Dolmar 7900 issue

[Wilhelm]

I don't think your 7900 is vapor locking. It's flooding. Easiest way to start them is hold the throttle wide open and start pulling. I think the needle in the Zama leaks fuel by resulting in a flood condition. Mine hasn't done it since I swapped an HD12 in from a 372xp.

Is a Walbro HD12 carb a direct bolt on for a Dolmar PS-7900?

They're like 11-20$ from China on eBay, maybe worth considering?!

 

[Khntr85]

I used to think that my PS-7900 is flooding, pulled the plug a dozen times just to find it to be bone dry.
I no longer pull the plug, just engage throttle lock and start yanking till she comes to life - takes easily 15-20 pulls.

I'm no 2-stroke engine expert, maybe it isn't vapor lock what bothers my PS-7900.
All I know she didn't have this issue before, and with time passing it is getting worse.

I am at a point where I am considering a carb rebuild.

This issue is non-existent during winter operation.

Wow that sucks...that's definitely unexeplable....

 

[huskihl]

Is a Walbro HD12 carb a direct bolt on for a Dolmar PS-7900?

They're like 11-20$ from China on eBay, maybe worth considering?!

I'm guessing that if you were to hold the throttle wide open, rather than the hi idle, which is about 1/8 throttle, it would start in 3 pulls.


Yep. Direct swap, but make sure you also get the throttle cable insert cam thingy. Not all carbs come with it. Not sure if an AM option will alleviate all of the problems though. Worth a try

 

[Poleman]

It is a direct switch and does perform good. I still find a modded Zama or a AM copy (if you can find a twin jet carb) will out perform it. For everyday and reliability it's great!!

 

[Wilhelm]

I'm guessing that if you were to hold the throttle wide open, rather than the hi idle, which is about 1/8 throttle, it would start in 3 pulls...

I'll try give her WOT next time the issue arises.

 

[Simondo]

I used to think that my PS-7900 is flooding, pulled the plug a dozen times just to find it to be bone dry.
I no longer pull the plug, just engage throttle lock and start yanking till she comes to life - takes easily 15-20 pulls.

I'm no 2-stroke engine expert, maybe it isn't vapor lock what bothers my PS-7900.
All I know she didn't have this issue before, and with time passing it is getting worse.

I am at a point where I am considering a carb rebuild.

This issue is non-existent during winter operation.

If your finding that the plug is "Dry" and the saw is not flooding then what is your reason for not using the choke position...even for a pull or two... instead of going straight to the warm start position ?
I hope iv not misunderstood what your finding and mentioned something already thought over . Sometimes i find a warm saw needs a pull on full choke to fire up.

 

[Wilhelm]

If your finding that the plug is "Dry" and the saw is not flooding then what is your reason for not using the choke position...even for a pull or two... instead of going straight to the warm start position ?
I hope iv not misunderstood what your finding and mentioned something already thought over . Sometimes i find a warm saw needs a pull on full choke to fire up.

Good question!
One pull with choke engaged doesn't seem to do squat, two or more pulls on choke floods the saw to a point that fuel comes out the muffler.
I no longer engage choke on a warm/hot saw to prevent extreme flooding which usually requires pulling the plug.

The behavior of my PS-7900 when the issue arises is weird to say the least.

I know how to counteract the problem, don't turn the saw off directly after a WOT bucking or noodling cut AND restart the saw within no longer than 2-3 minutes after having her shut off and blip the throttle a few times.
Also, don't let the saw run out of fuel - if she pulls fumes into the carb she will not restart after refueling.


I know my saw has a issue that I should track down and fix , as said before I am considering a carb rebuild.

 

[Simondo]

Good question!
One pull with choke engaged doesn't seem to do squat, two or more pulls on choke floods the saw to a point that fuel comes out the muffler.
I no longer engage choke on a warm/hot saw to prevent extreme flooding which usually requires pulling the plug.

The behavior of my PS-7900 when the issue arises is weird to say the least.

I know how to counteract the problem, don't turn the saw off directly after a WOT bucking or noodling cut AND restart the saw within no longer than 2-3 minutes after having her shut off and blip the throttle a few times.
Also, don't let the saw run out of fuel - if she pulls fumes into the carb she will not restart after refueling.


I know my saw has a issue that I should track down and fix , as said before I am considering a carb rebuild.

Well that is a odd one as you say ! definitely not something i have had to deal with . You may have come to a logical conclusion yourself about it being "time for a full carb rebuild" with "attention" to the metering side of the carb and will give you a chance to find if there is any crap inside the metering cover on the atmosphere side . Its not right that the speed and severity of flooding should happen.