vacuum leak likely locations on stihl 4mix?

[r7000]

I have a KM90r weed trimmer that is breaking my balls.
I assume the 4mix engine in it is the same as many others... when I bought a replacement carb off amazon it says:

Hipa FS90R Carburetor C1Q-S174 for STIHL FS90 FS110 HT101 FS130 FS100 FS110R KM110R KM130 HL100 HT100 FS100RX FS130R FS87 FS100R KM90 KM90R KM100 KM110 KM130R FC90 FC95 FC100 FC110 HL90 Trimmer

so i assume the intake and most of the engine is also the same, the only thing changing is bore diameter for the bigger stuff.


My KM90R started with being able to start, but wouldn't stay running for more than 10 seconds at idle, and would have no power under throttle, and would die out after greater than half throttle. If you feather throttled you could get the rpms up past ~3000 into the full rpm range.

trimmer is personal use, not abused, got 10 years or whatever on it. has very good compression. The exhaust screen was clean.

• I put in a new cam wheel and resealed the cam cover, neither of which looked to be the problem.
• started up seemed better at first but then quickly went back to the same problem and was worse.
• I then bought the amazon carb, did not help. Nor did a new NGK spark plug.
  
• got the valve cover + washer + feeler gauge kit. Pretty sure at this point my original valve cover gasket was a problem. I am sure now the valve cover is not the source of the vacuum leak. Valve lash is also set, and I don't believe was ever a problem.
  
• It now won't start on first few pulls, but it will start, then will go to fast high idle and stay like that fairly good, such that the weed trimmer head is spinning fast enough and you could use the trimmer just don't give it any throttle it won't accelerate and will die.

being a "four stroke" it's running up in rpm just like a vacuum leak would on a v-8 for example. The large hose at the bottom center of carb looks in good shape but I can't see where it connects to the crankcase.... where else would be a likely source of a vacuum leak that I am missing?

for those of you knowledgeable on these units what are the common problem items... in regards to vacuum leaks? don't tell me crank seal

 

[huskihl]

I would think any air leak that would matter in a four cycle engine would be in the intake between the carburetor and head. But I am not familiar with how the fuel gets into the bottom end of the 4 mix engines to lubricate them. @lehman live edge slab might know something about them

 

[Stump Shot]

I have a KM90r weed trimmer that is breaking my balls.
I assume the 4mix engine in it is the same as many others... when I bought a replacement carb off amazon it says:

Hipa FS90R Carburetor C1Q-S174 for STIHL FS90 FS110 HT101 FS130 FS100 FS110R KM110R KM130 HL100 HT100 FS100RX FS130R FS87 FS100R KM90 KM90R KM100 KM110 KM130R FC90 FC95 FC100 FC110 HL90 Trimmer

so i assume the intake and most of the engine is also the same, the only thing changing is bore diameter for the bigger stuff.


My KM90R started with being able to start, but wouldn't stay running for more than 10 seconds at idle, and would have no power under throttle, and would die out after greater than half throttle. If you feather throttled you could get the rpms up past ~3000 into the full rpm range.

trimmer is personal use, not abused, got 10 years or whatever on it. has very good compression. The exhaust screen was clean.

• I put in a new cam wheel and resealed the cam cover, neither of which looked to be the problem.
• started up seemed better at first but then quickly went back to the same problem and was worse.
• I then bought the amazon carb, did not help. Nor did a new NGK spark plug.
  
• got the valve cover + washer + feeler gauge kit. Pretty sure at this point my original valve cover gasket was a problem. I am sure now the valve cover is not the source of the vacuum leak. Valve lash is also set, and I don't believe was ever a problem.
  
• It now won't start on first few pulls, but it will start, then will go to fast high idle and stay like that fairly good, such that the weed trimmer head is spinning fast enough and you could use the trimmer just don't give it any throttle it won't accelerate and will die.

being a "four stroke" it's running up in rpm just like a vacuum leak would on a v-8 for example. The large hose at the bottom center of carb looks in good shape but I can't see where it connects to the crankcase.... where else would be a likely source of a vacuum leak that I am missing?

for those of you knowledgeable on these units what are the common problem items... in regards to vacuum leaks? don't tell me crank seal

I won't tell you "crank seal", however if one looks at the seal area and has oily junk all around, you can pretty much be able to tell yourself at that point.
This is the four mix death rattle...

