McCulloch Carburetors

[heimannm]

NOTE: Since the hack the photos were lost, I am trying to put this back together with some additional information I have picked up along the way.

Legal Disclaimer - what I am about to post is based on my experiences and research. I am a relative newb on the whole matter as I have only been messing with these for a couple of years. This comes with no promises except that I have done my best to describe things as well as I can. I welcome anyone to add their comments, suggestions, recommendations, etc.

Many are reluctant to work on the old McCulloch carburetors because they are not Tillotson HL's or other more familiar models. In addition, most of the old Mac's have a primer in place of a choke, which adds to their mystery and in many cases their tendency to be leaky. On top of that, the diaphragms that we can get today are pretty expensive, normally $30 for the set of metering and fuel pump diaphragms with their gaskets.

I have several saws with the old McCulloch carburetors and have been struggling with a few of them myself so lately I decided to just try and document step by step what I've been doing in hopes of helping others, as well as prompting some of you to jump in with better ideas and solutions where applicable.

I will be showing several different saws and carburetors as we go through this, but will try to make it flow in a reasonable logical manner to make it as easy to follow as possible.

Let's get started:

Here is my 795, this carburetor looks pretty bad, but the saw did run, just not very well.





On this saw I had to make a clip to hold the throttle push rod. I don't think the push rod was stock, or maybe the carburetor had been switched already. There should have been a little clip on the carburetor for the rod to slide through but the rod is so bent up it would never have worked anyway.



This is what the clip is supposed to look like. See the spring on the throttle shaft? More on that in the next post.



On this saw, there is no solid connection between the throttle push rod and the levers on the throttle shaft. When the push rod come forward it pushes the lever forward, and when the throttle is released it shuttles and pulls the other lever back.



Mark

 

[heimannm]

Many of these older McCulloch carburetors have an "idle speed governer", the throttle plate is off center so the velocity/pressure of the air moving through pulls the throttle shut. There is a tiny spring on the throttle shaft that pulls the throttle open. If you notice there are notches in the lever that allow you to adjust the spring to control how hard it pulls open, off setting the velocity/pressure through the carburetor.

At very low speeds, as the velocity/pressure drops the spring pulls the throttle open. It is a little unnerving at times to have an idling saw start to "hunt" speeding up and slowing down. Usually is speeds enough to move the chain a bit. I guess this was not a problem in the good old days when sawyers were men and lawyers were scarce.



Another Mac carburetor with the idle speed governor.



If you take the throttle shaft out, make sure the spring is to the left of the screw that hold the shaft clip in place.

The idle speed screw does ultimately control the minimum idle speed, the idle speed governer simply helps keep the engine running, sort of a built in throttle blipping device.

Mark

 

[heimannm]

While we are on the subject of governers, I also have a 740 with an air vane governer. On this saw the spring loaded air vane governer pulls the throttle open and a high speeds the air flow from the flywheel pushed the air vane closing the throttle. This is how it was done before the days of RPM limited electronic coils. Again the throttle linkage is not directly connected to the lever, but simply allows the spring loaded air vane to push the throttle open (or closed, depending on engine speed).

All together



This is a stock clip with the throttle push rod sliding through a small groove in the casting under the clip. Other saws even have a small felt wick in the casting that can be oiled to keep the shaft lubricated. Hey, that's a good idea, I will have to add that to this saw.



Here is a view of the arm of the air vane governer slipped over a lever arm on the opposite side of the throttle shaft.



Throttle closed, the spring on the throttle trigger pulling the rod back and overcoming the spring on the air vane trying to pull the throttle open.



Throttle rod extended forward, I don't know if you can see it but the rod has moved completely away from the lever on the throttle shaft and the spring on the air vane is pushing the throttle open.



Mark

 

[grandpatractor]

This is very interesting ,Mark!

 

[heimannm]

When removing these carburetors, I like to use a screwdriver and push the primer all the way forward and slip the linkage/push rod out.



To make a little more room to loosen the bolts, I was able to slip the fuel line off at the tank on this saw.



Other saws like my 795 have a molded fuel line coming through in the same spot and you just have to work around those. On the 795 I saw that there was a bit of leakage where the line went through the fuel tank. I read in another thread one way to solve this was to wrap a little teflon tape around the line, so I tried it and voila, no more leak.





Mark

 

[crmyers]

Good stuff, keep it coming.

 

[heimannm]

Now the carburetor is out, as I mentioned this one is pretty rough. This one has the fitting for the "fuel collector" hose that connects to the spit back shield at the back of the air box.



The idea is that since these reed valve engines will leak a little pressure back through the carburetor, the spit back shield collects the expelled fuel and the hose pulls it back into the carburetor. That's the theory anyway. The fuel collector line is disconnected from the carburetor in this view, that's another part of the story.



On the other side, is the infamous McCulloch primer.



Primer off, there are two o-rings that seal the primer to the carburetor body. Either standard #010 or #011 o-rings will work.



