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

[Ed Mullen]

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.

 

[tomalophicon]

I've heard of coils failing when hot. Might be worth trying another coil if you have one to hand.
OR after it stops pull the plug check for spark on the cylinder. You might have good spark when cold but nothing once it's warm (due to faulty coil).

 

[Woodslasher]

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.

Are you sure the carb is re-assembled properly? My 10-10 had a similar issue, I re-did the crank seals because I thought they were bad but it turns out I'd re-assembled the "flapper" side of the carb wrong. It could very well be the coil too, my 10-10 developed a no-fire issue after I did the seals, I thought the points were screwy but it may be that the coil has died.

 

[Jasonrkba]

Could be the gasket under the tank or the gasket under the carb. Id do the seals on a saw that old anyway.

 

[Woodslasher]

The crankcase seals have been replaced and the clamshell surfaces were well sealed

Could be the gasket under the tank or the gasket under the carb. Id do the seals on a saw that old anyway.

 

[Jasonrkba]

Going to have to spray carb cleaner in suspect areas when it's messing up. Sounds like a ton of stuff has been done. Its very easy to misaligne the carb gasket when installing the 10-10 carb. Ignition problems can mimic fuel issues. Is it a points saw?

 

[Ed Mullen]

The carb diaphragms and gaskets are in the correct order. On the fuel pump side: fuel pump diaphragm, check valve diaphragm and the gasket. The metering arm has been set level. On the metering side: gasket and then diaphragm. The fuel filter is under fuel. The fuel line is new and not collapsing. The inlet screen is clean.

I replaced all four manifold gaskets and made sure they were aligned perfectly with respect to the impulse hole. And yes - it's easy to get a partial hole blockage.

It's been converted to a chip by the owner.

I replaced both seals and sealed the crankcase with Motoseal. It passed vac and pressure test (when cold) with flying colors.

The saw runs great for the first few minutes. Then it starts to act like an air leak developed. Here is a video of it before it starts to misbehave.

If I'm going to spray the seals with carb cleaner when it's running - any tips on starting it without the recoil mounted? Or do I really have to start it and then try to remove the recoil on a running saw?

 

[heimannm]

I would either replace the chip with points and a condenser, or try another chip.

Mark

 

[GoBigRed]

I’ve had this happen on two 20-10 saws after complete rebuilds. I thought it was an air leak or carb as well. It turned out to be the carb after all, but my cleaning and rebuild did not go deep enough. Not sure if you said that you swapped in another carb to test that or not.

In both cases With the carb, I ended up popping the Welch plugs, and replaced the check valve. After replacing, the saws fired up immediately, idled, held their tune, and throttled up perfectly.
And as Mark said, installing a chip is a simplest and easy way to help these saws and eliminate any issues with setting points correctly.
Hope it helps and you get it running

 

[Ed Mullen]

I’ve had this happen on two 20-10 saws after complete rebuilds. I thought it was an air leak or carb as well. It turned out to be the carb after all, but my cleaning and rebuild did not go deep enough. Not sure if you said that you swapped in another carb to test that or not.

In both cases With the carb, I ended up popping the Welch plugs, and replaced the check valve. After replacing, the saws fired up immediately, idled, held their tune, and throttled up perfectly.
And as Mark said, installing a chip is a simplest and easy way to help these saws and eliminate any issues with setting points correctly.
Hope it helps and you get it running

I have an older 10-10 with a right hand recoil and an older version of the SDC 37A with an external idle adjustment screw instead of the integral idle adjustment.

I would have to drill and tap the case of the newer 10-10 to accept the older manual idle screw - otherwise I would have swapped the carb over.

 

[Ed Mullen]

I’ve had this happen on two 20-10 saws after complete rebuilds. I thought it was an air leak or carb as well. It turned out to be the carb after all, but my cleaning and rebuild did not go deep enough. Not sure if you said that you swapped in another carb to test that or not.

In both cases With the carb, I ended up popping the Welch plugs, and replaced the check valve. After replacing, the saws fired up immediately, idled, held their tune, and throttled up perfectly.
And as Mark said, installing a chip is a simplest and easy way to help these saws and eliminate any issues with setting points correctly.
Hope it helps and you get it running

Is the check valve part of the Walbro rebuild kit?

