Jonsered 70E problems after 15 min running great

[Overlooker]

I acquired the saw 9 days ago, and it was quite clean and had 135psi compression. Started and ran great until it didn't.

I discovered that the rear rubber handle mount had failed which affects the throttle linkage. I got a NOS mount installed today and headed out cutting. I had no problems with power or idle speed. After 15-20 minutes, it refused to idle down enough for the chain to stop moving, even after repeatedly revving and even making a couple more cuts. I even loosened the fuel cap in case I had a plugged breather. I shut her down, pulled the cover and found nothing amiss with the throttle linkage. Can a carb act up after many minutes of perfect performance? It's a Tillotson HS201C. Any ideas, or do I need to pull the carb and inspect the lower side? Thanks, O

 

[pioneerguy600]

Sounds like an air leak has developed, a vac and pressure test would rule that one way or the other.

 

[Overlooker]

I have considered the possibility of an air leak, and I have not ruled it out. When I have had air leaks with other saws, they lean out, increase RPM, and die. The saw has not done that at all. It never leans out and dies. I've replaced seals and gaskets on other saws, mostly Homelites, so that concern is high on my list, but this saw acts differently. If I pull the carb, I will definitely pressure and vacuum test everything. I certainly can't rule it out yet. Thanks, O.

 

[pioneerguy600]

I have repaired many that had air leaks, they just ran fast, higher RPM and could not be brought down to lower RPM by just richening the mix but they would all run and not die. Just run at higher RPM and continually turn the chain.

 

[Bob Hedgecutter]

Yep, if the idle screw does not do much to adjust the revs- air leak.

If you can turn back the idle until the saw near stalls and the chain is still running- clutch is poked.

 

[Overlooker]

I have repaired many that had air leaks, they just ran fast, higher RPM and could not be brought down to lower RPM by just richening the mix but they would all run and not die. Just run at higher RPM and continually turn the chain.

Well, that's certainly worth testing for. When you have encountered air leaks in the past, have you ever had a saw run and idle flawlessly for 15-20 minutes and then refuse to idle down? This saw has done just that both times I have cut with it in the short time I've owned it. Is heat building up and triggering the leak? Seems possible. Guess I need to pull the carb and do some testing, but how do I test it hot? Thanks for your input. T-A

 

[pioneerguy600]

I have had saws start and run but not flawlessly, they would run fast , high RPM at idle but run ok a WOT, just would not return to idle, chain would continue to turn and could not be slowed by tuning the carb richer. Air leaks were the number one reason I bought a Vac gauge setup and made a bunch of custom block off plates and gaskets, the vac gauge soon became my most used diagnostic tool. It could be a heat related problem causing an air leak but one of the first things I do myself is pull the muffler off and take a close look at the piston, a damaged piston will often allow the engine to run sort of ok before the engine heats up and both the cylinder and piston expands from heat, the engine will lose compression as the cylinder will expand enough the rings and piston will lose their sealing capability, the engine will sometimes shut down and not idle until the cylinder cools down and contracts somewhat, then it can be started back up and run again until heat swells the cylinder again. A carb can cause many problems if the internals are in poor condition,, stiff or worn diaphragms will not pummp fuel or meter fuel properly, clogged internal screens or passages cause erratic fuel delivery and thus erratic engine running.Clean the carb well and replace the diaphragms if suspect, stiff or distorted in any way, a pressure test afterward will tell if it is air tight, to find a leak it can be immersed under water with the pump still attached air pressure will show bubbles where there is a leak.

