HELP! HT131 new carb adjustment procedure

[Paul Lloyd-Jones]

I just put a new OEM carb on a Stihl HT131 polesaw. Can someone tell me how to adjust it or direct me to a thorough reference? The saw manual info is extremely brief. I have extensive experience with these saws, but very limited experience with carb tuning.

Probably unnecessary ramblings, but this was how it went for me: When I first tried starting it, it fired immediately, but didn't stay running. I messed with the low end for several minutes and the idle a bit and couldn't get anything but a sputter, and only if I kept the throttle all the way open. I was about to give up, but then I tried messing with the high a bit and got it running and idling well. It actually sounded pretty normal, but I held a tach to it and found that it was only getting up to 9000 rpms. I thought it was supposed to be closer to 12,000 (I'm seeing now that it's supposed to be 10,200) then messed with the high end screw a bit. I am confused at how much I was able to turn the screw with very little effect. I'm talking about like 8 full turns (I'm not sure I actually counted). Is that normal? Do these not come with limiters? The low turned a whole bunch too, but at least it seemed to actually be doing something. After I tried getting the high end up, I started adjusting everything again, and I just made it wonky. Finally gave up on it, figuring I'd be able to find a video or manual reference that would give me a whole tuning procedure or at least initial adjustment settings, but I spent a while looking and found nothing. I need help.

 

[stihl #1]

To do this right you need a tach.
I use this one from STIHL but any inductive tach that clips on the ignition lead will work.
HT 131 idle spec is 2800 RPM, top speed with bar and chain installed no load is 10,500, but be aware there is a rev-limiter in the module so it may not go over that even if you try.
In case you don't know the L screw is closest to the engine, the H screw closest to the air filter, and the idle screw is by itself and mechanically sets the angle of the butterfly. The mixture screws are tapered and backing them out richens the mixture by letting more fuel in and leans the mixture by taking away fuel flow.
To tune set both mixture screws at around 1 turn from lightly seated, then start it and gently rev it a little to warm it up.
You adjust by the lean drop off tuning procedure.
Set idle speed at 3300 RPM.
Turn L screw in and out until the highest RPM is obtained. This the lean drop off point, where the RPM starts going down if you lean it any further. So lets say it went up to 3700. Then back out the idle screw to 3300 again and then turn the L screw in and out until the highest RPM is reached again. If higher than 3300, then lower to 3300 again with the L screw. Usually 1 or 2 times get you there but I have had to do it even more on occasion.
Once you are at 3300 then back out the L screw to richen the mixture and slow the idle RPM to 2800. This will give you good acceleration with no bogging.

Once the L is set, hold at wide open throttle and see what the tach says then let off. Don't hold at WOT long, just get a reading. Lets say you see 9900 RPM, then you are too rich, so close the H screw 1/4 turn and recheck. It should go up to 10,300 or so. So close the H screw a little more and check again until you get to 10,500. If it goes up to or past 10,500 and hits the rev-limiter which will make the tach go crazy and you will hear a fluttering sound, then back out the H screw to richen the mixture, slowing down the WOT speed, and recheck.
This is a process that takes a little practice to master, learning to identify the sound of the engine and watching the tach so you get it to spec. If the carb does not adjust with sensitivity, then the carb is bad, or you have some other fault such as an air leak in the crankcase, or a restricted air filter or exhaust, as some examples. Stay away from cheap online carbs with no markings on them, they are nothing but trouble. The STIHL carb will have STIHL cast into the cover and some numbers on the side like 4180/02 or something similar.

 

[Paul Lloyd-Jones]

Thank you!!!
This is what I was looking for! I am going to give it a try. I'll let you know how it goes.

Quick update: H screw had come all the way out. That's why I wasn't getting any response and it was "spinning and spinning"!

 

[EFSM]

I just put a new OEM carb on a Stihl HT131 polesaw

I would be concerned if it doesn't run okay with the factory settings. I'd guess there may be a fuel line issue, broken or loose intake block, or vacuum leak otherwise. I just installed a new carburetor on a FS90 this morning and it wouldn't run right. Turns out the pulse line had a major crack in it where it was hard to spot. Then it ran pretty well on the stock adjustments, and the only thing it needed was the low turned out a quarter turn.

 

[Paul Lloyd-Jones]

Okay, so,
I pulled the H screw I had lost off of the old carb, so that I could start over. I followed the instructions from @stihl #1, and the lean drop off tuning procedure for the low end went great!
But when I went to adjust the high end, I could not get it above about 9,100 rpm. I played with it a lot. Any ideas?

 

[Paul Lloyd-Jones]

I would be concerned if it doesn't run okay with the factory settings. I'd guess there may be a fuel line issue, broken or loose intake block, or vacuum leak otherwise. I just installed a new carburetor on a FS90 this morning and it wouldn't run right. Turns out the pulse line had a major crack in it where it was hard to spot. Then it ran pretty well on the stock adjustments, and the only thing it needed was the low turned out a quarter turn.

