McCulloch Super 250 Bizarre Idle Fuel Issue

[camer_1107]

Hi guys I have a problem that seems strange to me but maybe some of you have encountered it before.

I have a McCulloch Super 250 with the McCulloch carburetor (not the tillotson) that I have been trying to tune after doing some carb work due to the saw running rough (original problem was not enough fuel). That in itself is a long story, but in brief: the diagrams were dried up, but I ended up replacing the top section of the carburetor along with the seals/diaphragms, only because I screwed up the tiny internal check valve and the needle and seat on the old one, as well as that the throttle linkage on the old one broke due to being so worn.

After much screwing around with the new and improved carburetor, I have been able to get the saw to rev up really nicely and to what sounds like top rpms. The problem is, once I let off the throttle, it will idle ok for about 5~10 seconds, then you can visibly see an overflow of fuel shoot into the carb before it sputters and drowns itself in fuel. If I give it some throttle (air) before it dies, it will pause then rev up, I'm guessing because the air has time to match how much fuel the carb is suddenly putting into the intake. This problem also sometimes makes starting it a pain, even when warm.

Here are some details on what I have currently replaced and what it is currently set at:
1. Top of carburetor replaced with other used one.
2. Carb gaskets and diaphragms replaced.
3. Didn't mess with needle and seat settings on new carb top because that is part of the reason I messed up the other one.
4. L screw turned 0.40 turns out.
5. H screw turned 0.50 turns out, haven't gotten far enough to start adjusting this one yet since I believe you need to actually be cutting to do it.
6. Idle governor screw is currently set to a more higher idle position because I was testing the current fuel issue to see if it would still happen at this high of an idle.
7. Compression is 133 psi.
8. Spark plug was replaced with a Champion J8C which has much better spark than the old worn out one.
9. Running 91 non oxy at 40:1 for oil with just a bit of sea foam in the mix.

After 2 hours I have decided to stop messing around because I am baffled and although I do have experience with 2 strokes in general, I am still pretty new to chainsaws and might be missing something. Therefore, I figured I'd go to the experts on this one, so if I missed anything, dont hesitate to ask/mention it.

Thanks in advance for any help, it is very much appreciated.

 

[heimannm]

If you know the metering lever is set correctly, I would look into the check ball. Flooding at idle speed suggests the high speed check valve/ball (Item 38) is not seating.



Mark

 

[camer_1107]

If you know the metering lever is set correctly, I would look into the check ball. Flooding at idle speed suggests the high speed check valve/ball (Item 38) is not seating.

View attachment 1103624

Mark

Ok thank you, I'll look into it after I get back from work today.

 

[camer_1107]

If you know the metering lever is set correctly, I would look into the check ball. Flooding at idle speed suggests the high speed check valve/ball (Item 38) is not seating.

View attachment 1103624

Mark

It turns out I didn't have any time to work on the saw last night, ended up being to busy. I plan on taking apart the carb and working on it tonight, and if I can't get it fixed, I have all day tomorrow too.

Do you have any tips for servicing the check ball/valve system? I know the valve seat is pressed in and I am just not sure how much I can do for cleaning because I don't want to risk using compressed air on it and end up ruining the parts.

Thanks

 

[heimannm]

You should be able to test it through the H needle opening. You should be able to blow through the port, but it should hold a vacuum if the check valve is working. The ball is nylon and pretty resilient so don't be bashful.

If worse comes to worse, I have the kits on hand.

Mark

 

[camer_1107]

You should be able to test it through the H needle opening. You should be able to blow through the port, but it should hold a vacuum if the check valve is working. The ball is nylon and pretty resilient so don't be bashful.

If worse comes to worse, I have the kits on hand.

Mark

So what causes the check valve ball not to seat? I tried the vacuum test on both carb tops that I have, and both couldn't hold a vacuum. One of them has the Welch plugs out and I can visually see that the ball is sitting in it's seat in the valve body, but it must not be getting a good seal somehow. I know it is super clean in there because this is the one I went crazy on with cleaning, so that is not the cause for it.

Does this just mean that the ball itself is worn/warped? If so, is there any way I can get my hands on a new check valve assembly? Maybe I'd be better off just switching to the tillotson carb for this chainsaw, from what I've been reading it seems to be better than this one.

Thanks for your help.

 

[heimannm]

I've had them stick closed before, but a shot of carburetor cleaner freed them up. I'm sure it is possible that there is some varnish or other debris in there, or it is possible that the ball itself or the seat have some damage. The seat is aluminum and could have suffered from corrosion if the carburetor was stored under bad conditions.

