Dolmar Chainsaw refuses to start

[lone wolf]

Thanks, gentlemen (and ladies if any here) for great suggestions. Yes, I first suspected and still suspect fuel to have been bab due to even my tank of saw Stihl MS 261 stalling at idle on the "fresh" fuel I got. I bought definitely fresh 93 fuel with ethanol after that (which pains my heart, but I cannot trust non ethanol station anymore) and I splurged $39.99 for MotoMix (which gave me even more chest pains). I know fuel is not to blame. I know the compression is not to blame. I know the muffler is not to blame because I verified that spark arrestor is clean - which it is, it still has shiny appearance since it is still new. And I tested starting with muffler off completely, so muffler possibility was eliminated. Two carburetors pretty much eliminates carb issue. I also gave one carb extremely good cleaning and testing. I still think I have a massive air leak. The saw died like it ran out of fuel, all the lines are patent and there is plenty of pressure in the tank that builds up almost instantly with cap closed. There is never vacuum there. With pre-carb delivery checked and carb itself cleaned and tested with alternate carb, I think the perfectly running saw stopping inside the cut was likely due to a vacuum leak, because I do not feel vacuum forming when I close the air filter hose with my thumb. I have received Mityvac vacuum and pressure tester, I just need to find time to test pressure and vacuum. How much PSI and how much inHg do I need to pump to safely test seals and gaskets? I think "Married with Engines" You Tube channel has a few videos posted, I need to check how much she pumped hers to.

Set to 7 on both and pressure and vac.

 

[ZeroJunk]

If the flywheel slipped in your situation it would be from accidentally hitting the chain brake running wide open, or the bar actually kicking back and the brake doing it's job.
The inertia or centrifugal force, I forget which is which, can spin the flywheel.
It happens.

Just so happens the other day I worked on a MS261 with the complaint being would not start.
I put an in line tester on it and would swear it had no fire until you really wound on it with the pull rope. Then it ran fine.

Could be with the plug out it is easier to get the RPM up. Or, Dolmar could be completely different.

I know this was touched on earlier.

 

[lone wolf]

First thing for the OP to check is does it have impulse. Put the Mitivac to the impulse line and see if it wiggles the needle back and forth! Get back to me with this before you do everything else.

 

[eugene89us]

Promise I will do some testing on Sunday. I am finally off after working every day for last 2 weeks, I will get up and test it first thing in the morning. Will give report on vac/pressure at impulse first, that should be fairly easy without any disassembly.

 

[blades]

Am I missing something here , " pressure in fuel tank"?

 

[eugene89us]

Do you not have pressure build up in the fuel tank? It is so hot in here that it does not take long for some pressure build up in the tank once you take it from the cool garage floor and into the sunny meadow. When you take the cap off, you can hear it releasing - hisssss. It is happening with my perfectly functional Stihl, so it seems to be the effect of the fuel expansion in the heat. I am sure I could cut the duckbill a bit to have it release some pressure also, but it is not excessive and has never caused saw to flood. It works as vacuum reliever, as no vacuum formed in the saw during use when it ran.

 

[blades]

someway somehow there should be some sort of venting of the fuel tank. and yes there is one on the 5105. If there is no vent or it is plugged up the carb pump can't pull fuel.

 

[eugene89us]

There is a vent, but the purpose of the vent is to act as a one-way valve. It is supposed to alleviate formation of vacuum in the fuel tank due to fuel depletion, but it is also allowed the tank to build up *reasonable* amount of pressure. Pressure will improve fuel delivery in the carburetor. On some Dolmars, this pressure can overdeliver fuel, resulting in flooding. But that is not my problem, mine appears to be opposite.

 

[Old2stroke]

There is a vent, but the purpose of the vent is to act as a one-way valve. It is supposed to alleviate formation of vacuum in the fuel tank due to fuel depletion, but it is also allowed the tank to build up *reasonable* amount of pressure. Pressure will improve fuel delivery in the carburetor. On some Dolmars, this pressure can overdeliver fuel, resulting in flooding. But that is not my problem, mine appears to be opposite.

Pressure will ALWAYS build up in a fuel tank with a one way vent valve, perfectly normal and all carbs should be able to prevent this pressure from forcing fuel into the carb. It's called the pop-off pressure (the point where fuel will be forced through the inlet valve) and should be a minimum of 10psi but might have to be higher for metal tank saws working in the hot sun.

 

[Ralf1e]

Thanks, gentlemen (and ladies if any here) for great suggestions. Yes, I first suspected and still suspect fuel to have been bab due to even my tank of saw Stihl MS 261 stalling at idle on the "fresh" fuel I got. I bought definitely fresh 93 fuel with ethanol after that (which pains my heart, but I cannot trust non ethanol station anymore) and I splurged $39.99 for MotoMix (which gave me even more chest pains). I know fuel is not to blame. I know the compression is not to blame. I know the muffler is not to blame because I verified that spark arrestor is clean - which it is, it still has shiny appearance since it is still new. And I tested starting with muffler off completely, so muffler possibility was eliminated. Two carburetors pretty much eliminates carb issue. I also gave one carb extremely good cleaning and testing. I still think I have a massive air leak. The saw died like it ran out of fuel, all the lines are patent and there is plenty of pressure in the tank that builds up almost instantly with cap closed. There is never vacuum there. With pre-carb delivery checked and carb itself cleaned and tested with alternate carb, I think the perfectly running saw stopping inside the cut was likely due to a vacuum leak, because I do not feel vacuum forming when I close the air filter hose with my thumb. I have received Mityvac vacuum and pressure tester, I just need to find time to test pressure and vacuum. How much PSI and how much inHg do I need to pump to safely test seals and gaskets? I think "Married with Engines" You Tube channel has a few videos posted, I need to check how much she pumped hers to.

I think you will find she pumps it to half a bar

 

[epo]

. I have received Mityvac vacuum and pressure tester, I just need to find time to test pressure and vacuum. How much PSI and how much inHg do I need to pump to safely test seals and gaskets? I think "Married with Engines" You Tube channel has a few videos posted, I need to check how much she pumped hers to.

The repair manual says 0.3 bar (4.35 psi).

 

[sean donato]

Pressure will ALWAYS build up in a fuel tank with a one way vent valve, perfectly normal and all carbs should be able to prevent this pressure from forcing fuel into the carb. It's called the pop-off pressure (the point where fuel will be forced through the inlet valve) and should be a minimum of 10psi but might have to be higher for metal tank saws working in the hot sun.

10 psi is a very low pop pressure for stock carbs. Its hardly above the oressure the internal fule pumos wilk make, usually between 5 and 8 psi. Typically it should be around 20 psi. Seen plenty of brand new walbros and zama pop in the 30 psi range.

 

[horv1869]

Gdje se nalazi ventil spremnika goriva na Dolmar ps 7900 ili Makita DCS 7900.

 

[ChuckMI]

Gdje se nalazi ventil spremnika goriva na Dolmar ps 7900 ili Makita DCS 7900.

Gdje se nalazi ventil spremnika goriva na Dolmar ps 7900 ili Makita DCS 7900.

Translation: Where is the fuel tank valve on the Dolmar ps 7900 or Makita DCS 7900?

 

[coffeebrk]

Any updates????