Any carb gurus? Walbro HD

[jockeydeuce]

I have no shame in admitting that I'm not super knowledgeable when it comes to carbs and although saw carbs are seemingly simple....They leave me frustrated often.

I have numerous Walbro HD carbs from 365/372 Husky's that either have intermittent idle issues or are extremely lean on the idle circuit. I've had welch plugs out run through the ultrasonic and they seem to be clean but no go.....I'm obviously missing something. Does anyone know the exact idle circuit flow in these?

 

[Terry Syd]

I don't know what you have, but if it is an EPA approved HD, then it will likely have a joined high and low speed circuit. If you have a spare carb to mod, we can try some mods.

If you have a spare carb, then push out the main discharge nozzle and see if it has TWO holes/inlets inside the main discharge nozzle tunnel. If so, then one of them will join up with the low speed circuit and be the source of fuel for the low speed circuit.

We can mod it one step at a time and see if you can improve your carb.

 

[srcarr52]

I'd try nipping the tip of the metering spring to lower the pop off pressure. Each flat wound coil is usually worth 3/4 psi.

 

[jockeydeuce]

I don't know what you have, but if it is an EPA approved HD, then it will likely have a joined high and low speed circuit. If you have a spare carb to mod, we can try some mods.

If you have a spare carb, then push out the main discharge nozzle and see if it has TWO holes/inlets inside the main discharge nozzle tunnel. If so, then one of them will join up with the low speed circuit and be the source of fuel for the low speed circuit.

We can mod it one step at a time and see if you can improve your carb.

I have at least 15 of these with varying degrees of idle issues that all the cleaning I can do won't cure them, so I have lots to play with.....The fact that some feed the low from the main discharge nozzle passage could be the issue with some of these and there's a plugged passage....I'll check into that....Thanks

 

[srcarr52]

I have at least 15 of these with varying degrees of idle issues that all the cleaning I can do won't cure them, so I have lots to play with.....The fact that some feed the low from the main discharge nozzle passage could be the issue with some of these and there's a plugged passage....I'll check into that....Thanks

Sometimes the nozzle won't be properly installed and may be partially blocking a passage.

 

[jockeydeuce]

Sometimes the nozzle won't be properly installed and may be partially blocking a passage.

That is entirely possible too...I don't imagine they all have the same issues. It's gotten to the point that I have to get these figured out if possible. These are expensive from the dealer and I'm about out of good ones for builds.....Most as I recall would lean out at idle and still have a full well on the metering side, so I don't think it's a "pop off" thing.

 

[srcarr52]

That is entirely possible too...I don't imagine they all have the same issues. It's gotten to the point that I have to get these figured out if possible. These are expensive from the dealer and I'm about out of good ones for builds.....Most as I recall would lean out at idle and still have a full well on the metering side, so I don't think it's a "pop off" thing.

Just because the well has fuel doesn't mean the fuel is getting into the engine. New fuel has to be coming in.

 

[jockeydeuce]

Just because the well has fuel doesn't mean the fuel is getting into the engine. New fuel has to be coming in.

Gotcha!

 

[Terry Syd]

If a carb is leaning out at idle, it may be a problem in the main discharge nozzle with the check valve. With a leaking check valve it becomes easier to pull air through the nozzle than fuel from the tank.

Since you have some extra carbs to work with, try pushing out the nozzle and see if there are two holes in the tunnel. If so, then you can block the passage to the low speed circuit by pushing some GB Weld in the hole. Clean out the tunnel and reinstall the nozzle. Then drill a 1mm hole from inside the carb wetside to feed the low speed needle. You have to screw the low speed needle in all the way and measure/mark the remaining needle that is outside the carb. Then remove the needle and place it on the carb to see where it seats. Drill the hole back from the seat (don't hit where the needle seats!). If the needle has a flat area without threads, that is a good distance to drill from the seat.

What the mod does is separate the two circuits. The idle should be more consistent and it will likely be more rich. You may want to use a stronger metering spring (or shim it with some washers) to lean out the idle and to take advantage of the extra fuel in the low speed circuit (for more torque down low).

 

[jockeydeuce]

Thank you for taking the time to share all that information @Terry Syd ....I'll try to get some time to play with some of these in the next few days. A lot of these have two pressed in brass jets in addition to the main nozzle....I'll get a pic tomorrow and maybe you can explain exactly what they feed?

 

[Terry Syd]

One of the jets will feed extra fuel to the discharge nozzle, it runs in parallel with the high speed needle. The other jet is an auxiliary jet for the low speed circuit. You can play with that jet to bump up the flow and increase the low speed torque.

I've never worked on a HD for a 372, so I don't have a clue what the mods should be for such an application. However, if we take it a bit at a time, we should be able to get something in the ball park.

Just as an aside, if the check valve is leaking just a little and the low speed circuit is feeding off the nozzle, then any variation in fuel flow in the nozzle will have a significant effect in the low speed circuit. Getting the two circuits separate will assist in regulating the fuel flow through both circuits.

 

[Philbert]

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)

 

[jmssaws]

I'm fighting these horrible peices of junk now on a couple 372's but the worst one is a hd17 on a 044 hybrid,it's either starving lean or bog and die fat at a idle and the tiniest adjustment of the low will make it one or the other.
For a good idle it will bog when you hit the throttle and sometimes die,for good acceleration after a couple seconds of idling it will start to lean surge and die.

I've raised pop off but maybe not enough?

 

[jockeydeuce]

I'm fighting these horrible peices of junk now on a couple 372's but the worst one is a hd17 on a 044 hybrid,it's either starving lean or bog and die fat at a idle and the tiniest adjustment of the low will make it one or the other.
For a good idle it will bog when you hit the throttle and sometimes die,for good acceleration after a couple seconds of idling it will start to lean surge and die.

I've raised pop off but maybe not enough?

The symptoms you describe at the end with coming back to idle and then get the lean surge and die, is a common one I run into and also had an HD on an 044 last week exactly the same.

I'm determined to get a handle on these if possible. If I can understand them, then the problems can be solved

 

[jmssaws]

I generally dig through my bucket of carbs and grab a zama but I don't have anymore so I have to make a HD work.

 

[jockeydeuce]

Hahaha.....That's how I roll too usually, but it's gotten to that same point for me. Slim pickings and buckets of carbs with tags marking known problems.

 

[jmssaws]

I've had 6 or 7 different carbs on the hybrid and they all do the same thing.

 

[jockeydeuce]

I've had 6 or 7 different carbs on the hybrid and they all do the same thing.

Geez man....That's a lot of dud carbs in a row with the same problem. I've gone through a few bad ones but the problems usually varied a bit. Sure would be a good time to have a known good carb on another saw to try.

 

[jmssaws]

They do vary but it's the same problem.
Some worse than others.

 

[1990Husky262]

have you tried to replace the check valves? If you look at the parts list they are available sepretly and relatively cheap. Out of curiosity are you sure your not having air leak problems? Like air leak from between carb and intake from a distorted intake flange from over tightening.