Who knows Walbro HDC carbs?

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I have a saw sitting on the bench that will not run on the low circuit at all. Still has decent power, but it just doesn't have enough ring seal to pump fuel at low rpm/idle.
Kicker is it has 165 psi. Doesn't sound bad right? It had 205 when I built it 2 years ago... Someone decided dad's chainsaw needed sand in the muffler....
 
I havent seen an ipl yet that doesnt show a disc for an HDC. Yes the 3 ports were clear. Do you recall the spec for an hdc with no disc?

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There was a similar discussion about a week or two ago where the posting showed a carb and the parts diag. for it with no disc. I'll see if I can dig it out. Just another thought for what it's worth, when the throttle is at idle position, is the throttle disc open enough to expose the idle fuel port to intake vacuum?
 
There was a similar discussion about a week or two ago where the posting showed a carb and the parts diag. for it with no disc. I'll see if I can dig it out. Just another thought for what it's worth, when the throttle is at idle position, is the throttle disc open enough to expose the idle fuel port to intake vacuum?
I can physically back the screw out till the blade is closed off...it just revs to the moon unless i choke it.

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I can physically back the screw out till the blade is closed off...it just revs to the moon unless i choke it.

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Sounds like something is seriously leaning out the idle mix, about what to expect from a faulty check valve that is stuck open or, in your case, perhaps been removed. Your new valve kit should fix everything when it comes.
 
If the check valve kit don't fix it you might look on flea bay for a HDC carb.
I see some HDC's for around $25 with a 30 day return.
I've never seen a HDC do the run away. I've seen them have idling issues. Just a thought: For one to do a fast rpm it would also need air to go with the excess of gas or it would flood.???????
I have couple Homie EZ's carbs that I have replaced the check valves in sometime back due to them not operating properly after all else inside the carb was eliminated as the issue and set them back and labeled as needs a test run. I flinched at the price of the check valve kit, but just wanted to see if it was the carbs issue. I probably won't do the HDC carb test runs until one of my old EZ's is back on the work bench for problems. (I don't like to fool around with something that is running good, things go SOUTH and then cave in on me usually, then struggle to get back to normal)
I keep a KNOW GOOD spare HDC carb around for my OLD Homie EZ's so as to make sure I can eliminate the carb as the saw's run issue.
What type of saw is the carb being used on?
 
HDC`s are one of the most problematic carbs I have encountered in chainsaw repairs, the circuit plates and their gaskets must match the carb they are being used on, that is one problem when repairing carbs previously stripped down by others, the metering lever sits very low to the circuit plate in them and of course the check valve can cause more problems than that repair/problem solver list Walbro has published, it takes a while to figure out this carb. I pull and repair/rebuild the check valves now that the price of them has got too high for what they are, old saws that use the HDC are tossed away due to the high cost of repairing the carbs now a days.
 
and as pioneerguy600 indicates in the previous post and especially if someone has already been flogging a HDC carb and possibly used a China clone kit that is not quite correct, keep a very close eye and heads up and inspect the new kit gaskets you are using and keep in mind that someone may have used a wrong diaphragm or gasket, especially the metering diaphragm with the little button for the metering lever. I've seen them different lengths and just close is not a good thing. Also keep in mind when installing the gaskets old to new comparison that someone may have used the wrong one, lay the gasket on the carb and look at the little holes and see if they match the carb's body. Strange but some of the carb kits now days are just not correct, even the welch plugs are wrong size on some carb kits. I do not pull a welch plug now days unless as a last resort and only then when I have a correct size replacement in hand. I have reformed old welch plugs if I could blow them out with air pressure. I suspect that some carbs, especially the smaller ones with check valves are actually ruined by using too much air pressure when cleaning.
I think I seen recently over on Leon's site, you tube where he was flogging a HDC or close type carb that the casting or cover plate was actually warped and he had some other tips about getting them sealed and bench testing..
Right when I think I'm good at repairing carbs one will get too me, put me in my place and sometimes a NEW China cheap clone will work, but don't be surprised if a clone is not really smooth as a OEM.
Summary: I feel your pain with them little carbs that look like Ronnie Milsap should be able to make them run, but, but.
 
If the check valve kit don't fix it you might look on flea bay for a HDC carb.
I see some HDC's for around $25 with a 30 day return.
I've never seen a HDC do the run away. I've seen them have idling issues. Just a thought: For one to do a fast rpm it would also need air to go with the excess of gas or it would flood.???????
I have couple Homie EZ's carbs that I have replaced the check valves in sometime back due to them not operating properly after all else inside the carb was eliminated as the issue and set them back and labeled as needs a test run. I flinched at the price of the check valve kit, but just wanted to see if it was the carbs issue. I probably won't do the HDC carb test runs until one of my old EZ's is back on the work bench for problems. (I don't like to fool around with something that is running good, things go SOUTH and then cave in on me usually, then struggle to get back to normal)
I keep a KNOW GOOD spare HDC carb around for my OLD Homie EZ's so as to make sure I can eliminate the carb as the saw's run issue.
What type of saw is the carb being used on?
Homelite 360

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If the check valve/screen is leaking that bad, or is missing, how is the saw over speeding with the throttle plate closed? Where is it getting the air from?

I thought you meant it was stalling at idle.
 
