McCulloch Mini and Small CC Chainsaw's

[banshee1]

I have had all kinds of issues with Mini Macs but never been zapped! It's such a basic ignition system, I bet when he tears it down he'll find a spot on the primary coil wire that rubbed through and is touching the chassis.
The bizarre thing is that it doesn't misfire, any time I've been dumb enough to introduce my body into the ignition system of a simple engine, I instantly start to do the jitterbug, and the engine stops firing!

Well that's what I was hoping to find. I looked and bent the plug wire all around and couldn't see a crack in the insulation. I put one wrap of electrical tape around it just to play devils advocate. I also replaced the kill switch wire. Because it's so cramped in there I ended up soldering the coil end of the wire to the blade, because no matter how I placed it, the terminal looked like it would hit the case

Put it all back together and still have the sparkling and shocking problem.

I didn't pull the flywheel to inspect the points like I probably should have, but I just didnt think they could be the culprit. Guess I'll take it apart again.

The whole no misfiring thing has me puzzled as well. Gotta be something stupid. Wish used parts were easier to find for these things around here, but if an older saw is running around here the owners think they have some rare saw and want more money then I'd pay.

 

[MacAttack]

Got the Mini Mac 25 back together after the basic carb rebuild, it started after a number of pulls once it primed with gas, but still not running right. Hi and Lo needles both one turn out as a baseline, throttle response seems ok but it won't idle down very well. Turning out the idle adjust seems to have no effect. Also it seems like I can't find a sweet spot on the Hi adjustment.
Others can maybe confirm but I'm suspecting it's the high speed check valve in the MDC carb, which i was trying to avoid replacing, is no good and leaking.
So that leaves me with a few questions...where do you guys source these check valves? Do I need any specialty tools?
I have a few Welch plugs, are the 2 plugs in a Walbro MDC carb the same?
It's too bad, it was a disappointing 2nd attempt for this saw.

 

[Manic84]

Got the Mini Mac 25 back together after the basic carb rebuild, it started after a number of pulls once it primed with gas, but still not running right. Hi and Lo needles both one turn out as a baseline, throttle response seems ok but it won't idle down very well. Turning out the idle adjust seems to have no effect. Also it seems like I can't find a sweet spot on the Hi adjustment.
Others can maybe confirm but I'm suspecting it's the high speed check valve in the MDC carb, which i was trying to avoid replacing, is no good and leaking.
So that leaves me with a few questions...where do you guys source these check valves? Do I need any specialty tools?
I have a few Welch plugs, are the 2 plugs in a Walbro MDC carb the same?
It's too bad, it was a disappointing 2nd attempt for this saw.View attachment 864772

I take it you've ruled out everything else and already done a pressure/vac test on the saw. But that does sound like a failing valve.
So to answer your questions:

No, the welch plugs are not the same. One is 1/2, the other is 7/16, that's the one with the valve under it.
The only tools you'll really need is: *something to pierce the plug, a **punch and a small precision flathead or pick for the circlip that holds the screen in place. Be careful removing the clip and screen, try to not pop a hole through the screen and keep track of the clip. (it can rocket off into the 8th dimension)

The check valve is part of the Walbro 86-523 kit. You can search for that on either Ebay or Amazon. The kit has everything you would need: The NBR (Buna) valve, the (pre assembled) brass seat, screen and clip... except for one thing - The welch plug included in the kit is 5/16 and will not work for the MDC. Make sure you have the right plugs (7/16) on hand from a left-over K1-MDC kit (if you don't have one already) or the Stens 615-675 plugs. (I had to file them down to more or less match the original plug thickness)

If you want to remove the brass seat you can, but it's optional.
Heimannm has used a ***tap to pull them, or you can used a blunted wood screw. You can fish out the check valve without futzing with the seat, but it takes a steady hand/patience and something small - like a precision pick. I made a post on this thread about how I made my own valve out of a pump diaphragm. If you have everything else: the plug, valve (homemade or kit), are careful removing the screen and just want to remove the valve and replace it, you can go this route. (just use something a bit more blunt when installing the valve)

That Mini Mac looks Awesome by the way

* You can drill it out, but be very, very careful doing it. Drill a hole then pry it out. An awl, ice pick, even a self centering punch could do the trick. Then just pry it out.
**I used the back end of a 5/32 punch to install the new plug.
*** Heimannm said he used either a 6-32 or 8-32 tap, I used a 8-32. The 6-32 was a bit small and didn't grab quite right. Or use a modded wood screw.

