McCulloch Carburetors

[burl1984]

I got the saw to Idle! I'm still not able to throttle up though. I rigged up the throttle with a section of stainless tig add rod and every time I attempt to throttle the saw ot adjust the high needle from the lightly seated position the saw shuts off instantly. I'm not sure if it's getting too much fuel during those times or if it gets too much air to fuel ratio and shuts off. Either way my investment is starting to pay off!

 

[burl1984]

Here is a video of it running but it's more like an audio clip, it got dark on me after work. But I'm glad it's running!

 

[burl1984]

Here it is throttling up. I'm going to take it to a local shop to have them help me tune it better and possible build a better cone pipe.


I think the previous BDC14 that I had installed on it was modified for Alky and was just getting way too much fuel.

 

[heimannm]

Nice to hear it run, congratulations on your perserverence.

Mark

 

[Sean McCarley]

this link helped me out very much. after i rebuilt my carb in my 895 it wasnt getting fuel, then i took it apart again and put the diaphram on the bottom and one gasket on top and it ran like like a boss. thank you very much marc for helping me out here

 

[ilikeurtractor]

Great post guys - thanks for taking the time to document. I picked up a McCulloch 380 with a flat back carb a couple of months ago and it refuses to pump fuel. If I take the pick-up tube out of the fuel tank and fill it manually while keeping it vertical it runs and idles like a champ (gravity feeding the carb). If I run clear tubing from the carb to a small jar of fuel below the saw it will only make bubbles when I turn the engine over (spark plug removed for ease of turning the motor over). So I'm guessing it is either the diaphragm for the pump or a check valve. Reading this post there seem to be talk of a check ball and spring under one of the welch plugs but I can't find specifics of what this check ball may do or if it's even there for sure. I'd hate to remove the plug(s) to not find anything. I'm going to start with a new diaphragm but if that doesn't work I'm not sure what to go to next. The existing diaphragm seems a little stiff compared to the rest of the layer material, but I can't tell if it's stiff enough to not function. I suppose this would hold true for the "duck bill" checks as well which also seem a little stiff. I'll let you know what I find out after I change it. Any information on what's under the welch plugs and the function in the metering diaphragm (main carb body) would be great. Thanks again and I hope I can add to the post!

 

[heimannm]

The check valve was only found on later versions, earlier one had a sintered metal "capillary seal", the purpose of either was for the high speed circuit flow control not the fuel pump.

Your case sounds like the fuel pump check valves on the diaphragm. Make sure the carburetor plates are clean and smooth where the check valve flap seal and that the flaps themselves are in good condition. I often find I can prime one directly down the throat of the carburetor and get it to start and run even when it won't pump fuel just by cranking it over with the rope.

Mark

 

[ilikeurtractor]

Mine would "kind of" run on its own when the fuel tank was full. It was very difficult to start and wouldn't idle very long even at that. When the saw was hot, the fuel tank pressurized and it would run better. After my first use of the saw I set it down in my garage and it sounded like a leaking tire. I wasn't sure what it was until I realized it was coming out of the cork-type plug in the fuel cap. I hope the saw doesn't continue to run hot after I get the pump fixed and the mixture screws set slightly rich.

 

[ilikeurtractor]

Here is a picture of one of the plates. Is the "capillary" sintered plug what I have shown in the picture? I also noticed a nick on the edge of one of the check flap holes. Hope this isn't my problem. If so, I'll try to seal it with some type of sealant.

 

[ilikeurtractor]

Well I installed new diaphragms and I guess in the end it runs better, but still seems like it should be better. I first changed the fuel pump diaphragm thinking that would get it. Then I tried the pump test where I tried to pump through clear tubing from a glass jar of fuel a couple feet below the saw. That time it didn't even seem like it would do anything, not even bubbles. Hooked it up to the tank and was the same as before - wouldn't start or idle. Then I changed the diaphragm associated with the needle and seat. Same results. Then I changed the gasket between the carb and intake port and that seemed to have helped some. I did test the small intake port path that goes to the fuel pump by holding a small piece of tissue over it and cranking the engine. This helped me confirm the port wasn't plugged and was operating correctly by blowing the tissue. Didn't really notice much vacuum though, just puffs of pressure. Maybe that indicates the crank case is not sealed like it should be, but I'm not sure how well vacuum would work with the tissue test anyway.

