Lawnmower problem - I need advice please

[DIYspanner]

I have a Qualcast Suffolk Punch 430S grass cutter which will only run on choke and stops as soon as I move the choke to partial choke.

I've put a new spark plug in and also totally stripped the carburettor yet the problem persists after all is reassembled.


Has anyone have a feasible suggestion to cure the problem please?

Thanks in advance

 

[HERTS P&D]

Have you cleaned the air filter and are you using fresh petrol?

Andy

 

[Burnerman]

To clean these carbs (Dell'orto I believe) you'll really need an aerosol can of carburettor cleaner and compressed air, if possible. Cloths and kitchen paper forbidden!
Did you find the emulsion tube (brass tube, maybe 20mm long, drilled both along its length and across its axis)......very hard to clean without air. It sits just upward from the main jet.
Set the idle jet screw to 1 1/4 turns out from fully in, and these machines don't like aftermarket filters in the fuel line.
Post back with a carb pic if you still have problems.
John

 

[DIYspanner]

I stripped the carburettor down and blew into all the holes that I could see.

What I assume to be the main jet, central inside the float chamber, seemed clear and the idle jet screw didn't have and debris on it.

I didn't see any other jets but I can strip it down again.

I put unleaded petrol from the local garage - wrong to do so ?

The air filter is clean although someone advised me to dowse it in new engine oil and wring it out prior to use.

When I strip it down again, I invest in some carb cleaner.

Thanks for the advice folks.

 

[Burnerman]

I'm afraid your breath won't dislodge much, if anything unfortunately.
The main jet is the one you see with the float bowl off. If its just a short (6mm) long thing then the emulsion tube is above that. Its possible that the main jet and emulsion tube are in one piece, in which case its 30mm or so long.
At the same time, pull out the pin that the float pivots on, complete with tipped needle, and check that the fuel inlet is clear.
Mark the position of the float bowl - some of them aren't symetrical and the float must be allowed to drop completely.
The needle valve point screws into a tiny drilling into the carb venturi, but there is also a fuel supply to it from the side.
You'll probably find a mower shop will charge you £15 or so to blow the carb clean if you take it to them off the machine.
Only oil the air cleaner if its the foam type - not paper.
Ordinary unleaded fuel is fine.
John

 

[DIYspanner]

I've stripped the carburettor again and noticed that where the main jet screws into the main casting inside the float chamber, there is a small drilling on the side of the jet housing and at angle to the jet so it allow petrol to pass from the chamber in to the jet chamber.

When the jet is screwed fully in its further into the housing than this small drilling.

Now as there are two sets of thread on the jet itself with a single hole between the threads, would I be right in thinking that this single hole should be adjacent to this drilling?

By the way - there is only the main jet and an idle jet screw (that I can see)

 

[Burnerman]

From the float chamber, fuel goes through the first drilling and then up through the main jet. The position of the main jet is fine, because the float chamber securing bolt uses the same threads.
The other small hole that you refer to will be on a part of the main jet / emulsion tube assembly that has a reduced diameter, so fuel can get there wherever it happens to be positioned.
The pilot jet may not be a seperate piece, it could just rely on accurate drillings through the aluminium carb body.
If you look at the end of the carb after removing the air filter you may see 1 or 2 small holes alongside the main venturi. This is where the pilot jet gets its air from so they too need to be clear.
John

 

[DIYspanner]

After a little investigation, I notice that when the main jet is screwed in tight, the end of the jet covers the drilling and as the neither of the two slots are aligned with the drilling, I believe this is causing fuel stoppage.

I had to loosen the jet slightly to align one of the slots with the drilling and on putting everything back together, got the lawnmower to run off choke!

Two questions -

Should there be an o ring on the jet at the other end?

How is the throttle / governor mechanism best adusted?

Many thanks

 

[Burnerman]

Wow you are really testing my memory here but you do seem to have sorted things out.
I believe - but not too sure - that there is a fat 'o' ring on the end of the main jet / emulsion tube....this would have the effect of lowering the jet slightly to achieve what you need.
If your carb has the sprung pin drain on the bottom of the float bowl, the carb is a Dellorto FHC and the gasket set part number is 52567.
You can get this from Eurocarb Ltd, 0118 943 1180.
As for the governer adjustment, isn't there a plastic nut attached to a spring that you turn to speed things up?
Post a pic if you can.
John

 

[DIYspanner]

I sorted the jet out with an o ring and managed to get the lawnmower to run.

My remaining problem now is setting the throttle/governor.

I have to push the throttle open with my finger because the governor rod is to strong for the throttle spring which it counters.

Regardless of how much I open the throttle by its lever before starting, as soon as the engine is started, the governor closes the throttle valve so perhaps a new throttle spring is required.