Help MS 311 won't run right after rebuild

[Gomofast]

Ok so my neighbor straight-gassed his MS 311. He had a friend (not a professional) put in a new piston & cylinder. I don't know the specifics of exactly what that guy did - whether or not he replaced crank seals, etc. But he couldn't get it to idle after afterwards, even with the LA screw all the way in (which I know is way too much).

I offered to look at it for my neighbor. The saw would fire and rev high for a second and then die. If you tried to give it gas, it would just bog and die. I was thinking fuel delivery problem or air leak, so I started with fuel stuff because that's easier for me... cleaned the carb, looks good. Then I checked the fuel filter and line - filter blows through freely and the line holds vacuum/pressure as best as I can test it with my mouth (I don't have a vacuum tester). So I thought maybe the carb is out of adjustment after the rebuild. I removed the limiter caps and tried richening up the L screw, but without much success. Eventually, I was able to get the saw to accelerate with little bog with the L screw way way out (like 5 or 6+ turns, which is ridiculous) but it still won't idle, and the screw is so far out that it feels loose. I tired a new plug too just for the hell of it.

At this point, I'm thinking that it must be an air leak and the way-out screw adjustment is just barely making it run by compensating for the air issue. But I don't have a vacuum/pressure tester to properly check for air leaks. Before I go that route of buying a vacuum/pressure tester, is there anything else I should check first? Anything I'm missing?

Thanks for the help.

 

[Sidecarflip]

One thing I've never been able to get my grasp around is how saw owners manage to straight gas any saw. I always keep my pre mix in a dedicated gas can and my straight gas for the 4 strokes in separate gas cans not anywhere near the pre mix can.

Working part time at my tractor dealer who is also an Echo dealer, I see a lot of straight gassed saws come in and I always wonder about the owners and their level of intelligence.

When the carb was done, was it rebuilt and just cleaned and were the internals reset properly?

 

[irhunter]

Saws which get new pistons really need P&V test before running for the first time. This is especially true for clamshell saws.

You seem like a saw guy, who is going to have saw repair in his future. You can get a tester now, or later, but...

Buy...or cobble one together.

Roy

https://www.arboristsite.com/threads/show-us-your-pressure-vac-tester-and-block-off-plates.368677/

 

[Gomofast]

When the carb was done, was it rebuilt and just cleaned and were the internals reset properly?

I cleaned and inspected then reassembled with same parts. Needle, diaphragms, etc. looked good.

 

[Gomofast]

You seem like a saw guy, who is going to have saw repair in his future. You can get a tester now, or later, but...

Yeah I'm thinking that's where I'm gonna end up. Just wanted to see if others agreed with my assumption that it's likely sucking a lot of air somewhere.

 

[Gord404]

well kinda the way I look at it if it ran good before it got rebuilt sounds like the rebuiler did something wrong and you have an air leak- treat yourslf to a MV 8500 and it does a lot of other things besides just testing for air leaks in the kit- about 100$

 

[Gomofast]

treat yourslf to a MV 8500 and it does a lot of other things besides just testing for air leaks in the kit- about 100$

Anybody recommend a cheaper option that still works well?

 

[irhunter]

A lot of people who work on their cars/trucks have vac pumps, and vac/pressure gauges already. I did.

Maybe, ask your friends/fam, if someone has what you need.

 

[Bob Hedgecutter]

Okay, so in labour costs alone- after two guys have fumbled around (I know- friend/neighbours saw so no labour charge) the saw is still faulty and we are resolved to playing guessing games?

Mityvac 8500 is the go to. Once only investment and pays for itself fairly quickly.
There are cheaper lesser quality units out there, but do they leak back through the pump unit and give false reads?
Or you can cobble one together with some tubing, a gauge and a blood pressure/ outboard tank primer type bulb.

If the replacement top end is of dubious quality, if the carb is also aftermarket- either could be at fault, but if the first guy to try being non professional and all was willing to cheap out on the top end- willing to bet he just swapped those two part to regain compression and called it good.

