First ever carb rebuild idles very fast

[dylanjustice]

I was given an old Husky L65, and told it was "a bit hard to keep running". It had great compression and started up easy after I cleaned it out, but it was indeed hard to set a low idle that wouldn't promptly stall.

I did my very first carb kit on it, and now it idles way too fast, even with the idle screw backed all the way out. I've checked everything I can think of and everything Dr. Google has suggested. I'm here looking for suggestions on what to check next!

Here's what I've checked:

- blocked the intake and outlet with rubber and leakdown tested (through the plug hole; there's no impulse line). Pressurized to 10psi, it dropped to 7 in maybe 2 minutes. I'm not set up for a vacuum test yet, but I will if that's indicated.
- triple checked order of gaskets, diaphrams, and flapper membranes
- pressure tested the new intake needle (and verified length/throw is pretty close to that of the old needle)
- carb is a Tillotson HS, the metering lever is supposed to be level with the flat surface of the carb. Bent it down 1-2mm below that, no joy.
- the throttle butterfly *appears* to be seating correctly

What else can I check? It feels like there must be air getting past the throttle butterfly, but how can I test (and/or fix!) that?

 

[J D]

Set the metering level back to where it should be & check pop off pressure on the fuel line.
What do you have the L & H set at? Have you tried winding the L out a bit?

 

[J D]

Also check the T screw is what the throttle is actually stopping against... if a spring or linkage or cable isn't right it may not be returning all the way once installed

 

[dylanjustice]

Thanks for all that! Metering lever is now readjusted, that was just a test to see if if helped things.

1. Pop-off pressure: Didn't know about this! It's about 15 psi. It seems like it's slowly closing by around 8-9 psi.

2. Mixture: I've been working with both needles at 1.25 turns out. I've played with the L screw, but no reasonable setting lowers the idle appreciably.

3. Throttle return: I'm working with the cover off and the linkages removed. I feel reasonable tension on the throttle return spring, and if I hold the carb up I don't see any light through the throat (except for the little half-moon at the bottom).

 

[J D]

Pop off is good.
L screw should definitely be having an effect... I'd be cleaning those passages out again as there's a good chance you still have a bit of something lodged in there.
Check needles too.
Did you remove any welch plugs?

 

[dylanjustice]

OK, I'll try harder to clean those passages. I wouldn't have guessed that blockages would cause high RPM, but I'm here to learn. And I actually *did* remove the low welch plug, things looked quite clean in there, so I didn't do anything, just sealed it back up.

The L screw has immediate effect, it's just so fidgety that I don't feel like I can set an idle with it that won't stall if the wind changes.

 

[SteveSr]

OK, I'll try harder to clean those passages. I wouldn't have guessed that blockages would cause high RPM, but I'm here to learn. And I actually *did* remove the low welch plug, things looked quite clean in there, so I didn't do anything, just sealed it back up.

The L screw has immediate effect, it's just so fidgety that I don't feel like I can set an idle with it that won't stall if the wind changes.

If you removed the welch plug it is likely that it may leak. even the factory can't seal them reliably without sealant and they have the proper tooling.

Best solution is a through clean with brake cleaner followed by bead of water thin superglue. Let the superglue cure in a humid place for a couple of days.

As far as the idle issue usually a high idle indicates an air leak, especially if the "T" screw hasn't been touched during the rebuild process.

 

[J D]

OK, I'll try harder to clean those passages. I wouldn't have guessed that blockages would cause high RPM, but I'm here to learn. And I actually *did* remove the low welch plug, things looked quite clean in there, so I didn't do anything, just sealed it back up.

The L screw has immediate effect, it's just so fidgety that I don't feel like I can set an idle with it that won't stall if the wind changes.

