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Discussion Starter #1
Greetings - got an older FS130 string trimmer I'm rebuilding and it runs rough or will not start.
Good spark with tester - plug in
Using a compression tester - it will get up to ~100PSI but rapidly looses pressure.
Using a vacuum pump (~-10psi) and closing off the intake and exhaust ports - it will hold a steady vacuum.

What should I check to find the compression leak? Any thoughts on why it will hold a vacuum but not pressure??
Thanks
 

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I believe you have compression gauge that does not have the required Schrader valve in the tip. If it doesn't, it won't work right. First, the hose then increases the effective combustion chamber size, giving a false LOW reading. Second, it won't HOLD the high reading as it's supposed to. If there is a valve in the tip of the hose and the gauge bleeds down, there's a leak in your test equipment.

The FS130 engine HAS A COMPRESSION RELEASE. So taking compression isn't totally accurate. The release works off the intake valve, and in order for it to work the valve lash must be set at 0.1mm, +/- 0.02mm.

The compression spec. for the 4180 series engine is 5 bar min., or 5 x atmospheric pressure U.S. (15 avg.) = 75psi. So you're reading of 100 is more than ample. "Design" spec. is 6-7 bar, or up to 105psi.

Your "no start" or "runs rough" is hard without more detail. I CAN TELL YOU that if you re-used the rocker cover gasket, if it leaks (they often do if re-used) the impulse required to operate the fuel pump in the carb. won't be sufficient. When those engine first came out, they used to give you the lash feeler-gauge and a rocker cov. gsk. with 'em.

As for you "rebuilding" it, if you have a CC leak a fuel pump problem can exist. If you split the CC, you must use a high-end anaerobic sealant, such as Dirko or Ultra-Copper/Black that's impervious to fuel, else you wasted your time and $$. A crank seal leak will also give you problems. The starter side one is usually the one that fails, albeit RARE.

As always, I suggest trying a spray-prime when an engine won't start, thus eliminating or identifying fuel delivery as a problem. Wet spark plugs suggest flooding or a poor spark issue, so you'll have to tell us more.

Ensure you are testing without an air filter. If the fuel tank vent is leaking fuel into the air filter box, that too can give you both the symptoms you describe.

I've worked on these since they came out 16 yrs. ago, just so you know where I'm coming from.
 

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Discussion Starter #4
Thanks Paul - very useful info!
1. Compression tester - I believe you have compression gauge that does not have the required Schrader valve in the tip. If it doesn't, it won't work right. First, the hose then increases the effective combustion chamber size, giving a false LOW reading. Second, it won't HOLD the high reading as it's supposed to. If there is a valve in the tip of the hose and the gauge bleeds down, there's a leak in your test equipment.
The model I have indeed has a Schrader valve in it - not in the hose but located in the side of the gauge housing itself. It is equipped with a pressure release button - I have checked that the valve stem is tight. Curiously it works fine with my FS 90 (holds pressure after several cranks). That model also has valves like the FS130.

The FS130 engine HAS A COMPRESSION RELEASE. So taking compression isn't totally accurate. The release works off the intake valve, and in order for it to work the valve lash must be set at 0.1mm, +/- 0.02mm.
I did check /adjust both intake and exhaust valves as indicated in the bulletin you attached. Made sure the cylinder was in the proper position prior to adjusting and used a Stihl feeler gauge for the 4 mix units. Agree with your comment about valve cover gaskets - I did notice a leak when I applied a vacuum check that went away AFTER I replaced the gasket with a new one. That was a new finding for me. If the intake valve is not set right will it keep pressure (no release)?

As for you "rebuilding" it, if you have a CC leak a fuel pump problem can exist. If you split the CC, you must use a high-end anaerobic sealant, such as Dirko or Ultra-Copper/Black that's impervious to fuel, else you wasted your time and $$. A crank seal leak will also give you problems. The starter side one is usually the one that fails, albeit RARE.
I had to replace the lower end of the CC due to extensive cracking of the housing. I used Yamma Bond 3 to reseal all areas - it is supposed to be solvent resistant. I have not checked for leaks around front or back bearing seals while under pressure - probably should do that.

Ensure you are testing without an air filter. If the fuel tank vent is leaking fuel into the air filter box, that too can give you both the symptoms you describe.
I removed both the carb and muffler and did the vacuum test via the impulse line. For the compression test the air filter was removed and the carb was WOT

As always, I suggest trying a spray-prime when an engine won't start, thus eliminating or identifying fuel delivery as a problem. Wet spark plugs suggest flooding or a poor spark issue, so you'll have to tell us more.

Yep -tried that also. Even after several tries the plug was bone dry. I repeated the process with another carb that was new thinking it was a carb issue but still the same issue. When the unit would start, it would idle fine but immediately bog out when WOT - tried to remedy this via carb adjustments (both L &H side) but could not fix the problem.

Appreciate all you time and suggestions - let me know if there is something else to check or if you believe I'm doing something wrong or different!

Thanks!!
 

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If a 4-stroke engine won't wet the plug with a spray prime, and the valve train is working properly and timed correctly, then I would figure the exhaust is restricted enough to keep the intake from coming in. Compression wouldn't suffer. One note, we've seen a few lately where the impulse hose on the bottom of the carb. has become hard, and leaks as a result.
Those engines are pretty bulletproof, so either I'm right, or at a loss.

As you seem to be quite familiar with the 4-mix motor, you know the intake port has a decent opening into the CC for lube transfer, so again any CC leak adversely affects them.