 

[MacAttack]

I take it you've ruled out everything else and already done a pressure/vac test on the saw. But that does sound like a failing valve.
So to answer your questions:

No, the welch plugs are not the same. One is 1/2, the other is 7/16, that's the one with the valve under it.
The only tools you'll really need is: *something to pierce the plug, a **punch and a small precision flathead or pick for the circlip that holds the screen in place. Be careful removing the clip and screen, try to not pop a hole through the screen and keep track of the clip. (it can rocket off into the 8th dimension)

The check valve is part of the Walbro 86-523 kit. You can search for that on either Ebay or Amazon. The kit has everything you would need: The NBR (Buna) valve, the (pre assembled) brass seat, screen and clip... except for one thing - The welch plug included in the kit is 5/16 and will not work for the MDC. Make sure you have the right plugs (7/16) on hand from a left-over K1-MDC kit (if you don't have one already) or the Stens 615-675 plugs. (I had to file them down to more or less match the original plug thickness)

If you want to remove the brass seat you can, but it's optional.
Heimannm has used a ***tap to pull them, or you can used a blunted wood screw. You can fish out the check valve without futzing with the seat, but it takes a steady hand/patience and something small - like a precision pick. I made a post on this thread about how I made my own valve out of a pump diaphragm. If you have everything else: the plug, valve (homemade or kit), are careful removing the screen and just want to remove the valve and replace it, you can go this route. (just use something a bit more blunt when installing the valve)

That Mini Mac looks Awesome by the way

* You can drill it out, but be very, very careful doing it. Drill a hole then pry it out. An awl, ice pick, even a self centering punch could do the trick. Then just pry it out.
**I used the back end of a 5/32 punch to install the new plug.
*** Heimannm said he used either a 6-32 or 8-32 tap, I used a 8-32. The 6-32 was a bit small and didn't grab quite right. Or use a modded wood screw.

Manic84 you are awesome, I couldn't have asked for a better write-up, I'm going to read and digest this again and start looking for that Walbro parts kit. Yep I'd be entirely content just replacing the valve and leaving the brass seat in place if that will get the saw running right. This Mini Mac has almost no running on it (which is why it's so clean), that valve is probably disintegrated.
Many thanks to you!
AJ

 

[PogoInTheWoods]

(it can rocket off into the 8th dimension)

Could you please use a smaller font? LOL

 

[MacAttack]

I took at my MDC parts, looks like I have one 7/16 plug, and one kit even came with the tiny screen and circlip, just no actual checkvalve.
Looks like that MM 25 will get disassembled a 3rd time, luckily I can have these saws apart in about 15 minutes. If there's one positive development, I can report that adding teflon tape to the nipples of the rubber T hose for the automatic oil pump seems to keep it from leaking onto oil onto the floor!

 

[MacAttack]

Alright, I've finally got around to posting about how I replaced the rubberized check valve with a homemade teflon one. I used a slightly modified leather punch to cut out the valve to size, and then cleaned up the edges with a razor blade.

I reassembled it and it passed the high tech sucky straw test... so far, so good. After start up it ran a bit rich at first (set it that way for peace of mind), but after some fine tuning, it idles perfectly, doesn’t act odd and has great throttle response.

After a few test cuts and running it kinda hard at times, it runs like a Mac should. But the SOUND! Dear God The Sound!!!
The neighbors are still giving me looks. After some more testing over a period of time, I'm very happy with how it performs.

I’ll seldom use the saw, so I expect the valve to last for quite some time. If I do run into any trouble, or it fails in any way (which I doubt) I will give an update.

To all those who gave me tips and ideas along the way - Thank you.

M

I'd be interested to know a little more about the leather punch you used and the Teflon material. In case I decide to try something similar, I have at a minimum 4 of these carbs that might need a new check valve. I think I've learned my lesson about rebuilding Mini Macs without replacing that check valve... which was another question I had. When you go through one of these carbs that has been sitting, is it your standard procedure to just always replace the check valve?

 

[Manic84]

I'd be interested to know a little more about the leather punch you used and the Teflon material. In case I decide to try something similar, I have at a minimum 4 of these carbs that might need a new check valve. I think I've learned my lesson about rebuilding Mini Macs without replacing that check valve... which was another question I had. When you go through one of these carbs that has been sitting, is it your standard procedure to just always replace the check valve?

