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Stihl blower stihl doesn't run

Discussion in 'Chainsaw' started by Kogafortwo, Oct 21, 2007.

  1. Kogafortwo

    Kogafortwo AboristSite Guru

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    I am restarting this thread since I put in about 8 hours labor on my blower. My old BG72 blower quit on me and since then, partly based on what y'all told me on this forum, this is what I did:

    - rebuilt the carb AGAIN, this time I soaked it in Coleman fuel and got the welsch plug out, blew all the passages clean
    - set the low and high mixture screws one turn out and also tried plus/minus another half turn
    - all gaskets from motor back to air box are new
    - fuel lines and pickup are new, tank is not cracked or anything
    - exhaust cleaned out, screen is clean
    - piston, cylinder, and rings look good, feels like plenty of compression on pulling
    - scrubbed the block good with Coleman and a toothbrush, could not see any leaks or gasket problems by looking at them
    - jug screws are good and tight
    - coil was set to one business card width from magnets on flywheel, spark is good, plug is good
    - I gave the blower some lovin' and cleaned it up to look like new, no part was left unwashed

    So, I reassembled it and gassed it up with fresh 50:1 mix and it does the same thing as before: about 30 pulls and it will rev up to full RPM, then run up and down in RPM for about 30 seconds until it dies. :bang:

    I am almost ready to spend the $140 on a new BG55, but I want to give it one last chance. What gives?
     
    Last edited: Oct 21, 2007
  2. Lakeside53

    Lakeside53 Stihl Wrenching

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    No impulse pressure.. Crank seals?

    Take off the carb, put a blob of grease on the impulse hole and crank. If the grease sucks in, you have impulse. If not.. you don't.
     
    Last edited: Oct 21, 2007
  3. Kogafortwo

    Kogafortwo AboristSite Guru

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    flywheel puller and crank splitter?

    Thanks Lakeside. I will try that. I'm thinking crank seals because I found alot of oily dirt down behind the flywheel. Must be coming out of there.

    To do this, I figure I need the crank seals, gasket for jug and crankcase, and I might as well replace the rings too.

    So now I need a flywheel puller and a crankcase splitter tool. Where can I get these at a decent price? Are they the same for all Stihl equipment in case I work on one of my other Stihl powerheads?
     
  4. Lakeside53

    Lakeside53 Stihl Wrenching

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    If I remember right... The BG 72 doesn't need a flywheel puller. Just a sharp tap from a brass hammer on the crank while holding the weight of the blower engine by the flywheel.


    PM me with you email address and I'm try to find a workshop manual.
     
  5. Kogafortwo

    Kogafortwo AboristSite Guru

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    Did the test, tore it down some more

    Hey Lakeside,
    I did the test with the grease over the impulse hole, and it pulled in the grease just fine. The blower ran for several second without the carb on it from the fuel it is flooded with. Never saw that before.:dizzy:

    I tore it down and got the flywheel off. Both the crank seals were dry and clean around them, but all around the cylinder gasket was wet. Then I pulled the cylinder and the gasket had a chunk missing. I thought I damaged it pulling it apart, but I don't know. So maybe the cylinder gasket was my problem, but can that be when the impulse hole was still pulling good?
    The rings, piston, and cylinder look good, and at always has good compression when I pull it.

    I will try a new base gasket and seal everything up. Will silicone RTV work OK as a gasket sealer, or should I assembe dry?
     
  6. Fish

    Fish Account Hold

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    Take your carb apart again, and remove the needle/lever and blow that passage out good, and hold it up to a light and visually verify that it is clear.
     
  7. Lakeside53

    Lakeside53 Stihl Wrenching

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    Don't use bathroom sealer RTV... Put a new gasket it, and put it in dry.
     
  8. Dennis Cahoon

    Dennis Cahoon Addicted to ArboristSite

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    Fish and Lakeside.......Where's Dean when AS really needs him! :popcorn:
     
  9. Kogafortwo

    Kogafortwo AboristSite Guru

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    Is the seat messed up?

    Hey Fish,
    the hole is clear all the way through. The screen is new and installed correctly too. Now I'm thinking:
    - Is there a burr or something on the seat itself that is holding the needle off and making it flood?
    - Or is the lever height set wrong, even though it is flush with teh flange around the hole like the instructions say?

