Dolmar 421 help

Tnarb44 · Jan 6, 2016

I just picked up a new Dolmar 421 and so far am very impressed with it. Today while going through its 2nd tank of fuel it died after running it 5-10 minutes and wouldn't restart. I noticed the primer bulb was very full and looked to have pressure in it so my first thought was vapor lock. Then I opened the fuel tank and it indeed had a lot of pressure in it. After that it started right up and I ran it another 15 minutes without issue. Question is do you think I got an issue with the tank vent? Or is it possible this is a 1 time thing? Thanks

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CR888 · Jan 7, 2016

Tank vent is my guess. When you opened the tank did the air come out or get sucked in? The fact you ran it 15 mins without issue also means vapour lock could be the issue. Vapour lock sucks!!

wde_1978 · Jan 7, 2016

I second the tank vent to be the culprit.

I had the same issue with my weed whacker ever since it was new, it would run about 10 minutes at full WOT and then start starving for fuel.
When opening the tank cap it would suck air IN and then run normal for another 10 minutes.
The first Year I just left the tank cap partially unscrewed to bypass the issue.
Last Year I got fed up with it and took the fuel cap apart and the rubber vent nozzle was indeed glued shut from the factory machining process.
I pushed a screwdriver through the rubber nozzle, put the cap back together and didn't have any problems since.

I don't quite (yet) understand the vapor lock phenomenon, but I believe my Dolmar PS-7900 suffers from it under certain circumstances/work conditions.
When I bucked my firewood logs last summer I encountered an thus far unfamiliar to me issue.
I would make a few long cuts bucking large logs, turn off my saw to clear my working area what would take 5-10 minutes, and then I would fail to restart my PS-7900. Engaging choke would flood it, not engaging choke would require like 10 or more pulls to just get a sign of life out of her. Opening the tank cap would show no effect.
Once restarted it would idle and/or WOT until it ran out of fuel.
This phenomenon is nonexistent on my PS-6400!
Outdoor summer temperatures were 30-40°C.
My Dolmar PS-7900 behaves "normal" at low to average outdoor temperatures (autumn, winter, spring, rainy days, lower temperature summer days).

That been said I don't think You have a vapor lock issue!
To my understanding "vapor lock" occurs when a saw has been ran and shut off while hot - not while the saw is running!
But I could be wrong.

NOTE:
My little Sachs-Dolmar 105 would ALWAYS boil the fuel in her little magnesium body when ran for a prolonged time while bucking firewood logs during summer - no vapor lock whatsoever.

A fuel tank vent only allows air to enter the tank to prevent vacuum, it will not allow gasses to exit the tank!

Vapor lock:
https://en.wikipedia.org/wiki/Vapor_lock

Tnarb44 · Jan 7, 2016

Thanks for all the help. If i had to guess I would say the air rushed in when it happened. I my concern it will be an ongoing issue is because this is only the 2nd tank on the saw, time will tell i guess. Is there a way I can check the vent to see if it is plugged?

wde_1978 · Jan 7, 2016

I never had to check the tank vent on any of my saws, thus I wouldn't really know whether they are accessible without mayor tear down of the saw or not.

But the good thing is a new vent valve is only about 6$.

By the look of the IPL it looks similar to the vent valve on my weed whacker.
There is a good chance that valve (which looks like a nozzle) is stuck on itself.

Here is a IPL picture showing the valve, the approximate location can be guestimated from it.
Hint, click the image to enlarge.