Project K970 Concrete Saw

[Automender]

I am totally new to 2 cycle engines but have never had any of my cars in the shop for 45 years unless it was a warranty service. I recently got on a kick to learn about 2 cycle engines and have a good running concrete saw. I purchased a Husky K970 16" cutoff saw with a 16 inch diamond blade. I joined the forum and read maybe a hundred or more threads to get a basic understanding.

The saw as bought would start and run but would not idle. Looked like the saw was flooding because the plug was always wet after the saw stalled out. I ended up ultrasonically cleaned and rebuilt the carb on this saw and set the needle valve level. Saw ran better and after adjusting the carb it was running pretty good but lagged a little on acceleration but I figured a 16 inch blade was hard to get started. I also replaced the cutter head because the bearing had ovaled out the bearing housing. I got a NOS for $35 on fleabay.

After I ran the saw for a short time I saw oily deposits around the base gasket. I check compression and it was only 75 psi. I pulled the jug to fix the base gasket and found the jug and piston in what I would say good shape. I inserted one of the piston rings into the bore and there was about 3/16 inch gap in the ring ends. I ordered a set of caliber rings and inserted those in the same bore and had only .0015 inch gap. Put the saw together with a touch of Motoseal on the base gasket. Check compression today and on the first pull it rose to 110 psi and couldn't pull again unless I pressed in the decompression valve.

Had a question on how people can pull the starter rope on saws with 150 to 175 psi compression. I was worried about messing up the recoil starter trying to test compression. this saw has a 56mm piston.

I attached a pic of both old and new rings in the bore. The pic of the piston shows some discoloration but no grooving in the skirt.

Any comments on the piston and cylinder conditions are welcome since I am new to this. I did buy after market cylinder kit from HL supply but may not use it right now. Just wanted to find out the effect of new rings. I know I should have did a pressure and vacuum test from the start but did have a tester. I recently bought one for future testing.

 

[Automender]

Saw started in three pulls and smoked until all the prelube on the rings, cylinder and bearings burned off. Seem to idle very good and after it ran for a few minutes the rpms rose much higher. I had the idle adjust screw turned in to max before since I could not increase rpms to get the blade to engage. Before the new rings I could not adjust it high enough to just engage it but after the rings and oil cleared I had to back down the idle. I was trying to think what could cause the idle to be too low. Maybe the impulse tube was not providing the carb with enough pressure to pump fuel. I am assuming the purpose of the impulse is the drive the diaphragm fuel pump. Any thoughts?

 

[Ford3000]

What do you mean by idle screw, are you sure you are not
adjusting the low screw on the carb.

 

[Automender]

What do you mean by idle screw, are you sure you are not
adjusting the low screw on the carb.

No it isn't the idle mixture screw it is the idle screw that holds open the throttle plate. I had to screw it all the way in before but after the re ring I had to back it out like it should be. It is the tapered one in the pic.

 

[Ford3000]

If you have the engine idling after start up, and the rpm
keeps rising, you could have an air leak, you need to do
a vac and pressure test, to rule this out, have a look on youtube
for how to vac test a two stroke engine.

Here is one such test on a Stihl chiansaw, same principles apply.

Had you the carb apart for cleaning, with the low and high jet removed.
It may be the angle but the gasket under the top plate on the carb looks
thicker on one side, are the screws all tightened up equally, not over tight or
you will strip the threads. Same with any other gaskets or diaphrams you removed.

 

[jacob j.]

Maybe the impulse tube was not providing the carb with enough pressure to pump fuel. I am assuming the purpose of the impulse is the drive the diaphragm fuel pump. Any thoughts?

If you're getting good impulse now then you probably were before. You are correct - the impulse actuates the fuel pump diaphragm. If you had to turn the idle screw to max before, that was likely due to low compression. A modern two stroke needs a certain amount of compression to even run.

I had a customer once who had a Poulan with low compression - he could start it and keep it running by keeping his finger on the throttle but it wouldn't idle at all. It had about 80 psi on my Lisle tester.

