McCulloch Chain Saws

[Maintenance supervisor]

The first picture is a check valve that turned bunchy and the second was flushed out with carb cleaner.

 

[Jethro 2t sniffer]

Maint sup that's very good handy dandy info there

 

[Maintenance supervisor]

Maint sup that's very good handy dandy info there

Well thank you , I struggled with certain saws knowing that they ran well and after doing some homework in the forums and asking questions about the sdc series carb I found that the check valve can be a serious mystery in tuning and frustration, half the time I think when we swap one carb out and say "wow this saw runs great !" Its because of the check valve being messed up in the original.
Changing them isn't easy either , I highly recommend baking them in the oven for an hour at 250° so the aluminum will expand and the brass insert will pop out.

 

[fossil]

Some of the HS carbs have capillary screen check valves.

 

[huskadolmar]

I have been told by my friend to ask vinny.

 

[huskadolmar]

No issues on high side...just low and the change up....22 degrees was temp...cold start. Warms up and drop from cut speed gets lean. Cut speed always dead right. Wont idle well hot and starts like a junker. Great compression. Cant seem to tune it out. Multiple carbs. Im stuck at this point. Cuts nice but a pain to keep running and a handfull to start.

 

[huskadolmar]

I should mention it vacum tested well

 

[Maintenance supervisor]

I have been told by my friend to ask vinny.

Ive had a saw run with the impulse hole partially blocked ran great high rpms because the venturi of the carb pulls fuel with vacuum but it would die at idle when dependent on the impulse Carb,block, insulator Gaskets all stacked up on each other could be the issue or a split in one. 10-10s had a split in one of the Gaskets did the same thing ,Remington super 754 had the impulse blocked did the same thing, also had a craftsman with a blocked impulse line same thing.

 

[huskadolmar]

Ive had a saw run with the impulse hole partially blocked ran great high rpms because the venturi of the carb pulls fuel with vacuum but it would die at idle when dependent on the impulse Carb,block, insulator Gaskets all stacked up on each other could be the issue or a split in one. 10-10s had a split in one of the Gaskets did the same thing ,Remington super 754 had the impulse blocked did the same thing, also had a craftsman with a blocked impulse line same thing.

Impulse clear but i will check again anyway. She blows hard through the impulse. Actually i had to put a ziptie on fuel filter because the impulse was blowing it off as i let off of full throttle

 

[huskadolmar]

Hate to tear it down again but i might just to be even more sure

 

[Gaspowered]

Ok the welch plug with the screen in it, there is a brass insert that holds the screen with a ring holding the screen , UNDER THE SCREEN there should be a flat peice of diaphragm material or a tan piece, usually its bunched up or turned to goo .... if you remove the plug and open the throttle while shinning a light through the bore you should NOT see light under that screen aka the check valve should be blocking it, now if its the tan material it may be opaque enough to see some dim light.
I have not had any luck removing the screen and stuffing the tiny material in place so a check valve kit is always on hand 86-523 walbro kit number.
Tillotson carbs do not have a screen hence no check valve/ screen = check valve.

When I opened up this carb I didn't see anything at all in there. Nobody had ever been in it before me. I guess I forgot to install it... I do have fuel pump diaphragm material and I could just stamp one out with a gasket punch. Now I vaguely remember seeing that little piece of diaphragm in the kit. That was probably the issue to begin with.

 

[Maintenance supervisor]

Impulse clear but i will check again anyway. She blows hard through the impulse. Actually i had to put a ziptie on fuel filter because the impulse was blowing it off as i let off of full throttle

Well that seems strange ? How far into assembly did you do a pressure check?
A strong impluse could suck a fuel line flat , blowing the fuel filter off seems counterintuitive to the design?

 

[huskadolmar]

Well that seems strange ? How far into assembly did you do a pressure check?
A strong impluse could suck a fuel line flat , blowing the fuel filter off seems counterintuitive to the design?

I checked it at carb mount level with vacuum. And though i may be wrong, it is my understanding that the impulse shoots pressure against the diaphram which draws fuel into carb. My 850 also pushes pressure out impulse when cranking and it runs perfect. Unlike husky saws that draw vacuum through impulse. Now i dont know if it changes while running. I dont know for sure. I find it strange that it produces positive pressure at filter and i cant figure out why. You may have a point. What i wonder could cause positive pressure at filter but allow full vacuum at a rev

 

[huskadolmar]

I wonder if it is flattening fuel line and backing up when i let off?

 

[huskadolmar]

Ive had a saw run with the impulse hole partially blocked ran great high rpms because the venturi of the carb pulls fuel with vacuum but it would die at idle when dependent on the impulse Carb,block, insulator Gaskets all stacked up on each other could be the issue or a split in one. 10-10s had a split in one of the Gaskets did the same thing ,Remington super 754 had the impulse blocked did the same thing, also had a craftsman with a blocked impulse line same thing.

Yup far as i can tell, the impulse is clear.

 

[vinnywv]

I have been told by my friend to ask vinny.

I have had more than one 82cc mac do that to me. 2 were pm800,s. On the 800's both times it was an air leak. Same on both it was the boot. It was not sealed well at the fuel tank. So back apart and i put some rtv around the top of the boot, not too much, and put it together. After several hrs of curing i checked them and bingo Idled fine and ran perfect. There was no holes or tears they just did not seal well against the fuel tank.
Same for a pm850. I make it a point now to just do that to save the aggravation. On reassembly seal it with something. I use red rtv cause i have it. I hope that may help you.

 

[vinnywv]

I cant answer the fuel filter thing...thats new for me. Id make double sure the carb was together in the correct order. That is odd.

 

[Maintenance supervisor]

I wonder if it is flattening fuel line and backing up when i let off?

Easy fix with a spring in it , an old transmission drive shaft seal has a good long spring in it.
The way the carb is designed doesn't allow impulse to reach the fuel the diaphragm is the barrier between air and fluid.
If you can run it with the cap off and watch the filter blow off there shouldn't be any bubbles, if there is something is definitely wrong.

 

[Maintenance supervisor]

This is interesting to me, thats an hl tillotson series base gasket on a ht series carburetor.
I might be able to fab it up for my 1-60? After impulse nipple added to the manifold port.

 

[huskadolmar]

I have had more than one 82cc mac do that to me. 2 were pm800,s. On the 800's both times it was an air leak. Same on both it was the boot. It was not sealed well at the fuel tank. So back apart and i put some rtv around the top of the boot, not too much, and put it together. After several hrs of curing i checked them and bingo Idled fine and ran perfect. There was no holes or tears they just did not seal well against the fuel tank.
Same for a pm850. I make it a point now to just do that to save the aggravation. On reassembly seal it with something. I use red rtv cause i have it. I hope that may help you.

When i have time, i will take it back apart and reseal the intake boot. It makes sense since it gets worse when its warmed up. Its worth a shot.