McCulloch Carburetors

heimannm

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I started a similar thread over at AS a while back, some folks found it helpful. I have learned bit since then and thought some of you over here might find it helpful as well.

I also want to say a special thanks to PogoInTheWoods for sticking with me and helping to get the photo posting issues worked out. I also want to thank Cliff for the Walbro literature he posted a long time ago.

75walbro.jpg

Many older McCulloch "large frame" saws came with the McCulloch flat back carburetors made by Walbro. Many of these were equipped with a primer rather than a choke but later saws, the Super 250, Super 550 and 895 in particular had a choke equipped flat back. The S250 and S550 have the stud to attach the AF cover, the model used on the 895 did not have the stud.

First off, to make it easier to work on the Super Series saws in particular, I modified a 7/16" combination wrench to fit in some of the confined spaces. This particular carburetor is the primer version from a saw with the "spitback" device. Notice the fitting circled in red, more to come on that later.

Mc1.jpg

And here is a walk around of the choke version. This happens to be a NOS one I got from Scooterbum a long while back. If you take a close look at the throttle lever on this one you can see some notches on the bottom and a spring wrapped around the throttle shaft. More on that later.

Mc2.jpg

Mc3.1.jpg

Mc3.2.jpg

Mc3.3.jpg

Mark
 
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heimannm

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Our first candidate, this one happens to be a 795. The "Super" series engines had the bracket in the top of the air box for attaching the AF cover rather than the stud like the front tank saws.

Mc3.jpg

As you can see where the carburetor attaches to the flange space is tight and the special wrench comes in handy.

Mc4.jpg

There is supposed to be a little metal bracket/clip on the carburetor body to guide the throttle rod (see the choke equipped carburetor above) but it was broken off of this carburetor. I fashioned a retainer from a piece of 1/2" wide steel strapping that works reasonably well.

Mc5.jpg

Here is a close up of what it is supposed to look like.

Mc6.5.jpg

Notice that there is no hard connection between the throttle rod and the throttle lever. When the trigger is squeezed the throttle rod moves forward and pushed the throttle open, when the trigger is released the rod pulls back and pull the throttle lever back but does not pull it tight against the stop to allow the idle speed governor to function.

Mc6.jpg

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heimannm

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This particular saw had a problem with a fuel leak where the fuel line passes through the tank. Based on advise from Hoss a long while back I tried a few wraps of Teflon tape around the hose and pulled it through again, no more leaks.

Mc7.jpg

Mc8.jpg

Other saws have a barb fitting in the tank so you can disconnect the fuel line before removing the screws holding the carburetor in place. Notice that this one has the spitback shield and the hose connected to the barb fitting on the side of the carburetor.

Mc11.jpg

This one also uses a different technique to guide the throttle rod, a small clip under the AF bracket hold the throttle rod in a groove cast in the air box housing. There is even a small felt wick in there to provide a little lubrication for the throttle rod as it shuttles back and forth. Note on this one the throttle shaft extends all the way through the carburetor and connects via a linkage to the lever arm from the air vane governor. In the days before electronic ignition with built in rev limiters, the air vane governor was a common way to try and prevent an engine from over speeding.

Mc14.jpg

On yet another saw you can see the groove for the throttle rod more clearly. The carburetor on this saw is not correct as it has the stud (not used on this one) and the hose from the spit back shield has no associated fitting on the carburetor.

Mc13.jpg

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heimannm

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A close up view of the throttle connection on the governor model, on these saws the throttle indeed is intended to pull the throttle lever back against the idle speed adjustment screw.

Mc15.jpg

And a closer look at the governor link on the other side. For what ever the reason, I don't have any photos of the air vane portion of the governor but as the name implies, there is an arm with an air vane (think small paddle like device) adjacent to the flywheel, as the engine speeds up the air from the flywheel pushed harder against the vane and pushes the throttle closed just like the governor on your old Briggs & Stratton powered lawn mower. (Side note, Robert McCulloch was married to Barbara Ann Briggs, yes that Briggs family...)