 

[Automender]

If you're getting good impulse now then you probably were before. You are correct - the impulse actuates the fuel pump diaphragm. If you had to turn the idle screw to max before, that was likely due to low compression. A modern two stroke needs a certain amount of compression to even.

I had a customer once who had a Poulan with low compression - he could start it and keep it running by keeping his finger on the throttle but it wouldn't idle at all. It had about 80 psi on my Lisle tester.

Thanks for the reply. I was wondering what happened to all the blow by from the worn rings figuring some gets down into the crankcase. I have to do some more tuning on the carb since the re ring. I think I was getting some major four cycling when leaving off the throttle which wasn't the case before or at least I think it is four cycling from what I read.

 

[Automender]

If you have the engine idling after start up, and the rpm
keeps rising, you could have an air leak, you need to do
a vac and pressure test, to rule this out, have a look on youtube
for how to vac test a two stroke engine.

Here is one such test on a Stihl chiansaw, same principles apply.

Had you the carb apart for cleaning, with the low and high jet removed.
It may be the angle but the gasket under the top plate on the carb looks
thicker on one side, are the screws all tightened up equally, not over tight or
you will strip the threads. Same with any other gaskets or diaphrams you removed.

That picture was just one I took before I took the carb off. I am confident the carb is back together right. Stupid me I ended up ruining the check valve in the carb that goes to the primer bulb thinking it was plugged but it was the rubber disk in what I thought was an orifice. I ended up buying a OEM used carb from ebay($20) and cut out the check valve and installed it in the original carb. So I had a carb to reference. My problem was too low of an idle with the idle speed screw fully adjusted to max. And I adjusted the mixture screw for max rpms which I know was not correct.

I don't think I have an air leak because when I pulled the plug it was heavily coated with carbon and the piston top was also layered with carbon also. I would like to do a pressure test but I was worried to pull off the muffler after my experience with replacing one on another TS400 which I had to torch off because the bolts were froze. Besides on these saws you need to pull the whole cutter head off to get clearance.

 

[Ford3000]

If the saw ran you would get deposits on the piston top,
and a wet plug if it was rich, a vacuum test is to test the
case for leaks, the case on a two stroke needs to have no
pressure escaping around the crank seals, the bottom of the
cylinder where it mates up with the case or past the rings on
the piston, that is why people suggested a vac and pressure test,
as the piston rises it will be pulling a vac in the case, if the seals
leaked the upward stroke of the piston would not pull in much fuel,
similarly the downward stroke of the piston should cause pressure
to build in the case below the piston and push the previously inhaled
charge up the transfer ports into the cylinder, if there is a pressure leak
this won't happen as it should.

 

[Ford3000]

Saw started in three pulls and smoked until all the prelube on the rings, cylinder and bearings burned off. Seem to idle very good and after it ran for a few minutes the rpms rose much higher. I had the idle adjust screw turned in to max before since I could not increase rpms to get the blade to engage. Before the new rings I could not adjust it high enough to just engage it but after the rings and oil cleared I had to back down the idle. I was trying to think what could cause the idle to be too low. Maybe the impulse tube was not providing the carb with enough pressure to pump fuel. I am assuming the purpose of the impulse is the drive the diaphragm fuel pump. Any thoughts?

Yes, the idle would rise when the rings bed in on the cylinder, and you would
have to reduce the idle accordingly.
If the saw is starting, running, idling and four stroking then it would seem your
build went to plan. If the cylinder was free of damage / transfer and the likes then
the saw should be fine. We could go into great detail like measuring the piston, the
cylinder bore to build a clearer picture regarding wear and future issues where said wear / clearances would play their part, but unless you have suspicions regarding your build there is no point, plus you have it all together now.

 

[Automender]

Used the saw this weekend to cut a five inch thick slab. It performed pretty good just seemed to slow down a few times when the blade was at it's full depth I'll be checking compression since the rings should start seating. Satisfied based on $175 for the saw, $14 for rings, $40 for new old stock cutter head and $35 for carb work. Total $270

I will start to work on a scrap TS420 I pick up for $50