10-10 Won't Stay Running - Air Leak When Warm?

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Ed Mullen

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Ok, I have to confess a story which is similar. I bought my first 10-10 for $35 a while back and rebuilt it to be my firewood saw. I ended replacing dang near everything, but when it came time to start it, it was the exact same problems you were having. I had $150 in to this $35 saw along with who knows how much time.
Issues with idling, stalling, and not starting plagued me for months. I finally put it on the shelf and walked away.
Then I stumbled across Leon’s chainsaw repair channel and he had that video about the check valve. So I bit the bullet, took the saw down, took my time, and learned how to replace it. First pull she fired, tuned it, and now she is my go to saw. It’s ported, running 24” bar, and she loves to eat

crazy how one little thing can do that.

so is 3/16 a good size that worked for you? That size is smaller than a 223 casing but larger than a 17 hmr. I’ll have to find one. I think it’s more common of an issue than people think. I’ve had to do 3 out of 13 macs that I have

I’m very thankful to have been able to help and it’s awesome that it worked out. You’re gonna love cutting with that saw. 👍🏻
 

Ed Mullen

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Ok, I have to confess a story which is similar. I bought my first 10-10 for $35 a while back and rebuilt it to be my firewood saw. I ended replacing dang near everything, but when it came time to start it, it was the exact same problems you were having. I had $150 in to this $35 saw along with who knows how much time.
Issues with idling, stalling, and not starting plagued me for months. I finally put it on the shelf and walked away.
Then I stumbled across Leon’s chainsaw repair channel and he had that video about the check valve. So I bit the bullet, took the saw down, took my time, and learned how to replace it. First pull she fired, tuned it, and now she is my go to saw. It’s ported, running 24” bar, and she loves to eat

crazy how one little thing can do that.

so is 3/16 a good size that worked for you? That size is smaller than a 223 casing but larger than a 17 hmr. I’ll have to find one. I think it’s more common of an issue than people think. I’ve had to do 3 out of 13 macs that I have

I’m very thankful to have been able to help and it’s awesome that it worked out. You’re gonna love cutting with that saw. 👍🏻
Yes 3/16" worked perfectly.

After scaling off the image for the McCulloch 83758 kit, the OD of the check valve is the same size as seat diameter of the brass insert where the spring clips goes. We measured that seat and it was about 0.1925" and 3/16" is 0.1875". So they were only 0.005" apart - close enough for me.

There is no way the check valve is the same size as the OD of the brass insert - it's considerably smaller than that. And that would make it a PITA to install too.

Moving forward I'm going to very carefully check the IPL on every vintage carb we work on. And if there is a check valve under a Welch plug - it's getting replaced. Period.

We just picked up a Partner 7000 Plus which according to the IPL has a Walbro WJ 1C and sure enough it has that check valve.

What really surprised me is that the check valve was missing completely from my carb. It must have dissolved over the years - maybe due to ethanol fuel. The one I installed was from a spare fuel pump diaphragm so with e-free mix, it should outlive me.
 

Simpy740

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I am reconditioning a Mac 10-10 Automatic. It starts easily when cold - it has a smooth idle, it has great throttle response and 4 strokes on the high end.

But after running for a few minutes - just sitting there the RPMs start to climb, it starts to race, it won't accept throttle (bogs) and then ultimately it stalls out. It's acting like it's starving for fuel or suddenly developed a bad air leak.

Holding the saw sideways (both ways) doesn't change the RPM much. But pointing the bar straight up immediately causes the RPM to climb like it has an air leak.

After it sits for a while I can restart it and it behaves exactly the same way again - runs perfectly for a few minutes, then starts behave like it has an air leak when warm.

Below is the background and work done on the saw:
  • The engine has good compression around 150 psi.
  • The crankcase seals have been replaced and the clamshell surfaces were well sealed.
  • All four intake manifold gaskets have been replaced. The impulse hole is totally clear from the crankcase to the carb.
  • The saw passes both vacuum and pressure tests - the needle is rock solid at 6-7 inches of vacuum for each test and doesn't budge for several minutes. I sprayed Windex everywhere under pressure and not even a single bubble anywhere.
  • The fuel filter is new and is completely under fuel when it misbehaves.
  • The fuel line is new. I even added a support near the carb where it bends so it doesn't pinch/collapse under suction.
  • The fuel cap is new (NOS), the duckbill is new, and loosening the fuel cap while the saw is running does not eliminate the problem.
  • The Walbro SDC 37A has been disassembled, cleaned in my ultrasonic cleaner, and rebuilt with a genuine Walbro kit. The fuel pump diaphragm, the check valve diaphragm and the gasket are in the correct order. The metering arm has been set level. The metering diaphragm and metering gasket are in the correct order. The inlet screen is clean.
  • The spark plug is new - Champion CJ8J gapped to 0.025.
  • The coil is original and I gapped it to 0.010. The points/condensor were replaced with a solid state chip years ago by the owner's grandfather. It has plenty of spark and sputters quickly on a cold start.
I've never experienced this sort of behavior where it runs fine cold and then starts to behave like a bad air leak after a few minutes.