 

[Overlooker]

I have had saws start and run but not flawlessly, they would run fast , high RPM at idle but run ok a WOT, just would not return to idle, chain would continue to turn and could not be slowed by tuning the carb richer. Air leaks were the number one reason I bought a Vac gauge setup and made a bunch of custom block off plates and gaskets, the vac gauge soon became my most used diagnostic tool. It could be a heat related problem causing an air leak but one of the first things I do myself is pull the muffler off and take a close look at the piston, a damaged piston will often allow the engine to run sort of ok before the engine heats up and both the cylinder and piston expands from heat, the engine will lose compression as the cylinder will expand enough the rings and piston will lose their sealing capability, the engine will sometimes shut down and not idle until the cylinder cools down and contracts somewhat, then it can be started back up and run again until heat swells the cylinder again. A carb can cause many problems if the internals are in poor condition,, stiff or worn diaphragms will not pummp fuel or meter fuel properly, clogged internal screens or passages cause erratic fuel delivery and thus erratic engine running.Clean the carb well and replace the diaphragms if suspect, stiff or distorted in any way, a pressure test afterward will tell if it is air tight, to find a leak it can be immersed under water with the pump still attached air pressure will show bubbles where there is a leak.

I have a Homie XL923 that was also a $5 dollar garage sale purchase that taught me a lot about air leaks.

Repairs included replacing fuel/oil lines, carb rebuild, replacing crank seals and cylinder gasket and, eventually, the rear handle/air box. My vacuum/pressure test equipment proved invaluable. Bottom line, it still was sucking air. I finally determined the brass throttle shaft had worn the aluminum carb housing enough to cause an intermittent air leak. Rather than spend $80-100 for a replacement carb, I used a rubber O-ring on the throttle shaft, held snug by a spring and a couple washers, to seal the gap.

It solved about 95% of the problem. I occasionally still have to jiggle the throttle to get it to idle down. What I've never seen before is an air leak that only occurs after the saw has been run for 15-20 minutes, but I don't doubt that it can happen. Anyway, my next step is to pull and test the carb. I'll keep you updated. Thanks for your input. T-A

 

[pioneerguy600]

I have a Homie XL923 that was also a $5 dollar garage sale purchase that taught me a lot about air leaks.
View attachment 1087818
Repairs included replacing fuel/oil lines, carb rebuild, replacing crank seals and cylinder gasket and, eventually, the rear handle/air box. My vacuum/pressure test equipment proved invaluable. Bottom line, it still was sucking air. I finally determined the brass throttle shaft had worn the aluminum carb housing enough to cause an intermittent air leak. Rather than spend $80-100 for a replacement carb, I used a rubber O-ring on the throttle shaft, held snug by a spring and a couple washers, to seal the gap.
View attachment 1087820
It solved about 95% of the problem. I occasionally still have to jiggle the throttle to get it to idle down. What I've never seen before is an air leak that only occurs after the saw has been run for 15-20 minutes, but I don't doubt that it can happen. Anyway, my next step is to pull and test the carb. I'll keep you updated. Thanks for your input. T-A

I have a 924 passed on to me by my late FIL, it starts and runs perfect but I hardly ever use it, like many of my vintage saws its a shelf sitter these days.Hope you chase the leak til you find them all.

 

[Cantdog]

Before you go overboard thinking the worst eliminate the easy stuff first. Check the cyl base bolts for tight. ...carb bolts as well. Check the isolation block to cyl gasket (rarely a problem). Also you need to make certain you have the correct carb to isolation block gasket. If someone has used the diamond shaped gasket that come with all Tilly kits that's an issue that needs fixing. Next I'd pull the top cover off the carb and check the final fuel filter/screen just under the cover. When these get clogged it'll make 'em run all kinds of funny. Have you tried to tune away from the issue with L&H jets? This is just stuff I'd check before tearing everything apart.

 

[Overlooker]

I have a 924 passed on to me by my late FIL, it starts and runs perfect but I hardly ever use it, like many of my vintage saws its a shelf sitter these days.Hope you chase the leak til you find them all.

I've got one more data point to add to the story. I was too busy to do any further diagnostics the past two days, but I did start the saw thto evening. After a 2-3 minute warm-up, it ran and idled (without the chain spinning) just fine, although I didn't have time to do any cutting. An air leak would likely explain the problem I'm having, but it's got to be heat related. I hope to tear into it further tomorrow. T-A

 

[pioneerguy600]

If it`s not an airleak then the starving out/leaning out of fuel delivery can cause the high idle symptoms also, clogged tank vent or crack in the fuel line, loose hose connection at the carb, loose elbow fitting has also caused me some deep searching a couple times.Impulse line needs to be tight as well.

