McCulloch 250... easy start... stalls immediately

[sliceoflife]

So I bought a mcculloch 250. Just finished cleaning the tank, fresh tank of 40:1 fuel in there... it was sold to me as starts right up with gas in the carb... but untested beyond that b/c tank was crusty.

Fuel line is in great shape, looks new. Fuel drips through it with saw inverted. Saw starts up and runs 1-3 seconds if I put fuel in the carb.... OR if I let it sit for a minute or so before pulling the chain. But never runs more than a couple seconds before it dies very abruptly... and if I try to start it again immediately after it dies it almost never will start.


Guessing this is a carb diaphragm problem most likely? Time for a carb rebuild?

I'll also note the low end carb adjustment was cranked in as far as it would go... symptom of failing diaphragm maybe?

Any thoughts on if this sounds correct or other problems I should look at?

 

[NSEric]

Im guessing the diaphragm is dried out and stiff in the carb.
There's two ways to fix it, a carb kit or keep dribbling gas in the carb with a squirt bottle to keep the saw running for a few minutes straight. Sometimes this will get them to come around and run.

 

[sliceoflife]

Thank you. I'll try option b and order a rebuild kit if it doesn't work.


Any tips on the best way to tune these once I get it running? Start it around middle and tune it leaner till it starts to sputter on full throttle and low throttle... or?

 

[heimannm]

Don't go too lean. It should clearly 4 stroke out of the cut and clean up when you put some load on it. L adjustment is best determined by snappy acceleration when you squeeze the throttle. Either too rich or too lean will result in poor acceleration off idle but the sound is different. Start with both the H and L adjustments out around 1-1/4 to 1-1/2 turns and gently lean them up from there.

Best to start by removing the carburetor and giving it a thorough cleaning, and if you haven't already replace the fuel filter in the tank.

Mark

 

[Backstage]

Yeah, definitely pull the carb and make sure the diaphragms and such are good. The word is "supple". Feeling it should make the lizard part of your brain go "Mmmmmm...yes."

Also check anything fuel delivery related. Fuel filter, fuel lines, squeeze bulb, fuel lines again, and the little fuel filter screen inside the carb.

 

[sliceoflife]

Ok so... I'm dumb time... there's no intake line or fuel filter in the tank... got so caught up cleaning the tank I didn't even notice that there wasn't one... I was just sloshing some fuel into the intake when I would pick it up.


Potentially all it needs is a way to deliver the fuel to the fuel line leading to the carb. So... fuel line plus filter en route.

 

[Okie]

Post up a model number from your carb.
Probably a Tillotson HL-63 (with a letter) These ID's are on the side of the carb, usually requires removal to see.
Quite common for the sintered gas filter in the carb to get restricted, but carb will usually idle but not rev up or starves for fuel at High rpms. Seeing gas in the air box is not a good thing. The old fuel line is usually hardened at the carb barb and will leak and the carb suck air when running and the Mac spring clamp usually has to be replaced with a small hose clamp even when new line is installed. I've replaced some of them old Mac fuel lines with tygon tubing with good results.
Be careful when removing the pump side plate (at the hose barb side of the carb). Some Tillotson kits do not have the correct size cork gasket and be careful trying to remove the sintered filter for cleaning. You can go to the Tillotson site and find service info for the carb. Also be really careful when re-installing the carb, if not aligned properly at the intake you will break a ear off the carb. (replacement carb's will surface on fleece bay every once in awhile, some good, but most replacement carbs are bad or Bubba has been inside.

Also that fitting on top of the Tillotson carb that holds the air filter cover in place is screwed into the top of the carb. It can be removed and use an adapter to test the fuel pump pressure when cranking the saw. (this info is in a service manual for the 1-40 through 250 Mac's) Might find info on-line about such. Briefly I removed that fitting and used a fine threaded stud and nut with a small hole drilled through the stud to connect a hose & 0-10psi gauge and you should see a low pressure of around 1/2 psi as you crank the saw to indicate that the carb is trying to produce a fuel pump pressure. Be sure the little Copper washer is on the stud for a good seal when snugged down.

https://store.chainsawr.com/ is a good place to look at pictures of parts for Mac's and others and they had a Mac factory service manual download available sometime back for the 1-40 through 250 series. After downloading the service manual for a nominal fee it's a good idea to send it to a CD or to a flash drive for future reference.

Most generally when starting them old Mac's cold or after storage I just dump couple spoon fulls of mixed gas into the muffler (this is easier than removing the air box cover and filter and priming in the carb throat and hold the throttle wide open until it starts.
Most of them old big Mac's USUALLY start better at a wide open or fast throttle. (even when warmed up)

 

[sliceoflife]

It's an HL63E

Thanks. A lot of good info there.

 

[Wood Doctor]

This snip is from the 025 IPL but it's probably similar to what you are missing. If this part is missing, I would look for others.

 

[Okie]

It's an HL63E

Thanks. A lot of good info there.

Their is a Tillotson Hl-63 A, D, E carbs that are interchangeable. (choke butterfly is made in the carb and spring loaded and all the linkages are the same for the choke and throttle.

Some of the old Mac's use a Tillotson 87D. It cannot be used in place of the HL -63 because it does not have a choke butterfly in the carb ((choke is part of the saws air box and the throttle linkages are different.
You got lucky. some of them old dogs used a Mac carb and some used a carb termed bullfrog carb. Kits are hard to find and expensive for these type. I have a bullfrog type carb that I removed and stored and converted the Mac saw to the HL-63 type carb.

Every once in awhile a useable Tillotson HL-63 A, D, or E surfaces on fleece bay. I've bought them for around $20-30 dollars for good ones but some are just junk and priced
too high.

