Peculiar Carburetor Issue(s)

[heimannm]

I posted a similar request over on AS but have not had many responses so I will ask here as well...

I work on a lot of different saws, most are just inexpensive homeowner type saws but none the less, I do have some practical experience with all kinds of saws. I am quite familiar with setting the metering lever to the proper height, and how to "tweak" it a little if need be depending on the conditions; for example on a fixed jet carburetor you can set them just a bit lower than book value to reduce the typical over rich high speed setting.

I have all three of the "special tools" two Walbro and one Zama to make sure I know the correct position of the metering needle when putting the carburetor back together.

I have run into this peculiar situation from time to time and wanted to know if anyone else has had this experience. The latest project was a Mini Mac with the Zama carburetor. I set the metering lever correctly, put the saw together and it would not pull fuel, it would run on a prime. Pulled it all apart (the only way to access the carburetor on the MM) and found the lever was flat against the body of the carburetor. I then reset the lever, put it all back together and the saw ran as well as a Mini Mac can. Set it down to work on something else for a bit and tried to run it again...now it was badly flooded. Pulled the spark plug and left it set overnight.

Next day it was clear that fuel was leaking into the crankcase so I took it all apart again and this time the metering lever was bent way up causing the flooding. Reset the lever again, and now the saw worked just as it should.

Can anyone help me understand how that metering lever is getting bent and out of adjustment when the carburetor is fully assembled. I don't recall this ever happening with an SDC that has a fork on both ends (diaphragm and needle) but I can't say for a fact it never happens.

Thanks for your input.

Mark

 

[67L36Driver]

FWIW:

I had a similar problem with a Power Mac 6.

The needle supplied with the carb kit was some longer (center) than the original? (right) I found in it.

I cut off the excess with a Dremel to match.

The replacement required the metering lever to be bent at an awkward angle. Top left.

What it ended up looking like assembled.

The thing is, it’s a Zama. I almost always have trouble with them. Tilleys and Walbro s, not near as much. [emoji43]‍[emoji94]

 

[67L36Driver]

I should also point out the saw’s fuel cap lacks a duckbill so the tank won’t build up any vapor pressure.

Now if the oiler worked, we’d be in fat city. [emoji58]

 

[heimannm]

The PM6 fuel cap is supposed to have a duckbill valve in it...

I know how to adjust the metering lever and I am very careful to check and adjust as required when reassembling the carburetor. In addition, I have a large supply of metering needles, levers, and pins on hand so I can replace any questionable parts with good ones before I put them back together.

What I don't understand is how they get so far out of adjustment after I've put the carburetor back together...

Mark

And P.S. I would much rather have a Zama on a Mini Mac or PM6 saw as the check valve seems to be far more reliable that the ones in the MDC's, just my experience.

 

[president]

FWIW:

I had a similar problem with a Power Mac 6.

The needle supplied with the carb kit was some longer (center) than the original? (right) I found in it.

I cut off the excess with a Dremel to match.

The replacement required the metering lever to be bent at an awkward angle. Top left.

What it ended up looking like assembled.

The thing is, it’s a Zama. I almost always have trouble with them. Tilleys and Walbro s, not near as much. [emoji43]‍[emoji94]

I cuss those Zama,s daily!

 

[Stump Shot]

Look at the "button" on the metering diaphragm, some are different lengths, and some kits do not recognize the shorter length button that presses the inlet lever. Had this happen to me and took some time to sort out. Luckily I keep a stash of all the left over parts from kits and had the right diaphragm to install.

 

[Gaston]

^^^^^^This.^^^^^^. Has gotten me more than a couple times.

 

[Mastermind]

Interesting thread Mark.

I got nada.

 

[heimannm]

Looking at a diagram of an SDC, I really can't understand how this can happen but then I've had the experience so I know it does from time to time.


Mark

 

[Shibby]

Interesting this came up, I'm also fighting issues with a PM6 carb. It's flooding and ultimately dies at idle. I'll have to check the stopper length and the button size.

 

[Al Smith]

Now if the oiler worked, we’d be in fat city. [emoji58]

--which most of the time they don't----On that I plugged off the impulse and bypassed the auto function and just used the thumb button .Mini's almost guaranteed to make a preacher cuss .

 

[IEL Fan]

Random thought....
is there an issue where the fuel pump diaphragm is not sealing(the little flap caught resting high on a small imperfection or grit). Fuel pressure from the tank, built up by expansion from a hot engine on the test rum, pushed the inlet needle back over night?
I have experienced saws that ran fine then intermittently flooded over night or a day or so later. I tracked a number of issues down to a slight build up on the needle seat. A small q-tip with mild lapping compound and "polished" where the inlet needle contacts the seat. Seemed to solve the flooding situation. This was more of an issue for me on Tillotson HL series carbs of older float type. Just a thought.

 

[Maintenance Chief]

Ok so my theory is that your carburetor is basically clean rebuilt and void of any fuild, being dry or at least empty upon start up the impulse is supposed to be only acting on the fuel pump diaphragm. With out any hydraulic braking of fuild any small air leak would act in both directions with the impulse both pushing and pulling on the needle diaphragm (assuming that a super tiny leak exists). Assembly oils in the impulse port would definitely make the pulse very strong.
Ok won't pull fuel ,you pull the carb reset the metering diaphragm height ( possibly fixing the tiny leak or oils have sealed the dry fuel pump now?)
I couldn't find a diagram of the zama model, but I'm assuming that the pump and metering systems are isolated from each other like other carburetors so being that they are designed to have fuel in them under the fuel pump diaphragm creates resistance from the impulse.
Even if the leak was so small that it was present only some times it could theoretically suck the metering diaphragm flat ,I just don't understand that particular carburetor enough to understand how it could go the opposite direction of the level being too high after reassembly?

 

[Shibby]

Figured out the carb issue on my PM6 and sharing here in case someone searches in the future they can hopefully find multiple answers in one post.

The lever for the needle stopper was bent up too much so the needle stayed open most of the time. I'm guessing the diaphragm didn't go high enough to let the needle down and close and essentially stayed flooded. Bent the lever to a normal position where the diaphragm would let it open and close small amounts and now it's running as expected.