fuel tank building pressure

[charlieh]

Hi,

I have done some searches on this and cant find an answer, :bang:

i have a 020t that wont start, but the fuel tank has pressure build up in it after i have pulled it over a couple of times, what could be causing this? :bang: :bang:

i cant see how pressure could be getting back through the carb, which has been stripped and cleaned, although it didnt have an overhaul kit fitted. It has had a new tank vent and filter fitted and sparks well, but it doesnt seem to be getting enough fuel to the engine. The rubber intake boot is in good condition as is the fuel and impulse lines.

the carb gaskets and diaphrams are in a good condition and dont appear to show any signs of wear. its got me stumped

as always any help is a bonus,

Charlie

 

[andrethegiant70]

Hi, charlieh, just a quick question. If you put fuel in the tank and DON'T pull it over, does pressure build in the tank? On a hot day, gas vapor will expand.... mine does. The question is, does the tank pressurize because you're pulling it over or are you just fretting over the saw in the hot sun? Also, how much pressure? Some of my saws give a pretty regular puff, but if the cap ricochets off your forehead that's different.

Just figured we'd rule a few things out.

 

[BostonBull]

If it wont start, and the tank is pressurized ONLY when just being fueled/opened and then trying to start....hows the tank vent look? 020's are known for these to go bad and there have been a few updates over the years on these.

 

[pgg]

If the fuel tank gets pressurized then the fuel tank breather is either stifled or completely blocked. End of story...

 

[andrethegiant70]

I can't say I'm familiar with all the saws, but aren't those gas caps essentially one-way valves? They are designed to let air IN to avoid vapor locking the saw, but I DON'T believe they are designed to let pressure out. Lets face it, if they let pressure out you'd spew gas every where any time the saw heated up. My Jonsereds 910 does that, to my great frustration, but it is a different system.

Now if the saw is holding VACUUM, that's different. Then it's easy, he's got a plugged vent.

So, I guess that's the question.. are you holding pressure or vacuum?? When you pop the cap off, does it suck or blow?

 

[stihltech]

vent

You are correct, vents let air in, not out. Some pressure is normal.

 

[Lakeside53]

Yep... pressure is normal and GOOD! It's just you agitating the fuel.


If you've looked at everything obvious, then it's time for a pressure and vac test..

 

[belgian]

I have an older hedge trimmer Husky 225H75 that does built up quite some pressure in the gas tank. I have replaced crank bearings/sealings a while ago while it developed a seal leak at the clutch side and accordingly, couldn't make it to run right.

Now I have noticed that I can make it run nicely with a full tank, but when the tank gets empty, the machine has tendency to lean out (high rpm at idle). Tank vent has been checked and works OK (I think). I wondered also it the tank pressure building was a good or bad thing.
Hmmm, Husky....

 

[charlieh]

Many thanks for the responses, the tank builds up slight pressure not huge amounts, but more than is normall on my other saws,i suspect however that this could be due to the different tank vent arangement on this compared to my other saws, in that the tank will let air in preventing vacume but wont release excess pressure.

I suspect that this saw will have to go the local stihl guys to be checked over, as i cant work out why it would be lifting fuel up to the carb fine but doesnt seem to put it into the engine?

Thanks again

Charlie

 

[andrethegiant70]

Hi, Charlie.. I guess it comes down to how intrepid you feel regarding working on the saw yourself. I have no experience with the Stihl top-handle saws, I know they are built a bit different than the others. Myabe someone could chime in on the particulars? I do know that the carbs themselves are fairly easy to work on and that doing a rebuild is fairly straightforward. There is a huge amount of information on carb rebuilds, accessible through the search function.

I seem to remember reading a few threads that indicated that the 020/200 has a pretty finicky intake system, but others on the site will be able to give you more information.

Here's what I would do (and have done many times): Use the search function (green bar at top of the page) and do a bunch of reading on your saw and carburetors in general.... within an hour or two you should have a pretty good idea of what you're looking at and whether you think its something you want to try yourself.