Based on the IPL for that carb, it looks like the metering side has a smaller Welch plug with a valve and seat assembly. Is that the one you replaced?

 

[GoBigRed]

Yes. The standard walbro kit does not have the Welch plug portion. I get all my carb kits from a place called discount marine parts. You can search that on the internet and find their store. They are also on eBay. Their kits are about $20 but they come with everything and also include a diagram for the carb and it’s parts. Then take your time and use Leon’s video on his YouTube channel. I have also made a check valve out of gasket material and used that with success. First one can be a booger but once it’s fixed it has always been my solution

 

[Ed Mullen]

Yes. The standard walbro kit does not have the Welch plug portion. I get all my carb kits from a place called discount marine parts. You can search that on the internet and find their store. They are also on eBay. Their kits are about $20 but they come with everything and also include a diagram for the carb and it’s parts. Then take your time and use Leon’s video on his YouTube channel. I have also made a check valve out of gasket material and used that with success. First one can be a booger but once it’s fixed it has always been my solution

I just checked and the K10-SDC kit has the welch plug, screen and the retaining clip. It doesn't have the check valve itself, but it looks like I can make one from a spare metering diaphragm as shown in this thread. Apparently it's a PITA to get the check valve installed but it's worth a try. Maybe that's why the saw speeds up with the bar pointed upward - the check valve would be under gravity at that point......

 

[GoBigRed]

I just checked and the K10-SDC kit has the welch plug, screen and the retaining clip. It doesn't have the check valve itself, but it looks like I can make one from a spare metering diaphragm as shown in this thread. Apparently it's a PITA to get the check valve installed but it's worth a try. Maybe that's why the saw speeds up with the bar pointed upward - the check valve would be under gravity at that point......

So when I first tried, you bet I was kinda intimidated regarding messing it up or doing it right. The biggest issue I had was getting the old check valve out. It actually went in fairly easy and once it gets past the small hole, it seems to spread out and lay like it’s suppose to

I cut one also from diaphragm material but I’m not sure it’s working quite like it should. I used a 223 casing and chamfered it so the opening had somewhat of a cutting edge around it. The 223 is too large and I had to cut it down to fit and doing it by hand is kinda shaky for me. I do think a casing from a 17 HMR might work better, but can’t confirm it

I guess what I’m trying to say is watch Leon’s video and then go for it. Maybe get some tools like a very fine set of small tweezers and/or a small pick with a very skinny head at the point.
Trust yourself and I really think it not going to be as bad as you think. Even getting the circlip in and out has gotten easier for me and I was really concern about getting it back in. But they seem to pop back for me without issue

 

[Maintenance supervisor]

It actually sounds like a crack or pin hole in the fuel line to me ? Especially if tipping the saw backwards or forward.
That should be on your check list, I've seen it smoke a piston if not discovered.

 

[Ed Mullen]

It actually sounds like a crack or pin hole in the fuel line to me ? Especially if tipping the saw backwards or forward.
That should be on your check list, I've seen it smoke a piston if not discovered.

Thanks for the input. Yes, it sure is acting like an air leak - agreed. I've replaced this fuel line a few times and the behavior didn't change and the filter is under fuel.

The odd thing is the saw runs great for a few minutes and then the idle starts to race and eventually it dies. And yes - if I don't figure it out it's going to smoke the piston. I've seen this enough times at this point to not let it play out - I'll just kill it if it starts to race.

I'm going to replace the check valve under the Welch plug today and see if that helps.

 

[tomalophicon]

Thanks for the input. Yes, it sure is acting like an air leak - agreed. I've replaced this fuel line a few times and the behavior didn't change and the filter is under fuel.

The odd thing is the saw runs great for a few minutes and then the idle starts to race and eventually it dies. And yes - if I don't figure it out it's going to smoke the piston. I've seen this enough times at this point to not let it play out - I'll just kill it if it starts to race.

I'm going to replace the check valve under the Welch plug today and see if that helps.

Have you tested spark immediately after it shuts down yet?