Good to know. I wasn't sure if the OEM carb comes pre-adjusted, but I guess you're saying it normally would. Well, it definitely wouldn't run immediately after installation until I played with that high end a bit. But, I do not know where the screws were set before I started messing with them, and I didn't know that they are supposed to be about one turn from snug. That was very helpful. Is that pretty universal, or just a 131 thing, or just a Stihl thing?
Also, I can recheck the fuel lines if I need to, but I did look them over and didn't see any issues, and the saw was actually diagnosed (as needing a new carb) by a mechanic at AgPro. I was skeptical enough to decide I didn't want to pay them $240 to do the work, when I could do it myself in 10 minutes with a $60 OEM carb, but not skeptical enough (or knowledgeable enough) to work out any other issue with it. Do you think the issue with me not being able to get the high end up where it needs to be could be related to anything you mentioned? I don't know what the intake block looks like and I'm not sure what a vacuum leak is.

 

[EFSM]

I don't know what the intake block looks like and I'm not sure what a vacuum leak is.

What I'm calling the intake block is the plastic insulator that the carburetor studs fasten into. The carburetor is not mounted directly to the engine crankcase because it would get too hot. That is not usually a problem unless it is dropped and damaged. A vacuum leak is simply a leak around the valve cover, timing cover, or at a crankshaft seal. On a normal 4-stroke engine with crankcase oil, that will result in an oil leak; however, on a 4-mix engine like this, a leak at a gasket or seal will throw the air/fuel mixture off. Make sure the valve cover is still tight and the impulse line to the bottom of the carburetor is intact. Did you check the valve adjustment?

 

[Paul Lloyd-Jones]

What I'm calling the intake block is the plastic insulator that the carburetor studs fasten into. The carburetor is not mounted directly to the engine crankcase because it would get too hot. That is not usually a problem unless it is dropped and damaged. A vacuum leak is simply a leak around the valve cover, timing cover, or at a crankshaft seal. On a normal 4-stroke engine with crankcase oil, that will result in an oil leak; however, on a 4-mix engine like this, a leak at a gasket or seal will throw the air/fuel mixture off. Make sure the valve cover is still tight and the impulse line to the bottom of the carburetor is intact. Did you check the valve adjustment?

Ah, okay. Maybe I'll pull the carb back off and have a look at that insulator. As crazy as this sounds, we have probably had 30 of these saws over the last 20 years (101s and 131s, and one stupid 103), and we currently have 8 of them, and I have never done or even been aware of valve adjustments. Maybe our one or two of our many saw shop mechanics over the years have done the adjustments here or there, but we just rarely have issues with the powerheads. Coincidentally, though, I just watched a video on it this morning and ordered a gauge off of amazon, but since I don't have it yet, I have't checked them. The kit with the gauge also has a gasket in case I end up needing that.
But yes, the valve cover is tight. Spark plug is also tight. Air filter also looks fine.

 

[Paul Lloyd-Jones]

BTW, Should this question have been posted in The Repair Shop?

 

[Woodpecker]

BTW, Should this question have been posted in The Repair Shop?

Your question will get more exposure here on the forum right where it is

 

[Shane¹]

I just recently repaired a couple 4 mix motors on trimmers and they both seemed like they had carb issues would not rev out and seemed low on power. Put new carb on one ran perfectly the other actually had a slightly worn camshaft lobe it would run perfectly just not rev up like the other one. If both carbs do the same thing I would suggest possibly just inspecting the cam it is not super hard to do

 

[Paul Lloyd-Jones]

I'm learning a lot about these saws, but probably getting too deep too quickly. This saw has always run fine. A couple months ago the crew placed it out of commission, and I am pretty much the only person addressing saw issues right now. When I finally got to this one, I found that it was decommissioned due to a stripped bar nut stud. I swapped the gear head with one from an old parts saw, and I went to start the saw, but couldn't get it to run. I wear a lot of hats these days and had a lot on my plate this particular day, and after swapping the spark plug and getting no change, I just sent it to AgPro. When they came back saying it needed a new carb, I was skeptical. Still, as I mentioned before, I figured I could do the swap myself, so I just had them order me the part and I picked up the saw.
All that to say, I would be really surprised if it were something like a worn cam lobe (not that knowing how to inspect that would be a bad thing). It's just that saw is not very old (probably less than 2 years). Also, it's not doing the same thing it was with the old carb.