 

[sawfun]

Vapor lock ? Coil issues ?

 

[Overlooker]

Vapor lock ? Coil issues ?

I took the saw into the timber yesterday, along with my 036/20in, and it ran fine. Before doing so, I checked cylinder bolts and carb mounting bolts. With a T-shaped hex wrench I was able to tighten all of them a little bit. The 70E ran fine and idled appropriately, but I never got the chance to keep cutting with it for 15 or 20 minutes straight. I had a job to do, getting a walnut saw log cut and hauled out of the timber. Once I had accomplished that task, I decided to run the saw for an extended period to determine if I had improved things by tightening bolts. I cut through an 18in section of walnut (horizontal) with WOT. As the saw idled down after my cut, it died, and I never got it started again. I'm pretty sure I flooded it. Fortunately, I know better than to got out into the timber with only one saw. I used the 036 twice to free up the 70E when I pinched the bar, and then again after the 70E refused to restart. I'm heading out now to try the 70E again. Thanks for your input. O

 

[steele_109]

Have you tried loosing the fuel cap when it won't restart,could be the vent in fuel tank.

 

[Overlooker]

Have you tried loosing the fuel cap when it won't restart,could be the vent in fuel tank.

Yes, I've tried that. Yesterday, I had a chance to investigate the carb. After removing it, I went through it and found no issues. All the gaskets and diaphragms were supple. I would have vacuum/pressure tested it, but I had cleaned and organized my shed a couple months ago. You know what that means...I couldn't find my tester until after I had run out of time! Eventually I remembered where it got stashed, and I will do more testing today. I'm more convinced that I've got an air leak, as the saw has become very hard to start and refuses to idle at all. Time will tell... Thanks for your input. O

 

[Cantdog]

When you had the carb off did you check the carb to isolation block gasket I mentioned earlier?
The gasket should look like the one in the center of the cyl base gaskets in this pic. The diamond shaped one that comes in all Tilly kits won't work as it does not completely cover the inpulse corridor shown in the second pic.

 

[pioneerguy600]

When you had the carb off did you check the carb to isolation block gasket I mentioned earlier?
The gasket should look like the one in the center of the cyl base gaskets in this pic. The diamond shaped one that comes in all Tilly kits won't work as it does not completely cover the inpulse corridor shown in the second pic.

View attachment 1088904View attachment 1088906

That generic diamond shaped gasket leaves the outer end of that impulse corridor open to the outside air so no impulse signal reaches the carb, I remember you showing that a time or two.

 

[Overlooker]

I have a 924 passed on to me by my late FIL, it starts and runs perfect but I hardly ever use it, like many of my vintage saws its a shelf sitter these days.Hope you chase the leak til you find them all.

After lots of testing, I'm quite sure I found the culprit. While the carb held vacuum at 15, and no soapy bubbles were blown on the flywheel side of the crank, the clutch side leaks like a seive. How do I pull that clutch? Three screws and a three-arm puller?

Thanks, O

 

[Cantdog]

That generic diamond shaped gasket leaves the outer end of that impulse corridor open to the outside air so no impulse signal reaches the carb, I remember you showing that a time or two.

Yes I have as it is an ultra common mistake made by folks not familiar with the true Jonsereds saws....from the 49SP through the 90 they all require the OEM shaped gasket. Either NOS or home made or a Husky 61-272XP gasket will work as well. But not the diamond shaped one. It's about the first thing to sort out on a newly acquired Jonsereds saw that has running issues. The next thing to look at is the cyl base gasket. They are very thin paper (0.007" when compressed) and if the cyl bolts get loose at all they can blow out easly. Not claiming either of these things are the issue on this saw but they are surely the starting point of trouble shooting.