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Cylinder Spark Plug Hole

Post in 'The Gear' started by thewoodlands, Oct 31, 2012.

  1. thewoodlands

    thewoodlands Minister of Fire

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    If the outside of the spark plug hole on the cylinder of the Stihl 390 has some oil build up, will this effect the way it starts?

    It's not farting until the 10 or 11 pull

    zap

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  2. fox9988

    fox9988 Minister of Fire

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    Sounds like you are losing some compression. That usually leads to hard starting. I would clean it up and reseat or replace the sparkplug.
    zap likes this.
  3. thewoodlands

    thewoodlands Minister of Fire

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    Thanks fox9988. I'll test the compression later tomorrow then post.

    zap
  4. thewoodlands

    thewoodlands Minister of Fire

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    What should the compression be 120 and up, it's about 5 years old.

    Still runs great in the wood.

    zap
  5. JoeyD

    JoeyD Minister of Fire

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    Sorry I can't help you with your problem except to mention my saw never farts, no matter how my times I pull it. Do I have a problem?
  6. thewoodlands

    thewoodlands Minister of Fire

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    Usually farts after the second pull, not sure if the 362 is different being a newer saw.

    zap
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  7. fox9988

    fox9988 Minister of Fire

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    120 psi is often consindered the min. Do you have a compression gauge? If not you could try a "poor man's" compression test. Suspend the saw's weight by the pull cord and compare the rate of fall to a saw that is known to be in good condition. I'm not sure how accurate this test is, I read about it on AS.
  8. HittinSteel

    HittinSteel Minister of Fire

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    de-comp sticking?
    MasterMech and LEES WOOD-CO like this.
  9. thewoodlands

    thewoodlands Minister of Fire

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    fox9988, I have the compression gauge.

    HittinSteel, no. I'll check the compression tonight, try starting it with the old spark plug (looks good) then put a new spark plug in then start again. I did stop at the local Stihl dealer, he said they gap it at .026 inches so I'll check the gap on the old spark plug and the gap on the new one.

    Zap
  10. lukem

    lukem Minister of Fire

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    Zap, it sounds like your plug may not have been in tight enough which caused fuel mix and/or exhaust to escape around the plug lowering your compression and causing that build up.

    I'd take the old plug out, clean everything up real good, and put it back in. If it still starts hard you may have a different issue.
    MasterMech, ScotO and LEES WOOD-CO like this.
  11. JustWood

    JustWood Minister of Fire

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    More than likely the issue. Have seen the decomps weep oil also.
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  12. thewoodlands

    thewoodlands Minister of Fire

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    lukem, it seemed to be in good and tight but will check it tonight.

    lees wood-co, it's cleaned up good(did last night) if the new spark plug won't change anything I'll check the decomp, it seems ok.

    Zap
  13. ScotO

    ScotO Guest

    Sometimes the plug seems like it's in the whole way, but the compression washer on the plug isn't seated. Make sure you tighten the plug down good (don't go crazy and torque the hell out of it, you'll strip the threads). Just make sure the compression washer on the plug is collapsed and seated good. I know if my saw sits with pump gas it in for a month or so, it takes upwards of 6 to 8 pulls to initially start. Modern gas is garbage, period.
  14. thewoodlands

    thewoodlands Minister of Fire

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    The first thing I did tonight was check the compression (the stihl 390 was cold) it tested 120 on six pulls, the second thing I did was take a new spark plug an gap it like the local shop does, damn thing never started (pissed) grabbed another new spark plug (never gapped it) put the second new one in, the 390 took off on the second pull.

    The 390 manual recommends 0.02 inch gap, the shop recommends a 0.26 inch gap. I'm thinking out of the box the new Bosch is gapped at 0.02 so I'll stick with that.

    zap

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  15. MasterMech

    MasterMech Guest

    Is that 120 psi the most it'll do or did you stop after 6 pulls? Should pull until the needle stops rising completely. Zip tie on the trigger will hold the throttle wide open and reduce the number of pulls it will take to obtain a max reading.

