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.