McCulloch Carburetors

[heimannm]

The blue port is the inlet coming from fuel channel connecting the fuel pump to the metering section and will be under fuel pump pressure when the saw is running.

The red port leads to a passage downstream from the venturi that directs the output from the primer into the intake of the engine. There is a check valve (ball and spring) in that port to ensure the output from the primer is discharged and not simply drawn back into the primer when the spring returns the plunger. This is a true primer that puts a charge of raw fuel directly into the intake.

The body of the primer provide enough resistance to prevent the fuel pump from forcing fuel past the check valve into the engine which would upset the mixture, or leaking past the primer when the engine is stopped which would lead to flooding while setting. Parker had this experience during his early modifications which lead to the search for the higher pressure spring for the ball check valve.

I think the issues with the modern bulb type "primers" is they would not have the needed resistance and would allow fuel from the pumping section to be forced directly into the intake, and the amount of fuel that is displaced by the bulb type primers is so great that one actual discharge cycle would badly flood the engine.

This is all speculation on my part...

Mark

 

[SawTuner]

Awesome write up Mark.
I have bench tested the primers before putting them on the carb and have had them hold well over 30 psi, testing from the fuel inlet port. The o-ring that goes in before you press the gland in needs to be just passed the inlet fuel hole in the primer body. You can see it as you are pushing it all together. If it is not passed the inlet hole I have found they will not hold pressure.
Hopefully this all makes some sense
Thanks
Parker Jameson

 

[Alaskan Stihl]

Eager Beaver Carburetor Issue:

So, it was my fault in regards of creating this catastrophic situation….

I am in the process of getting this McCulloch Eager Beaver back in running condition. After disassembly of the carburetor (Walbro MDC), I discovered that the Metering Lever clearance was too low.

So…you got it…I tried to lift it up a bit and it fractured and kinked the Forked End that slips into the needle valve.

Of course, none of the rebuild kits that I found on eBay comes with the Metal replacement lever.

Luckily, I have a stack of spare carburetor parts that I inherited. I dug through the pile and discovered the previous owner had several levers…which appear to be of aftermarket” construction.

I tried to fit in one of these levers, however it is too wide to fit into the recess area, however…looks like I might be able to file down both sides a bit and get it to fit correctly ….and luckily, I have several of these levers at my disposal so for trial and error until I get a modified one to fit properly.

Question: Has anyone attempted to use this type of lever and modify it to fit (see picture)?


Thoughts?
Alaskan Stihl

 

[heimannm]

Metering lever is on the way to Alaska...

I went through another batch of primers yesterday/today, it can take a long time to clean all of the old, decomposed rubber rubber from all of the working parts.



This time around, the two with the steel plungers (use 67116 v-seal) only needed o-rings, the v-seals were still fine. I did use an 010 o-ring in place of the v-seal on the first batch and that works as well.

As Parker mentioned, getting the gland pressed to the correct depth is important to ensure the plunger presses firmly against the o-ring to create a good seal when the plunger is pushed back by the spring. This will allow the primer to hold back the fuel pump pressure when the saw is running and prevent fuel from leaking past when the saw is setting, especially when warm which would lead to flooding.

With my new special tools the job went fairly quickly getting all the primers apart.

First, I modified a 1/2" deep socket by adding a set screw to lock the prime in place while drilling the pilot hole in the end. Just FYI, even cheap Menards sockets are very hard and tapping for the 10-32 set screw is quite challenging. There is a lot of variation in the dimension of the 1/2" square aluminum stock they used to make the primers so I put a shim opposite the set screw, some .003, some .005, some .006" to try and keep the hole centered



I used a small center drill in a drill chuck in the tailstock to make the pilot holes, then switch to a peck drill with the correct size bit to tap for the 10-32 screw to plug the hole. The center drill won't wander when you start the hole keeping it centered on the body of the primer and the peck drill provides very good feel as the drill goes through so you can avoid damaging the plunger or spring if you go too deep.



Disassembly as before - press the primer apart using the socket to hold it and provide clearance for the plunger to come out.



I did not take a photo but I just chucked a tap in the peck drill and tapped the hole by rotating the lathe chuck by hand, I tried to gauge the feel of the tap and not cut the threads full depth all the way through. This way the set screw will seat in the fairly thin body quite securely, the addition of a little red LocTite makes it quite leak proof.

