Ms390 annoying me

[Drew4522]

So I recently acquired a nice condition ms390 20" from a desparate friend for next to nothing in trade.. It sat for quite some time, so it was long overdue for service. I certainly know my way around engines, so I went into it thinking it would be easy.

After a piston and ring kit, carb rebuild, air filter, fuel filter, intake boot, plug, fuel line and impulse line, it has developed a low end bog and a quirky idle I cannot figure out. From a cold start it starts normal, as in choke till it pops, then half throttle for a few seconds, then to run position.. as any saw should; but most of the time will not idle without messing with the L and L/A screws. Every..single..time. Once I get it idling, it will run fine and cut, but will bog when throttle is squeezed for a quarter second until reaching high rpm. It seems to cut ok, not was I was expecting for a 64cc saw tho..kind of falls on itself. Yes I've dialed in the H screw (not with tach, by ear...). Muffler is opened up a bit, arrestor removed, nice white/blue spark on plug..shoes on clutch are good, and drum spins free..no, brake is not engaged.

My question is more for anyone who has ever had similar symptoms and what the end result was. I dont want to dump money into unnecessary parts if theres something I'm missing. Thanks!

 

[user 122190]

Sounds like the Carb Rebuild did not go well. AM carbs are real inexpensive. If you got more time then money, take the carb apart and look at everything real close (magnifying glass).

 

[Jasonrkba]

I had that same problem with my 025 when I rebuilt it and tried an aftermarket carburetor. I ended up taking the parts from the aftermarket carburetor (manifold, gasket, etc) and installing then into the original carburetor. Works like a champ.

 

[cus_deluxe]

Does sound like a carb issue, but are you sure you got the engine sealed up properly?

 

[Drew4522]

I'm sure its fuel related. Bought the carb parts from a dealer, its 5min away and I had it now, not waiting for shipping.. Thinking of replacing the carb itself with new guts from kit.

As far as case goes, I sealed it with Dirko the same way I've done dozens of other Stihl clamshells. It's worth taking back apart tho..had high hopes for the saw, given its condition.

 

[Wood Doctor]

Looks like things did not go well on the carb rebuild. I suggest that you buy a new aftermarket carb, install it, and see if the same thing occurs. It can't hurt, except for $12.

 

[Old2stroke]

If your carb has nozzle check valve in the high speed jet, the valve could be sticking open on an intermittent basis. This would cause the exact faults you are having and unfortunately carb kits do not include a new valve and the cost of a new valve is about the same as a new chinese carb (depending on where you live).

 

[Drew4522]

If your carb has nozzle check valve in the high speed jet, the valve could be sticking open on an intermittent basis. This would cause the exact faults you are having and unfortunately carb kits do not include a new valve and the cost of a new valve is about the same as a new chinese carb (depending on where you live).

Interesting.. Thinking I'll just order a new carb. What do I have to lose for $20.

 

[grizz55chev]

I'm sure its fuel related. Bought the carb parts from a dealer, its 5min away and I had it now, not waiting for shipping.. Thinking of replacing the carb itself with new guts from kit.

As far as case goes, I sealed it with Dirko the same way I've done dozens of other Stihl clamshells. It's worth taking back apart tho..had high hopes for the saw, given its condition.

Did you do a vac- pressure test?