    .006" isn't goint to make a huge difference on a 390. The fact that the saw starts up normally otherwise and runs well under load indicates the ignition system is healthy.

    If there's a whole lotta yankin' goin on here still I'd check the fuel line for very tiny pinholes or it's possible the fuel pump diaphragm in the carb is stretched a bit and just not pumping as strong as it used to. That said, double check the impulse path/line to make sure it gets a good strong impulse from the crankcase.

    My 034 takes 6-7 pulls before it farts if it's been sitting for more than a day or so. I'm with Scotty, modern pump gas is chit.
  16. StihlHead

    StihlHead Guest

    Stihl saws in that size all have the same plug and the same gap; 0.5mm = 0.02in.

    What the hell shop do you get this 'other gap' information from?

    120 compression is low too... should be higher (above 150). My recommendation for that kind of drop is new rings. They are cheap on Ebay and I get mine (Cabre rings) from a guy on Ebay in Greece for about $10. They bring life back to a tired saw. Clam shell engines like the 390 are a lot harder to pull and change the rings in though. Pro model Stihl saws are all really easy to change rings in.
  17. thewoodlands

    thewoodlands Minister of Fire

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    MasterMech, it maxed out after six pulls, I'll zip tie the throttle tonight.

    That's what our local shop gaps them at, the only reason he gave me was they run better gapped that way. ::-)

    How would you test for compression, I remember how I did it but would like to see if I missed something.
    1.Take out Spark Plug
    2..Screw in compression tester
    3.Push down compression button, Not sure if I did this or not but after seeing a few vids, if I did it's wrong.
    4. Throttle, not sure exactly what position I had it in.:oops:
    5.Pull

    zap
  18. MasterMech

    MasterMech Guest

    Now why would you do that, you are after all trying to see the maximum compression the saw makes right? ;) That button will get you a much lower reading. I would start it normally until it "pops" and then pull the plug and repeat your test.

    Throttle position doesn't really matter, so long as you max the psi reading on the gauge. But I do have a preference. :)

    120psi with the de-comp pressed in is pretty darn good. I still don't think compression/ignition is the issue here.
  19. thewoodlands

    thewoodlands Minister of Fire

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    So damn use to pushing that when starting, we'll see tonight. I had emailed Stihl about compression testing, they said the throttle should be in the I position and don't press in the decomp!

    zap
  20. HittinSteel

    HittinSteel Minister of Fire

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    Hopefully that is why you only have 120 lbs. of compression!
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  21. thewoodlands

    thewoodlands Minister of Fire

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    Two pulls and it was up to 120, another two 150, I stopped after five. It still started hard but I'll see how it starts again in the morning.

    zap

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  22. HittinSteel

    HittinSteel Minister of Fire

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    Good! 150 is very healthy. If it doesn't start well in the morning, I'd start with the second half of mastermech's post #15
  23. StihlHead

    StihlHead Guest

    More better. Your 390 is doing fine with that compression. I found that the 310s will not start as well if they are modified, as when they are all choked up from the factory. 4-5 pulls is on full choke is typical on my modified 310 here for it to rumble, and then another 1-2 at half choke. No decomp on a 310. Decomps can be the casue of flooding though, especially if you are not used to feeling the rumble of a full chike start and stall. Usually the decomp button will pop at that point, but not always. At that point you should flip the lever to half choke, or it will flood. Flooding is common when starting the 361 with the decomp button pressed in, and for that reason I do not use the decomp when starting the 361s when they are cold. The LA fire department put zip ties around the decomp buttons on all their 361 rescue saws to keep them from being used becasue of flooding problems starting the saws cold.
    zap likes this.
  24. thewoodlands

    thewoodlands Minister of Fire

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    I read your post, just came back in from starting the 390 on one pull without the decomp pressed in, I'll post how it runs tomorrow in some ironwood.

    Thanks for the decomp info.

    Zap
  25. thewoodlands

    thewoodlands Minister of Fire

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    Pulled the 390 out of the back of the Rhino once I was at the cutting area, pushed in the compression button, started on the second pull. Used it like that all day.

    zap

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