I did notice that the springs were not all alike, some are LH wound and some are RH wound. There did not seem to be any consistency to which ones had which spring (steel plungers were all LH, some of the aluminum plungers were LH but most were RH).



And finally, while most of the carburetors had a primer discharge port in the bottom of the throttle bore, at least one version (61857) discharges through the impulse port.



And before I forget, a 005 o-ring will make a good substitute for the square o-ring on the H and L adjust needles if the originals have lost their grip.



Mark

 

[SawTuner]

You've got quite the pile of primers going there!

 

[2broke2ride]

Metering lever is on the way to Alaska...

I went through another batch of primers yesterday/today, it can take a long time to clean all of the old, decomposed rubber rubber from all of the working parts.

View attachment 1004799

This time around, the two with the steel plungers (use 67116 v-seal) only needed o-rings, the v-seals were still fine. I did use an 010 o-ring in place of the v-seal on the first batch and that works as well.

As Parker mentioned, getting the gland pressed to the correct depth is important to ensure the plunger presses firmly against the o-ring to create a good seal when the plunger is pushed back by the spring. This will allow the primer to hold back the fuel pump pressure when the saw is running and prevent fuel from leaking past when the saw is setting, especially when warm which would lead to flooding.

With my new special tools the job went fairly quickly getting all the primers apart.

First, I modified a 1/2" deep socket by adding a set screw to lock the prime in place while drilling the pilot hole in the end. Just FYI, even cheap Menards sockets are very hard and tapping for the 10-32 set screw is quite challenging. There is a lot of variation in the dimension of the 1/2" square aluminum stock they used to make the primers so I put a shim opposite the set screw, some .003, some .005, some .006" to try and keep the hole centered

View attachment 1004801

I used a small center drill in a drill chuck in the tailstock to make the pilot holes, then switch to a peck drill with the correct size bit to tap for the 10-32 screw to plug the hole. The center drill won't wander when you start the hole keeping it centered on the body of the primer and the peck drill provides very good feel as the drill goes through so you can avoid damaging the plunger or spring if you go too deep.

View attachment 1004804

Disassembly as before - press the primer apart using the socket to hold it and provide clearance for the plunger to come out.

View attachment 1004805

I did not take a photo but I just chucked a tap in the peck drill and tapped the hole by rotating the lathe chuck by hand, I tried to gauge the feel of the tap and not cut the threads full depth all the way through. This way the set screw will seat in the fairly thin body quite securely, the addition of a little red LocTite makes it quite leak proof.

I did notice that the springs were not all alike, some are LH wound and some are RH wound. There did not seem to be any consistency to which ones had which spring (steel plungers were all LH, some of the aluminum plungers were LH but most were RH).

View attachment 1004810

And finally, while most of the carburetors had a primer discharge port in the bottom of the throttle bore, at least one version (61857) discharges through the impulse port.

View attachment 1004812

And before I forget, a 005 o-ring will make a good substitute for the square o-ring on the H and L adjust needles if the originals have lost their grip.

View attachment 1004813

Mark

Do you intend to offer the rebuilt primers for sale or perhaps on an exchange basis? If so, I would certainly be interested in at least a couple.
I have a few flatbacks that I would be running if I had working primers. The choke flatback in my Super 250 runs quite nicely. I swapped to a Tilly in my 550 due to an inop primer and I have a 450 on deck.

 

[heimannm]

That is my intent Kevin, sell outright or offer a discount on an exchange basis.

PM me if you would like to know more.

Mark

 

[MacAttack]

I know this contributes little / nothing to the conversation but it's fascinating to me that McCulloch ever used a complex primer system instead of a simple butterfly choke like most every other small engine before the primer bulbs appeared... do I have that correct? A primer in place of a choke?

 

[heimannm]

McCulloch wasn't the only saw manufacturer to build in an actual primer (not just the purge bulb like the modern saws) to put a charge of fuel either into the intake, or on some models directly into the crankcase to assist in cold starting. Everyone was looking for a reliable, easy starting saw and when they work properly the primers will provide that.

Mark

 

[MacAttack]

It actually makes a lot of sense, nothing gets a saw firing quicker than a shot of fuel straight into the crankcase or cylinder.

Especially if a saw has been sitting a while, there's nothing fun about having to get the carb primed up again.

Cool stuff.