One other thing about these HDC carbs I have encountered is the ones used on the Stihl 015 does not have the forked metering lever and the metering diaphragm does not need or have the little slotted button on it to hook onto the lever. Many mistakes have been made with kits in this carb as Walbro`s instruction sheet often calls for the rebuild parts to do so. Each carb must be examined closely to match the correct internal parts , most carbs that come to me have been rebuilt by others , incorrectly. I just finished up three different Stihl 015 saws that had ruined fuel delivery parts. Now there is three old Homelite saws sitting waiting for fuel related and lost spark problems.
 
If the check valve/screen is leaking that bad, or is missing, how is the saw over speeding with the throttle plate closed? Where is it getting the air from?

I thought you meant it was stalling at idle.
Good question, 17 years ive never seen a saw behave like this and ive seen em with massive case leaks. This bastard somehow ran with this stihl boot just sitting on the block and half sucked in around the carb...
de390e09467f2788a4b4c8ad8f4c86e6.jpg


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If the check valve/screen is leaking that bad, or is missing, how is the saw over speeding with the throttle plate closed? Where is it getting the air from?

I thought you meant it was stalling at idle.
Good question, 17 years ive never seen a saw behave like this and ive seen em with massive case leaks. This bastard somehow ran with this stihl boot just sitting on the block and half sucked in around the carb...
de390e09467f2788a4b4c8ad8f4c86e6.jpg


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We have some confusion here, are you saying that even when the throttle is completely closed the engine will race at high rpm or does it just race briefly after a cut when it should return to idle but then stalls instead?
 
I can start it...rev it up, it tachs out fine, no racing at full throttle. Let it off to idle and it drifts down...but doesn't really idle down. This is with the idle screw bottomed..ans L screw out 3 turns. As i back out the idle screw...it idles...up and takes off.

As long as i give it more fuel it will idle down fine. But the L needle has little to no effect

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I suspect it's sucking air. (above the piston instead of below the piston) for example somehow behind the carb at the intake manifold area, maybe.
Check that intake manifold rubber boot closely for a crack and it's sealing surfaces. I'm not familiar (no hands on experience) with that boot or the 330 Homie but you said you found the boot in your picture post distorted and out of whack when you first started flogging that Homie.
Where is the H (high) screw set? (and does it seem normal)
By normal I mean: Does the H jet seem to go through the smooth lean rev when holding the throttle steady and adjusting little more out and enter the 4 cycle (rich) as it's adjusted outwards. If the H jet seems to adjust ok, or if it don't ever get rich or 4 cycle it's a slight air suck , air being sucked above the piston instead of from below the piston in the block.

I've seen them HDC carbs L jet only want 1/2 turn out and very rarely over 1 1/2 (even though Walbro indicates ! 1/2 and if going out to 3 out is way out of tolerance. (this 3 out indicates a air leak and trying to compensate for too much air by adjusting more rich)

I've never seen one act like either but just going by it's sickness symptoms.
 
I suspect it's sucking air. (above the piston instead of below the piston) for example somehow behind the carb at the intake manifold area, maybe.
Check that intake manifold rubber boot closely for a crack and it's sealing surfaces. I'm not familiar (no hands on experience) with that boot or the 330 Homie but you said you found the boot in your picture post distorted and out of whack when you first started flogging that Homie.
Where is the H (high) screw set? (and does it seem normal)
By normal I mean: Does the H jet seem to go through the smooth lean rev when holding the throttle steady and adjusting little more out and enter the 4 cycle (rich) as it's adjusted outwards. If the H jet seems to adjust ok, or if it don't ever get rich or 4 cycle it's a slight air suck , air being sucked above the piston instead of from below the piston in the block.

I've seen them HDC carbs L jet only want 1/2 turn out and very rarely over 1 1/2 (even though Walbro indicates ! 1/2 and if going out to 3 out is way out of tolerance. (this 3 out indicates a air leak and trying to compensate for too much air by adjusting more rich)

I've never seen one act like either but just going by it's sickness symptoms.
Its almost 1 full turn out on the H needle, and yes if i hold it open and turn the H its very responsive, can easily flood it out or shut it off with the H needle. The L needle, really nowhere makes any change.

New boot, and pulled the manifold, inspected and sealed it and replaced the pulse line.

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When you say "bottomed", does that mean the idle speed screw is turned in as far as it will go and is holding the throttle as open as possible at idle?
Yes. If i run it down all the way it will get some fuel from the main nozzle and hold a speed, as i back it out...the saw speeds up and runs away

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All HDC's have the rubber flap in the check valve. Some are different PN's (The HDC 39 calls for an 86-520 kit)

One carb that does use only a capillary screen are some of the Tillotson HS carbs.

The saw will not run properly without that little flap in the check valve. They are only missing if someone has been in there as they turn to a gooey black mess when they have failed.
Once you have that in, it might be easier to diagnose the issue.
 
All HDC's have the rubber flap in the check valve. Some are different PN's (The HDC 39 calls for an 86-520 kit)

One carb that does use only a capillary screen are some of the Tillotson HS carbs.

The saw will not run properly without that little flap in the check valve. They are only missing if someone has been in there as they turn to a gooey black mess when they have failed.
Once you have that in, it might be easier to diagnose the issue.
I have pulled a few that only the fiber mesh was left in the check valve, it was completely porous, all the coating had been dissolved and gone.
 
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