Mc17.jpg

And here you can see the throttle trigger is squeezed and the throttle rod move forward letting the spring on the air vane governor push the throttle open. It can take a little trial and error getting the air vane governor arm and spring set just right to let the engine run up to speed and yet still effectively limit the RPM to prevent over speeding. If you block off the air intake on the flywheel cover when resting the saw against your leg you can here the RPM pick up since the air flow is reduced and the spring pushes the throttle further open.

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heimannm

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One of the first things I do when taking the carburetor out of one of these saws is disconnect the primer by pushing the plunger forward with a screwdriver and slipping the primer rod out as it makes removing the carburetor much easier later on.

Mc21.jpg

As noted earlier, if the saw has the fitting through the tank, disconnect the fuel line to give yourself a little more room to work when removing the carburetor.

Mc22.jpg

The carburetor is now out of the saw. If you look very carefully in this photo you can see the idle speed governor spring sticking through the notches in the bottom of the throttle lever about the 7:00 position.

Mc24.jpg

The way the idle speed governor works is: the throttle butterfly is off center on the throttle shaft. The spring pulls the throttle open, and air flowing through the carburetor pulls the throttle closed. If the engines slows down and threatens to die off, the spring pulls the throttle open a bit. I like to think of it as a self blipping device. On my 550 in particular, when the saw is setting at idle the engine really lopes like a V8 with an exotic cam. At times it speeds enough that the chain moves a bit but of course that was not a problem in the day since sawyers were men and there were a lot fewer attorneys with nothing better to do .

Mc25.jpg

On the other side of the carburetor, the fitting for the hose coming off the spit back shield.

Mc26.jpg

First step in carburetor disassembly is to remove the primer. There are a couple of O-rings that seal the primer to the saw, I find that either #10 or #11 will work and not sure which is in fact the right one.

Mc28.jpg

Many of these primers leak, and replacing the O-rings underneath the primer does no good, the problem is internal to the primer. I have not yet figured out a way to dismantle one to investigate the insides but I have destroyed a few trying.

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heimannm

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Under the primer is a check valve assembly consisting of a spring, check ball, and seat. The ball is simply and 1/8" diameter steel ball (don't believe some IPL's that mistakenly list it as 1/4") and the seat is in fact the same part as used in the Tillotson HL needle and seat on the metering circuit. I believe that the flaps in the fuel pump make up the other check valve needed to make the primer operational.

Mc30.jpg

The gasket that seals the carburetor to the flange on the air box is the same size and shape as the one used on Tillotson HL carburetors with one significant difference, the impulse holes are in slightly different locations. This has been the source of a lot of aggravation for many of us working on the big old McCulloch saws.

Mc32.jpg

Mc33.2.jpg

Mc33.5.jpg

Begin disassembly by removing the four screws on the bottom of the carburetor.

Mc33.jpg

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heimannm

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Removing the fuel pump and metering body from the carburetor reveals a fairly conventional metering lever, pivot, needle, spring, and seat. Under the 1" welch plug is a sintered metal capillary seal on the older carburetors, or a 3/32" diameter nylon ball and seat on later models. The nylon ball and seat were a service replacement part for the capillary seal and the parts can often be found if you dig around a lot of old McCulloch parts. The ball and seat are analogous to the high speed check valves found in many of the later cube style carburetors. Sorry but I don't have any photos of what's under the plug.

Mc34.jpg

The needle used in the MAC carburetors is the same as the one used in the Walbro SDC carburetors, the metering levers are different though. The McCulloch service parts included the needle and both levers in one kit.

Mc35.jpg

Here is another look at the primer and check valve bits in case you forgot what the look like or the arrangement of the parts.

Mc36.jpg

The spring is just over 3/8" free length.

Mc37.5.jpg

And the metering spring is right at 1/2" free length.

Mc40.jpg

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heimannm

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I like to remove the H and L needles when I have one apart and clean them out pretty thoroughly with carburetor cleaner. The original equipment seal / tension device on the needles is a square cross section O-ring, but I find that a conventional #004 will work if you are careful getting it fit into the carburetor body when replacing the needles.

Mc37.jpg

Mc39.jpg

When you put it back together, make sure the metering lever is even with the body of the carburetor. I have had a few problems with a few saw flooding so I try to set it 0.005 to 0.010" low.

