empty of fuel when it starts to run poorly?

new line will take care of any crack
cap is easy to check, unlikely it would suddenly get plugged though.
will look at carb and see how easy it would be to take apart. Not OK to blow air in then?
no bulb that I know of.

 

[D'Animal]

The cap does not get plugged.

The carburetor draws fuel from the tank. The cap is suppose to let air into the tank so a vacuum is not created.

When a vacuum is created it will run with the ckoe a little bit on, then 1/2 on and then all the way on. You are forcing the carburetor to overcome the vacuum unitl it just won't do it anymore. Let the units sit for awhile or take the cap off and the vacuum is satisfied. The unit will start and run like it did before. The more fuel in the tank, the shorter period of time it will run.

 

[e9999]

bummer

here is the latest.


changed the fuel line for a new one.
put the filter in the tank on again after blowing it clean (I guess) with compressed air (it's a stone like affair)

took the carb (Walbro) partially apart. Blew compressed air through the fuel intake. Cleaned the big blue plastic ultrafine mesh across the bottom and the metal mesh built in the carb. Put some carb cleaner in all the holes. (did not let it soak overnight though). Let's assume for now that the carb is clean unless there is some gunk way deep in there. We can safely assume the line is good.


Recap: it started to act up when I ran it without a filter for 15 mins or so. May be a coincidence I imagine.

Tried again. No difference at all.

It starts very easily cold with the choke fully on or with the choke 1/2 on when warm. One pull only when warm. 2 or 3 if cold.

I see fuel in the line. No indication of air or vacuum bubble ever even when it dies.

It runs fairly fast on 1/2 choke steadily for long periods of time with throttle fully on. If I go full choke it stops immediately. If I go to no choke it accelerates first then slowly dies down over a period of 5 or so seconds. If I let go of the throttle it dies.

Removing the tank cap makes no difference. Removing the air filter makes no difference.

basically it only runs at 1/2 choke.

What gives?

 

[D'Animal]

You have an air leak in the system.

 

[e9999]

You have an air leak in the system.

OK.

Like where?

 

[Blue77FJ40]

You also have this site for free small engine repair videos that might help.

Repair Videos

 

[rockcrawler]

I'd start at the beginning with this thing and take it from there.
Rip the carby out, clean it and replace the gaskets and any seals, new sparkplug, clean filters and the fuel tank and then set the mixture screw to default. I know the problem started with your fuel line breaking but replacing it and cleaning out the carb should have solved your problem if that's all the problem was.
When you take them to a shop for a service that's what they do to get them going unless there are major issues which it doesn't sound like with yours.

 

[e9999]

took more of the carb apart.

I'll probably get a repair kit with new gaskets and diaphragm.

Only one thing looked odd: the "plunger" that is actuated by a fulcrum arm -itself actuated by the big diaphragm- looks like it's not moving when the fulcrum arm is moved up and down. Since I imagine this is the fuel pump, that was odd. Doesn't look like that does anything. It just is sitting there (not frozen). It just has a rubber tip. Maybe that's just a valve and not a pump?

 

[D'Animal]

That is the metering diaphragm you are looking at.
The metal think with the rubber tip is the needle or inlet needle. The rubber tip is what seals against the seat.

What do you think it is suppose to do?

took more of the carb apart.

I'll probably get a repair kit with new gaskets and diaphragm.

Only one thing looked odd: the "plunger" that is actuated by a fulcrum arm -itself actuated by the big diaphragm- looks like it's not moving when the fulcrum arm is moved up and down. Since I imagine this is the fuel pump, that was odd. Doesn't look like that does anything. It just is sitting there (not frozen). It just has a rubber tip. Maybe that's just a valve and not a pump?

 

[e9999]

That is the metering diaphragm you are looking at.
The metal think with the rubber tip is the needle or inlet needle. The rubber tip is what seals against the seat.

What do you think it is suppose to do?

makes sense, sure didn't look like a pump device. Although my plunger still doesn't look like it's doing much.

I was looking for a pump gizmo, thinking about chainsaws, but maybe there is no pump then since a wacker is normally used upright and could be entirely gravity fed?

No air leak found so far.

 

[D'Animal]

The square shaped diaphragm is the fuel pump diaphragm. It has two little "flappers" that actually pump the fuel. The cover plate is generally held on with 4 screws. This is were the screen is.

The big diaphragm that you were looking at is the metering diaphragm. The side without the metal piece on is is exposed to the atmosphere. Atmospheric pressure is what pushes against the diaphragm. When the diaphragm is pushed in, the inlet needle opens up. The chamber is filled with fuel until the fuel need is satisfied.



No difference between a line trimmer, chain saw, hedge trimmer, leaf blower, etc carburetor. They all function the same. Some have a higher fuel pressure calibrated into them. It depends on if the tank is mounted above the carburetor or below the carburetor.

 

[e9999]

ah, very interesting, this pumping diaphragm must be actuated by crankcase pulsing pressure from the piston I imagine.

gotta go back in there and see where everything is. Should try and find a schematic of a Walbro (this is a WA199) carb someplace, that would help.


On the top side, the metering diaphragm that you mention and under that a rubber gasket in a moon shape flat device held onto the carb by a screw.


Good news is that if it's a diaphragm that does the pumping, maybe that's all I need to change to fix things.




added: found schematic. http://wem.walbro.com/walbro/product2.asp?partnum=WA-199-1&Series=WA
Good to see what everything is.
Looks like a lot there on the Walbro site.
Will look for service manual too. Found one: http://wind-drifter.com/technical/WalbroServiceManual.pdf

 

[D'Animal]

ah, very interesting, this pumping diaphragm must be actuated by crankcase pulsing pressure from the piston I imagine.

.

Exactly!


That is why when you have a crankcase leak (air leak) the fuel pump does not work or work correctly. Unit runs lean, hard to start, runs with choke 1/2 on, etc.

It is not getting the correct pulse signal from the crankcase to achieve the lambda of 14:1 ratio air to fuel ratio.

 

[e9999]

Exactly!


That is why when you have a crankcase leak (air leak) the fuel pump does not work or work correctly. Unit runs lean, hard to start, runs with choke 1/2 on, etc.

It is not getting the correct pulse signal from the crankcase to achieve the lambda of 14:1 ratio air to fuel ratio.

yikes! crankcase leak now... more to look for...