Vapor locked chain saw?

[loganj01]

Is it possible...or maybe I should ask if it's likely...that a chain saw can vapor lock? My Dolmar 7900 will sometimes refuse to start if it's hot and low on fuel. Even if I fill it up, I can pull it 30 times and nothing. Pull the plug and it's bone dry. Check the spark and it's nice and blue. It won't even hit if I pour a capful of gas down the carb throat. Let it cool off, choke it, and it's running in 2 pulls. It'll do this sometimes if I run it out of gas too. This saw has always been peculiar about starting after it's run out of gas, but not this bad. I was evidently running it too lean from the time I bought it and fried the piston. Now I have a Bailey's big bore kit on it and have pulled the muffler a couple of times in the last 20 hours or so of run time...piston looks great so I think it's a little rich if anything. Should I be looking at carb diaphrams? Fuel lines? AFAIK, it doesn't leak fuel although it will leak at the cap if you don't tighten it pretty tight. It runs and cuts great when it's running...idles well. I put in a new comp release when I installed the Bailey's kit too. Anyone got a suggestion?

 

[jimbo1490]

Replace the fuel pump diaphragm and flapper valves.

Jimbo

 

[loganj01]

That's kind of what I was thinking too...just wanted to see if anyone else had the same thoughts. Thanks.

 

[HiOctane]

It looks like vapor lock case the way you describe it.Hard starting after you stopped to refuel or stop the saw for few minutes.Try to run your saw a bit richer to cool the engine,be sure the saw is clean and keeep the chain sharp.Dont think replacing diap could be the problem.

 

[CedarRock]

Is the tank vent clear?

 

[alderman]

Just thinking out loud as I really don't know, but if it is an issue with the tank vent, wouldn't the saw die before he shut it down?
Also vacuum would be removed when he refuels so it should restart.

 

[trappermike]

Vapor-locking is a very real problem on some chain saws,the older ones were more prone,and it usually happens in hot weather. Saw is running great,then you shut it off for a few minutes,then it wont start. It is usually heat build up around the carb and fuel line. If this is the actual problem there is simple mods you can do to cure it.
I'm not familiar with the design layout of your saw off the top of my head,so it's tough to describe exactly what to do.A couple photos of the engine top cover and carb cover would help.
But a cure is usually like this- Drill 2 or 3 1/2" holes from the fan area to the carb box so cool air can blow in around the carb from the fan. Then drill 3 or 4 1/2" holes at the back of the carb cover,above and behind the carb. Thus you have cool air blowing from the fan thru the carb box and out the back keeping the carb cool and hot air building up around the carb.
Usually cures the problem and adds a little power too.:msp_smile:
Hope that works,let us know...

 

[bigwhiteash]

I have the 7300 version. From what I recall in the manual, there's a re/moveable window at the carb wall that you can change over for winter use, I assume, to let warm air to the carb area. Maybe you're set up for winter?? HTH

 

[trappermike]

Yes,if the saw has a "Heat door" be sure it's closed.

 

[loganj01]

I didn't know about the heat door...I'll look at it and post the results. Thanks.

 

[loganj01]

The heat door was in the correct position for warm weather operation so I'm back to square one. I'd think that if there was some sort of design issue, other people with 6400-7900 Dolmars would have had a problem. I really like the performance of the saw but I'm starting to wish I'd bought something else because of this issue...it doesn't cut worth a crap when the engine won't run. :confused2:

 

[trappermike]

I saw the vapor-locking problems many times and in new model saws too. Today's blends of gasoline contribute to the problem too.
Modifying the saw as I described works. But what you can try,just to see if it's a carb heating,fuel line heat problem is to try running the saw with the carb cover off(Just be careful not to poke holes in the air filter with branches) and see if it doesn't work better. I don't know what your saw looks like but I'm assuming it has a separate carb cover like a Husky or Stihl. In the case of saws with a one piece carb/cylinder cover it must stay on,for the cyl. cover to duct fan air over the cylinder.
The Dolmars are usually very sturdy built saws,I wouldn't give up on it too easily.

 

[loganj01]

I'll try that...the Dolmar has a carb cover that has a screen to filter out the chunky stuff before it gets to the breather filter. It'll run without it. I had been planning to buy the "professional" filter from Baileys as soon as I had the spare cash and that might solve my problem. It uses a much larger paper element filter and comes with a larger cover as well. I won't give up on the Dolmar...I was just ranting a little. I suspect most of us have had a moment when we would almost pay someone to get a saw or other uncooperative piece of equipment out of our sight :hmm3grin2orange:

 

[trappermike]

I know the feeling! As a mechanic there were a few saws and other types of machines that I absolutely hated to work on. As my own boss now,I can just reject such machines,as I know they are a waste of my time and someone's money.
My guess is your Dolmar will be fine.:msp_smile:

 

[Mad Professor]

Try 100 LL as fuel.

Should not vapor lock at 30, 000 ' +

 

[albert]

If the saw was vapor locked, a few drops of mix should get it to fire. A cap full may be too much on a hot saw. It's a good pratice to let the saw idle for a short time before shutdown to let the engine cool a bit. You may other issues.