bar tolerances and metering fulcrum tolerances

[purdyite]

I bought a replacement chain for a bar on a Poulan saw; the bar is #530044693, 18 inch, for #91 chain which is 0.050" gauge. Caliper measurement is a consistent 0.060" along the length of the bar, on both sides. I assumed that the chain gauge should be 0.058"--so does that mean that bar width tolerances are .005 either side of the drive link? My concern is because wear on the bar looked as if the chain gauge might have been too small.

Second question: I have struggled with the metering needle adjustment on several carbs. It seems that it is more critical to be at or below the prescribed height of the lever ( I have the Zama and Walbro gauges) to prevent severe flooding. So if that is the case, how low can you set it before it's a problem the other direction? What is the tolerance setting (if any) on the metering lever? My brain thinks that the diaphragm probably stretches a little from the start, so pressure would cause the lever to be pressed a little farther than when the diaphragm is brand new, which means there has to be some wiggle room there. Or am I just overthinking this? Appreciate any helpful response.

 

[Philbert]

Bar groove sounds loose to me for .050 chain. Still have the receipt? Want to buy some .058 chain?

This illustration is from Oregon for judging bar groove wear - should apply for you as well.



Philbert

 

[grizz55chev]

I bought a replacement chain for a bar on a Poulan saw; the bar is #530044693, 18 inch, for #91 chain which is 0.050" gauge. Caliper measurement is a consistent 0.060" along the length of the bar, on both sides. I assumed that the chain gauge should be 0.058"--so does that mean that bar width tolerances are .005 either side of the drive link? My concern is because wear on the bar looked as if the chain gauge might have been too small.

Second question: I have struggled with the metering needle adjustment on several carbs. It seems that it is more critical to be at or below the prescribed height of the lever ( I have the Zama and Walbro gauges) to prevent severe flooding. So if that is the case, how low can you set it before it's a problem the other direction? What is the tolerance setting (if any) on the metering lever? My brain thinks that the diaphragm probably stretches a little from the start, so pressure would cause the lever to be pressed a little farther than when the diaphragm is brand new, which means there has to be some wiggle room there. Or am I just overthinking this? Appreciate any helpful response.

Don't know about out there in middle America but bar and chain combos for your poulan run somewhere around 25 bucks at Wally world, Tractor Supply, etc.

 

[purdyite]

I thought it was a little much also. I have the Pferd bar shaping thing and brought the bar back to good shape. I can get a chain pretty easy here.

 

[grizz55chev]

I thought it was a little much also. I have the Pferd bar shaping thing and brought the bar back to good shape. I can get a chain pretty easy here.

Those are throw away bars, not worth your time reconditioning.

 

[pioneerguy600]

As for your carb question,the spring under the metering lever should hold the diaphragm up preventing it from opening the needle off the seat as long as the diaphragm is supple and flexible. The metering lever can only travel downward so far until it bottoms out so no matter if the diaphragm is new or a bit stretched from age they will only move the needle off the seat a limited amount. If the carb will not function at the specified metering lever height then it likely needs to be thoroughly cleaned and a new kit installed along with a new spring for under the metering lever. However if the carb will function better as is with the lever set a little lower that should not cause any mechanical problem with the engine in a stock non modified condition.

 

[alexcagle]

When I rebuild carburetors on saws, I perform what I call a "bump test".

This is to verify that the carb needle's fulcrum spring is strong enough to resist the diaphragms slight movement from the inertial force of being bumped or jarred around.

Also during the rebuild, as I'm closing it up, I will squirt a little WD40 around the needle area to wet the tip of the needle so it can seal during the air pressure test.

The test is a light tap on a work bench, with the carburetor oriented in it's normally installed position, and under 8psi of pressure .

I start with a light tap, and each successive one is a little stronger, but not more force than, say... if you were to drop it from 3" hight on a wooden bench.

If the pressure drops, then see if several cosecutive tests give the same results. It may be necessary to lower the fulcrum spring lever height in to compensate for the diaphagms slight"bounce", to give it a buffer so it will hold pressure while being transported.

If the diaphragms material is too flexible, and it moves enough to overcome the fucrum spring pressure, then it is more likely that transporting the saw could cause the carburetor to leak fuel into the engine.

Over time the pliability of the diaphragm can change and affect the performance of the carburetor.
A diaphragm that that is crinkly like cellophane should be replaced because they tend to tilt back and forth during movements instead of in straight linear movements.

 

[Chris-PA]

Those are throw away bars, not worth your time reconditioning.

I disagree - if I can spend a few minutes repairing something that will still be useful it beats driving to the store and spending money. And it doesn't take long to crimp the rails closed a bit and file off the surface.

Look at some old Popular Mechanics issues from the 30's to the 50's, and there were always tips on repairing all manner of things people just throw out now.

 

[grizz55chev]

Believe me when I say I'm not one to throw something away that can be fixed. My truck has 294k on it, my wife's car has 234 k on it, I don't own a saw that I didn't put together from parts, I fix anything I've ever owned. If the bar in question can be fixed without spending half the day to machine and massaging, Then yes by all means I'll use it. The chain alone will cost you around 15$, so for 25$ I'll get both and call it a day.

 

[purdyite]

Thanks Alexcagle for the description of the interesting carb test. That tells me that there should be some room for play, and to err on the side of the lever being low rather than high. I'll have to check that out.

Around my place, if it doesn't work it can't stay. But I spend a lot of time giving things the benefit of the doubt--the saw is a cheap Poulan, the bar is stock, and not very worn. It's dressed and ready to go after about 5-10 minutes of TLC. I fix stuff all the time rather than dump it and buy new.

 

[GeorgiaVol]

If it's a Poulan bar check your local Wal-Mart. I got 3 14" bar and chain combos from them recently that were on clearance for $11 each.