I use a small dental pick to work the "o-ring" out, this is in fact a square cross section rubber plug that the .125" diameter steel ball seals against. The spring keeps the ball seated until the primer is depressed, then the fuel discharged from the primer squirts directly into the space on top of the reed valves.

Here are the parts in order:



Mark

 

[heimannm]

The gasket that seals the carburetor to the air box is the same one used on the Tillotson HL carburetors. Notice that there are multiple holes for the impulse signal that runs the fuel pump. For whatever reason, the McCulloch (Walbro) and Tillotson carburetors have the impulse ports in a different location. Keeping this in mind can save a lot of headaches when you are switching carburetors.



Flip it over and remove the four screws holding the carburetor together. You may notice this is not the same carburetor as shown in the other photo's. I compiled this from several different carburetors I have been working on. Don't let this distract you too much.



The meter lever and needle are very similar to the Tillotson HS and Walbro SD type carburetors, the shaft is held in by a small 4-40 screw.



Here are the parts disassembled in order, the seat has a 5/16" hex just like the Tillotson HL series, needle, lever and pivot, spring, and the screw.



Mark

 

[heimannm]

The metering diaphragm & gasket. The new one is on the left, and an example of the old one on the right. It just occurred to me looking at the photo's that the new one has a "button" in the center, the old one was flat. Later on we'll see the metering lever is to be set flush with the body of the carburetor, how is that supposed to work if the plate on the diaphragm is flat?



The new gasket and diaphragm in the relative position they will be installed. So far so good but look, the new diaphragm has some additional holes around the perimeter.



And guess what, on some of the carburetor bodies the hole overlaps the O.D. of the bowl over the diaphragm. It took me about 6 tries on one carburetor to figure out where the awful leak was coming from. On another body, the dimensions were different and the hole was sealed. Look carefully and you can see the hole is partially uncovered in this case.



My solution? A little Seal & Stick solvent resistant adhesive/sealer. Just a little dab to fill the holes and stick the gasket and diaphragm together to prevent leaking past the hole.



What I am learning since first posting this thread some while back is the metering diaphragms with the "extra" holes are really for the kart carburetors, BDC models, etc.

Mark

 

[eyolf]

Many years ago I was parts and service guy for a small midwestern lawnmower disributor. Near the end of my employment, the Jacobsen company came out with a new ridng mower called the RMX, and it had a few serious problems in the drive system.

I stumbled onto a solution or two, and wanted to put together a pamphlet with nice photos and instructions. Cost was breathtaking. You've put together a presentation that puts what I had in mind, then, to shame. And the cost was, well, lets say "affordable"

Aint technology great?

And great job, BTW!

 

[heimannm]

I suppose at some point I should mention that the whole thing needs to be cleaned up while everything is taken apart. I usually remove the H and L needles as well as the metering needle and try to clean all of the bores and passages with aerosol carburetor cleaner. So far I haven't run into many that needed more than that in the way of cleaning the gunk out.

Before installing the new metering needle, I thought I'd measure up a relatively new one for the sake of comparison. Here is the lever and pivot, spring, and needle. The new spring is 1/2".



And here is the metering needle installed in the body of the carburetor. Now I see how the flat plate on the original diaphragm works, sorry but I got confused for a minute as I did an SDC carburetor today as well. Look at one of those (SDC carburetor) and you will see the the lever is in a sort of notch in the body of the carburetor and only the "button" on the plate attached to the metering diaphragm will activate the lever.



The meter lever is to be set flush with the body of the carburetor. I have had a few problems with flooding if the lever gets a bit too high. I try to keep it flush or .005 to .010" low.



Mark

 

[heimannm]

Thanks eyolf, I am hoping that others with more and better information will jump in and supplement what I am trying to do here.

Now for the fuel pump. If you get the parts from Bob, chances are the fuel pump will be three separate loose pieces, two gaskets and the valve/diaphragm piece.

The Bob kit, metering diaphragm and gasket on the left, fuel pump diaphragm/valve in the center, fuel pump gaskets on the right.



The OEM part was a sort of sandwich, with the diaphragm/valves between the gaskets, special little slots for the valves to fit through, and stapled together. OEM kit on the right


When I first started struggling with these carburetors, I tried all sort of combinations, making a sandwich like the original, using only one gasket with the diaphragm flat against the fuel pump body, two gaskets on one side with the diaphragm flat against the fuel pump body, O.K. you get the idea.

What did I learn? If you prime the carburetor manually, any combination will work. I have only had success with the McCulloch primer working if the diaphragm is flat against the fuel pump body with the OEM type diaphragm and gasket set up. I am not sure if that is real, or if it just worked out that the "sandwich" attempts had other problems with the primer circuit. Anyone else out there have any ideas/experience?

The Bob diaphragm/valve laying flat on the fuel pump body.



The Bob diaphragm and gasket, diaphragm below and gasket on top.



