The pump diaphragm only works at idle. If you need more fuel at WOT then you may want to drill the auxiliary main jet a bit larger.
Sometimes you need to drill the needle seat to flow enough fuel to keep the diaphragm full.
Thanks for the links phil. Cant beleve i missed this originally but better late than neverIs there a good 'Carburetors for Dummies' type book?
I understand the most basic stuff about carbs, but not all the circuits, how to diagnose, etc. Any good references that focus on the types of carbs used on chainsaws? Thanks.
Philbert
EDIT: Found these on YouTube. First 2 are pretty clear and helpful. Third is pretty long, so still working through it.
How A Carburettor Works/STIHL/Two Stroke/Fully Explained (TheRepairSpecialist)
Walbro Carburetor Service (Walbro)
12 Reasons Why Two Stroke Cycle Engine may Bog Down (CRASH COURSE) (TheRepairSpecialist)
the damn thing won’t run for more than 5 seconds.
5 seconds? Crikey, you mean the saw won't even idle for 5 seconds?
EDIT: If you can start the saw on choke and it will run if you keep the RPM up, but it will soon die if you drop it back to idle, then it sounds like a really bad check valve on the nozzle. The bad check valve is allowing air to be sucked back into the wet side.
There is one way to eliminate the 'faulty check valve' issue - make up a 'dummy' nozzle. Find an old nozzle and fill it in with GB Weld so that the nozzle is blocked off and won't work. If you put the 'dummy' nozzle in the carb and the saw will now idle, then you have diagnosed that it was sucking air back through the check valve.
Of course, I'm sure you already checked that you put the pump diaphragm back in the carb when you put it back together and that the impulse line is hooked up.
Just FYI, over on A/S, I read a thread about the cheapo $10-$15 Chicom carbs they sell on ebay and it seems like a LOT of folks are using them instead of messing with their carbs, and they work well most of the time...
and spent over an hour making micro adjustments and the damn thing won’t run for more than 5 seconds.
When you mention making 'micro adjustments', I'm wondering if when you drilled the low speed fuel access hole you may have got it too close to the seat and nicked it when you drilled the hole.
If you have an old Zama C3M sitting around, you may want to send it off to Poleman. Not only will you get a lot stronger running engine from that 7900, but it will have the Zama idle circuit that is super easy to tune. You can always go back and forth to the HD-12 to play around with.
the Zama and modifying it, I have no problem sending it to Poleman but would prefer to do it myself.
Got it! Ain't nothing like doing it yourself! Sh!t that has got more of a buzz than someone handing it to you on a platter.
You need a micro-drill set with a pin vise. The idle/transition jet needs to be pulled and the first transition hole needs to be drilled out from .5mm to .55mm in order to make the carb idle easily.
Drilling the jets is a bugger. The first run through the jets takes a bit of time and patience, the jets are .25mm from the factory. Using the .3mm ream it takes about 30 minutes to get through without breaking the drill. After that the later sizes are about 5 minutes.
Drill the idle/transition jet to .30mm - then pull it (make up a diagonal pliers with a notch on it so you can TWIST it as you pull). Then drill the first (closest to the idle port) transition hole to .55mm.
Replace/tap in the idle/transition jet and then move to the auxiliary low speed jet. Again, the first run will take you some time, but after you get through with the .3mm, then start moving up and take it out to .40mm.
That should put you in the ballpark. You may want to 'shim' the metering spring if it is a bit rich off the bottom. Some like that, it is a matter for you, just put a washer or two under the spring if it is a bit rich.
Its a bit of a fiddle, but Crikey, it sure makes the saw into a whole different WORK saw when you're finished.
View attachment 126561
The jet in the lower left is the idle jet. The jet on the other side of the Welch plug is the auxiliary. Ant the brass top one is the nozzle that will need filled ( indentation on top) is jets are drilled above .40 to lean out the feed and make/allow the H needle to be adjustable. I use JB Weld.
The idle jet is the one you will need to pull to drill below it.View attachment 126562You'l see this once you pull the jet. Drill the lower hole that feeds into the ventura View attachment 126565 to .55mm, then tap the jet back in.
This is what I use to pull the jet. I modify the pliers by gringingyhe bottom of the jas thinner and flatter to get the gripping surface below the top of the jet. Firmly grab the jet but don't crush it. Just a firm grip so it doesn' slip. I than rock the jet very slightly until I feel it giving then a little harfer and it comes right out. With this method I've found I can remove them without creating marks ( or very little).
other than some small glitches I will need to work out at idle
That looks nothing like the HD12 I have.