Carb diagnostic help needed

[atpchas]

Try replacing the fuel lines. A slight crack or pinhole might make it so the carb can't draw/ punp fuel. It would also explain why it happens with both carbs.

Excellent suggestion but they passed a pressure test. Thanks!

 

[river of smoak]

fresh gas ?

 

[atpchas]

Years back a neighbor asked if I would look at his Stihl string trimmer. Similar issue. The block behind the carburetor that also has ports for impulse and fuel had a crack that prevented cold starts.

that is possible, gasket could be installed in wrong orientation.

I've been burned by that before so I double-checked this one - not an issue, either crack or misalignment. Good ideas, though - thanks!

I'm making progress. Decided to give the AM carb a second chance and it appears to be working. Still too many pulls to start cold, but the machine is starting. The carb is also adjustable in minor way. Tweaking that is my next avenue of attack. Waiting hours between start attempts is getting old...

 

[atpchas]

fresh gas ?

Yes, that's the first thing I check every time - and is the issue more often than any other. Thanks!

 

[Old2stroke]

Number one cause of cold start problems with chainsaws is the operator is trying to start with closed throttle. Does that trimmer have a method of latching the throttle open to a fast idle position for cold starting, or are you just supposed to hold the throttle open while pulling it over like most trimmers? Trimmers never seem to have the throttle interlocked to the choke lever.

 

[atpchas]

Number one cause of cold start problems with chainsaws is the operator is trying to start with closed throttle. Does that trimmer have a method of latching the throttle open to a fast idle position for cold starting, or are you just supposed to hold the throttle open while pulling it over like most trimmers? Trimmers never seem to have the throttle interlocked to the choke lever.

This trimmer does not have a detent for the throttle, it's either set a high idle with the screw or manually hold throttle open while cranking. So far it seems that manually holding open the throttle is a hindrance, not a help. Perhaps I'm holding it TOO open but I'm not dexterous enough to hold it open only a little while pulling rope.

 

[dboyd351]

I don't think you should have to give it much if any throttle when your cold starting it with the choke on
It ought to fire with just the choke and no throttle. The opposite of choking a two stroke is to turn the choke off and hold it wide open. So yes, I think it's possible to try to give it too much throttle while choking it.
Wide open throttle can be affective in starting a flooded engine but you say it isn't getting gas, so it isn't flooded.
If you aren't getting fuel through the carburetor, no amount of choking or giving it throttle is going to help.
Maybe try taking one of the carbs apart and spraying carburetor cleaner through every orifice to make sure they are clear.

 

[dboyd351]

ultrasonic cleaners are very effective at carb cleaning if you gave access to one

 

[stihltech]

Not real familiar with these as they usually do not get repaired at the shop. BUT a 2 stroke is a 2 stroke.
There has to be a way for a fast idle, a choke or enrichment mechanism for a cold start, a good pulse, an air tight bottom end, good spark and fuel. And the exhaust has to get out (mud daubers love exhausts on trimmers!)

 

[atpchas]

I don't think you should have to give it much if any throttle when your cold starting it with the choke on
It ought to fire with just the choke and no throttle. The opposite of choking a two stroke is to turn the choke off and hold it wide open. So yes, I think it's possible to try to give it too much throttle while choking it.
Wide open throttle can be affective in starting a flooded engine but you say it isn't getting gas, so it isn't flooded.
If you aren't getting fuel through the carburetor, no amount of choking or giving it throttle is going to help.
Maybe try taking one of the carbs apart and spraying carburetor cleaner through every orifice to make sure they are clear.

ultrasonic cleaners are very effective at carb cleaning if you gave access to one

So far the AM carb is giving consistent results - cold start in 3-4 pulls with choke and no throttle. I can live with that.
Thanks for continuing to offer help.

 

[atpchas]

Not real familiar with these as they usually do not get repaired at the shop. BUT a 2 stroke is a 2 stroke.
There has to be a way for a fast idle, a choke or enrichment mechanism for a cold start, a good pulse, an air tight bottom end, good spark and fuel. And the exhaust has to get out (mud daubers love exhausts on trimmers!)

That's a good checklist for potential issues. My experience is that most (maybe all?) of the rotary or barrel carbs lack a high idle setting. This machine definitely lacks that feature. That may have to do with how fuel is supplied - butterfly carbs get fuel through various drillings in the venturi whereas all the fuel in rotary model comes in through the needle in the center of the barrel. What I've read is that the rotary design allows for a more even air/fuel mixture throughout the rpm range.
Thanks for the suggestions

 

[stihltech]

What is the make and model of the original carburetor? I am doing some research and need to know this info as different make rotary carburetors do a cold start in different ways.
Does your carburetor have another button on it somewhere? That is one method.