If you're "short on time" and really need the saw quickly, the dealer is probably your best option. If you don't mind taking some time to do a little research here, and you have the patience to flail a bit before you get it right, then doing the project yourself sounds like an option. The folks on this site have helped me through a LOT of saw issues.

 

[charlieh]

I am fairly happy doing the work, i have replace the gaskets and diaphrams on several of my saws, so i might give that a go, for what it costs and whilst i have the carb in pieces.

I suspect that if this doesnt solve the problem then i will have to take it to the dealership to sort, its not that i dont want to learn, more being afraid of doing something wrong! i am fairly mechanically minded, but the though of stripping the whole thing down doesnt appeal to me, i know where my limits are!

from the posts it looks like the pressure in the tank probably isnt that much of an issue, so the problem is else where, question is where? carb is lifting fuel but not putting enough out to run the saw, it has occasionally fired but never run. The jets are clear and the carb is set to factory spec, the induction line appears to work fine when the engine is pulled over by hand. I suppose a carb service kit is the place to start

 

[andrethegiant70]

Charlie:
Compression feels good? If you can get a look at the piston, check for scoring. If it is scored up your problem is a bit more complex.

Another way to check for carb/fuel trouble involves removing the carb (you have) and putting a teaspoon (or maybe half for your little saw) of mix down the carb throat (hold the throttle trigger down to allow it to flow into the cylinder.) If you now pull the saw over and can get it to start and run for a few seconds (I think you may have mentioned it popped) you've probably ruled out potential ignition and compression woes.

You said your fuel line and impulse line look ok... be sure about them.. fuel lines that get soft can collapse on themselves and impulse lines often develop cracks that you can't see until they are stretched.

Lastly, carb membranes are cheap and may look good, but they may still loose their spunk. The carb is apart? At $12 bucks, it's an easy rebuild. Make sure you get the membrane order correct!! My advice to you is to do some reading on carb rebuilds and changing the welsch plugs.

 

[charlieh]

the piston has some very very mild scoring, nothing i would start to worry about, and the compression seems fine, I will try the half spoon of fuel mix and see what the result is, I have to say this is the hardest saw to have a tricky problem with, it seems there are so many things that can cause problems on this saw!

also what are welsch plugs? the only websites i seem to be able to find are australian car racing sites? or am i just being thick?

Charlie

 

[WRW]

also what are welsch plugs? the only websites i seem to be able to find are australian car racing sites? or am i just being thick?

Charlie

Not meaning to butt in, but it's welch plug. Should come with the carb kit.

 

[pgg]

Well i've used various 020t/200t's over the years, don't recall ever seeing one that wouldn't actually start, even if they were starting to run like s##t, sounds almost as if the pulser tube isn't connected?? Magneto gap nice and tight? Spark lead not worn and arcing to the cylinder head? Carbs fuel strainer not 'crusty'? No tiny split in the rubber inlet manifold? Fuel hose not kinked? Take it there is a fuel filter in the tank?? No splits in the fuel hose where it plugs to the fuel filter? No1 'problem' with 020t/200's - the muffler screen quickly chokes solid with carbon. Ditch the screen. No one way valve lids on these saws, they've got a tank breather tube setup instead. They do clog sometimes but pretty rare. I assume they also let air OUT as well as in, tiny rubber duckbills, they block petrol but seem to allow a reverse air flow when the saws not running,(the mechanics here probably know more on that) also the later models have a nuisance 'blocked filter' compensator setup, (stihl got bored and started adding unnecessary bells & whistles) Also the crank seals DO start to leak pressure on 'high milage' 020/200's..

 

[andrethegiant70]

I was hoping a 200T guy would pitch in, thanks, PPG. Great post.

 

[Sprig]

Yep... pressure is normal and GOOD! It's just you agitating the fuel.

If you've looked at everything obvious, then it's time for a pressure and vac test..

Yes Andy!

And if the only complaint is a 'hiss' from the cap, nothing but nothing.

 

[pgg]

Also the carbs' fuel needle tang often needs tweaking upwards when installed and replaced new from a carb kit. Good luck!