Dolmar 7900 issue

[ffchewy17]

I had a job today to get a dead live oak down. 42" diameter at the base, 50' tall. Working in Central Florida, 96 degrees today and full sun. Started the cut with a 7900 28" square ground. Got the tree on the ground and started bucking rounds, cut 5 rounds then had to noodle a section at the first major branch junction due to size. Saw ran out of fuel at the end of the noodle section. Went to fuel up, gas can was in partial sun, running 89 E-free @ 40:1 K2. After fueling saw started to finish next buck section and saw would not hold RPM. The saw felt like it was starving for fuel, surging at wide open throttle.

I had to pull out the backup saw MS461, and finish the day with out any issues. I've never had any issues with the Dolmar before. Pulled the exhaust off at home and no scoring on the piston. Have not tried to restart saw yet. I tune with a tiny tac. 7900 runs HD filter set up and dual port gutted muffler.

Could this be vapor lock? Have never had an issue before with any saw in the heat. Ran two more saws on the same mix for the rest of the day with no issues. I really like the 7900, but I kinda lost confidence in it today. Hopefully it's something I did and not a saw issue.

 

[backhoelover]

i would check tank vent and carb. to test tank vent pull vac on the tank. it should not hold at all

 

[Wood Doctor]

Possibly a clogged fuel filter. I had a Stihl MS 361 do the same thing last year. I changed the fuel filter the next day and it ran like it was new.

 

[ffchewy17]

Will do. Thanks for the ideas. It was a first for me, I think that's why it's so frustrating.

 

[Chainsaw Jim]

It almost seems like the fuel filter got lodged up at the top of the tank when refueling.

 

[Khntr85]

How long have you had the saw, how many hours on it....

Sounds like the 461 is a good one...how long/how many hours are on the 461???

 

[huskihl]

Check the impulse hose. They melt occasionally

 

[ffchewy17]

The 7900 is 7-8 years old, I've re-ringed it once. I have no idea on the hours, it's one of my front line saws, but I rotate it with (2) 461 & WWS 390xpw.

I ordered a new fuel line, filter, tank vent and impulse line. Hopefully that covers the bases.

The 461 has never skipped a beat and its on its 3rd piston.

 

[ffchewy17]

I did leave the 7900 sitting out for two hours after the initial surging problem and it still did the same thing. If it was vapor lock would it go away after sitting for a while?

 

[mdavlee]

It should clear itself after it cooled and let fuel flow again. Pull the fuel line loose at the carb and see if it squirts fuel. Those 7900 seem to have pressure months later.

 

[ffchewy17]

I drained the fuel tank yesterday at home. It find have pressure behind the fuel cap Wednesday while it was sitting. The 461 also had pressure behind it, but it didn't seem to effect the saw.

 

[Rob Stafari]

It almost seems like the fuel filter got lodged up at the top of the tank when refueling.

^This... Just last week I had this happen with the oil suction line on my dolkita. Quit oiling while I was cutting up some mulberry. Needless to say the bar tip didn't like this one bit, it decided to go on strike and quit.

 

[Wood Doctor]

^This... Just last week I had this happen with the oil suction line on my dolkita. Quit oiling while I was cutting up some mulberry. Needless to say the bar tip didn't like this one bit, it decided to go on strike and quit.

I recall that happening to me once with my Echo 3900 that had a bar tip give up from heat and stop the saw dead. The engine would start and run but the chain was frozen. I greased and oiled the tip and the saw was back in business.

 

[Simondo]

Do you have one or these in the tank ? Dolmar fit them in many of there saws over here. That little disk is inside and i have had fuel starvation causing a similar symptom to yours without any visual reason to suspect the filter ...

 

[ffchewy17]

I have that same filter.... have not pulled the filter yet. Is that small disk inside the filter?

 

[Simondo]

I have that same filter.... have not pulled the filter yet. Is that small disk inside the filter?

It is....I have found over time they .."Can" .. stop flowing so easily . Might be worth checking out.

 

[ffchewy17]

Thank you for all the info everyone

 

[Brian Carlson]

Might pull the carb and look at the screen in the carb to make sure it's not clogged up with the super fines. Had this happen a couple times with other saws. Watch when pulling the carb apart cause you can damage the diaphrams. Might have a pin hole in a diaphragm causing it to not pump fuel correctly. Just some other ideas for ya to consider

 

[Khntr85]

The 7900 is 7-8 years old, I've re-ringed it once. I have no idea on the hours, it's one of my front line saws, but I rotate it with (2) 461 & WWS 390xpw.

I ordered a new fuel line, filter, tank vent and impulse line. Hopefully that covers the bases.

The 461 has never skipped a beat and its on its 3rd piston.

Damn man you are getting somengoodnuse out of our saws....

When you put a new piston in do you touch up the cylinder any....

Do you replace the bearing and seals, or just put in a piston if all else is good???

 

[Ronaldo]

About the fuel filters... I have had them look fine, not appear dirty etc, but try to suck or blow through them and they are completely plugged. Something to check.

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