I can post videos if that would be helpful.

Any insights from the McCulloch pros would be greatly appreciated. Thanks all.
Check and clean fuel filter and small vent hole in cap it sounds like it's running out of gas also check fuel line for softness it could bad otherwise take care apart and pull out screen and clean carb some times debris from gas tank get built up behind screen and restrict fuel
 

Ed Mullen

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Check and clean fuel filter and small vent hole in cap it sounds like it's running out of gas also check fuel line for softness it could bad otherwise take care apart and pull out screen and clean carb some times debris from gas tank get built up behind screen and restrict fuel
Thank you for posting. The problem has been solved. The carb was missing a check valve under the welch plug. Fortunately the rebuild kit had a replacement welch plug and the check valve was easy to make.
 

sledhead34

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I am reconditioning a Mac 10-10 Automatic. It starts easily when cold - it has a smooth idle, it has great throttle response and 4 strokes on the high end.

But after running for a few minutes - just sitting there the RPMs start to climb, it starts to race, it won't accept throttle (bogs) and then ultimately it stalls out. It's acting like it's starving for fuel or suddenly developed a bad air leak.

Holding the saw sideways (both ways) doesn't change the RPM much. But pointing the bar straight up immediately causes the RPM to climb like it has an air leak.

After it sits for a while I can restart it and it behaves exactly the same way again - runs perfectly for a few minutes, then starts behave like it has an air leak when warm.

Below is the background and work done on the saw:
  • The engine has good compression around 150 psi.
  • The crankcase seals have been replaced and the clamshell surfaces were well sealed.
  • All four intake manifold gaskets have been replaced. The impulse hole is totally clear from the crankcase to the carb.
  • The saw passes both vacuum and pressure tests - the needle is rock solid at 6-7 inches of vacuum for each test and doesn't budge for several minutes. I sprayed Windex everywhere under pressure and not even a single bubble anywhere.
  • The fuel filter is new and is completely under fuel when it misbehaves.
  • The fuel line is new. I even added a support near the carb where it bends so it doesn't pinch/collapse under suction.
  • The fuel cap is new (NOS), the duckbill is new, and loosening the fuel cap while the saw is running does not eliminate the problem.
  • The Walbro SDC 37A has been disassembled, cleaned in my ultrasonic cleaner, and rebuilt with a genuine Walbro kit. The fuel pump diaphragm, the check valve diaphragm and the gasket are in the correct order. The metering arm has been set level. The metering diaphragm and metering gasket are in the correct order. The inlet screen is clean.
  • The spark plug is new - Champion CJ8J gapped to 0.025.
  • The coil is original and I gapped it to 0.010. The points/condensor were replaced with a solid state chip years ago by the owner's grandfather. It has plenty of spark and sputters quickly on a cold start.
I've never experienced this sort of behavior where it runs fine cold and then starts to behave like a bad air leak after a few minutes.

I can post videos if that would be helpful.

Any insights from the McCulloch pros would be greatly appreciated. Thanks all.
gasket off the impulse correct and lined up ?? i saw one of these that the gasket slid out during install and it wasn`t impulsing right ran good for away but acted up in short order
 

Ed Mullen

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gasket off the impulse correct and lined up ?? i saw one of these that the gasket slid out during install and it wasn`t impulsing right ran good for away but acted up in short order
Thank you for posting.

The problem has been solved. The carb was missing a check valve under the welch plug. Fortunately the rebuild kit had a replacement welch plug and the check valve was easy to make out of carb fuel pump diaphragm material.

Regarding the gaskets - yes, we made new ones and installed them with the impulse hole properly aligned. I can see how this could easily create problems as there are multiple gaskets and it's easy to get the impulse hole plugged.
 

TRTermite

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to the OP
I will be reading this several times because one of these days I will probably be dealing with some of the same issues when I pull my 10-10 automatic off the shelf and see what needs to be done (it has spark)just to many "WANNA DO" projects.
Thanks for starting this thread You already know how much frustration you are saving everyone down the line
 

Ed Mullen

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to the OP
I will be reading this several times because one of these days I will probably be dealing with some of the same issues when I pull my 10-10 automatic off the shelf and see what needs to be done (it has spark)just to many "WANNA DO" projects.
Thanks for starting this thread You already know how much frustration you are saving everyone down the line
The real hero of this thread is GoBigRed who pointed me in the right direction. I would have never found the problem without his help - and now others can benefit. That's the whole point of this hobby - sharing information and increasing our knowledge base.
 

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