Stihl 038 avs very high idle

[Ollie_M]

On choke it’ll take about 10 pulls before it pops, choke off it’ll then start and instantly take off to what sounds like about 10,000 rpm or definitely up there. I’ve cleaned the carb through with carb cleaner, split the rear handle off and checked the pulse line and intake boot neither are split which rules out air leak I think? The only thing I find weird is when I take the fuel line off the carb you don’t get that little squirt of petrol coming out you would normally get. So my only idea is to get a carb rebuild and see if that helps.

 

any suggestions and help would be greatly appreciated 

[Muddy42]

40 minutes ago, Ollie_M said:

On choke it’ll take about 10 pulls before it pops, choke off it’ll then start and instantly take off to what sounds like about 10,000 rpm or definitely up there. I’ve cleaned the carb through with carb cleaner, split the rear handle off and checked the pulse line and intake boot neither are split which rules out air leak I think? The only thing I find weird is when I take the fuel line off the carb you don’t get that little squirt of petrol coming out you would normally get. So my only idea is to get a carb rebuild and see if that helps.

 

any suggestions and help would be greatly appreciated 

 

Is the carb H and L at factory settings, idle screw not engaged at all?

 

Is the fuel line split, that could leak air too? Pull it out and try and blow through it and bend it around a bit to look for cracks.

 

It would also be worth doing a pressure test of the crankcase.  I find the best way to do this is remove the muffler, block up the intake and exhaust ports with blocks of wood and inner tube. I then send air through the impulse line.

[Ollie_M]

Carb is at factory settings. I can suck fuel up through the line no problem, doesnt seem cracked.  Don’t have the kit to do a crankcase pressure test, is there a kit someone would recommend I buy?

[Muddy42]

50 minutes ago, Ollie_M said:

Carb is at factory settings. I can suck fuel up through the line no problem, doesnt seem cracked.  Don’t have the kit to do a crankcase pressure test, is there a kit someone would recommend I buy?

 

I have the mityvac pressure and vacuum kit and would recommend it. It also has a good range of bungs and tubes to help get the test set up. The key is you want to block all but one port into the crankcase which you use to introduce pressure and vacuum.  I generally introduce pressure through the impulse line. I put the spark plug in.  I use rubber inner tube to block the intake and exhaust port.  The screws/bolts that attach the muffler or carb can be really helpful.  Also dunking the pressurized saw can help reveal the source of an issue.

 

https://www.amazon.co.uk/Mityvac-MV8500-Silverline-Pres-Metal/dp/B0002SQYUA

 

 

[spudulike]

Well...the theory is that if the throttle valve is fully closed and the saw is still over revving, albeit at no load, then the motor must be getting its air from a leak. So...in layman's terms, take the carb off and make sure the throttle valve plate is closing completely....adjust the idle screw to make sure you get very little gap around the brass plate.

Ensure the spring is holding the throttle shut firmly and then refit the carb, ensuring that the throttle mechanism isn't somehow being held open. Start the unit and if the revs are still flat out then you have a serious air leak in the engine as if the carb is shut tight....the saw shouldn't even run.

[Ollie_M]

46 minutes ago, spudulike said:

Well...the theory is that if the throttle valve is fully closed and the saw is still over revving, albeit at no load, then the motor must be getting its air from a leak. So...in layman's terms, take the carb off and make sure the throttle valve plate is closing completely....adjust the idle screw to make sure you get very little gap around the brass plate.

Ensure the spring is holding the throttle shut firmly and then refit the carb, ensuring that the throttle mechanism isn't somehow being held open. Start the unit and if the revs are still flat out then you have a serious air leak in the engine as if the carb is shut tight....the saw shouldn't even run.

idle screw is adjusted so theres very little gap, i forgot to say which might be quite key is that if i choke it once it starts it will idle at a somewhat normal speed - something i find real weird as it obviously restricts air flow significantly but i dont get what it does differently to the throttle valve

Edited February 12 by Ollie_M

[Muddy42]

11 hours ago, Ollie_M said:

idle screw is adjusted so theres very little gap, i forgot to say which might be quite key is that if i choke it once it starts it will idle at a somewhat normal speed - something i find real weird as it obviously restricts air flow significantly but i dont get what it does differently to the throttle valve

 

That would make sense if you had a serious air leak somewhere in the crankcase as as spud says.  This leak makes the engine over rev at idle, putting the choke on will cut off the air, compensating somewhat.

 

Block off the saw as described above and pressure test the saw under water. You'll find the leak easily.

[spudulike]

I reckon that putting the choke on is pulling through a lot of fuel which has a similar effect to how you adjust the H screw to keep the revs down at flat out running. The very rich mix will stifle the engine at idle thus keeping the revs down. 

You have a real big air leak so a split impulse line, a crank seal that is seriously worn/degraded, loose cylinder, damaged crank case etc. It will be that big that it should be quite visible.

[Ollie_M]

25 minutes ago, spudulike said:

I reckon that putting the choke on is pulling through a lot of fuel which has a similar effect to how you adjust the H screw to keep the revs down at flat out running. The very rich mix will stifle the engine at idle thus keeping the revs down. 

You have a real big air leak so a split impulse line, a crank seal that is seriously worn/degraded, loose cylinder, damaged crank case etc. It will be that big that it should be quite visible.

Impulse line and intake boot checked all ok, cylinder is tight, will give it a good clean and check for physical cracks as I know it’s had hard use. It’s an old saw that sat for a good 10 years so I feel crank seals are a good possibility, for an air leak this bad the seals would be visibly buggered I would’ve thought? Don’t really want to spend out for a mityvac for how often I’ll use it but if I can’t visually find this air leak I may just have to get it.

[Muddy42]

18 hours ago, Ollie_M said:

Impulse line and intake boot checked all ok, cylinder is tight, will give it a good clean and check for physical cracks as I know it’s had hard use. It’s an old saw that sat for a good 10 years so I feel crank seals are a good possibility, for an air leak this bad the seals would be visibly buggered I would’ve thought? Don’t really want to spend out for a mityvac for how often I’ll use it but if I can’t visually find this air leak I may just have to get it.

 

No need necessarily for a mityvac if its a big leak.  Leaks in the crank seals and not always visible. Try a bicycle pump or blowing air from your mouth through a tube into the impulse line with everything else sealed off.  Dunk the saw under water to identify the leak.