 

[Stump Shot]

I would think any air leak that would matter in a four cycle engine would be in the intake between the carburetor and head. But I am not familiar with how the fuel gets into the bottom end of the 4 mix engines to lubricate them. @lehman live edge slab might know something about them

In a normal four stroke engine, yes. These little goomers the charge goes through the case just like a two stroke for lubrication, therefor it can't suffer leaks like a two stroke, this as best I could ascertain from wrenching on Stihl back pack blowers and having to junk a few.

 

[lehman live edge slab]

Well I haven’t done much on them other than set the valves but when I was researching them after i ended up with some to just look over and set valves just because I wanted to know how they operate. They do rely on a sealed case because your still using piston motion on the case side for pressure and vacuum. They in a sense added a valve train to a 2 stroke saw engine because it runs on a similar principle. Piston goes up draws gas into case where oil lubes the bearings then on the down stroke it pushes the charge up a port sealed by a reed up to the valves where the next down stroke and intake valve opening allows the charge into cylinder. So I’d think any leaks at crank seals or that reed not sealing off the charge from case would affect running. I also wouldn’t 100% trust an aftermarket carb if that was one off Amazon and it’s still on the trimmer. I do have a friend that I’ll put a text into for some ideas and also check my 4-mix service manuals after work today.

 

[r7000]

my problem was my new carb was out of adjustment, in addition to me working in the dark late evening in haste. Next day when I focused on the trimmer I got it running well with the new amazon cheapo carb adjusted.

Then for kicks I put the original carb back on, what is supposed to be 3/4 turn open on both the L and H circuits. The L is open about 3/4 turn and idle is great. The H i had to open 2.5 turns to get it to run acceptable, and gonna get as much use out of the original carb now as I can I guess.

 

[r7000]

back to not running, will only idle, on original carb.
put the new amazon carb on and it ran worse.
so gonna reseal everything with permatex motoseal.

shop said it sounds like it is getting air, when you throttle it from idle it dies and you have to feather the throttle and 50% chance you can get it to spin up to near full rpm but not for more than ~3 seconds. shop said the plastic manifold/spacer between carb and aluminum cylinder could be cracked. Is that possible? I don't see how. I've replaced all the carb gaskets with no luck.

 

[Lcfd1124]

Check carbon on the valves. That is a major issue on the four mixes. Pull the plug, put piston at top dead center and fill it full of any decarbonizer, sea foam, chevron techron etc. Let soak for a few hours and watch the junk come out

 

[Stump Shot]

Check carbon on the valves. That is a major issue on the four mixes. Pull the plug, put piston at top dead center and fill it full of any decarbonizer, sea foam, chevron techron etc. Let soak for a few hours and watch the junk come out

I've seen one that the carbon stuck the valve open, while I had it apart, I soaked the valve stem in two stroke oil which is what the carbon was made from and softened it up and let the valve close normally again. If it still could run some seafoam in the fuel mix would have done the trick surely. A:so cleans the fuel system up nicely.

 

[Ketchup]

The cam wheel can be worn even if it looks okay. Look for shreds of plastic or rub areas in the housing. If it’s worn then your valves aren’t traveling through the full range. It’s possible the new cam worked better for a short period, but then also wore down. They’re designed to be the point of failure so the valve stems don’t bend. Definitely de-carbonize your valves. That’s the most common problem.

But who knows on these Stihl 4mixes? Sounds like you’re problem comes down to too much air and not enough fuel. A stuck intake valve would definitely do that, but a leak is possible too.

 

[r7000]

resealed cylinder to crank with permatex motoseal, and new crankshaft seals. It may have been the crank seals because the old ones slid off real easy. The piston top was clean and shiny, no carbon anywhere, very clean. I do have valve grindind compound, and i did each valve, not that I believe they were any part of the problem but they are definitely 100% now.

with original stihl carb, engine starts right up easier than before. give it gas it dies. I put new china carb on, runs better can get to full throttle. I have dremelled this new carb so I can adjust the protected H/L set screws. I think it was mostly original carb but gasket or crank seal leak was the problem. Need daylight and free time to finish adjusting china carb to see if it's all set.

I believe this always used stihl oil in the gray bottle - so if that oil was said to be bad and caused carbon problems there was no evidence of that here at all.

 

[Ketchup]

Have you tried re-building the OEM carb?

 

[r7000]

"rebuilding" original carb - no, only opens up and sprayed with brakeclean and compressed air, just new front & back mounting gaskets. New stihl carb is on the way. The new china amazon carb allows the trimmer to run up to full speed, where the original stihl carb does not. but there is still obvious performance problems with the new china carb, which i dremelled to be able to turn the H/L screws which helps but I cannot get expected performance and reliable throttle response.