MS460 wont start. Please help

[jw0807]

Hello all
Recently i bought a second hand 460 magnum, and it ran fine apart from it idled a bit rough and died sometimes. The idler screw had also been snapped off
So i got a new carb for it, chucked it in with new fuel line, filter and spark plug and fired it up but couldnt get it to run right. It would still die after idling sometimes for a minute.
After trying to tune it for a while, it died and wouldnt start again, and havent been able to get it to start since. I reset the carb to a setting i would think would make it at least fire (1 turn out on both screws) but still nothing.
It has spark and fuel and this is with air filter and spark arrestor removed. I sprayed a tiny bit of brakeclean through the carb and it fired once but nothing else. Ive tried to start it each day for the past 4 days.
Any and all help is appreciated

 

[gumby707]

Air leak??

 

[jw0807]

How would i diagnose an air leak?

 

[ballisticdoughnut]

Pull the muffler and inspect the piston. If it’s scored you’ve got bigger problems.

 

[jw0807]

Have looked at piston and cylinder walls and they look good

 

[OH_Varmntr]

How would i diagnose an air leak?

Pressure/vacuum test the crankcase. This will test the integrity of the crank seals.

Also check the impulse hose that comes off the carb and connects to the crankcase just below the cylinder. They get hard over time and will crack and cause a lean condition by not pumping enough fuel.

Also check the carb boot as these also get hard over time and will crack.

Did you buy a cheap replacement carb off ebay for it or an OEM carb? If it were me I'd swap the broken idle speed adjustment screw out with the new one (after noting setting) and throw a rebuild kit in the original carb (if the original is the actual OEM original).

 

[Ozhoo]

Start easy, pull the plug and see if it's wet. With the plug removed turn the saw upside down and crunk to see what pours from the cylinder.

 

[lone wolf]

Well first off was the carb you put on it OEM or knock off junk?

 

[user 122190]

If you can get it to pop or run slightly with starter fluid, and not without, then you have a gas problem. At this point fiddling with the screws is not going to solve the problem. Was the carb from Stihl or was it an aftermarket purchase?

 

[jw0807]

Yea carb was aftermarket crap which could be the problem but then I thought it wouldn't start at all but it did start right after I put that carb in. Impulse hose and carb boot feel good and it is fresh fuel.
After trying to start it, the plug is wet because I choked the first 2 pulls, and there is some fuel that comes out when I flip it and crank.
Sorry it isn't the idle screw that is broken, it's the H screw

 

[jw0807]

Just had a look at the carb side of the piston, and it doesn't actually look too flash. I guess this could be the problem?

 

[ballisticdoughnut]

I’d recommend doing a compression test.

 

[thenne1713]

Maybe, but also says trash getting past AF?

 

[Derrick Sawyer]

Needs rebuild and oem carb rebuild etc,

 

[user 122190]

Yea carb was aftermarket crap which could be the problem but then I thought it wouldn't start at all but it did start right after I put that carb in. Impulse hose and carb boot feel good and it is fresh fuel.
After trying to start it, the plug is wet because I choked the first 2 pulls, and there is some fuel that comes out when I flip it and crank.
Sorry it isn't the idle screw that is broken, it's the H screw

Take the carb apart. Probe all holes and check the metering. Usually the metering is flush with the carb body. Do a search (here) for what that all means.

 

[user 122190]

Just had a look at the carb side of the piston, and it doesn't actually look too flash. I guess this could be the problem?

Maybe. IMO OEM carbs are what you need to stick with. I had an older John Deere mower this past summer that I had to rebuild the carb several times. They can be very tricky to get just right. Don't skip on anything and do your very very best. Once they get goobered up they can be a real pain to get right again. But they are the best starting point.

 

[cookies]

Do not go probing the carb holes. Start with a compression test, that piston looks scored like it was run without the air filter likely needing the cylinder cleaned and a new piston.
Low compression also means low vacuum meaning the fuel is not drawn in correctly and atomized into the air charge.

 

[stihltech]

Hold the saw up by the starter handle. How fast does the saw go down? The piston looked scored from the picture.

 

[Derrick Sawyer]

Piston is scored from run too lean (not enuf fuel at high rpm), poor oil mix-use 1:40 with echo red armour or Stihl or husky and high octane low ethanol fuel, air leak, etc, don't use brakeclean or starter fluid, it will wash oil away

 

[Vintage Engine Repairs]

Pressure and vacuum test it then swap out the piston and clean the transfer. Don’t go AM on the carb, rebuild the original. Inspect fuel and impulse lines, fuel filter and manifold.