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Stihl 08 Still Won't Rev

Started by jteneyck, May 16, 2010, 10:13:43 AM

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jteneyck

This is a follow-up of several months ago regarding my neighbor's 08 that idles fine but won't rev up.  I finally got all the parts (diaphrams, Welsh plug, needle and seat, and ball check main nozzle) for the Tillotson HL 168B carb. and put them in last night.  Started right up this morning, but the problem remains, it won't run up above mid range rpm's.  

Other things I have checked:  
Gas tank vent:  OK.  No difference running off alternate tank.  
Muffler:  OK, no blockages
Centrifugal governer:  OK, not the problem
Piston/rings:  Visual check on outlet side = OK.  Abrasion below lower ring on inlet side - assume due to dirt through screen air filter.

It blows a lot of smoke when I open the throttle, likes it's running extremely rich, but adjusting the hi speed jet did not make any difference.  I have not done a compression check yet, but plan to when I get a kit/adapter to do so.  Does anyone have any ideas as to what the problem might be?  I'm stumped and could sure use some advise.  Thanks in advance.

Rocky_J

If you were unable to lean it out adjusting the high end jet, then my first guess would be that you were not actually adjusting the high end jet. That would only be my first guess though.

jteneyck

Hi Rocky.  What's your second guess? 

SwampDonkey

"No amount of belief makes something a fact." James Randi

1 Thessalonians 5:21

2020 Polaris Ranger 570 to forward firewood, Husqvarna 555 XT Pro, Stihl FS560 clearing saw and continuously thinning my ground, on the side. Grow them trees. (((o)))

Rocky_J

Have you tried draining and flushing the bar oil tank, then running it? I'm not sure how the old 08S oil pumps work, but many of those old saws had diaphragm pumps running off crankcase pressure. If they leak then bar oil runs into the crankcase.

jteneyck

The chain does not spin at idle unless I set the idle screw too high, so the clutch must be working OK.  Bar oil leaks out the bottom of the saw pretty badly, and the chain throws a lot of oil when it's running even at low revs.  So if I get your thought correctly, Rocky, if bar oil is leaking into the crankcase it could either be preventing the pulser from working correctly or making the mixture very rich, correct?  I'll drain and flush the bar oiler and try running it with no bar oil.  I'll look at flushing the crankcase as well if I can do it from the muffler port.  Thanks for the advise so far.  Stay tuned.  Other ideas? 

sablatnic

The 08 has a limiter, that closes the choke when the rpm gets too high. Could it be acting up? I can't remember, but it must have a spring somewhere, that maybe is too slack.  boobap has a list for the 08s. You could ask him to mail one to you.

jteneyck

Rocky, I drained and flushed the bar oil resevoir, as well as flushed the crankcase as best I could through the carb. mounting block.  The saw actually ran a little better for awhile, with the occasional hint of revving up, but never really getting there.  I played with the HI speed jet some more.  I can easily make it too rich, but when I make it lean it eventually gets too lean w/o ever getting up to high revs. 

It sure looks like the saw is running way too rich even at lean Hi speed settings.  Lots of exhaust smoke and fuel mix dripping out the muffler.  So taking your idea one step further, Rocky, if the gas tank is connected to the crankcase, which it appears to be, and there is a leak then maybe fuel is getting into the crankcase.  Does anyone have a schematic for a Stihl 08?  If so, I'd be grateful if you could send it to me, or point me towards it.  Thanks. 

Rocky_J

Quote from: jteneyck on May 16, 2010, 02:17:55 PM
  Does anyone have a schematic for a Stihl 08?  If so, I'd be grateful if you could send it to me, or point me towards it.  Thanks. 

Just scroll down a couple threads on the chainsaw forum page.
https://forestryforum.com/board/index.php/topic,44008.0.html
8)

SwampDonkey

With the idle screw, what I was getting at is, it might be in too far as well as the jets turned in too far. You can do that and have the saw bog a bit and also when the throttle is let go have the saw still rev high for 2 or 3 seconds before coming down. I've read that the 08 jets are set to 1-1/8 turn each to the left from close. The idle screw just wants to touch the throttle arm and adjust as to stop spinning or not stall. You might need to turn in the low jet just slightly to stop stalling. A smidgeon turn goes a long way.
"No amount of belief makes something a fact." James Randi

1 Thessalonians 5:21

2020 Polaris Ranger 570 to forward firewood, Husqvarna 555 XT Pro, Stihl FS560 clearing saw and continuously thinning my ground, on the side. Grow them trees. (((o)))

jteneyck

Thanks for the link Rocky, and for sending me the schematic, Boobap.  Much appreciated.  Looking at the schematic I'd say it's not very probably that fuel could leak from the tank into the crankcase.  If that ain't the problem, I'm back to the beginning so I'm looking for more ideas, guys. 

To recap, this is what I think I know:
Carb. rebuilt so it should be OK
Saw idles fine. 
Fuel tank vent OK.
Muffler clear
Runs rich even with the HI speed screw set lean.  When set too lean (3/4 turn) the saw dies w/o ever revving up.  Runs best at 1-1/8 turn open. 

