Hot starting problems Stihl Brushcutter

[arbclimber]

Hi, As title suggests really, got a stihl FS120 strimmer/ brushcutter. Starts fine from cold and will run fine for as long as you like. trouble is as soon as you turn it off it wont restart. I have found the only way to get it to restart is to take the plug out and give it a go with a wire brush. Pop the plug back in and it starts first pull. Its had a new plug, pretty sure the mixture is about right, any suggestions gratefully received.

[Les Cork]

Hi, As title suggests really, got a stihl FS120 strimmer/ brushcutter. Starts fine from cold and will run fine for as long as you like. trouble is as soon as you turn it off it wont restart. I have found the only way to get it to restart is to take the plug out and give it a go with a wire brush. Pop the plug back in and it starts first pull. Its had a new plug, pretty sure the mixture is about right, any suggestions gratefully received.

 

Hi

Sounds like vapor lock to me, although your plug could be breaking down under heat which plug are you using, a 6 or 7 rating ? i'd use a 7

 

Vapor lock occurs when excessive heat transfer causes the petrol/air mix to vaporise in the carb/emulsion block before it reaches the cylinder and ignition point from the plug.

First check air filter, exhaust [especially spark arresters i always remove them unless your working in oil refineries etc] then check carb settings and back off high speed jet without losing too much performance.

Fuel oil mix is important what ratio and what oil ie. semi synthetic or mineral are you using. Next check tank vent and tank pick up filter

Every 15 minutes the temperature increases so when you stop the engine and the fan on the flywheel stops cooling, the engine gets hotter and vapor lock can occur.

Using a fuel additive in every gallon will help as it puts back in what the petrol companies have taken out.

 

This was a common problem when unleaded fuel came in in the 80's.

 

I hope your wire brush is a brass one as a standard wire brush scratches the electrodes allowing extra carbon build up which can make your plug run erratic.

Also check is the primer bulb draining ? and when hot is there a difference in the compression of the engine ?

 

Let us know how you get on.

[arbclimber]

Hi

Sounds like vapor lock to me, although your plug could be breaking down under heat which plug are you using, a 6 or 7 rating ? i'd use a 7

 

Vapor lock occurs when excessive heat transfer causes the petrol/air mix to vaporise in the carb/emulsion block before it reaches the cylinder and ignition point from the plug.

First check air filter, exhaust [especially spark arresters i always remove them unless your working in oil refineries etc] then check carb settings and back off high speed jet without losing too much performance.

Fuel oil mix is important what ratio and what oil ie. semi synthetic or mineral are you using. Next check tank vent and tank pick up filter

Every 15 minutes the temperature increases so when you stop the engine and the fan on the flywheel stops cooling, the engine gets hotter and vapor lock can occur.

Using a fuel additive in every gallon will help as it puts back in what the petrol companies have taken out.

 

This was a common problem when unleaded fuel came in in the 80's.

 

I hope your wire brush is a brass one as a standard wire brush scratches the electrodes allowing extra carbon build up which can make your plug run erratic.

Also check is the primer bulb draining ? and when hot is there a difference in the compression of the engine ?

 

Let us know how you get on.

 

Many thanks for your comprehensive reply:thumbup1:

The plug is an ngk bpmr7a. It doesn't seem to get a carbon build up on it but for some reason giving it a quick brush seems to be the only way to get it going again. Top tip about the brass wire brush, I honestly didn't know that:blushing:

I use a standard 50:1 mix using the standard Stihl (red) oil.

Ill have a play with it over the next couple of days and keep you posted regarding results.

Once again, many thanks.

[ScotClayShooter]

Inside the carb there is a little gauze filter and these can choke up and stops it drawing fuel.

Wher you use the purge it wont fill back up and seems to be full of air.

 

If its a few years old it might just be time for a carb kit the new diaphram and fuel pump can work wonders.

 

Letting it idle for a little while before you shut it off can help as well.

[arbclimber]

Thanks for the advice mate:thumbup1:

[ScotClayShooter]

Forgot at the time but also is the needle lever set at the correct height then next is the diaphram in the right way around and the gaskets.

 

Seen some really good ones in the past.

 

We have Makita strimmers and from the factory the L is set 1 turn out and you cant get them to rev up from cold 3 turns out and everyone thinks your a genius lol