TS420 Project Cutoff Saw

[cookies]

What does a pop off test accomplish? The sealant is a heat activated epoxy. Haven’t seen anything yet that affects it.

pop off test confirms the needle will open if the fuel tank over pressurizes, look at the needle tip under 10x magnification.
to reduce squish I like a glass table top and 8-10 inch circular sticky backed 220 grit. It can take a half hour to carefully cut .010 using this method but I find it works well at adjusting out of square base to squish band cylinders and too much squish without needing a lathe. You can go rougher at 120-150 grit then switch to 220 for finishing touches.
Stick with the hand held pump gauge as the last thng you want or need is a oopsy 90 psi shot into the case not to mention I like to leave it for half hour rotating the crank a little every so often often during lunch breaks with the saw mostly assembled to make sure intakes and impulses do not leak.
That carb is just nasty!

 

[Automender]

pop off test confirms the needle will open if the fuel tank over pressurizes, look at the needle tip under 10x magnification.
to reduce squish I like a glass table top and 8-10 inch circular sticky backed 220 grit. It can take a half hour to carefully cut .010 using this method but I find it works well at adjusting out of square base to squish band cylinders and too much squish without needing a lathe. You can go rougher at 120-150 grit then switch to 220 for finishing touches.
Stick with the hand held pump gauge as the last thng you want or need is a oopsy 90 psi shot into the case not to mention I like to leave it for half hour rotating the crank a little every so often often during lunch breaks with the saw mostly assembled to make sure intakes and impulses do not leak.
That carb is just nasty!

I used an ultrasonic cleaner to clean up the carb before I ended up opening it up. I capped both ends off so the water based cleaner would not penetrate the carb. It cleaned up nice. I would have not opened it up but I could not get the needle valve to pop open and didn't want to go over 25 psi. I figure the needle was struck, but it wasn't. Since I have no history on this saw I figured I should just do a rebuild.
When I pressure tested the case I actually had pressure on it the next day but I didn't turn the crank. I like that idea.
I have a lathe but it would take a long time trying to chuck up the cylinder right without making some sort of adapter. The cylinder bolts don't go through the top, they come in from the bottom of the case. The jug isn't a flat bottom, It had tabs that go into the case. That makes the sandpaper method harder. I guess you can cut a hole in the glass and sandpaper.

 

[Automender]

What does a pop off test accomplish? The sealant is a heat activated epoxy. Haven’t seen anything yet that affects it.

I figured the test would determine if the needle valve was leaking and also if it was struck shut without opening the carb. The service manual doesn't actually have a spec on the pressure like on a Walbro carb on a Husky saw.
Is there a substitute for that epoxy, like Motoseal or just regular epoxy?

 

[sawfan123]

I figured the test would determine if the needle valve was leaking and also if it was struck shut without opening the carb. The service manual doesn't actually have a spec on the pressure like on a Walbro carb on a Husky saw.
Is there a substitute for that epoxy, like Motoseal or just regular epoxy?

You’re doing an awful lot of work to not open carb. Even just replace it.
As for the sealant, no. Nothing lasts.

 

[Automender]

You’re doing an awful lot of work to not open carb. Even just replace it.
As for the sealant, no. Nothing lasts.

Pressure test only takes a minute so that seemed worth while to verify function. But that didn't work so I learned an important lesson. Not all carbs can be tested for needle valve function. Maybe I will just keep increasing the pressure to see if it eventually pops. Yea, since this is my first total rebuild it is a learning experience and I am doing more than it is worth. $30 will buy me a new aftermarket carb and I have $10 into a carb rebuild kit.

 

[Automender]

Finally got back to my winter project. Everything back together and finally filled tank with fresh mix. Tried to prime the carb but would pull any gas into the carb. I am a little confused on my findings. I connected the primer to the carb which I removed from the saw and was able to pull gas into the carb through a hose attached to the fuel inlet and into the tank. I found the new fuel line was a little long and may have been creased. Shortened the line and still no fuel.
Decided to pressure test the top of carb with my mityvac connected to primer connection and could hear an air leak. Placed carb in water and noticed air coming out of the main jet which I think is normal. When the carb was installed I hooked up the mityvac to the primer connection and tried to pull a vacuum to get gas into the carb but no luck. No vacuum and no fuel. I need to spend a little more time troubleshooting.
Need to restart troubleshooting. Need to try connecting a fuel line to the inlet of carb and primer bulb to the primer connection and try again. If it does pull gas then possible that the fuel filter is totally plugged. If it doesn't then I have a air leak in the lines somewhere. I did pressure test the fuel line from filter to needle valve and that was air tight.
Does the main jet check valve have anything to do with the primer circuit? I am thinking it does.

