Poor/No idle = Air leaks. Usually bearing seals? Best/adequate fix?

[ijpom]

Saw in question is the "free to me " kind. Craftsman 2.4 from 1999, not strato-charged.

P&C looks good, fuel system refreshed (new line and carb rebuilt), and starts ok when cold, and will get to full speed when off-cold.
When warm, running at full speed is good, but will idle down and die if not revved. Adjusting low mix screw and idle setting does not make this better.

My reading tells me it's an air leak and probably through the bearing seals.
I don't have pressure testing equipment for leak down, so that leaves me with trying to squirt soapy water on the seals while keeping it running?
Or just replace/repairing the seals, and see if that makes it better.

So the question is regarding the fix. Best fix would be to source new seals. Yes.
Does anyone just RTV the entire seal to housing interface with success?

For reference, the following picture is when I overdid the RTV on the Craftsman stratocharged 42cc after a P&C replacement. Don't know if seals were good or bad going in. Result is good.

 

[TBS]

Air leaks will make it idle high or rev up, it sounds like to much fuel at idle.

 

[ATpro]

Try picking it up by the crank cord off the floor, if you can't pick it up off the floor then its a compression problem, you should be able to pick it up and the saw should cycle down slowly. If it cycles down to fast its probably a compression problem. It sounds like the crank seals, had an old saw that did the same thing and it turned out to be the crank seals. Run like a scalded dog top end, but would loose some torque in the cut, and wouldn't idle if not revved.

 

[Vintage Engine Repairs]

A pressure and vacuum tester is an important diagnosis tool for 2 strokes (and 4 in certain applications).

As mentioned above if the saw loads up at idle that’s commonly a rich L symptom. When you pin the trigger from idle, does it blubber / splutter? Or does is bog / dip in revs before going to WOT? how many turns out are your screws (H & L) on your carb ?

 

[ijpom]

Try picking it up by the crank cord off the floor, if you can't pick it up off the floor then its a compression problem, you should be able to pick it up and the saw should cycle down slowly. If it cycles down to fast its probably a compression problem. It sounds like the crank seals, had an old saw that did the same thing and it turned out to be the crank seals. Run like a scalded dog top end, but would loose some torque in the cut, and wouldn't idle if not revved.

Hey AT,
Heard of this method before and have no data behind backing up my tests, so did I all the saws that were in arms length:

BlueMax 45cc, runs great, falls slowly when lifted.
BlackMax, 38cc, new saw (never run), falls slowly when lifted.
Craftsman 42cc strato-charged, New P&C, runs great, falls slowly when lifted.

Craftsman 42cc non-strato, can't lift saw with cord. Making it worst of bunch by a margin.
From exhaust side, P&C looks great. Is there another place where these saws typically loose their compression? Intake side?
Thanks

A pressure and vacuum tester is an important diagnosis tool for 2 strokes (and 4 in certain applications).

As mentioned above if the saw loads up at idle that’s commonly a rich L symptom. When you pin the trigger from idle, does it blubber / splutter? Or does is bog / dip in revs before going to WOT? how many turns out are your screws (H & L) on your carb ?

Hi VER,

Tried starting out at about 1 turn out each needle, but worked my way to much heavier settings for top end and idle. 1.5 turn for L, 2 turns for H. I assume this is too heavy to be normal.
That was part of my indication about air leak.

Less than these settings, pinning trigger makes engine bog. At these settings, idles Ok until hot, then won't.

Compression? as suggested above.
I'm probably going to re-tear into carb again, and then use for parts if compression loss is fatal..
Thanks

 

[Vintage Engine Repairs]

Nothing wrong with 1.5 on L.

2 on H seems a little more than normal, but every saw, carb and manufacture has slightly varying needle positions. That or it just could be that you’ve set it a bit too rich.

One saw I worked on was 4.5 turns out on H and it was a walbro carb. It was dependant. So, use the screw positions as an indication of what may be the issue, but don’t rely on it as your primary means of diagnosis alone.

Can you upload a video of it starting, running and idling and what happens when you pin the throttle? Is it an original carb or is it a Chinese aftermarket?

 

[SteveSr]

Tried starting out at about 1 turn out each needle, but worked my way to much heavier settings for top end and idle. 1.5 turn for L, 2 turns for H. I assume this is too heavy to be normal.
That was part of my indication about air leak.

You can also aim brake cleaner with the straw to suspected leak locations while the saw is idling. If the saw dies you have a leak.

 

[MacAttack]

Air leaks will make it idle high or rev up, it sounds like to much fuel at idle.

I agree with this...when I've had saws that wouldn't idle it was often too rich on the low end. So even turning the idle adjust all the way in wasn't enough air. This is why it doesn't sound like crank seals / air leak to me.
You mentioned carb rebuild, did check the metering lever / needle adjustment?
When the saw is running, and you are turning the low speed needle out, is it acting rich and smokey? Or lean?

 

[ijpom]

Nothing wrong with 1.5 on L.

2 on H seems a little more than normal, but every saw, carb and manufacture has slightly varying needle positions. That or it just could be that you’ve set it a bit too rich.

One saw I worked on was 4.5 turns out on H and it was a walbro carb. It was dependant. So, use the screw positions as an indication of what may be the issue, but don’t rely on it as your primary means of diagnosis alone.

Can you upload a video of it starting, running and idling and what happens when you pin the throttle? Is it an original carb or is it a Chinese aftermarket?

