SDC-1 carb tuning

[dieselnut]

Hello all,

This will probably be a long post, so please bear with me. I'll try to give as much info as possible.

What has anyone's experience been tuning the Walbro SDC-1 carbs? I do a lot of mechanical work, so I am quite familar with all of this. That being said, I'm questioning the tuning of the XL-101 that I'm working on. I also don't have any experience with small carbs that have accelerator pumps. I have rebuilt the one on my XL-101 completely, including the accelerator pump diaphragm. I carefully cleaned all of the passages, and pressure tested it during reassembly. I also did a pressure and vacuum test on the engine which tested good, and have roughly 140 p.s.i. of compression cold and 135 hot. I have read the service manual regarding carb adjustments, setting both low and high to 1 turn out, which is about mid-range for the Homelite manual. According to the manual, if there is a lag in acceleration for an accelerator pump equipped carb, there is a problem with the accelerator pump diaphragm or passageways. They are completely clean, though. I understand that those are starting adjustments and tuning will be done from there.

I'd like to add here that prior to replacing the accelerator pump, the saw would idle and run fine with the idle screw at roughly 1 3/4 out and the hi speed at roughly 3/4 out. When tipped forward at idle, though, it would usually die, indicating that the idle was too rich. If I adjusted it so that it wouldn't die when tipped forward, it stumbled badly on acceleration, sometimes dying.

After replacing the accelerator pump diaphragm, it now seems to like about 1 1/2 turns on the idle and 3/4 on the high speed screw and does not seem to load up like it used to, as described above. That is what most of the non-accel carbs tune at, in my experience.

Am I missing anything or overthinking it? When comparing to the manual, it seems a bit off, but is that just the way it is? I'm just trying to reconcile it all in my head.

[xl130]

I’m not professional or expert but I have worked on a good amount of these saws. I had a XL101 with the SDC carb. I rebuilt it fully as well.

I recall it ran a bit funny at first but levelled out after a short bit of running. In let needle height can play a big role on how lean or rich a saw idles I have found. Very small height difference can make a noticeable difference in idle.

From my experience a carb runs the way it wants to. I start every one the same after rebuilding and adjust as needed. 1-1/4 to 1-1/2 on both needles and set the adjustment screw do that it’s barely touching the throttle lever.

Clutch springs, small air leaks and even points ignition systems can have symptoms similar to fuel system problems. I have a kill switch drive me crazy for about a year. It would start and run but die when you would throttle up. Was convinced it was an air leak or fuel system problem. One day I tried unplugging the switch. Ran amazingly!!

I have adopted the policy of going through everything on a saw now that is new to me.

[dieselnut]

xl130, thanks for the reply.

What gets me is that I can richen the idle mixture from the initial 1 turn out setting to about 1 1/2 turns until it accelerates smoothly and seems to run like a scalded cat. The high side can be no more than about 7/8 out, otherwise, it's way too rich. It cuts, idles, the whole bit,for a full tank of fuel, but dies or significantly slows (rpm-wise) when tipped forward, indicating too rich. As I mentioned in my first post, the new accelerator pump diaphragm seems to have cured the load-up problem, which leads me to believe that there may have been a pinhole in the diaphragm, requiring the idle side to be too rich. However, as I'm unfamiliar with the accelerator pump models, I could well be incorrect. It seems like a very simple circuit, with the fuel being injected into the low side, assumedly past the tip of the idle needle and into the main jet. That's how I traced it out, anyway. This carb just doesn't seem to tune the way most other ones do, but I'm sure I'll nail it down. As suggested, I'll re-check the metering lever height again, just in case. I also found out that the high side on this carb contributes fuel to the low side, as the engine dies if the screw is seated.

As far as everything else you mentioned, I did check all of it after getting it home, as I like to go through everything first, as you said that you did, as well.

New spark plug, tested for air leaks, cleaned points & reset the gap, cleaned and rebuilt the carb, new fuel line (thank you Leon for the fuel line and acc. pump diaphragm, among other things), compression test, spark test, cleaned the air filter, and tested the oiler. It runs great and cuts well, it's just the tune that's a thorn in my side for now. I have an Oregon ignition module on the way to try out, also.

I'll keep tinkering and let you know what I come up with. Thank you again.

[dieselnut]

Welp, I installed the ignition module and gave it a rip. It started running VERY strangely, backfiring, lots of throttle lag, and it wouldn't idle very well, dying after 10 seconds or so of running. I took it down again and discovered 2 broken reeds. After an unsuccessful search for some OEM reeds, I ordered the setup that nighthawk had posted a few years ago and has done a you tube video about. Don't know if it will work, but I'll report my findings after they come in.