Stihl FS250R Delema

hogwild12

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I have a Stihl FS250R that will not start and for the life of me I am truly puzzled as I have rebuilt many 2 stokes but just can’t find the no start reason on this one. Any suggestions will be appreciated!
I have performed or checked the following:
Has great compression (~120psi). Carburetor is good, also installed it on another machine and it performed well. Carburetor mount is in good shape, no cracks and gaskets are good. Fuel lines and fuel filter are good. Ignition module (coil) is good, also installed it on another machine and it performed well. Air cap is correct on coil. Has spark at the spark plug so kill switch etc. are ok. Spark plug is good. Flywheel key is in tack so timing is correct. Muffler and spark arrestor are clear.
If I manually give it a shot of gas into the carburetor it will fire (not strongly though) but has the symptom of starving for gas or even a badly clogged spark arrestor.
I’m at a loss on this machine. Any suggestions? I would like to get this work horse back to running.

I look forward to you thoughts and suggestions.
 

bertsmobile1

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Two compressions to worry about on blue smokes.
If the crankcase seals are stuffed then there will be no induction pressure drop to pull the air through the carb ( OK yes the almosphers pushes I did do physics ).
he ings can be showig good secondary compression ( cylinder ) but leaking the other way.
That one took a while to work out.
So you really need to do both a compression and vaccuum test on both the cylinder and the crankcases.
I have had engines that happily hold 100 PSI in the case but would not hold 1" Hg of vacuum.
Similarly I have had cylinders that held better than 100 PSI of compression but got a 0 vacuum so they were leaking crankcase to cylinder thus 1/2 the fresh charge got lost before the induction port was open.
I might tell you that one sat in the shop for over a year driving me mad till I decided to do a vacuum test and found it leaked like a seive.
New rings and a happy customer.
 

hogwild12

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bertsmobile1,

Your comment make every bit of sense and actually a suspicion I've had. I think I'll take your advice and pop in some new rings.

Thanks!
 

bertsmobile1

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bertsmobile1,

Your comment make every bit of sense and actually a suspicion I've had. I think I'll take your advice and pop in some new rings.

Thanks!

Just lie bearings go with new rings in your car, crankcase seals go with new rings in blue smokes.
Found that out the hard way by having to do the job twice several times till I twigged that the nice new rings put extra pressure on the old worn seals.
SO now they come as a package, rings & seals

There is also a nice upside that ckeasning the shafts spotless, a labour charge that is not part of the schedules fees, can be ignored till after the crank is out when it is a 3 minute job as trashing the seals is not a problem when they are going to be replaced.
 

hogwild12

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So now I'm even more puzzled. I put a piston, rings and cylinder on this machine with no noticeable change! If I manually put some fuel into the carb/cylinder it will try to fire but will not go more then 3-4 revolutions. I don't have the equipment to do a proper vacuum test but with a new top end it should be good right? So why is this thing not pulling fuel to fire up? It has now become a challenge to find the problem if for nothing more than to understand what the issue could possibly be.
 

bertsmobile1

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NO
Primary compression and vacuum are required to make a blue smoke to run.
The crankcase has 2 seals on each end of the crankshaft.
If they are not sealing then it sucks air from outside into the crank case rather than through the carb , or if you get enough vacuum , it can blow the air/ fuel mix out the seals rahter than into the cylinder.
Also if the exhaust is choked off you end up with a constipated engine so it will only burn a very rich mixture.

There are a lot of really cheap vacuum/ pressure testers about the price of 2 or 3 carbs that got replaced in place of the $ 2 seals.
 
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