 

[Ed Mullen]

I have an update - GoBigRed was 100% correct.

I drilled a hole in the Welch plug, used an easy out to remove it, removed the spring clip and the screen - and peeked inside the brass seat.

The check valve was missing completely.

I scaled the size of the check valve from this eBay listing (see attached pic) for the check valve kit. And then used my Mayhew 3/16" hollow punch to make a new check valve from a spare fuel pump diaphragm (0.2 mm thick).

Following the guidance in this related thread, we carefully slipped the new check valve under the brass insert, replaced the screen and then replaced the spring clip.

The Walbro K10-SDC kit does have the replacement Welch plug. We installed the new Welch plug, tapped it into place with a drift punch so it was below flush and put the SDC-37A carb back together.

We put the carb in the saw, reset the mixture screws to factory starting recommendation and damned if the saw didn't start right up.

We got the L screw and the idle set and that sonofabitch sat there idling for 10 minutes. I went bar up, bar down, sideways left, sideways right and it didn't skip a beat. I couldn't believe it - I was just stunned.

We set the H screw rich to start and then we tuned the saw in a big piece of sugar maple - 4 strokes unloaded, 2 stokes in the cut and lots of power without overheating. This saw rips nice for its size and is the the vintage muscle saw I was hoping for.

By the way, I found this 10-10 IPL and on pages 3-4 is the Walbro 37A with a really nice exploded view showing all parts, including that check valve kit (Ref # 30 - PN 83758). This was very useful when rebuilding the carb as I have never encountered two diaphragms on the fuel pump side of the carb and they do need to go in the correct order.

I am beyond shocked this tiny little check valve completely solved the carb/tuning problem and I'm forever indebted to GoBigRed and this forum for helping me. I have about 40 hours of troubleshooting into this saw and I was totally stumped and literally ready to drop this 10-10 off a bridge. THANK YOU ALL!!

 

[thenne1713]

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.

I had a little Poulan w/ primer that had similar symptoms that drove me crazy; SHORT version of LONG story is FUEL MUST be able to free-flow, and trimming 1/8" from 1-1/8" long fuel line carb/primer CURED the issue; the extra 1/8" gave just enough crimp to line to allow prime, but STOP free-flow; trimming straightened fuel line and restored free-flow.

 

[GoBigRed]

I have an update - GoBigRed was 100% correct.

I drilled a hole in the Welch plug, used an easy out to remove it, removed the spring clip and the screen - and peeked inside the brass seat.

The check valve was missing completely.

I scaled the size of the check valve from this eBay listing (see attached pic) for the check valve kit. And then used my Mayhew 3/16" hollow punch to make a new check valve from a spare fuel pump diaphragm (0.2 mm thick).

Following the guidance in this related thread, we carefully slipped the new check valve under the brass insert, replaced the screen and then replaced the spring clip.

The Walbro K10-SDC kit does have the replacement Welch plug. We installed the new Welch plug, tapped it into place with a drift punch so it was below flush and put the SDC-37A carb back together.

We put the carb in the saw, reset the mixture screws to factory starting recommendation and damned if the saw didn't start right up.

We got the L screw and the idle set and that sonofabitch sat there idling for 10 minutes. I went bar up, bar down, sideways left, sideways right and it didn't skip a beat. I couldn't believe it - I was just stunned.

We set the H screw rich to start and then we tuned the saw in a big piece of sugar maple - 4 strokes unloaded, 2 stokes in the cut and lots of power without overheating. This saw rips nice for its size and is the the vintage muscle saw I was hoping for.

By the way, I found this 10-10 IPL and on pages 3-4 is the Walbro 37A with a really nice exploded view showing all parts, including that check valve kit (Ref # 30 - PN 83758). This was very useful when rebuilding the carb as I have never encountered two diaphragms on the fuel pump side of the carb and they do need to go in the correct order.

I am beyond shocked this tiny little check valve completely solved the carb/tuning problem and I'm forever indebted to GoBigRed and this forum for helping me. I have about 40 hours of troubleshooting into this saw and I was totally stumped and literally ready to drop this 10-10 off a bridge. THANK YOU ALL!!

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.