 

[Coralillo Lo Pro]

I've rebuilt and ultrasonically cleaned carbs and still had them not work right. Sometimes they're hopelessly gummed up somewhere or need a lot more cleaning than an average clean. So I wouldn't entirely accept the carb is okay. Leaking welch plugs also escape people's notice on a simple clean and reassemble. One more reason for the Mityvac. An obvious one that stumped me on my last piston and cylinder replacement was the cylinder bolts being loose. Got it running a little and 3 of 4 the bolts loosened up and would barely run or not at all ever after. I wiggled the cylinder a bunch of times to make sure it wasn't loose but never thought to check the bolts. The one tight bolt deluded me into thinking the cylinder was on okay. Drove me crazy trying to find the leak, changed the crank seals and still the same. Mityvac just wouldn't pump any pressure at ll, and it was staring me in the face the whole time, stupid loose bolts.

 

[Gord404]

If i can't get a reading and the saw is all sealed up I will dunk it in a tank of water- doing a 3120 lsat week- turns out they were using the K crank in it and leaked out the hole in the crank shaft under the clutch bearing area, never thought a fairly new saw would have the wrong crank in it- that hole is to lube the sealed clutch for the K from the inside out of the crankcase- in the tank it showed up.

 

[muddy42]

P&V testing is the backbone of so many types of test and the starting point for fixing a saw. Just get one.

Then you can:
- vacuum test the fuel tank air vent
- pressure test the fuel line
- pressure test the carb (gently)
- p&v test the crankcase.

Apart from the last one and sealing up the crankcase, these tests take literally seconds to perform and are so useful.

 

[stihltech]

At work I have the Stihl vacuum and pressure tester and all the adapter plates. At home I have the Harbor Freight vacuum and pressure tester. Would I want the Harbor Freight at work.? No, not enough quality for daily work. Does it work at home? Quite well actually.
You need a tester to repair this saw, unless you can actually see something leaking.

 

[HarleyT]

Most folks blame the straight gas thing, when it is not.

 

[muddy42]

At work I have the Stihl vacuum and pressure tester and all the adapter plates. At home I have the Harbor Freight vacuum and pressure tester. Would I want the Harbor Freight at work.? No, not enough quality for daily work. Does it work at home? Quite well actually.
You need a tester to repair this saw, unless you can actually see something leaking.

Crankcase leaks cant hide if you dunk the saw under water during a test. Even the most basic bicyle pump kit will show up leaks then.

But wont do vacuum tests and the smaller tests need a pressure guage.

I started with an ebay job, but quickly realised I needed a mityvac.

 

[rutterbe]

I have rebuilt 4 clamshell ms 290s and could not get them to run right I wasted 10 hrs on one of them before I got a vac /pressure tester. I was thinking that I did not seal them right So i took them apart and resealed them again. no results. on the 10 hr saw I replaced everything except the cradle that the motor sat in except the carburetor. guess what the problem ended up being.The guy that I was doing these saws for would test them when i rebuilt them and they ran great. The following week they run like crap. all of these saws had scored pistons so I am concluding the the carbs are failing. We replaced the carbs on all four saws and now they run great.

 

[Gomofast]

Update: I bought the Mityvac. Saw holds pressure and vacuum just fine. Fuel line holds pressure. Spark arrestor is clean. Fuel filter is good. Disassembled carb and cleaned it twice. But the saw still does the same thing... starts, revs for a second or two and then dies. And no amount of adjustment on the L screw will make it idle. I'm thinking the carb might just need to be replaced. Anything else I should try first?

 

[Coralillo Lo Pro]

Update: I bought the Mityvac. Saw holds pressure and vacuum just fine. Fuel line holds pressure. Spark arrestor is clean. Fuel filter is good. Disassembled carb and cleaned it twice. But the saw still does the same thing... starts, revs for a second or two and then dies. And no amount of adjustment on the L screw will make it idle. I'm thinking the carb might just need to be replaced. Anything else I should try first?

Yeah, I'd try a new carb.

 

[stihltech]

Does fuel push out of the fuel line when you remove it from the carburetor? Had this happen twice lately that the fuel hose and or filter were plugged. Usually an A/M hose, but anything is possible.
If all good, possible stuck high jet nozzle in carburetor.
I apologize if this is a rerun.
New carb is probably the best bet, but recheck your fuel supply and pulse first. Won't work with out it.

 

[Gomofast]

Does fuel push out of the fuel line when you remove it from the carburetor?

...recheck your fuel supply and pulse first. Won't work with out it.

Yes, has good fuel flow. And there is no separate pulse line on this model. Only one fuel line. Air pulse is built into the manifold. And I tested pressure/vac through the manifold, it's good.