But whatever problems still exist, if any, it will be good enough to cut the amount of wood I plan on using it for so I'm going to leave it as-is and move on. For a 40+ year-old saw I guess I can't have too high of expectations for it and it does run. The carb does seem "finicky" to adjust but this may be due to other issues affecting it. I did notice that fuel seems to spit out the throat of the carb when the saw is gunned when I had the air cleaner off for testing. I could feel it hitting my right wrist when I throttled it up. Not sure if that is normal or not. Maybe this indicates faulty reed valve(s)? I'll be glad when saws are fuel injected some day

Thanks again for the post. If it wasn't for this I might not have messed with it much and I was able to learn a little bit amount along the way.

 

[ilikeurtractor]

It turns out the root cause was the crankshaft seals were bad. I replaced those and now it idles fine. I have some other vintage saws that are exhibiting the same symptoms (having to constantly tune the carburetor to apparently compensate for air entering into the crankcase.) One is a Homelite C-series. Unfortunately, the flywheel seal on those is not easily accessed from the crankshaft side as the housing is integral with the crankcase. The McCulloch design made it easy to change seals. Hopefully I will be able to pull the seal from the flywheel side without busting down the entire saw, but it is a rather thin seal making it hard to use a seal puller on.

 

[Eccentric]

Most saws are built like that (with the seal fitting into the crankcase casting rather than a bearing/seal carrier). Not many designs around with removable FW side seal/bearing carriers anymore. Also, the PTO side seal on both that Mac and the Homelite seat directly in the drivecase (C-series Homelite) and crankcase (large frame Mac).

The wandering tune isn't so much from the engine pulling air past the seal (although that can be a factor). More of the problem comes from a weak/varying impulse signal (crankcase pressure pulses) going to the pump section of the carb as a result of that leaking seal. The end result (wandering tune) and the root cause (a leaking seal or seals) is the same in any case....

 

[ilikeurtractor]

Thanks for the clarification on the symptoms of a leaking seal. That is consistent with the saw not being able to pump fuel as I first experienced. I just got out of the garage and by some grace of God I was able to pull the seal out of the Homelite from the flywheel side. I drilled a couple of 1/16" holes in the face of the seal and was able to scoop it out with some dental hook tools. I really didn't think that would work like that. It showed me the seal was not very tight. The new one was similar and I put a thin coating of silicone around the outside perimeter before I drove it in. I'm not sure if that is a good idea or not but I'll take my chances if it helps seal things up. I greased the seal ID and shaft. I noticed that there is plenty of room in the seal area. I have a couple of C saws that probably will need seals and I think next time I'll just drive a new seal on top of the old and skip and chances of causing damage to the shaft or housing. I believe there is enough room. With some grease on the new seal and between the old one I'm thinking it will be fine. Breaking the whole saw down, pressing out the crank and then having to heat the flywheel bearing housing upon reassembly like the repair manual says just seems like a daunting ordeal to change a little seal out. They also say that when you heat the housing be careful not to heat it too much as you might damage the new seal. Wouldn't it be better to install the seal after heating, assembling, and letting it cool down? If the housing had a step in it so the seal couldn't come out the flywheel side then I could understand this but the bore is straight.

 

[Bob95065]

Where do you get the rebuild kits for McCulloch carbs? Can someone please post a link?

 

[ilikeurtractor]

I found them on eBay but I see the seller (customchainsawparts) doesn't have any listed at the moment.

 

[heimannm]

Contact Bryce (customchainsawparts) and he can get them for you. They are made by Joe Salva at Sugar Creek Supply but it is very
hard to contact Joe.

Mark

 

[Mastermind]

Bumping this thread to show my appreciation.

Thank you Mark for an excellent and most informative thread.

 

[sawfun]

I had to find the correct needle spring to prevent flooding. Mine needed a stiffer spring as there are several lengths and stiffnesses. Set the lever at .010 below the gasket surface and run 1/4" line to allow full flow for no delivery choke points. If you use the Custom Chainsaw kits on Ebay it should work well on a BDC 14 or 24.

 

[BIGD4DICE]

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

View attachment 333604

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.

View attachment 333602

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.

View attachment 333603

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.

View attachment 333602

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.

View attachment 333605

Mark

I have this carb on my 797. At idle after only 10 seconds it runs up and stays high idled with a pulse. Im not savvy to saws a lot but have heard it may be crank seal leaking. I'm taking it to saw shop in the morning for vac test. I don't have much for tools. Hopefully something simple. My second 797 and not having much luck. Has good compression. Will test it tomorrow too.

 

[chy_farm]

View attachment 333664--------

All the contents here are very informative Heimannm, to me the most amongst them is this maximum velocity of rotation for stock 740. this information surely help me calculate another better pipe for that series. Thank you for sharing!
Chy