I have a few of the check valve kits, I will check the prices and let you know on Monday when I am back in the shop.

The Tillotson HL carburetors can replace the Walbro/MAC carburetors but in my opinion you will give up a little performance edge.

Mark

 

[camer_1107]

I've had them stick closed before, but a shot of carburetor cleaner freed them up. I'm sure it is possible that there is some varnish or other debris in there, or it is possible that the ball itself or the seat have some damage. The seat is aluminum and could have suffered from corrosion if the carburetor was stored under bad conditions.

I have a few of the check valve kits, I will check the prices and let you know on Monday when I am back in the shop.

The Tillotson HL carburetors can replace the Walbro/MAC carburetors but in my opinion you will give up a little performance edge.

Mark

Ok thank you, let me know.

 

[camer_1107]

I've had them stick closed before, but a shot of carburetor cleaner freed them up. I'm sure it is possible that there is some varnish or other debris in there, or it is possible that the ball itself or the seat have some damage. The seat is aluminum and could have suffered from corrosion if the carburetor was stored under bad conditions.

I have a few of the check valve kits, I will check the prices and let you know on Monday when I am back in the shop.

The Tillotson HL carburetors can replace the Walbro/MAC carburetors but in my opinion you will give up a little performance edge.

Mark

Did you end up finding out how much those check valve kits are?

 

[heimannm]

The 61155 kits are $19.95 plus shipping. You would have to make the installation tool as shown in the illustration below.



Mark

 

[camer_1107]

The 61155 kits are $19.95 plus shipping. You would have to make the installation tool as shown in the illustration below.

View attachment 1106829

Mark

Ok thank you for finding that out. I will let you know if I decide to go that route, I'm pretty busy these days and dont have much time to work on the saw unfortunately.

 

[Okie]

Couple tips from experience with the Mac, Tillotson Carbs Mods
I've done the mod from Mac to Tillotson carb sometime back with good results. I think maybe it was a 1-53 saw.
I think I got the stuff from chainsawr.com. The carb had a return warranty and I also got a kit just in case. All worked out great. They were good with questions about such also. I also bought a OEM MAC SERVICE MANUAL DOWNLOAD FROM THEM FOR LIKE just a few dollars that covers the 1-40 through 250 saws and then I burned the download to a CD for future reference in case the computer crashed and made copies for the shop. It's a really good MAV SERVICE into that shows all FACTORY service tips for the three type of carbs, Mac, Tillotson, and I thing one was termed something like Bullfrog carb.
I got a carb and the linkages in the chainsawr kit mod.
Just a hint also. I seen couple of clean looking Tillotson HL63's over on fleece bay few days ago. I do not buy any carbs that the screws are really buggered up.
I've heard rumors like Mark indicated about the 250 might be little bit less in performance but on the 1-40 thru 1-50 series of Mac the Tillotson is the way to go.
Keep a heads up when going to the Tillotson carbs, some do not have the choke butterfly built into the carb. The HL63a, B, C, D, E have the spring loaded choke butterfly.
I think the Tillotson HL87 is the ones that does not have the butterfly, it's a plastic cup on the air box itself.
The Tillotson kits are readily available and the Tillotson.com site has good Service tips.
Also when cleaning a Tillotson do not use any air pressure over about 20 lbs and do not direct inject into the passages or the welch plugs WILL go air borne

 

[camer_1107]

Couple tips from experience with the Mac, Tillotson Carbs Mods
I've done the mod from Mac to Tillotson carb sometime back with good results. I think maybe it was a 1-53 saw.
I think I got the stuff from chainsawr.com. The carb had a return warranty and I also got a kit just in case. All worked out great. They were good with questions about such also. I also bought a OEM MAC SERVICE MANUAL DOWNLOAD FROM THEM FOR LIKE just a few dollars that covers the 1-40 through 250 saws and then I burned the download to a CD for future reference in case the computer crashed and made copies for the shop. It's a really good MAV SERVICE into that shows all FACTORY service tips for the three type of carbs, Mac, Tillotson, and I thing one was termed something like Bullfrog carb.
I got a carb and the linkages in the chainsawr kit mod.
Just a hint also. I seen couple of clean looking Tillotson HL63's over on fleece bay few days ago. I do not buy any carbs that the screws are really buggered up.
I've heard rumors like Mark indicated about the 250 might be little bit less in performance but on the 1-40 thru 1-50 series of Mac the Tillotson is the way to go.
Keep a heads up when going to the Tillotson carbs, some do not have the choke butterfly built into the carb. The HL63a, B, C, D, E have the spring loaded choke butterfly.
I think the Tillotson HL87 is the ones that does not have the butterfly, it's a plastic cup on the air box itself.
The Tillotson kits are readily available and the Tillotson.com site has good Service tips.
Also when cleaning a Tillotson do not use any air pressure over about 20 lbs and do not direct inject into the passages or the welch plugs WILL go air borne