Ideas???

SwampDonkey

Turn the idle out so it just touches the throttle arm on the carb., plus a couple turns in and turn hi and low jets out to 1-1/8 turn. Then, fine adjust the idle and low jet clockwise. Baby steps. ;D. Also, have you checked the throttle cable to see if it opens fully when depressed on the trigger, might need adjusting if there is a way to adjust it. On a brush saw there is a ribbed sleeve the cable goes into and it can be tightened or loosened with a flat screw driver in a sliding slot when assembled. Maybe there isn't anything to adjust on a chainsaw in that manner.
"No amount of belief makes something a fact." James Randi

1 Thessalonians 5:21

2020 Polaris Ranger 570 to forward firewood, Husqvarna 555 XT Pro, Stihl FS560 clearing saw and continuously thinning my ground, on the side. Grow them trees. (((o)))

jteneyck

Played with it some more and found that just running the saw at WOT it will eventually spike to high revs then back down to mid revs.  It will do this a few times and then really spike to high revs for a sustained period of time, maybe 30 seconds or more.  Then suddenly it goes back to mid range revs, and then the whole sequence will repeat.  The centrifugal speed limiter is not causing this as I've disconnected it.  

I drained the fuel tank and ran on a separate, open vented tank.  Same thing, so fuel is not leaking into the crankcase.  I took off the air filter and saw fuel at the carb. throat, so I took off the muffler to check to be sure the rings are not broken.  They look fine - springy when you press on them with a screw driver tip.  I verified the muffler is not plugged by blowing compressed air through it.  

So, I have fuel.  I have air.  I have an open exhaust path.  Something with the ignition?  This saw probably has a points/condenser system.  Bad condenser?  Is there some kind of centrifugal spark advance that could be doing this?   Any ideas are appreciated.  Thanks.

Rocky_J

I was 15 years old when electronic ignition was introduced on small two cycle power equipment and by the time I was 17 the stuff with points ignition had all but disappeared from the market. I've never owned anything with points ignition (other than my first pickup truck) and know nothing about them other than it was a serviceable part that needed to be replaced on a regular basis in order for the ignition to work properly.

With that in mind, the sporadic rpm issue unrelated to carburetion  definitely sounds like an ignition problem. You need three components for an engine to work. You need fuel, spark and compression. Diagnostic procedure means you narrow it down to one of those three categories and then trace down the issue. You've eliminated compression and completely gone through the fuel system, so that leaves ignition.

boobap

what about sucking air and temperature differentials...i am just throwing stuff out there, but then you said it is smoking a lot...did you run it without bar oil? i had a little homelite that used crankcase pressure to "pressurize" the oil reservoir...i accidently switch the two hoses...it ran, but it smoked so much, i couldn't see down the street... :o i have hardly worked with points, but it is weird that a pattern like that would continue with points...they, for the most part either work or don't...you could try and replace the entire points and condenser with a CDI...my saw shop carry's them for around $15. it completely turns the ignition into "digital" technology...just some thoughts.  :-\

chainsawr

2 things I don't think I saw mentionied above.

1 Have you put in a new fuel filter?

2 The 08 is governed to to 7000rpm.  Maybe it is running right up to that point and sounds like it is only in mid range since you are used to newer saws that spin faster.  Most new saws spin up 12-14k.  The 08 is almost exactly half that, right in the middle.   
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joe_indi

Quote from: chainsawr on May 17, 2010, 07:38:01 AM

2 The 08 is governed to to 7000rpm.  Maybe it is running right up to that point and sounds like it is only in mid range since you are used to newer saws that spin faster.  Most new saws spin up 12-14k.  The 08 is almost exactly half that, right in the middle.  
Chainsawr has raised a very valid point there.
By the way are you trying out the saw with a guide bar and chain?
From what I have seen on Youtube, these old saws four stroke quite a lot without load.And put out a smokey exhaust too.
The 08S does have an oil pump.The oil tank is located on the sprocket cover (and seems the oil pump )
So theories on crankcase pressurized  oiler systems' leaks are out.
Maybe you should check the condition of the saw "in the cut".
I have to check  the stroke of the crankshaft, it must be pretty long because the bore shows two versions 47mm and 49mm.
And yet this saw was happy running a .404 chain.
It must have a lower rpm compared to modern saws like Chainsawr has pointed out.
It probably needs  to run with some  load.

The only thing I have seen that comes close to the 08S is the Stihl P840 water pump that uses the engine of a 08S with most of the chainsaw parts sawed off.
That pump just loves running with a high head of water pressure.

jteneyck

Thanks for the feedback everyone.  Some points to clairify:

The oil pump is externally located in the clutch/bar cover, as you said Joe.  When I ran it with no bar oil, the chain/bar started to smoke when it spiked to high rpms, so I put oil back in the tank.  It ran the same so the oiler is not part of the problem. 

I have run the saw on an external fuel tank, ChainSawr, so it can't be a bad fuel filter since I had none with this set up. 

