Stumped...air leak

[singinwoodwackr]

I have a Shindaiwa 577 I got together last week. Cut with it yesterday and it did well. It sputtered, however, so back to the bench.
Plug shows lean condition so v&p tested, hmm.
I have nailed the leak down to the mating surface of the carb and boot using a rubber gasket and plate in place of the carb. Holds fine.
I've tried 3 carbs including 2 brand new.

Bubbles form at the back of the carb.
There is no gasket but I even tried making one, no joy.
I wondered if it was leaking through the throttle shaft so tried a little Hylomar Blue sealer around the outside of the shaft...no difference. Brand new carb made no difference either so I doubt the shaft is the source.

Carbs all tested fine

 

[lone wolf]

What about the intake boot where it mates to the carb? Any parts missing?

 

[alexcagle]

You probably have rubber stiffness/shrinkage.
Either replace the boot, or remove a little in the noted area in my screenshot.

 

[alexcagle]

That's the tech "overtighten preventer"

 

[alexcagle]

Air testing "through a carburetor" is a new one on me.
Throttle shafts leak even when new.

 

[singinwoodwackr]

What about the intake boot where it mates to the carb? Any parts missing?

No, it is similar to other saws…

 

[singinwoodwackr]

Air testing "through a carburetor" is a new one on me.
Throttle shafts leak even when new.

Good point, and I had thought of that. The carbs all passed tests by themselves so are fine. I’m most likely overthinking this…as usual.
The crankcase holds vac and pressure without the carb…I should leave it there. Put it back together, try a new carb and see if it still sputters.
Will try another fuel filter as well.

All of the symptoms didn’t start until it got to cutting. Prior to that it idled fine and reved up normally.
I may also have leaned it out too much as spec on this saw is 12000…much lower than I’m used to.

 

[singinwoodwackr]

View attachment 1168138
You probably have rubber stiffness/shrinkage.
Either replace the boot, or remove a little in the noted area in my screenshot.

Boot is soft and playable …thought of that since the saw hadn’t been run in probably 20 yrs

 

[muddy42]

As above, your pressure test doesn't look right. Basically you need to test the bottom end plus the crankcase - so seal behind the muffler, behind the carb and apply pressure either through the impulse line or spark plug (and seal the other alternative). The best way to pinpoint bubbles from a hard to see spot is to dunk the saw under water.

 

[singinwoodwackr]

As above, your pressure test doesn't look right. Basically you need to test the bottom end plus the crankcase - so seal behind the muffler, behind the carb and apply pressure either through the impulse line or spark plug (and seal the other alternative). The best way to pinpoint bubbles from a hard to see spot is to dunk the saw under water.

Yea, I was over thinking it, duh.

 

[david newbie]

Yea, I was over thinking it, duh.

If you need a real expert, just ship it to me......

 

[Vintage Engine Repairs]

Make sure the intake manifold is protruding the seat of the partition wall. Over time, they lose their ability to return and thus you get that leak between carb and manifold. This is why it’s essential to use the correct test adapter for the intake and not rubber strips between the carb and manifold…

 

[singinwoodwackr]

Make sure the intake manifold is protruding the seat of the partition wall. Over time, they lose their ability to return and thus you get that leak between carb and manifold. This is why it’s essential to use the correct test adapter for the intake and not rubber strips between the carb and manifold…

That's what it looks like to me. However, I added rubber spacers that should have "pushed" that out enough for a good seal but did nothing.

 

[stihltech]

HMMMM interesting.
The rubber gasket needs to go between the carburetor and the intake. There are other things that can leak on the carburetor. The shaft at the back of the carburetor, the accelerator pump. I have tried a gasket in this space to compensate for a warped intake mount (YESS I KNOW- not recommended, but useful if parts are not available). Make sure the pulse to the carburetor is open if you do.
Actually, looking at it again, the flat plate needs to go at the intake with no gasket and no carburetor. If it seals, then you do not have a pressure or vacuum leak.
Fuel tank vent and fuel? Had an 028 today that would not pass vacuum. Problem solved.

 

[singinwoodwackr]

HMMMM interesting.
The rubber gasket needs to go between the carburetor and the intake. There are other things that can leak on the carburetor. The shaft at the back of the carburetor, the accelerator pump. I have tried a gasket in this space to compensate for a warped intake mount (YESS I KNOW- not recommended, but useful if parts are not available). Make sure the pulse to the carburetor is open if you do.
Actually, looking at it again, the flat plate needs to go at the intake with no gasket and no carburetor. If it seals, then you do not have a pressure or vacuum leak.
Fuel tank vent and fuel? Had an 028 today that would not pass vacuum. Problem solved.

right, I used just the plate bolted to the carb mount/boot and it sealed just fine (no rubber). But, it won't seal like this with a carb...weird. I wonder if these HDA29 carbs are just leaky suckers
I ran the saw again today. fuel filter was replaced. the sputtering from the other day cleared up. I had used the original OE filter as it looked brand new. I'm assuming that it had residual crud left over from the minimal use the saw got before being shelved...probably 20yrs ago. Throttle isn't as snappy as I'd like so will rebuild the carb...hadn't done that as, again, it looked brand new inside Even the gaskets were perfect. I got K-22 kits in today so will sonic tank it and try a new kit.
A fun saw to use. It has so much torque with a 24" bar I'm going to try an 8t rim instead of the 7 next time and see how the increased chain speed does. It might lose all that nice torque...we'll see. I know it would do very well with an 18" bar and 8t, that's for sure. It is nice using an 18#, 24" saw, however.
I'll experiment with different bar lengths...have lots to play with.

 

[singinwoodwackr]

Make sure the intake manifold is protruding the seat of the partition wall. Over time, they lose their ability to return and thus you get that leak between carb and manifold. This is why it’s essential to use the correct test adapter for the intake and not rubber strips between the carb and manifold…

It protrudes but not nearly enough, IMO. Using just a plate with no rubber sealed fine.
The back of the carb is flush with the plastic of the tank...hence the leak, I believe. I have tried to add a homemade, 1mm, rubber gasket but there was no difference. I also tried stuffing an O-ring behind the boot to push it out more...nope.

I wonder about tracing the back outline of the carb onto the face of the tank where it mounts and then removing some of the plastic so the carb would then sit back further into the tank.
Ah, fun with old saws