Yeah I actually bought an HL-63F from eBay last week with the intention of swapping over, I even bought a carb rebuild kit as well. However, after getting it, I quickly realized that the impulse port for the fuel pump on the carburetor was nowhere near to lining up with the port on the saw itself. I'm not sure if there is a way to mod one way or another to get the two to line up, or of there is a different version HL-63 to buy. All I know is that this roadblock has led me to looking back at the frustrating peice of work that is the McCulloch carburetor. Any advice is appreciated though, thanks Okie.

I will post some pics of the different ports in a second here.

 

[camer_1107]

First picture is the tillotson carburetor, the second is the McCulloch.

 

[heimannm]

There is an adapter that needs to be installed between the flange on the airbox and the carburetor.





Mark

 

[Okie]

I'll look at my Tillotson HL63's. I've used the HL63D, E and A with no problems replacing them in place of the HL87D (has no choke butterfly) and used on the 1-43 series. Might be day or so I'll have to dig for the Mac carb and the metal big shop is like 120F in the daytime. I've never seen a F series Tillotson carb. I'll compare mine to your picture.

I seem to think that on my Mac 1-40 through 1-53 saws their is a adapter plate about 1/8 inch thick between these carb's and the manifold with a thick gasket on each side. Have to be careful and keep a heads up make sure the lip on the inside of the adapter towards the manifold is aligned properly or it's would be possible to break a ear off off the carb and or the manifold when snugging up the mounting bolts if the lip is not aligned properly. I'll also look to see if the Gasket towards the manifold might have a elongated opening so as to get the pulse hole to the carb to align.
Heimannm might know if you need a adapter per my description.
I have not installed any of the carbs on a 250, just 1-40 series through 1-53 but I know the kit I got from chainsawr to replace the Mac carb on these models fit with no issue.

There you go. Heimannm was typing at the same time as me and beat me to the punch.
Stay aware of what I said about the adapter or you can break your carb's ears or damage the adapter or break the intake manifold ear.. I've never broke a carb/intake ear but have seen pictures of guys that are trying to run their carb with only one mounting bolt with poor success and also seen pictures of Mac's on fleece bay of saws for sale and the ears broken off and not mentioned sometimes.

 

[Okie]

I failed to mention that I've also seen pictures of the magnesium ear of the intake manifold engine side ear broken off due to the adapter not being aligned properly when tightening the bolts and Bubba would be trying to mod a bolt and large washer to get a clamp.
These magnesium cast parts do not bend they break.

 

[camer_1107]

Thanks guys, I appreciate the help; I had a feeling an adapter was going to be the answer. Just found one on eBay using the part number from Mark.

 

[camer_1107]

Ok so I got the adapter plate for the tillotson carburetor and was able to put it in and run the saw. I have to say, it runs a lot better than it ever used to, I didn't realize the saw had the capability to start and idle so reliablely. This just gives more points to tillotson in my book. I've dealt with their carburetors in older mercury i4 and i6 outboard engines, but I never realized how well designed they were on a wider spectrum until now. All I had to do was put some new diaphragms in the carb and set the screws and the saw idled till the tank was empty. The only thing I'm trying to figure out, since I'm still new to working with this style carburetor, is setting the high speed/full throttle rpms; right now it seems like the saw has more to give while in the cut. If somebody has some advice regarding that it would be appreciated, but if not I can probably figure it out in time.

Thanks for everybody's help in this thread, I greatly appreciate it. I know I'll have some great times with this saw in the future thanks to you guys; I already have some things lined up to use it for and I'm glad it runs reliablely now.

 

[Okie]

Can you explain your question?

AND are you still using the 404 chain?

Be sure you keep the H jet adjustment on the rich side. (rich is CCW for the 4 stroke)

What gas/oil ration you using?

You say the saw idles good for long period of time. That is a really good thing. Seals are good and no vacuum/pressure leaks. You are gaining ground.