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Any carb gurus? Walbro HD

460cixy

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Will be drilling larger when I get a chance to get it apart again also think it may be sucking air between the carb and intake boot cheap China *s-word
 

460cixy

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When I get a rush of blood ile do some more drilling I have another saw running nitro and it was not half as hard to set up but needs a lot less fuel too
 
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Any updates on these carbs? I did the described mod (sealed the low speed circuit hole under the nozzle and drilled a hole from wet side to L needle, verified the nozzle was serviceable) on the HD12 on my 7900 as it seemed to idle erratically and be running out of fuel on the top end. Now it runs even worse than before. I started .25 out on L/ .75 out on H and spent over an hour making micro adjustments and the damn thing won’t run for more than 5 seconds.
 

countryhog

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Is 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)
Thanks for the links phil. Cant beleve i missed this originally but better late than never
 

Terry Syd

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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.
 
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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.

I verified the check valve when it was pressed out using a piece of fuel line on it with my vacuum pump. It appeared to be good.
 

RedGas

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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...
 
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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...

I’m one of those people who want to know “why” things fail or go wrong. Before I simply toss a new carb on it I want to know what is going on with this one. Not poopooing you, just my curious mind.
 

Chainsaw Jim

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Regardless of what goes wrong with a stock carb on a stock saw, it's highly unlikely that jets will ever need plugged or drilled out for a fix.
Once you rule out the carb kit there's only a couple things that can be wrong...a warped cover plate, bad casting on the pump side, bad check valve, corrosion blockage, or damage from wear or misuse.
 

Terry Syd

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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.
 
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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.

I made a TON of measurements and ensured I would not hit the needle seat. If anything I nicked the threads. Using the bit I also verified the location after the hole was drilled, just in case I screwed it up. I also ensured it was properly cleared out of chips once completed. I’ll have another C3 shortly to put back on the saw. I read the other thread that you guys were dissecting the Zama and modifying it, I have no problem sending it to Poleman but would prefer to do it myself.
 

Terry Syd

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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.
 
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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.

You lost me in the technical terms.... Its a lot to ask, but do you have pictures for each of the ports you are talking about?

I have a micro drill set and pin vice already. I also have a couple different metering springs, but no washers. I’m pretty sure I can make a few washers with a hollow punch, I have some small pieces aluminum, stainless and copper sheeting.
 

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20150901_175547.jpg
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.20150901_175419.jpgYou'l see this once you pull the jet. Drill the lower hole that feeds into the ventura 20140831_175430.jpg 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).
 

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Oh.... BTW, I have used an Aftermarket HD carb and drilled it as Terry discribes....I tested it on a 440/460 Hybrid and it was outstanding other than some small glitches I will need to work out at idle. I think it is mostly related to pop off y what it' doing.
Throttle responce and top end were very good....
 
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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).

That looks nothing like the HD12 I have.
 

Terry Syd

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other than some small glitches I will need to work out at idle

Yeah, that's where dialing in these HDs becomes a pain in the azz. I'm trying to work my way into the idle issue on this HD-5 of mine slowly. I want to get it leaner without moving the low speed needle from where I've got it. I've tried notching the throttle plate, but it looks like I'm going to have to go back into the carb, pull the welch plug again and then take the front air bleed hole out to the next size.

Even after all that, I still may end up with some sort of compromise with the low speed needle.

It's like there were two designers for the HD. A really qualified bloke designed the venturi and the position of the air bleed holes - then they gave it to the 'work experience' bloke to design the idle/transition circuit...
 
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Terry Syd

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That looks nothing like the HD12 I have.

The Zama 'twin jet' is completely different to the HD. What makes it such a great carb to mod is that every function; idle, transition, low speed circuit flow and high speed flow can be SEPARATELY tuned.

Rich (Poleman) has a lot of experience in modding them for different applications. The Chinese replicas are no longer available and OEM carbs are starting to get hard to find. If you can get your hands on one, I'd suggest sending it to Rich to mod rather than risk messing it up.
 
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