Hi Ver,
Thanks for your input.
Tried to make a video for the purposes of audibly diagnosing idle and bog down, but could never get it to idle well enough to get a video. Even cold. Forever pumping the throttle.

At this point I'm assuming low compression

I agree with this...when I've had saws that wouldn't idle it was often too rich on the low end. So even turning the idle adjust all the way in wasn't enough air. This is why it doesn't sound like crank seals / air leak to me.
You mentioned carb rebuild, did check the metering lever / needle adjustment?
When the saw is running, and you are turning the low speed needle out, is it acting rich and smokey? Or lean?

Hi Mac,
Thanks for your input.
I did end rebuilding the carb again, but despite a replacement of a warped valve gasket, there was no discernable improvement in idle performance.
I'm no expert on measuring the metering lever, I just put in the new pieces (needle, lever, spring, etc) and give it a rough once over to ensure the lever in kind of level with the plane of the housing, and that pushing the lever allows the needle valve to open.

It did lead me to a discovery of an air leak from the primer bulb, and other unknown place.
Tank to carb: solid fuel flow with no air bubbles
Carb to primer: air bubbles in fuel (picked up air in carb; unexpected by may mean nothing?)
Primer to tank return: air bubbles in fuel, explained by the cracks in primer bulb.

Don't know if that is indication of anything?

I'm probably going to get some kind of basic compression tester, and test this saw for what I now assume is low compression. Saw is drained of fuel for winter test and disassembly as required. Since it's just an 'extra' saw, no big effort will go into rebuilding it, instead it will be donor parts for other saws.

 

[SteveSr]

I'm no expert on measuring the metering lever, I just put in the new pieces (needle, lever, spring, etc) and give it a rough once over to ensure the lever in kind of level with the plane of the housing, and that pushing the lever allows the needle valve to open.

BAD Move. New metering levers are NEVER adjusted properly. That is your job when you replace it. You can also reuse the old one which may be set correctly but ALWAYS check it as someone may have monkeyed with it before you got it.

It did lead me to a discovery of an air leak from the primer bulb, and other unknown place.
Tank to carb: solid fuel flow with no air bubbles
Carb to primer: air bubbles in fuel (picked up air in carb; unexpected by may mean nothing?)
Primer to tank return: air bubbles in fuel, explained by the cracks in primer bulb.

There should be NO bubbles in the fuel coming from the tank. This would indicate that you are sucking air somewhere. You can test by removing the fuel pickup and pressurizing the fuel line. Carb / fuel line should hold 7-10 psi indefinitely. If it leaks down you have a leak to find and repair.

 

[ijpom]

BAD Move. New metering levers are NEVER adjusted properly. That is your job when you replace it. You can also reuse the old one which may be set correctly but ALWAYS check it as someone may have monkeyed with it before you got it.


There should be NO bubbles in the fuel coming from the tank. This would indicate that you are sucking air somewhere. You can test by removing the fuel pickup and pressurizing the fuel line. Carb / fuel line should hold 7-10 psi indefinitely. If it leaks down you have a leak to find and repair.

Hey Steve,
Can I get a clarification on your experience with air bubbles in fuel lines. I would replace line that added air to the carb run, but these secondary lines seem less critical, and I'm told that a bad primer bulb has no bearing on a running engine.
Thanks

 

[SteveSr]

Hey Steve,
Can I get a clarification on your experience with air bubbles in fuel lines. I would replace line that added air to the carb run, but these secondary lines seem less critical, and I'm told that a bad primer bulb has no bearing on a running engine.
Thanks

Correct, purge valve discharge is less important unless it is leaking out all over the tool. Purge bulbs are also optional as long as the check valves in the purge assembly are functioning properly. All the purge does is pull fresh fuel from the tank through the carb and then discharge it back to the tank.

 

[ATpro]

The fact the saw failed the pick up test tells you that it is a compression problem plain and simple. Before you spend time rebuilding carb's and such you have to find the compression problem. Messing with anything else is a waste of time. If it was the carb or carb settings it wouldn't loose compression with the pickup test. With the saw failing the pickup test you will never be able to adjust the carb because the low compression affects the function of the carb and any adjustment you make won't make any difference. Find the compression problem and fix that first, then you can make adjustments to the carb as needed.

Compression problems could be in the spark plug seal seat, wore out power head, broke rings, leaking cylinder gasket, hole in piston, wore out rings and leaking crank seals. My bet is crank seals or wore out power head or rings. The problem with inspecting the cylinder through the exhaust port it will only tell you what one side of the cylinder bore looks like.
Don't know exactly who made the craftsman saw but Poulan made many of them with the craftsman name on them and parts for them were interchangeable with Poulan saws.

 

[Old2stroke]

Agree completely with lack of compression being the main issue. Air bubbles in the fuel line from carb to primer can mean that the main nozzle check valve is not closing properly, a fault that can also lean out the low speed to the point where it won't idle.

 

[Okie]

I did not read all of this thread but if you are going to LOGICALLY repair a chainsaw you need to think about buying or building yourself a pressure tester and a compression gauge.
One of the first thing I do is pressure test the block if one is having idle issues. No need in flogging the carb if the block has a air leak.
You can build a pressure tester for few bucks using a blood pressure bulb of buy a pressure/vac Mityvac. If you piddle around much trying to repair mechanical stuff, knuckle down, hump up and get a Mityvac.

I very rarely do a compression test unless it feels weak and is hard to start, but I do remove the muffler and see if a saw has cancer chewing on the piston/cylinder.
Lots of good info on this site about building or buying both.