It's better to replace them. The MDC valves have/had a high failure rate - if it works now, it might not work later.
Give yourself peace of mind and replace them. But let's face it, if you're looking at a 40 year old saw, the check valve probably isn't that well off.

This gets into detail and will be a long one.

I worked with what I had and my method was redneck as hell, so bear with me here.
The leather punch I used was one of these.



But if you want to do this, you'll probably get better results with one of these in 7/32. (The approximate size of the check valve)
This is called a "Hollow Punch". (I wish my impatient ass knew about these earlier)



This was the process:

The punches were conical in shape, so I used a bench grinder to shorten one of the larger punches to about 7/32 in diameter. (filing it was taking way too long) Once I got it close enough, I would just eyeball the thing next to a drill gauge to where it looked about the right size, (It doesn't have to be exact, just close enough) and manually file it all around bit by bit to get it flat and uniform.
After I had got the size to more or less where I wanted, I took an old 5/32 round file to take off some of the burrs and "sharpen" the edges inside of the modded punch.

Again, you have to make sure the stub is flat enough to make contact with the material evenly, file it until it's even and go back in with the 5/32 as needed. (when doing it on a grinder, leave a bit of material for some leeway) After I was happy with the result, it was go time.
I didn't use the punch in the usual way, but more like a cookie cutter. I'd lay the material on the bench, set the punch to the modded stub, and bear down on the thing with all I had in a straight down twisting motion. (told you it was redneck)
After a few earlier attempts, it was clear the Teflon was very resistant to the process, so the force I used was justified.

The Teflon material I used was a Teflon pump diaphragm from a leftover Walbro D/K20-WAT kit. There will almost always be little
"hairs" on the edges of the cut out valve, so just trim them off with a razor or X-acto knife. You can make the valves out of the rubberized diaphragms as well, but I used the Teflon because it has a higher resistance to ethanol and wear.

Keep in mind that the Teflon is a bit more stiff than the rubberized stuff, so it could bend after installing it using the bypass method. To try and counter this, you can use a small pick or a piece of stiff wire bent to shape and make sure the valve is sitting right, then give it shot of penetrating oil... WD-40, Liquid Wrench etc. to wet it, then move it around - it should come back to it's shape.

After installing it and everything checks out, it should work just fine and last longer than the old valve did.


M

 

[PogoInTheWoods]

Or just use a spent .22 long casing and trim to fit. (7/32 = 0.2188)

 

[Manic84]

Or just use a spent .22 long casing and trim to fit. (7/32 = 0.2188)

I remember the post where you suggested that. I'll admit I thought about doing that, but backed out of it at the last minute.
Just had to do things the hard way I guess.

 

[heimannm]

Is this a "rare" edition of the Mini Mac?













Mark

 

[MacAttack]

I haven't seem one with those decals, but it physically looks similar to a later "Mini Mac" type McCulloch 160 that I was given. (The saw next to the Mini Mac 30).

 

[Maintenance supervisor]

Got the Mini Mac 25 back together after the basic carb rebuild, it started after a number of pulls once it primed with gas, but still not running right. Hi and Lo needles both one turn out as a baseline, throttle response seems ok but it won't idle down very well. Turning out the idle adjust seems to have no effect. Also it seems like I can't find a sweet spot on the Hi adjustment.
Others can maybe confirm but I'm suspecting it's the high speed check valve in the MDC carb, which i was trying to avoid replacing, is no good and leaking.
So that leaves me with a few questions...where do you guys source these check valves? Do I need any specialty tools?
I have a few Welch plugs, are the 2 plugs in a Walbro MDC carb the same?
It's too bad, it was a disappointing 2nd attempt for this saw.View attachment 864772

I often bake the carburetor at 200° for about a half hour to remove the brass insert for the check valve. Makes it pop out alot easier.

 

[MacAttack]

I'm going back and forth in my mind about making some new check valves versus buying them. I REALLY don't want to keep taking Mini Macs apart more that I have to, but it's tempting to attemp making them myself.
I found some spare Walbro diaphragms in a kit that I'll probably never use, is that blue one made of Teflon? The lighter colored one is some strange material, and the black one is a thin rubbery material. Wondering if one of these would be good to make a check valve disk from?

