Tuning old saw low on compression

[Grubs]

I have a 25-30yr old Stihl 010AV (little 14" home-owners saw) passed down from father to son that has done a *lot* of work and is getting a bit low on compression. I can *just* lift it by the starter cord if I have a steady hand! The saw was essentially retired but I'm tinkering with it again for some light bush clearing duties that are "beneath" my MS361's jurastiction.

I tune the saw first by ear, but then by throttle response. After tuning by ear the saw has a massive flat-spot on takeoff from idle. I make the idle richer until the flat spot goes away and the saw accellerates smoothly. The idle is then a little rough (though it will idle for an extended time without stalling ). In terms of setting the screw between the rich and lean stall points its more like only 1/5 of the way up from the rich stall point and 4/5 away from the lean stall point (hope that makes sense).

Is the flat spot I'm seeing indicative of worn rings/low compression or is my tuning by ear simply "off"?

Does worn rings/low compression lead to a leaner idle/starting mix or lack of fuel to start? Certainly the richer idle is easier to start.

FWIW - Spark is great and I installed a new carby, fuel line/filter, spark arrester approx 12 months ago so I'm quietly confident that my starting/accelleration issues are compression (or lack of it) related rather than anything else. I've been wrong before though.

I should replace it with an MS180 but im still recovering from my MS361 purchase (AU$1300). I like old machines... my car is old too - 37 yrs.

 

[romeo]

Yes, your rings are too worn. You are having to keep it rich to get enough fuel and oil on the rings to make compression. I am willing to bet its probably around 110 pounds of compression. I would just put a ring in it if the piston still looked good.

 

[Grubs]

Thanks for the reply. I found rings on ebay but whether shipping US to AU + the cost of a small cylinder hone makes it worth doing is another question.

Can anyone confirm that 36 x 1.5mm rings are the correct size for this 010AV (early version - orange body without chain brake)?

 

[romeo]

According to Mike Acres library it is a 36mm piston. I dont know about the ring thickness though.http://www.acresinternet.com/cscc.nsf/ed1d619968136da688256af40002b8f7/6f738e18994f7b8f88256ba200490c1d?OpenDocument

 

[timberwolf]

Best to pull it down and look at things before hunting for rings.

Hones are not the magic answer to worn cylinders. In general they make the problem worse. Minor scuffs can be cleaned up with fine sandpaper, aluminium stuck to the bore with muriatic acid.

If the rings are that far gone, the bore and piston might not be worth it. Check first.

 

[Scooterbum]

Best to pull it down and look at things before hunting for rings.

Hones are not the magic answer to worn cylinders. In general they make the problem worse. Minor scuffs can be cleaned up with fine sandpaper, aluminium stuck to the bore with muriatic acid.QUOTE]

Ditto.

Rings are correct 36mm X 1.5mm

Cylinders on the 010-011 were pretty tough,usually just a clean up.Careful with the muriatic around the ports.

 

[timberwolf]

Careful with the muriatic around the ports.

Good point, also watch right at the top 1/8 of an inch of the bore, often the plating is thin there and acid can find it's way under the plating.

 

[Grubs]

Thanks fellas. Much appreciated.

Originally I was planning to just keep tuning her richer and richer until she coughs her last cough, but after inspecting the bore and seeing that it all looks clean and smooth I'm investing the grand sum of $10 for rings.

As far as saws go it isnt much to write home about... but dad gets a kick out of seeing me still using it after all these years so I like to keep her running as a backup saw.

 

[Grubs]

R.I.P. little orange saw.

I installed the new piston ring today. Internals looked great for a 30yr old saw (010AV)

After installing the piston ring I turned her over a few times by hand to ensure everything was smooth and then with a new lease on compression started her up for the last time.

The conrod roller bearing completely disintegrated

I shut it down almost instantantly. The jug looks perfect but a small portion of the piston skirt broke off adding to the bearing debris that is now rattling around in the crank case. Perhaps a new conrod bearing and piston could rebuild her...but then I'd worry about the main bearings... where do you stop?

My local dealer has the MS180 on special this month.

 

[RED-85-Z51]

Wonder why the conrod bearing went? Did you remove the piston to put on rings?

If you can fish out all the rollers, a new bearing would be inexpensive and pretty easy to install.

The piston, id have to see it to make a decision, but if its just a tiny piece of the skirt, you may be able to dremel it smooth and be certain it is smooth in the cylinder, and round.

Ive torn down 4 cycles that had peices of the skirts sittin in the sump, only had one fail and it was a high high hours Kohler V-twin, large peice of the skirt broke off and the piston hooked the cylidner and it stopped.

 

[Grubs]

No I didnt take the piston of the rod. I just slid the cylinder off, and installed the new piston ring by hand and then slid the cylinder back on using my fingers to compress the ring. The cylinder has a taper at the base so it was easy to slide on. The piston doesnt have a locating pin for the ring so no probs there either. I put the gap on the side facing the exhaust but it is free to rotate.

Here are the pics - the damage to the skirt is more than I thought. The broken side is away from the exhaust port if that makes any difference.

I counted 12 rollers from the bearing. I cant find any trace of a bearing cage/race apart from the fine shrapnel that is all through the case and looking at the crank I cant imagine how this bearing is installed? Do the little rollers just sit in the grove on the crank with the rod just floating? (if so why did they jump out?) ... what does the bearing look like and how do you install it?

 

[bama]

If this saw is the same as the 011, they sit in the crank groove free(no cage). I learned to use grease to keep the rollers in place(between the rod and the groove) until the cylinder is back on the saw. Do all the rollers look good? Is that just debris from the skirt? Is it possible that the rollers came out a little bit during installation and then when the bearing fell apart, the skirt was damaged? Those rollers were FUN to get in place when I rebuilt the 011.

 

[RED-85-Z51]

If this saw is the same as the 011, they sit in the crank groove free(no cage). I learned to use grease to keep the rollers in place(between the rod and the groove) until the cylinder is back on the saw. Do all the rollers look good? Is that just debris from the skirt? Is it possible that the rollers came out a little bit during installation and then when the bearing fell apart, the skirt was damaged? Those rollers were FUN to get in place when I rebuilt the 011.

Its even more fun when there are 30 of em and they are 1/6 the size, on a craftsman top handle saw....:censored:

 

[Grubs]

Lazarus!

Thanks for the guidance bama and RED.

I flushed the crank case with 2-stroke mix to remove the fragments of piston skirt. Inspection of the roller bearings and crank/rod surfaces showed no damage so I reassembled the saw (after smoothing the jaged edges at the base of the piston where the skirt broke off). I used sticky bearing grease to hold the rollers in position and managed to get them all in place without dropping any back into the crank case.

1/2 a tank of gas through it already - mostly re-tuning it but a few cuts went well too.... Its ALIVE!