I thought perhaps the three piece Bob Johnson parts were actually for the kart carburetors, like some of the metering diaphragms you might get from Bob, but looking at the carburetor book I see that all of the MAC and BDC carburetors use the same fuel pump diaphragm/gasket combination, PN 57274. Anyone else out there have something to add?

Mark

 

[heimannm]

I thought as long as I had it apart, I would measure the primer spring as well. Looks to be just over 3/8" long. I measured the steel ball and confirm that the IPL's that describe it as 1/8" are correct, some incorrectly call out 1/4" diameter. Also confirming the o-rings that seal the primer to the carburetor can be either #010's or #011's.





Also confirming an earlier mention, the gaskets that seal the carburetor to the air box are same shape as the Tillotson HL, makes sense since the carburetors will also interchange, but the impulse ports are in a different location. I have not made mention that the impulse to run the fuel pump in the carburetor comes through the flange on the front of the carburetor. In every case make sure the gasket has a hole that aligns with the carburetor and airbox.





One more thing while I remember, the OEM o-ring on the needles is also a square profile, but I have found an standard #004 o-ring will also work pretty well.



Mark

 

[hexen71]

I have nothing to add but keep up the good work because I find it very informative and will be using this gide in the next few weeks as I just acquired 5-10e

 

[crmyers]

This should be a sticky.

 

[heimannm]

Do the carburetors work after I got done messing with them?

As I mentioned, I can't make the McCulloch primers work with the Bob fuel pump gaskets/diaphragm/valves, but on the 640 with the OEM type it works fine. Since I don't use these saws for regular cutting, I don't mind priming them manually to get them to start, and once started they will restart if they don't set for too long.

I am not sophisticated enough for video, and I don't have any logs to cut anyway so you will have to trust me on the photo's of the tach being representative. You could also ask my son Jeff since he came to help this afternoon and saw all three (640, 740, 795) running.

The 640 WOT unloaded - this one really surprised me. If I remember correctly (it has been a long time since I first started on this thread) I did have to go back through that carburetor and reset the metering lever height to keep it from leaking fuel when it was setting.



The 740 idle and WOT





The 795 idle and WOT, it started to run out of fuel just as I was trying to snap the photo but I swear I saw it go over 9,000.






Well guys, that's it for tonight. I hope some of you find this helpful, and other will pipe up and let us all know how we can do even better.

Oh, does anyone know where I can get about 6 of the metering needles for the McCulloch carburetors as well as a few extra primer seats? After all this I still have a few leakers and it appears to be coming out of the carburertor throat while the saw is setting not running.

Thanks!

Mark

NOTE: I am 99% sure now that the metering needles from the SDC carburetor will work in the MAC carburetors. I did manage to find some of the seats as well, now if I just had time to stay home and work on saws...

 

[Justsaws]

Very good tutorial.

It looks like you would have to mount the AM fuel pump the way that you did to get it to push prime. Is it necessary to use both gaskets on the AM kit to get the needed clearance for the pump and flaps valves to function?

I missed the part about not being able to get the primer to work with the AM kit. Any chance that using the gasket with the slots from the OEM kit with his pump would fix that?

 

[heimannm]

I have nothing to add but keep up the good work because I find it very informative and will be using this gide in the next few weeks as I just acquired 5-10e

Is your 5-10E an electric start? Cool beans, I have a 3-10E and a 5-10. The 3-10 has a Walbro SDC carburetor, the 5-10 had the McCulloch flat back/bullfrog but someone beofre me converted it to a Tillotson HS I believe.

The 10 Series flat back/bullfrog is another animal altogether. I have a little experience with them so far, more to go there certainly. Perhaps as some point in the future I will gain some additional practice with the 10 Series and try to do something with those as well.

Good news is the primers are very good if you have all the right parts. Bad new is the little duck bill check valves are virtually impossible to find anymore.

Mark

 

[heimannm]

Very good tutorial.

It looks like you would have to mount the AM fuel pump the way that you did to get it to push prime. Is it necessary to use both gaskets on the AM kit to get the needed clearance for the pump and flaps valves to function?

I missed the part about not being able to get the primer to work with the AM kit. Any chance that using the gasket with the slots from the OEM kit with his pump would fix that?

I have not been able to locate the OEM fuel pump kit, only the parts I have received from Bob. B200 told me he got some from Bob and they were the OEM style. Bob has lots of stuff and his inventory changes a lot so you get what he has when you order.

Mark

 

[Justsaws]

I have not been able to locate the OEM fuel pump kit, only the parts I have received from Bob. B200 told me he got some from Bob and they were the OEM style. Bob has lots of stuff and his inventory changes a lot so you get what he has when you order.

Mark

It is late and I might not being seeing the obvious answer in the photos. What I was asking about is whether or not you could use the old OEM gasket with the slots in it and the new AM pump diaphragm from Bob's kit in order to get the full up down motion restored to the fuel pump.