Lawn mower hunting/surging

[Hpd]

Hi all I've got a Challenge Xtreme lawnmower with a Chinese SV150 engine.

It starts well but hunts/surges really noticibly. I have stripped the carburetor down and cleaned it thoroughly with carb cleaner but no difference.

I have tried new fuel which didn't change anything either.

The engine has a governer that isn't pneumatic but I don't know if/how this is adjusted etc.

Any help would be awesome

 

[Nige F]

If it's any consolation I've got a Hayter with a B+S engine that does that too. Burnerman is the guy who knows - hopefully he'll be along soon

 

[Burnerman]

Really a source of compressed air is vital to really get the carb clean, but no matter....please don't adjust the governor!
The surging is due to a weak petrol mixture - fuel starvation, if you like. The engine realises its going to slow down, so the governor tries to speed it up again.....so the revs go up and down.
So - float chamber off, check the needle valve for cleanliness once the float pin has been withdrawn. Then, undo the main jet, and try to extract the emulsion tube which is brass and just above the main jet. This must be clean, and the cross drillings are very tiny!
There is a small drilling just to the side of the main choke which also supplies the pilot jet with air.....squirt some carb cleaner in there (pilot jet removed) and see where it exits.....the pilot jet is usually set to 1 1/4 turns out from fully in.
If the engine doesn't tick over then its a pilot jet issue which needs to be sorted first.
If you want to post up a few pics of the carb we can maybe help you more, but if you really get stuck then GHS Ltd do a pattern new carb very cheaply.
John

 

[Hpd]

Thanks for the help I'm going to remove the carb right now and get some photos up!

 

[Hpd]

Here's some pics

 

[Hpd]

I've labelled this picture to try and make things easier. I'm quite confident I've got everything as clean as possible but could do with some clarification on how to position all the parts I've labelled. Thanks!

1) idle screw?

2) is this the pilot jet that needs 1.25 anti-clockwise turns from closed?

3) not sure

 

[Burnerman]

Pic 1 shows the mechanical tick over screw, partially covering the brass pilot jet. ...unscrew the jet for cleaning. Pic 2 shows the spring loaded idle mixture screw, typically 11/4 turns out from fully in. Pic 3 shows the emulsion tube, only fits one way! Pic 4 shows the needle jet that lets the fuel through. Final pic, shows the tick over speed screw, 2 is the idle jet in brass and 3 is the idle mixture screw. Hope this helps!
John

 

[Hpd]

Thanks for the help! Unfortunately I've put it all back together and the mower is no better off.

 

[Burnerman]

See what happens when you give the engine almost full throttle, and then add a little choke. Does the engine stabilise?
John

 

[Hpd]

Sorry to sound really stupid but how do I do this? There is no actual throttle adjustment so do you mean to push the throttle open on the carb? Also how do I give it a little choke? Just back off the throttle?

 

[Burnerman]

With the engine running and the air cleaner out of the way, just see if you can just close off the main venturi of the carb......even by using a thumb. It simply restricts the air that the engine can access.
Another thing to check.....with the fuel pipe off, see that there is a good flow of fuel from the tank.
Does the engine run happily at low speed (tickover?)
John

 

[Nige F]

All good advice John, but mine is exactly the same - cleaned etc. needs a bit of choke all the time. Would getting the carb ultrasonically cleaned be of any use ? As to the Chinese engine I heard from a friend that our local mowerman said "Chinese carb problems? get a new carb " What are your thoughts on these possibly drastic actions ? Bearing in mind my mower cost £50 and doesn't hunt too badly. Unlike Hpd's . Many thanks

 

[Burnerman]

For sure it is a difficult one - I have had the odd problem that has defeated me too, and I have a Honda pressure washer that refuses to run nicely unless its on full bore and pumping and its just the same with a replacement carb
Ultrasonic cleaning costs around £20 up here and that's really to remove any gum and varnish from very old fuel so I don't do that unless the carb is a unique classic item.
I'd recommend new gaskets though if the carb has been on and off a few times, especially the front one - this has to be in good nick to allow for the primer bulb to squirt fuel in for a cold start.
Check out GHS Ltd for really good mower supplies!
John

 

[Hpd]

Okay so with lots of throttle the engine runs smoothly and by trying to give it some choke like you suggested it does seem to still run without hunting yeah. Any thoughts on what to do try now?

 

[Burnerman]

So the engine seems to behave when running at a high speed - is this correct?
Next, set the throttle stop screw on the top of the carb so the engine is running as evenly as it can on a high tickover. Then, gently turn the idle mixture screw slowly in each direction. Turning it anticlockwise should allow excess fuel in - the engine may chuff black smoke and start to slow. The setting is correct when the engine starts to speed up again. Turning the screw in weakens the mixture and may cause the engine to speed up a little....however, it will eventually stall. 1 1/4 turns out from fully in is usually the guideline but often the screw can be up to 2 1/2 turns out before the engine is happy.
Once the tickover seems to be stable, set the throttle stop screw on the carb top to the idle speed that you want.
John