 

[Shane¹]

I'm learning a lot about these saws, but probably getting too deep too quickly. This saw has always run fine. A couple months ago the crew placed it out of commission, and I am pretty much the only person addressing saw issues right now. When I finally got to this one, I found that it was decommissioned due to a stripped bar nut stud. I swapped the gear head with one from an old parts saw, and I went to start the saw, but couldn't get it to run. I wear a lot of hats these days and had a lot on my plate this particular day, and after swapping the spark plug and getting no change, I just sent it to AgPro. When they came back saying it needed a new carb, I was skeptical. Still, as I mentioned before, I figured I could do the swap myself, so I just had them order me the part and I picked up the saw.
All that to say, I would be really surprised if it were something like a worn cam lobe (not that knowing how to inspect that would be a bad thing). It's just that saw is not very old (probably less than 2 years). Also, it's not doing the same thing it was with the old carb.

If it is different with both carbs you should be on the right track I just wanted to share my experience I have worked on a lot of normal 2 strokes but not many of the 4 mix motors and the one I had sounded just like a carb issue and it took me a minute and swapping a carb to figure out the worn cam. What I have gathered from reading it seems like if they get overloaded the plastic cam gear gets hot and melts.

 

[DND 9000]

Which OEM carburetor exactly? That`s importand to know.

 

[Paul Lloyd-Jones]

If it is different with both carbs you should be on the right track I just wanted to share my experience

Okay. good to know. And I'm sorry for how my response came off. I didn't mean to seem ungrateful. I really appreciate any ideas or input.It seems like it's difficult to find people who know these motors well.

it seems like if they get overloaded the plastic cam gear gets hot and melts.

That interesting that the cam lobes are plastic. I guess that makes sense. Wouldn't have considered that.

 

[Paul Lloyd-Jones]

Which OEM carburetor exactly? That`s importand to know.

Carb box says 4180-120-0619.
That picture of the actual carb is the original. I didn't look into the saw again to see if they are identical, but it was the dealer that did the order for me (not that that's any guarantee).

 

[DND 9000]

It is the right carburetor.

The base setting of 1 turn out on both screws is not right for these. I don`t know the exact base setting but it is around H=4,5 and L=2,5. Try to use this as basic setting and tune it from there with a tachometer and the tuning method that @stihl #1 mentioned, or let your dealer do it.

Idle: 2.800 rpm
High: 10.500 rpm

 

[Paul Lloyd-Jones]

Great! I'll give that a shot. Thank you!

 

[stihl #1]

My bad, the base setting is 4 to 41/2 on the H. DND was right. I also saw mention of the cam wear issue. Depending on how worn it is it may let it idle but not rev up. If the valve clearance is excessive the compression release will not be working so it will take a lot of effort to pull the rope, so that is a clue. If so take off the valve cover and remove the spark plug and pull it over slowly watching the rocker arms, and when one is holding a valve open see how loose the other one is, then do the same for the other rocker arm. if the clearance is really loose it may be the cam. If just a little loose then adjust the valves.

You need the special gauge that has a narrow tip to adjust. 4180 007 1005 is a kit with the gauge and gasket.

If you take off the recoil, shroud, starter cup, and then the 4 screws the cam cover comes off, sealed with RTV silicone. I have seen the block worn and the dowel pin loose in it's bore. and also wear in the matching bore in the cover.

Here is a cam that is worn. This one would idle but not rev up and the compression release didn't work.


If you go this far and find the cam or the cover worn, replace the cam and cover (1,4), and be sure to line up the timing marks with the piston at TDC.
As with all things, a job is easy once you know how to troubleshoot, and have done the job a couple of times. If you don't have the right tools and you are intimidated by all this find a dealer with a certified tech and let them do it.
HTH
Eddie

 

[Paul Lloyd-Jones]

Wow, Eddie! Thank you so much! That is very thorough! I really appreciate it.

Oddly enough, I tried the H at 4ish and the L at 2.5ish, and it didn't work well. I couldn't use the lean drop off method with those initial settings because as I would lean it out, it would just rev higher and higher (I went as high as 8500 before I stopped). I tried setting the initials to about H3.5, L1.5 and H5.5, L3.5, and neither of these worked either. (This is probably much easier when you have a better understanding of what all these changes entail and how they effect each other, but since I don't speak engine yet, I just have to play with it.)
Anyway, after that, I decided to hook the tach up to a few other pole saws at the shop. I was pretty shocked when I found that not one of them revs up past 9,200, and in fact, they mostly averaged around 7,000 wide open. Then I started thinking that maybe the they were all underperforming maybe because they all need valve adjustments that have never been done. So then, I pulled out a brand new one to put the tach on it and confirm (since my gauge hasn't come in yet so I can't really try just adjusting the valves). Same story, even with the brand new one. This actually made me feel better.
I still don't understand, but it kind of makes my problem go away. So, I went back to about H1.5, L1 initial setting, used the lean drop off procedure (which again came out almost perfect on the first try), adjusted the H to bring my RPMs up a tiny bit, and then I put it in wood.
It was doing okay, but would bog down a little bit when pushed. Then when I would try to turn it upside down and cut with the top of the bar, it would bog all the way down till it stalled very quickly. I richened the H a bit (maybe 1/2 turn?) and it corrected and seemed to be cutting great!
What do you think?