I have done a search and am looking for anyone modifying a single needle carb. What is behind the long stud located where the high speed needle would normaly be?
I have a Zama on a Homelite and am wondering if the high side can be richened up as it definately needs it.
Has anyone taken apart the carb and found what's behind the tamper-proof needle?

Inquiring minds want to know.

T.L.A.R. Why not just swap out the carb for a two needle? Seems like you are going to wast a lot of time and energy the other way. I just got a brand new Walbro WT from Ebay for about $7. plus shipping.

I may ultimately switch it out for a two needle Walbro, but I am challenged by the unknown. I like to learn how things are built and how they can be modified.
I guess it is kinda how the Homelite conversion got started.
$7 plus shipping is hard to beat for a two needle carb.

the single needle regulates both idle AND main jet. Very unsatisfactory for our use wit non-fixed loads. Been there, wasted my time.

Interesting, that explains some oddities trying to adjust it. Allso explains why two fairly large holes are needed in the throttle plate.
It would appear they over-simplified the carb for a very narrow rpm range.
This explains alot, thanks for the info.
Looks like a two needle is in order.

YUP!

I don't want to beat a dead horse, but my curiosity got the best of me this afternoon and I tore apart the Zama carb.
It seems they did away with two separate fuel controls [needles] and drilled the high, medium, and low speed passages common to one needle. Then they drill two fairly large air bleed holes in the throttle plate. I mounted the carb back on the Homelite 25 and run it again after I plugged the two throttle plate holes to slow down the idle.
If the mixture is adjusted for a good idle, the engine accelerates to about 2700 rpms and dies. If the engine is adjusted to high speed, which it never really reaches as the orifices are too small and it is still lean, then the engine wont idle because it is too rich.
I allways wondered how they kept rpm down with a string trimmer with no load.
As most of you allready know, they have found a way to lean out a carb and keep a string trimmer allmost fool proof.
I just purchased a Walbro two needle 11.1 mm for just over $6.00 plus shipping.
Into the junk bucket goes the Zama.

FWIW, I know how they work and maybe it might save someone else some frustration.

You just went one step further than I did, and it is good information, thank you.
It seems the carb works like this:
The regulating needle is adjusted to full throttle as usual.
when the valve closes, the vacuum over the idle jets increases, and fuel draw would be too high for the limited amount of air that is admitted to the engine. This can only partly be compensated for by the idle jet sizes, so: This is compensated by air being drawn through the high jet, thus feeding the idle jets with a mixture of fuel/air. (supposing the main jet has no check valve)
Extra compensation is done by limiting the amount of closing of the throttle valve. (the bleed holes you closed)

This would probably work well for one engine only, so a different engine would need a new carb type (new number, different jet sizes and bleed holes).

I think this explains why so many different carb numbers. Then add to the mix that some have primers, some have choke plates, and they ALL have different throttle shafts and arms to fit the manufacturers item, no wonder we have so many different numbers. Then they are all different venturi sized to boot!
I allso discovered the primer bulb uses the pump diaphram to manually move fuel to the carb. Clever. I think that answers the question if a two line primer bulb carb can be used as a single feed line carb. Blocking off the return line should allow this feature. If I am wrong someone correct me.
I think they purposly keep the carb lean to prevent over-revving and for emmisions and fuel economy. As evident by the over abundance of used trimmer/blower engines, the engine/unit is cheap and expendable. They aren't made to run hundreds of hours.
Sorry to carry on about something that is probably common knowledge to others, but maybe this will help others that get frustrated with the single needle carb.

Let's hope not all engines will be doomed to the single needle approach, and dual needle carbs will be available for years to come.

Many weedwacker motors have undersized venturies to limit the RPMs when the string breaks and the throttle is wide open.