Blockages cause hi rpm because it restricts the fuel causing the saw to run leaner. A leaner mix will run faster (but have less power & potentially cause overheating).
In the process of tuning you find the sweet spot where the idle revs are the highest turning the L screw, then richen it up a touch (generally another 1/8 - 1/4 turn out, just enough that you get good throttle response), then you should be able to use the T screw to adjust the idle down to where you want it.
The H is then set at WOT.
Madsens website had a good guide on basic tuning. Tom @vintageenginerepairs has some good instructional videos on here & on YouTube

 

[stihltech]

Why are we not looking for a vacuum leak? Get some brake cleaner or carb cleaner and spray around spots with gaskets and/or seals while running as low an rpm as you can go. You will find a leak.

 

[samaritan]

Why are we not looking for a vacuum leak? Get some brake cleaner or carb cleaner and spray around spots with gaskets and/or seals while running as low an rpm as you can go. You will find a leak.

Yup, I had that problem on a Husky , It was an intake boot leaking, sucking in air

 

[dylanjustice]

Why are we not looking for a vacuum leak? Get some brake cleaner or carb cleaner and spray around spots with gaskets and/or seals while running as low an rpm as you can go. You will find a leak.

A couple of people *have* nudged in that direction, but since it passed a leakdown (pressure only!) test, and seemed to have been running OK before my carb kit, I was convinced it was something I'd done in the carb rebuild. Yesterday I figured out that the other saw I got from the same guy has a compatible carb. Swapped it in with no change in behaviour, so if I'm reading the signs correctly, that really doesn't leave much except an air leak.

I didn't know about spraying carb cleaner at the gaskets, that makes a ton of sense, thanks! I'll try that next time we get a break in the rain here.

 

[Gord404]

where on van. island do you live- got a lot of parts for those

 

[dylanjustice]

where on van. island do you live- got a lot of parts for those

Thanks, I pm'd you! I really appreciate it, and I'll hit you up when I figure out what's going on here.

 

[dylanjustice]

@stihltech gets the trophy -- it faltered when I sprayed the left side of what I now know is an intake spacer. Sure enough, looks like the last guy didn't have enough sealant to go all the way around. Or maybe he was just being a little too cautious about the impulse port.

The sealant that's left on there is like fibercrete though! Is there a solvent that will soften it up but can be trusted to not dissolve my (plastic? resin?) intake spacer? I've tried goo gone and mineral spirits with no joy, but don't want to progress to toluene etc without someone's opinion that it's unlikely to wreck things further.

Photo below to illustrate the dried sealant. Forgive the mess -- I figured I'd confirm I could get the saw going before I put the time into cleaning it.

 

[cbfarmall]

Check that gasket between the carb and manifold. If you used the new one from a Tillotson kit, it is the wrong gasket and leaves part of the impulse port uncovered resulting in an air leak. Hopefully, you have the old one to compare.

 

[cookies]

remove the spacer and very carefully using a razor blade or even sandpaper on a flat surface to remove the goop, you can try carb cleaner to see if it loosens or softens it. A fingernail can help a lot on the little hard spots to remove them.

 

[J D]

+1 on Cookies advice...
After everything else a sheet of sandpaper on a piece of glass or something similar works remarkably well

 

[dylanjustice]

@cbfarmall, you nailed it. The new gasket looked like a perfect match against the carb... but I somehow hadn't looked closely at it against the intake spacer, and you're absolutely right, it doesn't cover the whole impulse channel. This also solves the mystery of why my leakdown rate was OK -- my rubber leakdown covers were big and they *did* seal the whole impulse channel.

The old gasket is oversized and ragged where it sticks out past the edges of the carb, and frankly looks like it was cut out of a piece of inner tube or something, but now that it's back on, the saw is idling as about as nicely as I could hope to expect. As embarrassing a miss as this was, I sure do love it when there's a big obvious single oversight causing the problem.

I've learned a bunch about the saw in this process, and I really appreciate everyone's help. Thanks very much to everyone who participated here!

 

[dylanjustice]

I just wanted to come back and say thanks to everyone who contributed here. I had a big cherry come down and my little battery saw wasn't up to the job, but the L65 is running great, and didn't seem to mind the work at all.

Site image upload not working, so here's a link to the aftermath. Didn't have a banana, but there's an ATV for scale.