    My wife is telling me to be quiet about the old blower and let the evil MIL buy me a new one for Christmas. But I just can't let it go...:deadhorse:
     
  10. OLD CHIPMONK

    OLD CHIPMONK AboristSite Guru

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    Mind blowing . Worked on a chaisaw one time, started,ran wide open, then quit. Can't remember but either low or high speed needle tip had broken off & stuck in the seat. Got it out from the carb throat with a stiff pick (dental type) installed new needle, problem solved .Good luck.
     
  11. J.Walker

    J.Walker Addicted to ArboristSite

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    BG 72 Blower

    Could a hole in the primmer bulb be causing the problem?
     
  12. Kogafortwo

    Kogafortwo AboristSite Guru

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    Primer bulb is good, this has to be an air leak somewhere, I just don't know where yet.
    I am going to put a new base gasket on, put some sealer around the case split, and maybe change out the crank seals, then reassemble.
    I have way too many hours and a few beers into this thing now.

    Thanks for the tips.
     
  13. Kogafortwo

    Kogafortwo AboristSite Guru

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    I give up

    After all the other stuff I already did, today I replaced the flywheel side crank seal (even though I still had good impulse), put in a new head gasket, and then I re-checked the carb to make sure I didn't miss anything on the last TWO REBUILDS.
    Then I tried setting the needle lever above spec and below spec, stihl nothing.

    Time for a new BG55.

    But one last question (are yhou out there Lakeside?): would a new carb DEFINITELY fix this?

    If so where can I get the best deal on a Walbro WT53 for it? The Stihl shop up the road wants $90 bucks, no way when the new blower is only $140.
     
  14. rxe

    rxe AboristSite Guru

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    Stinks of air leak - have you actually pressure tested it?

    My 070 that I've been fixing was just the same - a pig to start, then screamed at high speed before cutting out. Turned out to be multiple air leaks - oil impulse, cylinder gasket and carb gasket.

    Try a quick and easy test. Tip some mix directly into the intake, and try to start it. If it starts easily....then screams...it is so air leak.
     
  15. Kogafortwo

    Kogafortwo AboristSite Guru

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    Yup, it will start and scream if I dump fuel in. But I have sealed up everything I could think of except crankcase split and recoil-side crank seal. All the other gaskets are new or at least have some gasket sealer on them.

    Maybe it is the crankcase split. I could try running some gasket sealer around the outside of it since I don't plan to split the case.

    Thanks for the tips.
     
  16. Lakeside53

    Lakeside53 Stihl Wrenching

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    The only way to know is to simply pressure and vac test it.. A bicycle pump and bucket of water will suffice....

    Sealer on the outide of a crankcase is not going to work, and in any case, it's unlikey your leak is there.
     
  17. rxe

    rxe AboristSite Guru

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    I'd get or make the test flange for the carb - part no 1128 850 4200 from my tools booklet. They are available on fleabay for $20 or so, and that one is used for a lot of stihl kit, so is a useful item. Seal the exhaust with a rubber strip, then bolt the exhaust back on.

    I use a bike pump, but I draw the line at a bucket of water - I brush on soapy water which bubbles up a treat.

    Based on my recent experience, I use the following to determine whether I need to worry about an air leak:

    - Saw is hard to start, doesn't draw fuel well into the carb, even if the carb has been rebuilt with a new diaphragm and fuel lines are sound
    - Saw starts quickly if fuel is tossed into the carb directly
    - Saw has a very high, uncontrollable idle. The kind of saw where the idle is very fast, yet the throttle screw is backed right off
    - Idle jet needs to be way richer than factory settings
    - Saw doesn't respond to the throttle, just screams

    It is amazing just how much air can be drawn in. My 070 would tach at about 4000 rpm with the throttle totally closed.
     
    Last edited: Nov 12, 2007
  18. Kogafortwo

    Kogafortwo AboristSite Guru

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    This is exactly what my blower is doing. I will hold on to it to try and fix it as a spare or sell it cheap, or I might just cannibalize it for parts for my other Stihl powerheads.

    Gotta hand it to Stihl, a lot of their equipment shares pieces like airboxes, filters, fuel caps, plugs, and the like. And that stuff adds up when you have to buy it new.

    The Mother In Law is buying me a new BG55. Maybe I can get her to run it for me too.
     
    XJWoody likes this.
  19. conbano

    conbano New Member

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    work shop repair manual for stihl BG72

    hello Lakeside,

    any chance if you could please send me a link or email for the the work shop repair manual of the stihl BG72

    thank you in advance!
     

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