 

[Manic84]

I'm going back and forth in my mind about making some new check valves versus buying them. I REALLY don't want to keep taking Mini Macs apart more that I have to, but it's tempting to attemp making them myself.
I found some spare Walbro diaphragms in a kit that I'll probably never use, is that blue one made of Teflon? The lighter colored one is some strange material, and the black one is a thin rubbery material. Wondering if one of these would be good to make a check valve disk from?
View attachment 866101

The rubberized diaphragm is what the OEM check valve is/was made of. (even the new kit)
The tan colored one is made out of teflon.
I'm kinda against the blue one, it works alright as a diaphragm, but I'm leery of using it for anything else.

It all comes down to which one you want to use.


Here, this will give a breakdown of the properties of each diaphragm.



How many Minis/MDCs do you have on the back-burner at the moment MacAttack?

 

[MacAttack]

The rubberized diaphragm is what the OEM check valve is/was made of. (even the new kit)
The tan colored one is made out of teflon.
I'm kinda against the blue one, it works alright as a diaphragm, but I'm leery of using it for anything else.

It all comes down to which one you want to use.


Here, this will give a breakdown of the properties of each diaphragm.

View attachment 866108

How many Minis/MDCs do you have on the back-burner at the moment MacAttack?

Thanks for the info!

Right now I have the Mini Mac 25 that needs a check valve, then I have Mini Mac 6 and Mini Mac 35 that I am planning on doing the check valve before I put them together. Then I have a MM 30 that eventually will probably need a check valve. I also have some other part saws that if I had some parts could make another Mini Mac from. This is why I need to learn how to fix these valves, can't be lazy any more.
So you like the Teflon or the Rubberized material? I might just order a few check valve kits to get started anyway.

 

[Manic84]

Thanks for the info!

Right now I have the Mini Mac 25 that needs a check valve, then I have Mini Mac 6 and Mini Mac 35 that I am planning on doing the check valve before I put them together. Then I have a MM 30 that eventually will probably need a check valve. I also have some other part saws that if I had some parts could make another Mini Mac from. This is why I need to learn how to fix these valves, can't be lazy any more.
So you like the Teflon or the Rubberized material? I might just order a few check valve kits to get started anyway.

Mini Mac Madness!

I favor the Teflon for two reasons: It's more resistant to ethanol laced fuel and wear in the long run.
And it's a bit easier to punch out then the rubberized stuff. (hollow punch or you use pogo's *rimfire idea)
I'm not knocking the NBR valves and diaphragms, they'll perform just fine, I just question it's durability over time.

It's not a bad idea to order up a few kits to have them on hand or just in case something goes wrong.
But try fiddling around with how to go about replacing the valve and making your own to build your confidence for future projects.
Study it and work out a plan, it's not difficult, but you must be careful - Clear your mind and focus.

*The .22 casing is good, but the edge will roll and it'll go oblong after a few whacks. So have a few if you want to make more than a couple of valves.
I think anything would be easier than what I did.

 

[MacAttack]

Mini Mac Madness!

I favor the Teflon for two reasons: It's more resistant to ethanol laced fuel and wear in the long run.
And it's a bit easier to punch out then the rubberized stuff. (hollow punch or you use pogo's *rimfire idea)
I'm not knocking the NBR valves and diaphragms, they'll perform just fine, I just question it's durability over time.

It's not a bad idea to order up a few kits to have them on hand or just in case something goes wrong.
But try fiddling around with how to go about replacing the valve and making your own to build your confidence for future projects.
Study it and work out a plan, it's not difficult, but you must be careful - Clear your mind and focus.

*The .22 casing is good, but the edge will roll and it'll go oblong after a few whacks. So have a few if you want to make more than a couple of valves.
I think anything would be easier than what I did.

If I try making a few, I will invest in that hollow punch you shared, that looks like the ticket!
But yeah, will probably get a few of the Walbro kits as well. Was it Heimannm that was trying to source sheets of that Teflon material?

 

[jp233]

Mark - Hope you're enjoying the PM3200-14 ! I sure haven't seen many Macs with those goose logo stickers.

Funny that just after I shipped it, had a huge wind storm and put some small trees down. I used my Eager Beaver 2.0 mini mac (that you fixed) extensively... it worked fantastic

And took care of a tree that was leaning near a power pole at my kids school. It was only leaning on the guywire, not the pole or any wires. Shade tree operation on a ladder, but it worked perfectly. Lots of smaller branches able to get down before taking the big one. For the bigger stem though, a PM610 ripped through it like a hot knife through butter

 

[heimannm]

Mark