Help with High Speed/Load Cut Out Problem on my 038 Magnum

Some of you might remember that I traded my "like new" 250CB-E to my sister for her "moderately used" 038 Magnum last fall. She needed a small easy to start saw, I needed the 038 like a hole in the head ................. anyways, I helped her out!

The saw seemed to run "fine" although it took about a dozen or more pulls chocked to get that first pop, more than normal. I put about 5-6 tanks through it during the winter then started developing a problem. The saw would cut out and quit when cutting under full load. It would cut fine till just under full throttle under full load, or cut fine under full throttle under light load. Just not when its taken to the max. I could theoretically think of a lot of things that could be wrong ............ ignition, fuel supply/tank venting, carb, bad impulse, and seal leaks all come to mind. I decided to start with the Bing carb and rebuilt it, a good cheap fix for an old saw. The metering diaphragm looked bad so I figured I'd found it. Also put a new OEM air filter and NGK spark plug on it.

I tried to start it and no go, after many pulls checked the plug and it was dry. Ok, after rebuilding the carb it might be dry and won't flow, so I shot a little gas directly into the cylinder and it fired right up after a few pulls. It ran steady and strong and was obviously now drawing gas and running. I tuned it from my original start points and it definitely seemed to be running better than before. Very crisp response and run up.

Went out to cut and it was working fine on some smaller stuff, 6-8" stuff, full throttle and load (20" bar). I then moved to some bigger stuff about 14" and the saw started the high end cut out again. Just died suddenly if you kept it pegged full throttle/load, but would recover if you backed off it fast. If you do kill it, it starts right back up immediately to idle/rev fine. I experimented around and found that I could cut that 14" stuff at about 3/4 throttle without it cutting out, but if I gave it full throttle it died. Also tried breaking the gas cap to equalize any possible vent issue but no luck.

Right about then is when it proceeded to blow its muffler cover ....................... but that's another post (loose screw).

You can see the pattern. Its running better with the carb rebuild, but it did not affect the problem. I'm guessing impulse line and seals next, more cheap fixes/extended service I can do myself.

I'm changing out the seals on my 024 right now (and a new set of Caber rings). Anyone got any ideas on my 038?

 

Fuel line collapsing. Dirty filter. Metering lever too low. I had a Jred that would fall on it's face when you really pushed it, the a/v mounts were bad and it was pinching the fuel line.

Food for thought.

 

+1 on the fuel line. I bought a 024 real cheap because the saw shop had replaced the oem line with a piece of tygon that was a bit too long and was getting pinched by the air filter... Guy didn't was tired of dealing with and sold it to me. I Cut the line a little and re routed it and all was good.

 

Had at it last night, broke it all down for an complete overhaul. I figure new rings, seals, impulse/fuel lines, fuel filter. Going to take apart that tank vent and clean it too, its just one of the tube types with the threaded set screw in it. From a semi-working saw to a pile of dirty parts!




Got the seals out this morning. I do it the "poor man's" way with a little orange stihl screwdriver and a pair of really big 90 degree needle nose pliers that have one of the tips ground down very thin. I use the little pieces of tongue depressor wood to put between the pliers and the saw when I pry the seal out so as not to damage the saw surface.




Bearings look and feel really nice!





Time to order some parts!

 

I've got my new seals installed and I'm just waiting for my $13 (shipped) Caber rings from Greece to arrive. I really didn't need the new rings in terms of compression, but as long as I had the rest of it tore down I figured I would replace them too. The piston and cylinder had a decent layer of carbon built up on them, but they cleaned up to the nicest set I have seen yet in any of the used saws I have worked on. The old rings were not in very bad shape at all.

If the new seals, impulse line, and gas line/filter do not solve the full throttle/load cut out problem, then I figure the problem has to be back with that Bing carburetor. I rebuilt it, but it could still be bad. I see that I can pick up a brand new carb for $20 shipped!!! Probably should have done that in the first place!!!

 

Nice pics... good job on those seals. I've removed a couple but it's easier to bring 'em to my dealer and pay a few bucks for removal and install. With all the new rubber parts and a good carb work-up, she should be running fine! The good thing about taking it all the way down is you'll inspect everything and will verify flywheel gap, intake boot, impulse line, etc.

 

Plus, you get to clean it all up like new. Especially those lower cylinder fins that are usually plugged with stuff causing overheating!

 

What the heck is bing?? Only heard of Zama and Warblo. Even on china knock offs you see one of those two usually??

 

The ones I was looking at were Tillotson, and appear to be OEM on some of the 038 lines of saws? I thought those were made in Ireland. There are definitely a lot of even cheaper Chinese ones available for about $17 shipped, but they all have a weird compensator end cover that does not match up with mine (I know you can replace that with a blank cover though)..

I think Bing is of German manufacture, at least my carb side cover said Bing - Germany.

 

Tillison sounds more familiar. I have only built a dozen or so diaphragm carbs. Not an expert here, and dont claim to be...just never heard of them.

Seems like I have built a ton of boat carbs in the various motors I have owned ...learning and screwing up or trying to get one going. Also built a ton of 4stroke small engine carbs over the years.

I am no MasterMechanic though thats for sure.

 

Oops, you are right, that $20 one was aftermarket. I read that ad again and it mentions Tillotson several times but then has a statement below that says "that the OEM name is just for reference, its a "high quality" replacement".

However, a little more net searching around and I did find genuine new OEM Tillotson HE5A shipped for $29.95!

Also, seeing all the rebuild kits for sale again made me remember something. The rebuild kit I bought came with two identically configured pump diaphragms. One appeared made of the same type material as the metering diaphragm, and the other was made of a "harder" type black material like the "blue pump diaphragms" in many Walbro kits. Know what I mean? I used the harder material diaphragm to rebuild it. If it still does not run right after this work I'm going to try switching to that "softer material" pump diaphragm to see if it makes a difference. Got nothing to loose!

 

I hate when you buy a generic kit for some carbs you get enough stuff for 2 nearly identical carbs. And sometimes the replacement diaphrams look so close but are not...I had one one time where one of those really thin pieces looked close but nothing was a match so the original piece went back in??

 

Got a patient to operate on? I'll bring my "custom expensive tool" to the GTG.

 

Success, yes!!!

I'm not totally sure what the problem was, but replacing the fuel line/filter, the impulse line, the seals, and cleaning the gas tank vent solved it!!! (the shotgun approach on an old saw)

I also put new rings in it so its good to go for the long haul again. I ran about 3/4 of a tank through it today before the rain started without it missing a beat. I also replaced that dual port OEM muffler that had blown its cover with a nice aftermarket one. I like it much better as it uses two "Stihl style" machine screws in the upper corners of the cover instead of the old OEM cover with just the single sheetmetal screw in the center. This cover is not going to vibrate loose. That cover has dual ports to the left, they just don't show in the photo.




Ready to Rock'n'Roll again ................ although I'm probably going to sell it, and two others, to finance my next move .................

 

Nice job Boog!

I'm using the same approach on an old 154se. Compression is about 170 lbs, but it runs like shiite. Figure its worth new parts from the seals on up.