How does 2 stroke primer work please?

[JOEPRO]

Talking about small 2 stroke equipment like con saws etc.

I have 2 now with primer issues. A BR600 blower which was taken apart by someone else so I need to understand the system and check fuel lines are correct way around etc. And a Stihl hedge trimmer which won't prime.

Do these generally all work the same way? Do they draw fuel under vacuum through the carb to clear any bubbles? Or do any of them require or work under pressure in the fuel tank?

Any clear guidance greatly appreciated, never really fully understood how they work and would like to!

Take this one for example, best to test the lines with a vacuum tester? Assuming it works by drawing vacuum that is.

 

[addjunkie]

 

[Seadog]

I thought they pushed. On my Partner chainsaw and Mcallister hedge trimmer I've discovered that they pull

 

[Migmac]

It’s technically a purge bulb, pulls the fuel to the carb and pushes air back to the tank

 

[JOEPRO]

So I guess in terms of diagnosis then using the primer bulb on a cup of fuel should show if it can draw fuel in and purge the air.

If it can then the next step would be to test on the engine in question. I think if it draws air then either the carb or fuel lines upstream must then be suspect?

For the BR600 I wonder is the priming issue a carb issue. Pulling air form somewhere. In which case thinking about how to rule that out maybe it's as simple as dunk the carb in a jar of fuel and re - run the test. If it can then fill the primer bulb and purge air then the carb must be allowing air in.

If that makes sense - in real terms of how to test this.

 

[redlig]

So I guess in terms of diagnosis then using the primer bulb on a cup of fuel should show if it can draw fuel in and purge the air.

If it can then the next step would be to test on the engine in question. I think if it draws air then either the carb or fuel lines upstream must then be suspect?

For the BR600 I wonder is the priming issue a carb issue. Pulling air form somewhere. In which case thinking about how to rule that out maybe it's as simple as dunk the carb in a jar of fuel and re - run the test. If it can then fill the primer bulb and purge air then the carb must be allowing air in.

If that makes sense - in real terms of how to test this.

Don't forget the drop of Acetone into the carb air inlet just to prove everything else is working.

 

[Seadog]

Remember that it only works in one direction. That may be obvious, but could catch you out.

 

[JOEPRO]

So for the HS56C hedge trimmer I just got it in the post today. I bought it spares or repair basis from flebay due to it not priming fuel.

I found this first



It couldn't be that easy right?

It wasn't.

First off the primer bulb wouldn't do anything. Took off one hose to carb and primed again. It squirted fuel out. Towards the carb... that doesn't seem right.


Put it back on and same thing. Pumped up hard and would not push fuel into the carb at all.

Reversed the hoses and then it would prime as normal. That is, with the fuel hoses pulling fuel through the carb.

So somehow someone must have been in there before and messed up the hose direction?

Anyway still no life in it and this is what I believe is the real reason! Also has great spark. But compression is too low for it to fire IMO.



So I'll have to tear it down, hopefully the original cylinder walls are not damaged and I will get away with a quality aftermarket piston set for less than £20. Golf brand.

 

[angellonewolf]

Snap I've got one I'm playing with

Mine I think is carb related it's fires when fuel down bore And did run ruff for more than a few seconds so with luck I've got a carb kit on the way should be here in a few days

 

[Kram]

I have a cheap Mcculloch saw that was intermittently not primkng and sucking air through the bulb. Its currently in bits.

 

[Seadog]

I have a cheap Mcculloch saw that was intermittently not primkng and sucking air through the bulb. Its currently in bits.

Mine's a McCulloch, not Mcallister

 

[dieselsamurai]

I think at most points during the year I have some POS 2 stroke in bits shame they cant be 4 stoke

 

[JOEPRO]

Snap I've got one I'm playing with
View attachment 466412

Mine I think is carb related it's fires when fuel down bore And did run ruff for more than a few seconds so with luck I've got a carb kit on the way should be here in a few days

Would be interested in the compression reading from that one if you had a chance. Although I know mine is waayyyy too low in any case.

 

[angellonewolf]

Would be interested in the compression reading from that one if you had a chance. Although I know mine is waayyyy too low in any case.

should be able to do over weekend ruff as s**h at mo only managed a 3/4 day at work today had to come home

 

[8ob]

Would be interested in the compression reading from that one if you had a chance. Although I know mine is waayyyy too low in any case.

On the small Stihls you will struggle to get them running on anything less than about 80psi, ideally more than 100psi.

Bob

 

[JOEPRO]

should be able to do over weekend ruff as s**h at mo only managed a 3/4 day at work today had to come home

No bother and I hope to get up around 6 tomorrow and have this one tore down and parts ordered before work

 

[JOEPRO]

On the small Stihls you will struggle to get them running on anything less than about 80psi, ideally more than 100psi.

Bob

Yes prob should be right around 150psi brand new. Which is what my re pistoned MS201 was putting out as an example and many others all seem around that with new cylinders/ rings.

Then they wear down and under 80 psi is the trouble zone as you say. I looked with a borescope and from what little I can see of the cylinder walls there is no damage. So will see if the original cylinder is salvageable as that is always the preference quality wise.

 

[Kram]

What compression tester do you use for these small engines? I have an old one for cars but doubt its any good in 25cc 2T's.

 

[angellonewolf]

Yes prob should be right around 150psi brand new. Which is what my re pistoned MS201 was putting out as an example and many others all seem around that with new cylinders/ rings.

Then they wear down and under 80 psi is the trouble zone as you say. I looked with a borescope and from what little I can see of the cylinder walls there is no damage. So will see if the original cylinder is salvageable as that is always the preference quality wise.

ive had quite a few under the 100 but only just they run fine 90/95 much under that its rings or pistion and rings if the bore is ok you can get a few marks out with wet and dry wiping round and round the bore if they are very small ive done a few in the early years dont bother now

 

[JOEPRO]

What compression tester do you use for these small engines? I have an old one for cars but doubt its any good in 25cc 2T's.

This one here

Post in thread 'Today I acquired...........' https://www.mig-welding.co.uk/forum/threads/today-i-acquired.50192/post-2411561

Works great and very accurate repeatable results. The adopters in the box work for everything