Mc41.5.jpg

There are a few different options for gaskets and diaphragms for the MAC carburetors. Parts come up on e-bay, Bob Johnson in New York has had them in the past, and Joe Salva at Sugar Creek Supply makes a decent reproduction set. Customchainsawparts (e-bay seller) normally has the Sugar Creek Supply reproductions listed for under $30. You won't find a "carburetor kit" with everything like the needle, check valve, and welch plugs.

Here are the parts I have gotten from Bob Johnson, I didn't realize it at the time but this metering diaphragm is in fact for the BDC carburetors used on the kart engines and needs a little work if it is to be used in the saw carburetor.

Mc41.jpg

This is what the OEM type parts look like.

Mc43.jpg

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heimannm

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The OEM fuel pump diaphragm/gasket arrangement is a sandwich, with the flap seal sticking through slots in the gasket to insure a good seal with the body of the carburetor.

Mc44.5.jpg

To simulate this with the parts from Bob I have tried all sorts of configurations, sandwich, diaphragm directly against the pump body with one or two gaskets on top, etc. If I prime the saw I have found any of the combinations will start and run but I have not been successful getting the primer to work with anything but the OEM parts arrangement.

On the metering side, I pointed out the diaphragms with the extra holes are for the BDC carburetors and won't work as they are with the saw carburetors.

Mc44.jpg

The BDC diaphragm (66092) is in the middle, the OEM saw diaphragm (55067) on the right.

Mc45.5.jpg

The gasket is correct, from all of the IPL's I have looked at including many different saws and the McCulloch Carburetor guide I have never seen it listed as a separate part but always a part of the diaphragm in spite of the fact they are loose parts.

Mc46.jpg

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heimannm

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The problem is not obvious, but it is shown here.

Mc47.jpg

You can't really see the problem here either.

Mc49.5.jpg

The extra hole in the BDC diaphragms at 12:00 and 6:00 are not completely sealed when fit in the body of the saw carburetors.

Mc49.jpg

Mc51.jpg

Mc53.jpg

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heimannm

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Not realizing I had the wrong diaphragms, I "fixed" the problem by sealing off the holes with some Seal & Stick I had on hand.

Mc55.jpg

Back to the fuel pump for one last look. Three piece diaphragm and gasket on the left, OEM sandwich combination on the right.

Mc59.jpg

I lay the diaphragm directly against the body of the carburetor...

Mc61.jpg

and put the gasket on top.

Mc63.jpg

Then stack them up and screw it together.

Mc65.jpg

Mc66.jpg

Mc67.jpg

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heimannm

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One last time, make sure your gasket has the impulse hole in the proper location and that it matches up with the impulse port in the flange.

Mc69.jpg

This is the best photo I could find in a pinch showing the impulse port location on the flange where the carburetor attaches to the air box.

Mc70.jpg

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heimannm

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Do they work?

Here is the 640 at idle and WOT, I was surprise to see that one running over 11,000 RPM. Don't worry Aaron, I didn't hold it there for long.

Mc73 640 Idle.jpg

Mc74 640 WOT.jpg

Here is my 740 at idle and WOT, I would say 9,500 for the old girl was going pretty well.

Mc75 740 Idle.jpg

Mc76 740 WOT.jpg

The 795 idle and WOT, it was running out of fuel just as I was finishing up but I swear it went over 9,000 as well.

Mc77 795 Idle.jpg

Mc78 795 wot.jpg

And there you have it.

I will do some digging to see if I happened to get any photos of the BDC14 I had apart last summer working on my kart saw project.

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heimannm

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Nice Randy, a front tank flat back with no idle speed governor and a 9/16" gear drive sprocket.

I failed to mention earlier, the H and L settings for the flat back carburetors while similar to the Tillotson, are not exactly the same.

I generally start with the L 1-1/4 to 1-1/2 turn open and make a fine adjustment from there.

The H should start with 1 turn open and generally ends up between 3/4 and 1 turn open. In my experience, it is rare for the H to be adjusted more than 1 turn open. Just thought I should include that information.

Mark
 

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