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????

 

[Smedlow]

I have a 5100s that was running great with dealer adjustments on the carb that quit running and wouldn't start at all. I finally tore it down and took the cylinder off the piston only to find that the ring had been knurled into the groove in the piston. My theory is that it was running to lean, got hot, and melted the aluminum piston so that it riveted the ring into the groove. Anyway, I honed the cylinder, got a new piston, tuned it a little richer, and now it runs at normal temperature with plenty of power. My only problem with it is that if I try and start it choked, it floods. So, I start it in run position. Anyway, I don't remember if you said that you had compression. I didn't, which you can imagine if the ring wasn't doing anything. Something to look at.

 

[thenne1713]

Good morning. I am a bit at loss currently. As I posted previously, I started using NOS Dolmar 5105 S chainsaw within the last month. So far, it went through about 2 tanks of fuel. She was ripping through wood like there is no tomorrow. In fact, I posted a video here of her starting, idling, and cutting.

I was bucking one of the younger oak trees that was recently dead. The saw was going strong, very hot MS afternoon. As I was bucking its last cut, the saw turned off in the cut. I opened the gas cap - the tank was not completely empty, but it was almost there. Figured the fuel filter was not fully submerged anymore, must have been low tank. I also recently got fresh 93 octane ethanol free gas that I use with all my saws. Mixed 50:1 with Red Armor as I usually do. The saw refused to restart.

I got my Stihl chainsaw - my primary saw. Finished bucking with it. Once it ran out of fuel, I refueled it as well and it ran kinda slow on that gas. It would die at idle, though would restart at fast idle. Of course, here I have confidence to say that I must have gotten old gas from the station. It is $4.6/gal (vs. ethanol 93 octane runs around $1/gal cheaper). I dumped gas from both. To be on the safe side, I went ahead and splurged on Moto Mix. Stihl chainsaw perked up immediately, would idle beautifully again without issue. But even with bad gas, it would still start at fast idle, but it would not idle well on its own.

Trying to rejuvenate Dolmar has not been as straightforward. I dumped all the fuel, replaced with Moto Mix. No start. I took the carburetor out, cleaned it out. Later on, I even put my spare new carb on it - so I know carburetor is NOT the issue. Checked the fuel line and filter - perfectly new, no restrictions. The tank holds some pressure when closed, no vacuum is formed there. Compression at spark plug is right at 180 psi. Took the boot off and inspected the intake boot as well as impulse line - both perfectly healthy. Both hold pressure and vacuum when tested with my mouth on one side and finger on the other. Checked the coil. The plug sparks nice blue spark when out of the cylinder. Well, I have a spare new backup coil. It also sparks when grounded, no avail - saw does not restart with spare coil. So that eliminates coil issue. Used a business card to gap the coil. Found a perfect one that shows 0.0100 inch on my caliper. If I push the caliper tighter, I can get 0.0098. I think that is as perfect gap as you can make it. Oh yeah, replaced spark plug with new NGK BPMR7A, gapped at 0.02. Inspected piston when the saw was disassembled. Brand new looking. Does not even have any carbon on it, all factory markings there without a scratch on both intake and exhaust sides. I took a photo of brand new out of the box flywheel. Currently, when piston reaches TDC, the magnet position is spot on where it was when it was brand new, so getting flywheel out of time is also not the case.

At this point, I am completely at loss. The saw that was cutting through wood like it is nothing is now a paperweight. I know it has compression, I know it has spark, I know the carb is good. I even took the muffler off to test it to eliminate muffler or spark screen issues. After so many deperate pulls, the muffler opening gets wet, so I know the saw is getting fuel. Once in a while, it will sputter a bit. Yesterday, in fact, it almost started. Ran for about 2 seconds and died again. My only last option is to check the seals. I will need to order vacuum and pressure tester to do so and then test in case there is a leak at the seals or gaskets. But that makes it unlikely after only 2 tanks of gas.

Searched forums. A couple folks were in same predicament. Some had coil issues that did not spark at all - mine sparks well, and tested 2 coils anyways. One ended up trading it back to dealer after not finding solution with same symptoms as mine. It is a simple engine, I am just not sure where else to look. Any ideas would be appreciated greatly.

I had little craftsman top handle was given to me, drove me crazy many hours, would prime, start. die after 15-seconds; replaced fuel line/ filter, later replaced carb= same. Finally noticed a slight crimp in 1-inch long line. Had installed new line same length as old... duuuuhhh... fuel must be able to free flow. So is your fuel reaching spark plug? Will it flood if you kill IGN and crank a time or two? Yes, I too have had gas that would not run... but you sound like you covered that possibility too? Good Luck.

 

[Tim Carroll]

My 5100 started running like crap last year and would start and die in the cut. Once it quit, it didn't have spark. I bought a coil for it, new plug, and a carb kit. I could not get it to run at all after I replaced all that. I put some mix down the yap and it would fire but not idle after it burned that off unless I held throttle open. I checked tank vent, checked hoses, checked intake boot and replaced the fuel filter. I finally pulled the carb back off, cleaned the carb out again, checked diagram to make sure carb was put together correctly, cleaned the base flange that the carb mounts to, and tightened it back up well without stripping anything but tighter than I assembled it the first time and it is running fine now. My thought is that I probably had an air leak at base of the carb. I know these things can be maddening and hopefully you figure it out.