 

[frank_]

i usually wet the filter with fuel, to rule out any other issues, and to confirm its a carb problem

 

[Isidro]

That's definitely detonation pitting on the piston crown. Either it was run too lean or the timing was too advanced or it overheated (i.e. blocked air flow, dirty fins). Or more than one. It led a hard life.

In general terms, if saw is overheating the timing is too far advanced? If the saw is overheating and timing is correct then you should retard the time? Lets assume that this saw has no other issue accept timing and overheading. Not sure that is realistic an assumption.

 

[Automender]

More testing today yielded a good purge bulb and fuel hoses but still wouldn't pull enough vacuum to prime the carb. Not sure of what the item is called but the check valve that is in the venture but it leaks vacuum when I use the primer. I can't get access to the part because it is covered but a plug, not a welch plug but a solid metal one. Ordered an aftermarket carb from Amazon and it will arrive tomorrow. Didn't want an aftermarket carb but didn't want to layout $130 for an OEM, aftermarket was $25 with fuel, air filter and sparkplug. See if it works!!

 

[Automender]

Got the aftermarket carb and installed it. Carb looked of good quality but see if it works. Saw came to life after being committed to a scrap pile and strip of parts. Looked like the one bearing cage was the real issue but the carb main jet check valve I believe was the issue that condemned the saw. Couldn't prime the OEM carb but AM primed easy. The only issue is the idle is too high and can't adjust it down to disengaged the clutch. Ended up spraying the inlet boot area and idle dropped down. Looks like a boot seal issue or impulse tube. I pressure/vac tested engine but not with the boots or impulse tube. Since the saw revs up smooth and runs smooth my guess it isn't a leaky carb. P/V tested the carb also.
I did notice the carb bottoms out on the inlet plate so not sure if the bottom inlet boot is getting the proper squeeze. Maybe the OEM boot has lost it thickness and doesn't seal right.
Not sure if anyone has experience with the Stihl boot clamps but they seem to be tightened to the end of the travel which bothers me a little also. On my Husky K970 I added a ring of card stock to the inner diameter of the clamp to give it more squeeze. May also do that to the two on this saw. Hate to re test the saw with the boots and impulse tube installed but that is probably the right thing to do. Need to make a test plate.

 

[Automender]

Getting closer to pressure testing to find out the high idle issue source. Made a test plate from 1/4 inch aluminum and a old carb inlet tube. I drilled a hole and heated the plate and inserted the tube and when cooled it was in really tight. I added a little epoxy just for good measure. Maybe tomorrow I will slip a inner tube piece of rubber in between the muffler and the cylinder. Should have made this plate when I did my first pressure test.

Not sure why the factory test plates have the impulse tube and carb holes cut into them.

 

[Automender]

Decided to pressure test today but had problems putting the intake boots back in to the carburetor plate. I just pulled the boots off the cylinder and was going to pull them into the plate and then push the boots on. I found a nice little crack in the bottom boot. I had ordered a bottom boot because the old one didn't seem to have a thick flange to give a good squish to the lower carb face. I replaced the boot and decided not the pressure test the saw. I also removed the spacers on the boot clamps so I could tighten them a little more. Popped up on the third pull and fired up on the first pull without the choke. Idle was low enough to stop the blade and I tweaked the idle mixture a little to make sure it was a little rich. Need to do some cutting to really get it right.

 

[stihltech]

Stihl does not recommend a pop off pressure. Pump up to 7 PSI and it should hold. I will tap the inlet lever and see if it holds after dropping. The important thing is that the needle seals and the inlet lever moves freely.

 

[Automender]

Stihl does not recommend a pop off pressure. Pump up to 7 PSI and it should hold. I will tap the inlet lever and see if it holds after dropping. The important thing is that the needle seals and the inlet lever moves freely.

Thanks for that input. I guess I found out that there was not a pop out pressure even past 15 PSI. A walbro carb I worked on for my K970 cutoff saw did have a pop off pressure. I had to replace the TS420 carb because that check valve in the main "jet" would not seal when I primed the carb. I may try to push out the jet but it is not replaceable. I think I would have to pull out the throttle shaft to get at the right angle.

 

[Automender]

Stihl does not recommend a pop off pressure. Pump up to 7 PSI and it should hold. I will tap the inlet lever and see if it holds after dropping. The important thing is that the needle seals and the inlet lever moves freely.

I see your credentials and thought you may known the answer to this question. I replaced all the bulk cut fuel hoses on this TS420 and they seemed to have the habit to kink where as the old hoses were stiff and would not kink. I used OEM hose so do they have a tendency to kink?