Since your compression is okay, I figure your valve timing is ok. But, to re-hash the cam is timed at TDC, lobes down, horizontal line on cam horizontal as seen through the cover opening. Have seen worn lobes present problems, albeit rare.
 

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Discussion Starter #6
Thanks Paul! Appreciate your continued advice and trouble shooting. I think I have identified one issue - - appears that the exhaust valve is not fully closing. Verified this via a long leak down test (using both vacuum and air @ ~10psi load). In either case a leak was detected only when the exhaust port was allowed to be open. Confirmed that cylinder was @ TDC. Upon extensive surgery to the unit (splitting open the CC) it looks like the exhaust valve is "OK" but the seat around the valve is heavily carbonized. The intake valve area is squeaky clean. I'm guessing that the carbon build up is preventing complete closure of the exhaust valve??
Since I have the unit completely apart - what a good way or solvent or approach to de-carb the cylinder with out scratching/scoring the walls or valve seat area? Piston and cylinder walls look good, Rings were OK but I will replace anyway.

308838


Was thinking about soaking the cylinder in diluted simple green and then placing in a sonication bath to see if that will remove some of the residue. Your thoughts??
 

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We still have a "5" gallon bucket of Safety-Kleen cold parts immersion cleaner, which does wonders with cylinder carbon deposits. It appears they've discontinued that, and if you could buy it today it'd easily be over $200. STIHL used to make a de-carbonizing fluid, since discontinued but we still have a case or two of it specifically for the 4-mix motors. Else a glass-beader would do just fine, being careful not to hit the cylinder wall. Else I use a std. screwdriver and carefully scrape most off, and if you have a die-grinder with a soft wire brush you'll do great.

Have seen your symptom at times, and using the STIHL de-carbonizing fluid (bring to TDC and fill chamber) and letting it soak for 2 days, then flushing actually works great. Since you have the cylinder off you have other options. Pretty rare problem though.

I have never done a valve job/valve work on these, as the it's too labor intensive against the cost of short-block or replacement unit. A short-block is very affordable (and cost-effective for a DIY), so next time you may wish to consider that avenue.

I know many out there, as I, don't like it when a mechanical device "beats" them. Sometimes you spend the extra time and get a lesson in trade. Or, you could just put the whole thing in a log splitter and have a beer.
 

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Discussion Starter #9
Thanks Paul - agree and understand your comment about tear down vs. short block. Where I am located typical Stihl dealers charge ~$60-$75/hr + parts. In the case of the FS 130 - it's like a old friend that I hate see do nothing. Had it for many years and it served me well. Parts are getting harder to find and a limited search on short blocks prices was in excess of $300. It's a challenge to get it running and something to occupy my time during the strange times we are in. After soaking the cylinder housing 1-2 days in diluted simple green looks like no effect on removing the carbon deposits or making it easier to scrape off. I used a dremel tool equipped with a cotton pad and a long extension and some very fine grade polishing compound to clean up as much as possible around the valve seats and the valves. Once the rings come in the mail I will re-assemble and see what happens. May/may not make a difference but what the heck. It's not ready for the log splitter yet but I'm ready for a beer!(y)
 

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S/Blk 4180 020 0201 MSRP $151.49. Short-discount item for dealers, I can easily see it being sold for more, but not more than say $205.
Parts availability should be no problem, Stihl states that they make parts for 10 yrs. after a unit is discontinued, you have about 6 yrs. left. Further, they often continue many items after that.
 

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Discussion Starter #11
Got the FS 130 back together - replaced the piston rings (broke one during cleaning), plastic valve camshaft (it appeared worn on one side and the metal plate on the back was very loose), replaced all gaskets including the camshaft seals, polished valve seats and valves. I did notice that although the intake/exhaust valves appeared identical the had two different IDs on them - one had a "B" the other "I". One was magnetic the other not. I placed the "B" labeled valve on the intake side as per Stihl repair manual. Reassembled with Dirko sealant. After adjusting the valves several times compression was about 120PSI. You were correct about the compression release - the unit still gradually lost pressure after max. Did a leak down check - no issues. So far the unit runs well - I also replaced the clutch assembly since it was not working (even at ~2800rpm the shaft kept spinning). Readjusted carb with digital tach at high and low end.

In total about ~$100 in repair parts and a fair amount of my time - but hoping this will continue to serve me well for several more years. Appreciate your expert advice Paul !
 

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Good to hear. For future reference, Stihl clutches are guaranteed for life against defects (not the springs), and, we do see all brands of trimmer/edger/etc. clutches seize up from corrosion (not a warranty condition). Normally all that is needed is to press the pivot bushings out, wire brush them and the bosses in the shoes, a little never-seize or grease on the pivots and back in business. FS clutch shoes are brittle, the boss area must be supported well else you'll break that area off...guess how I know that!?
 

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Discussion Starter #13
yep been there done that. This clutch was heavily rusted up - -While the shoes look OK suspect the spring was also not good. Do you happen to have a part # for the 2 "metal grommet" and the thing spacer rings (parts 1 & 2 below)?
The clutch on the FS90 is also acting up and I broke the metal grommets trying to take them out

308987
 

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If you've ever followed Louis Rossman on youtube, I'm cut from the same cloth. Stihl doesn't believe in DIY, and I understand to a point. There area idiosyncrasies and things you won't be aware of if you don't have the OEM training. I'm telling you this as I'm on the DIY side and will provide a full IPL if you need it, just msg. me.

I'm all for DIY, but the OEM's, sometimes due to compliance issues, have proprietary systems. John Deere is a big one on that, if you look at the forums. And with good reason, I agree but to a point. Again, watch Louis Rossman.

Here's the IPL you need...
 

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