Eager Beaver Carb Issue

[Alaskan Stihl]

Ok…I acquired a McCulloch Eager Beaver (boat anchor). Of course, right off then bat, it doesn’t start properly and once started…runs extremely rough.

I ordered a rebuild kit (Walbro MDC-16) and rebuilt the Carburetor. After the rebuild, I recalled that a “pressure bench test” should be conducted. Here is what I did:

#1. Threaded in both H and L screws
#2. Attached PSI Gauge to fuel outlet nozzle.
#3. Pumped up to 7PSI

I found that it bled off quite rapidly. After squirting soap around top plate; I discovered small bubbles. I then tightened up the (4) screws a little more and tested again. It bled down slower…but still bled down.

I was going to say the heck with it and try putting everything back together to see if the Piece of Crap would run…when luck would have it… I broke the plastic throttle lever while I was trying to get the metal lever snapped into place.

At that point; frustration took over and I tossed the damn thing in a plastic bin and threw it on a shelf!!!!

After calming down…

I ordered a new throttle lever (it is now on the way)…however…from what I read, those models of Carburetors are extremely temperamental and I should probably resolve any issues still in question.

Which brings us to those questions…

Number #1: I have read where these Walbro MDC Carbs are JUNK. I have a McCulloch 110 that has a ZAMA M7-2YP. Could I replace the Walbro MDC-16 and purchase & install a ZAMA M7-2YP in place?

Number #2: Did I conduct the Pressure Test correctly? Is the Walbro MDC-16 accommodating to this kind of “Pressure Bench Test” or is it designed in a way that allows it to bleed off? I watched a YouTube in which the Mechanic was having the same problem with the Carb failing the pressure test…(he cut the video off before resolving the issue).

Number #3: Finally, if everything is…as it should be….and it failed the pressure test due to fault, then what is that fault? Could it be the Check Valve (supposedly, there is a check valve behind one of the “Welch Plugs”).

Question #4: My Carb Rebuild Kit came with new Welch Plugs…however; it did not come with a check valve assembly. Where could I find a new a new Check Valve Kit for this Carburetor?

Thanks to anyone who can help!

Alaskan Stihl

 

[SteveSr]

Number #2: Did I conduct the Pressure Test correctly? Is the Walbro MDC-16 accommodating to this kind of “Pressure Bench Test” or is it designed in a way that allows it to bleed off? I watched a YouTube in which the Mechanic was having the same problem with the Carb failing the pressure test…(he cut the video off before resolving the issue).

Yes, you will need to find and fix the leaks. A carb should hold 7-10 psi indefinitely.

Number #3: Finally, if everything is…as it should be….and it failed the pressure test due to fault, then what is that fault? Could it be the Check Valve (supposedly, there is a check valve behind one of the “Welch Plugs”).

The fault is likely bad gaskets as you found but more importantly the needle and seat. Welch plugs, check valve, and mixture screws are after the needle/seat so are irrelevant.

BTW, I don't mess with welch plugs unless I can confirm a blockage underneath. Getting t hem to re-seal is dodgy. Even the factory can't do it reliably with special tooling and resorts to proprietary sealant as a stop gap.

 

[heimannm]

The high speed check valve in an old MDC is almost always bad. I've pretty much given up trying to rescue the MDC's and opt for the Zama M1-M7, they almost always perform as they should. I am convinced the material used for the MDC is so bad it always corrodes and caused problems.

Mark

 

[Lightning Performance]

You need some tanerite (sp)

 

[MacAttack]

Those MDC carbs are in fact junk, which I am entitled to say after the countless hours I've spent inside them over the past few years. I had an entire multi-page saga of a Mini Mac 25 that I had apart 5 or 6 times trying to get it to run right. Id take a Zama any day over an MDC.

Here is my personal experience with the MDC. I never did a sophisticated pressure test, just tried blowing air into the fuel line. The check valve has no effect on that, the check valve keeps air from entering the high speed fuel circuit at low speeds. I have 3 small Macs with the original check valves still working (I think), I don't let them sit and idle much.

When I've had small Macs with MDCs that ran like trash it was the metering lever height causing problems. Make sure that is correct first, the top of the metering lever should be close to flush with the body of the carb. The picture I attached is one I had where the lever was WAY too high, you should be able to lay a metal ruler on the carb mating surface and the top of that metering lever just touching it.

 

[Alaskan Stihl]

Question:

Could I replace the Walbro MDC-16 (Eager-Beaver) and purchase & install a ZAMA M7-2YP (Mac 110) in its place? If memory serves, they look pretty close to identical…? Of course the problem is ….finding a spare ZAMA M7…

 

[heimannm]

Yes, the Zama is a direct replacement for the MDC. In fact even the gaskets and diaphragms are so close to being identical that you don't realize the wrong part won't fit until you try to install them.

Mark

 

[MacAttack]

Does that include throttle linkage? Fuel line?
I have a roached Eager Beaver 2.0, and at least 2 or 3 Mini Macs to rebuild. Maybe I'll try installing the Zama carb on one of the Minis.

 

[heimannm]

Yes to all.

Mark

 

[MacAttack]

Thanks Mark!

Without a doubt, the Zama is much easier to get running correctly. My SP40 has the Zama, it even has the fixed main jet, it seems like a much less troublesome design than the Walbro.

 

[heimannm]

The SP40 and the PM510/515 use a Tillotson HU carburetor, not sure what the Walbro exchange is for those saws. I can't say that I have encountered a Zama on one but I do know many of the 300 Series saws did use a Zama.

On the 10 Series a lot of guys avoid the Zama carburetors (often found on the PM800 variants) but even there I find the Zama carburetors to be quite good.

Mark