Have to be careful when the sellers use the word Vintage Mac Kart parts. Some of the old go karts used a modded Mac engine and the term is used to jack up the price of usually vintage Modded junk.

heimannm on this site is also full of good useful Mac info.​

He has helped me with lots of good useful info about getting them up and running.
Patience is your friend when flogging them Yellow ones..

 

[Wood Doctor]

I apologize. My mistake -- wrong MFG.

 

[sliceoflife]

So fuel line and filter are on and it runs...


Not being familiar with 2 stroke motors of this size, it feels to me like I'm having to run it very rich (adjustments are almost all the way in/clockwise... maybe 1/2 a turn out on each... also the adjustments feel VERY stiff, and squeaky... like they're pressing into very hard rubber), revs sound very high where I've got to put them to keep it from stalling... also the idle throttle is having to be kept in engaged a decent amount... and it doesn't idle smooth at all, revs up crazy high then tapers off and almost stalls then revs up high again, etc, etc.

I'll try and record a video/audio of it running next weekend (wont be home this weekend), but I'm assuming ordering a carb rebuild kit would probably be a good idea at this point?

 

[sliceoflife]

Videos (nothing worth seeing but hopefully the audio will help diagnose)

Both Carb adjustments are almost fully in and the idle throttle is engaged a significant amount too. Middle video is me trying to back out the carb adjustments and how it fails when I do.

 

[heimannm]

Your description makes it sound as though it is very lean, even to the point of running out of fuel at high speeds. I normally start with both H and L out about 1-1/4 turns and work from there, acceleration should be fairly crisp when you hit the throttle and it should just begin to 4 stroke when running wide open unloaded. Often they need to be quite rich (really 4 stroke unloaded) to supply enough fuel under load.

Mark

 

[sliceoflife]

So I have it idling fairly well now with the filter on and a good bit of idle throttle (clock I’ve been made to understand it’s called). This is with the L adjustment 100% in… I can back it out about 1 full turn before stalling the saw, but it sputters.

Right now H adjustment is about 3/4 turn out, but I don’t have the chain on. Once my safety gear arrives, I’ll put the 16” chain on and adjust that.

Also have a rebuild kit en route for when I have time and the mood takes me.


I’ve seen this phrase 4 stroke a lot around these forms, do you have a good description or example of what it is/sounds like in reference to these engines?

 

[Backstage]

Agree with Heimann. It’s running very RICH. Turning the carb screws in reduces fuel delivery.

Your problem might be a carb needle valve that’s too high or diaphragm post that’s too long. Either way, it’s giving too much fuel.

 

[Okie]

The Tillotsons HL63D, E, A and the 87D use a RK-88HL kit.

You will get extra gaskets and diaphragms so be sure you match the ones used to your old ones.



I’ve heard of some Mac 250’s that use a Tillotson HL63F which uses a RK106HL kit. (but I’ve never seen one)

You can find a kit on flea bay.

Go to the Tillotson site and print off a IPL of your carb and pay attention to the CORRECT gasket and diaphragm arrangements.

Do you know how to do a pressure vac test of the block?

Most generally the first thing I do on a chainsaw that does not idle correctly is a pressure/vac test of the block because if the crankcase is not sealed good you will be wasting time and money working on the carb. The engine won't idle correctly.

Also I've found the crank seal on clutch bar side of the Mac's be bad, engine would almost run ok with the bar off, install the bar and snug up the chain and the idle jet would change and get erratic. I would confirm the seal was bad with a pressure test of the block. (using water/soap spray around the seal and not over 7 psi) I've carefully replaced the crankcase seals WITHOUT taking the crankcase apart.

Another test for bad crank seals or a hint of such is if you get the engine running fair and you tip the saw sideways or upside down and the engine changes it;s running or idling.
Crank is slightly changing positions on a old leaking seal.

 

[sliceoflife]

I'll try the latter test tomorrow... looking up the seal test on YouTube it looks like I'll need to buy a hand pump with pressure gauge and some misc hardware to plug up a few pieces of the saw, so that may be a bit before I can round up those things.

 

[Okie]

If you do your own mechanic work, chainsaws and automotive a Mityvac 8500 is a good tool for pressure/vac. ($65 plus) Also good for pressure testing carb's.

A flea bay blood pressure bulb @ $10 and a automotive compound gauge @ $15 that does 0-10 lbs pressure and 0-30vac will do for pressure testing a 2 cycle with pressure only.
I usually make block off plates using 3/16 aluminum plate or 1/8 inch steel plate and old inner tube or gasket material for a seal.. 1/8 inch aluminum sign plate is not rigid enough.

Also just a warning. Keep a heads up when compression testing chainsaws. Most generally a Automotive compression tester will indicate that the compression on a small two cycle engine is low and bad. You can review such on-line. Most generally if a chainsaw wants to try and start easily the compression is ok. Low compression can sometimes be felt by not much kickback on the pull rope when cranking. I do not pay much attention to compression tests but I do remove a muffler and take a peek at the piston BEFORE spending any time or money on one. If the piston is scored bad no need in doing other repairs first because it has lung cancer and heart disease..

Also you indicate that the carb jets cannot be out CCW to normal for tuning and tend to need to be CW or in. CW indicates that the engine may be flooding (wants less gas mix through the carb for better running.) Check the needle/seat and the metering lever height adjustment and keep a heads up if using a new needle from a kit. (some kit replacement needles are just not the right size or length as the one being replaced) close but no cigar so compare carefully.

 

[sliceoflife]

Thank you.

Seller did compression test it and said it was good. I do not recall if they had pictures proving that, but I'll dig up the listing and check. If not, I will work on getting the block plates and other tools you suggested.