Stihl TS 420 no start

christophert

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Aug 1, 2017
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I have rebuilt 2 Stihl Cutoff saws that were extremely abused (ran with no air filters, ran with too lean of air/fuel mixture for example..). I replaced the piston, jug, carbs, coils (firing intermittent) and the saws stubbornly refuse to start. Both are firing, have fuel going to the primer bulb, and will sometimes pop in the exhaust enough the pop the decompression valve. I had training on Stihl's but it was 20 years ago and these saws are a lot different than the ones I trained on to repair. Any help would greatly be appreciated!!!
 
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bertsmobile1

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Did you replace the crank case seals ?
These saws run in very dusty & abrasive enviroments and eat seals for breakfast lunch & dinner.
Quick & not particularly reliable test.
Pop a little starter fluid into the plug hole and pull the cord
repeat down the carb throat.
If it fires better ( or only ) when the fluid is put in the plug hole, suspect the seals.

Second test.
Remove the plug and squirt some heavy oil down the carb throat, pull it through till the plug hole no longer blows oil.
Pop the plug back in.
Engine fires again suspect the seals

The best thing of course is to do a vaccuum & pressure test on the crank but very few people have a vaccuum gauge & pump.

Lots of do dads now days but the same principles apply now as they did then.

If you bought the magneto units from a cheap on line vendor then they are to be considered suspect.
A lot of cheap stuff on line is the parts made for Stihl that Stihl's QC has rejected as not being of servicable quality so the makers on sell them to the scrap dealers who then resell to the distributors who sell to the on line vendors who sell to you.
Genuine OEM parts does not mean "Genuine good quality OEM parts".
When you are running a plant on less than 20% mark up you can not afford to toss out anything.
 

christophert

Member
Joined
Aug 1, 2017
Threads
15
Messages
37
Did you replace the crank case seals ?
These saws run in very dusty & abrasive enviroments and eat seals for breakfast lunch & dinner.
Quick & not particularly reliable test.
Pop a little starter fluid into the plug hole and pull the cord
repeat down the carb throat.
If it fires better ( or only ) when the fluid is put in the plug hole, suspect the seals.

Second test.
Remove the plug and squirt some heavy oil down the carb throat, pull it through till the plug hole no longer blows oil.
Pop the plug back in.
Engine fires again suspect the seals

The best thing of course is to do a vaccuum & pressure test on the crank but very few people have a vaccuum gauge & pump.

Lots of do dads now days but the same principles apply now as they did then.

If you bought the magneto units from a cheap on line vendor then they are to be considered suspect.
A lot of cheap stuff on line is the parts made for Stihl that Stihl's QC has rejected as not being of servicable quality so the makers on sell them to the scrap dealers who then resell to the distributors who sell to the on line vendors who sell to you.
Genuine OEM parts does not mean "Genuine good quality OEM parts".
When you are running a plant on less than 20% mark up you can not afford to toss out anything.

Thanks for the reply! No, it slipped my mind about the crankcase seals (its been 20+years since I worked on cut-off saws). I did put genuine Stihl parts on them as the guy who orders my parts did order "aftermarket" stuff the first time around which was a frustrating experience because I tore them down only to have wait for new/ "new" replacement parts because of poor quality/fit. Again, thanks for the input!
 

christophert

Member
Joined
Aug 1, 2017
Threads
15
Messages
37
Did you replace the crank case seals ?
These saws run in very dusty & abrasive enviroments and eat seals for breakfast lunch & dinner.
Quick & not particularly reliable test.
Pop a little starter fluid into the plug hole and pull the cord
repeat down the carb throat.
If it fires better ( or only ) when the fluid is put in the plug hole, suspect the seals.

Second test.
Remove the plug and squirt some heavy oil down the carb throat, pull it through till the plug hole no longer blows oil.
Pop the plug back in.
Engine fires again suspect the seals

The best thing of course is to do a vaccuum & pressure test on the crank but very few people have a vaccuum gauge & pump.

Lots of do dads now days but the same principles apply now as they did then.

If you bought the magneto units from a cheap on line vendor then they are to be considered suspect.
A lot of cheap stuff on line is the parts made for Stihl that Stihl's QC has rejected as not being of servicable quality so the makers on sell them to the scrap dealers who then resell to the distributors who sell to the on line vendors who sell to you.
Genuine OEM parts does not mean "Genuine good quality OEM parts".
When you are running a plant on less than 20% mark up you can not afford to toss out anything.

Thanks for the reply! No, it slipped my mind about the crankcase seals (its been 20+years since I worked on cut-off saws). I did put genuine Stihl parts on them as the guy who orders my parts did order "aftermarket" stuff the first time around which was a frustrating experience because I tore them down only to have wait for new/ "new" replacement parts because of poor quality/fit. Again, thanks for the input!
 

christophert

Member
Joined
Aug 1, 2017
Threads
15
Messages
37
I did what you suggested. Shot some starting fluid into saw#1, it ran about 2 minutes then lost power and died. I was going to do the vacuum test but I do not have the block off plate for the intake that will cover both ports on the intake side. Saw#2 popped with a shot of starting fluid but it wouldn't start. So, I ordered new seals and I am waiting on them. I should have put them in when I had the saws torn down, but it was my oversight. Again, thanks for the help.
 
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