 

[stihltech]

Also are the starting instructions missing on the air filter cover? I know, dumb question, but a pretty common issue in a saw shop is a bad starting procedure.
We don't need no stinking owners manual!

 

[atpchas]

What is the make and model of the original carburetor? I am doing some research and need to know this info as different make rotary carburetors do a cold start in different ways.
Does your carburetor have another button on it somewhere? That is one method.

Here are pictures of the carb that was on the trimmer when it came to me. The online IPLs I've seen are not very helpful with only a part number and no manufacturer name.
Not clear on what you mean by "another button."

 

[atpchas]

Also are the starting instructions missing on the air filter cover? I know, dumb question, but a pretty common issue in a saw shop is a bad starting procedure.
We don't need no stinking owners manual!

I find it starts more easily if I ignore step 3 which asks for WOT.

 

[stihltech]

#1- Thank you for very good pictures.
They even want a fast idle to start. Too cheap to make a detent for it.
My guess, there is not enough fuel being drawn cold with the throttle open. For some reason the circuit is restricted or there is an air leak keeping it too lean. Pulling the needle up in the jet bore should allow more fuel. This is why other manufacturers use a choke or start button for cold enrichment.
Once running there is much more vacuum to pull flow.
Are there other holes in the bore of the carb to draw fuel? are they open? Kinda hoping I find a dead one no to dissect.

 

[Vintage Engine Repairs]

I'm working on a Ryobi trimmer (RY253SS). It refuses to start or even pop when cold. If given a shot of mix into the carb, it starts with 1 or 2 pulls, runs well and restarts readily. After sitting for a couple hours, again no hint of firing until a shot of gas is provided. It has spark (observed and hell, it runs), good compression (160 psi), internals of carb (diaphragms, screen) look good. A replacement carb (Amazon AM) behaves identically. What would cause it not to pull enough fuel to pop? It's not flooding, plug is dry as a bone when pulled after 10-15 pulls.
Thoughts?
If it matters, the carb is a rotary style, not butterfly.

From memory, the Ryobi machines use some
type of glass reinforced nylon intake manifold that can warp. This can cause a lack of impulse to the fuel pump drilling on the carburetor to move the fuel in the carb especially at lower rpm on starting.

Check if that intake manifold has warped and if you’re still getting good contact between the manifold, gasket, and carb.

If you put the new carb on and the new gasket that comes with it, you may find that for a while it will work better, until the gasket no longer makes good contact or the manifold warps more.

Something a bit like this - I’ve recently gotten a bit more “fancy” doing it on a lathe…

 

[Oletrapper]

Good morning Atp, I have read the entire thread, twice, and thinking the problem may not be the carb. You state lightly used 6-7 years, but you really have no way of knowing that unless perhaps the unit is yours. 160psi is in my humble opinion, very very high. 90 to 110 is normal on a brand new unit. You might want to pressure and vac test the crankcase. Leaking seals would cause loss of vacuum required to draw fuel into the cylinder. Once running all may be fine but first start attempt may be a problem. Also might want to remove the muffler and check piston/rings and cylinder walls. The barrel carb is a very simple carb, again in my humble opinion. Let us know what you find.
OT

 

[Old2stroke]

I find it starts more easily if I ignore step 3 which asks for WOT.

The problem with these carbs is, with the throttle closed, the intake vacuum can only pull "idle" fuel out of the main jet, not enough for the rich fuel fog required for cold starts. Step 3 only works if the choke is COMPLETELY closed and intake vacuum is correct, then the vacuum will be present right back to the choke and will pull a huge amount of fuel out of the open main jet. This also makes it easy to flood the engine as no air is getting past the choke so at the first sign of firing (or 4 or 5 pulls with no firing), the choke has to be opened. I have 5 trimmers, one with this type of carb and have worked on many other makes, the only one I can remember having a method of latching the throttle open for cold starting is the Stihl but ALL of them require the choke to be closed and the throttle partly or fully open for cold start.

 

[muddy42]

It sounds like not enough fuel is getting to the carb.

- You say the fuel lines hold pressure. can you describe this in more detail, are you applying pressure right back at the fuel filter or just from the purge bulb?
- Have you tried with the fuel filter removed or replaced the filter?
- Is the fuel tank vent working. Does it make a difference if you start with the fuel cap loose?
- have you tried a pressure and vac test on the crankcase? This is totally different from your compression test. A broken seal can cause all kinds of issues, this test should always be performed on any tricky repairs and before buying any new parts.