When I said it won't rev above the mid range, except intermittently, I was being kind about mid range.  I don't have a tach, but I don't think it's anywhere near 7000 rpm, probably not even 5000.  My neighbor says it only sounds right when it does spike up to high rpms. 

When I last ran the saw I forgot to mention that I set the HI speed jet down to about 3/4 turn open.  The blue smoke is gone and this is where I got the longest hi speed duration.  I'm guessing it's really too lean at this setting since when it did spike up it was really screaming. 

Boobap, your point about the cause being temperature related makes sense, but I can't find any obvious air leaks.  Still could be an issue.  It behaves like the crankcase pressure is low except when it spikes.  Is there an easy way to check crankcase pressure w/o tearing the saw down? (There's no pulser tube on this design, so that can't be the problem.)  And I'd agree that points usually either work or don't, but I'm out of ideas, unless I got something wrong in the carb. rebuild (but I don't think so). 

Thanks again.

joe_indi

Check the exploded view diagram of the carb.
Something might be out of sequence
Joe

jteneyck

I took the carb off again and verified that everything was reassembled in the correct order - it was.  I also verified that the inlet control lever was flush with the body.  As I put the saw back together I further checked the pulser path between the carb. and the crankcase and found no obstructions.  The saw started right up like before, idles fine, but just won't run up very high regardless of how I set the HI speed jet. 

So, as far as I can tell, the carb is fine, I have fuel, I have a clear exhaust path, the rings are good based on visual inspection. 

I just know someone has had this problem before, and I'm hoping you'll help me out because this is killing me.  So keep those ideas coming 'cause I'm way to young to die.  Thanks.

John 

joe_indi

John,
I was reading your first post on this thread.
You mentioned carb model 168B. However on the parts list it is 166C.
Could there be any difference in any of the parts........................?
Just a thought.

Here is something else I just remembered. A  customer brought me a water pump with a Tillotson using similar diaphragms.
The problem was it would start alright and idle smoothly.The minute you touched the throttle, the engine would belch some smoke and die.
The cure was a bit drastic.
Every gasket and diaphragm got a coat of liquid gasket to prevent air leak.
This cured the problem.
Like I said, just a thought
Joe

jteneyck

Hi Joe,  Thanks for the info.  The carb. really is a 166B, I mis-stated it as a 168B in the first entry.  Sorry about that.  The diaphrams and gaskets are positioned just like the schematic you sent me. 

The saw doesn't die when I try to rev it up.  It's just that it only revs up part way.  When I let off the throttle it returns to idle with no problem.  I made sure all the old gasket material was cleaned off before I installed the new parts, so I don't think there are any air leaks around them. 

Since rebuilding the carb did not change how the saw runs, at this point I have to believe the problem is somewhere else.  Any ideas along those lines?  And thanks again.

John



joe_indi

John,
Check the ignition timing.
I dont know how you are going to do it, but, the only thing left is that.
From what I understand from going through the data at hand:
Ignition timing should be 2.0 ... 2.4 mm B.T.D.C.
cb point gap  3 - 4mm
Joe

boobap

How do you check the timing of a chainsaw? Is it even possible to adjust? Maybe I am missing something, but the only way to adjust that would be to "clock" the flywheel forward or backward, which most of my flywheels are keyed. Am I missing something? I know that the gap can be adjusted on the pickup... ???

joe_indi

Quote from: boobap on May 19, 2010, 02:33:06 PM
How do you check the timing of a chainsaw? Is it even possible to adjust? Maybe I am missing something, but the only way to adjust that would be to "clock" the flywheel forward or backward, which most of my flywheels are keyed. Am I missing something? I know that the gap can be adjusted on the pickup... ???

Before the days of pointless(read electronic) ignition, there was cb point ignition (contact breaker).
Reducing or increasing the point gap had some influence on the timing.
Advance with wider gap and retard with a closer gap.
The key factor with the flywheel is the woodruf key. If its damaged the flywheel could move out of true position thereby altering the ignition timing.
How does one check the timing on the chainsaw?
If you are asking with what I think a xenon timing light could be used.
Power it from a battery source and use the impulse from the ignition lead to trigger it.
You will need to mark the flywheel , in this case from TDC in reverse from 0 to 5mm.
To mark the flywheel in mm instead of degrees, you will need a print out of a timing wheel.
One of the members on this forum had pointed me quite some time ago in the direction of a website which had such a wheel.
Download the image, print it out, cut out the image of the wheel, hold it against your flywheel with TDC aligned and mark the degrees.


Here are some examples :
http://www.modelgasboats.com/images/stories/issues/Nov08/PortTiming_Squish/free_degree_wheel.jpg

http://www.gabma.us/tools/DegreeWheel.bmp

http://www.smellofdeath.com/lloydy/images/tmgdsk.gif
I dont remember how you get the degrees.
Logically, if you know the stroke (32mm in the case of the 08S) BDC to TDC is 32mm degrees would be 180.
Simple maths says 1 degree = 0.18mm
Or 1mm =5.63 degrees
Maybe I am wrong on this.Maybe somebody else here can explain it better and correctly.

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