Mountfield ride on stalls when engaging blade

[Jez650]

Hi all,

After getting my Mountfield 725M with Briggs and Stratton engine out after the winter it struggles to keep running during engaging of the blade. The lever has to be pushed forward carefully and sometimes the motor will stall. This is not due to safety switches.

I suspect the blade brake. I asked a shop who said that it is more likely to be the running rather than the blade brake and that I should bring it in for a service. I don't have access to a trailer. So I had a go myself.

Things I have tried:

Changed plug,
Changed air filter,
Changed engine oil,
Removed and checked carb,
Changed the blade,
Adjusted blade brake.

All these things have helped a little but I am sure something has suffered while it was standing over winter. Any thoughts out there?

 

[Burnerman]

Are you able to ascertain that the blades are able to revolve with belts and the brake released?
If everything seems ok there, can you check that the governor is actually throttling the engine to accept the increased load?
If you're happy enough with that, the usual cause is a weak mixture due to a partial carb blockage.
John

 

[Jez650]

Hi John, thanks for your post.

1. Drive belt and brakes I will check tomorrow (weather permitting) although I feel everything is ok there as I have recently changed the blade.

2. The governer I know nothing about except that it is to do with the two little springs above the carb. Should I just visually check that the throttle is mechanically increased somehow while running or is there some other method?

3. Funny you should mention weak mixture. That was my first thought as the engine seems to have a high pitched rattle from the exhaust (the exhaust is not loose). I wondered if this is pinking as it runs on sometimes too. I removed the carb recently but couldn't really do much as I wasn't able to strip it far. The main jet looked ok. During reassembly I damaged the gasket between the carb and the air filter housing. It is sound though. Also the plug tip is brown and the circular edge is black which does not suggest weak mixture to me.

Maybe you now see my confusion. I will check all these things as soon as I can.

Please keep the ideas coming, one and all.

 

[Burnerman]

It's interesting to hear what you say about the engine running on.......it's fairly unusual with a Briggs motor, but it is due to the mixture being a
little on the weak side. To prevent this, Briggs incorporated a fuel shut off solenoid on the bottom of the carb float bowl, recognisable as there is an electrical connection there. It isn't used on all engines though!
As for the governor, this keeps the engine running at a fixed speed under load. So long as the rods leading from the carb to the back of the engine are free to move, all should be ok there.
Your plug check is sound, and don't worry about the damaged gasket.
John

 

[mikecdowling]

Hi, I had a similar problem with a Yamaha ride on mower, I stripped down the engine, adjusted the PTO etc. In the end I checked the fuel filter which was pretty full with crud, changed this and the problem went away. Might be worth checking the filter on your model and replacing. If there is a lot in there it may be worth flushing the tank through to clear it out.

 

[Jez650]

Another brilliant idea gratefully received. Thanks, Mike.

The weather is a bit rubbish here in sunny Norfolk at the moment and looks like continuing this way for a couple of days. As I have to work on it outside by kneeling on the grass I am a bit stumped for now.

In the meantime I have ordered a fuel filter (I have no idea how old mine is) which I will fit as soon as it arrives. Then if necessary I will strip the carb (after checking the other things that John suggested).

If any of this cures the fault and I can categorically identify the cause I will let you all know.

 

[Jez650]

Looking at the suggestions from John and Mike above here goes with the continuing saga.

The blade moves freely when the brake is disengaged.
The governor does do something (will discuss later).
I changed the fuel filter. The old one did not seem to restrict flow but I changed it anyway.

Regarding the mixture I spoke to a chap at the machinery section of the local garden center. The upshot of this conversation was that I checked the fuel and found it has no water in but did have some bits in it (very small bits and not that many of them. I have seen worse) so I changed the hose (filter too as above). I also asked him to clean the carb using their ultrasonic cleaner. This removed the paint brilliantly but I was surprised to see, when I got home, there was still fuel in the float bowl. I may take it back for another go minus float bowl and floats etc as he said it may take several attempts.

Anyway there is no real improvement in the original issue which was stalling when engaging the blade. In fact after all of the above the engine now 'hunts' quite badly unless I turn in the throttle stop to a stupidly high setting. What happens (at low revs and high revs) is that the revs slowly drop then the governor 'comes in' and picks up the revs. Then it 'drops out' and the revs fall again then the cycle repeats. (I have a movie of the governor mechanism in operation. PM me with your email addy if you wish to see it). The engine seems to run much better with the air filter on.

As an aside, I mentioned plug gaps to the guy in the shop. I set mine to 20 thou and it now starts on the electric. Previously it was 30 thou and would only start on the rip cord. Changing the gap did not seem to affect the running. The guy recommended I set it back to 30 thou and suggested the coil/magneto could be on it's way out.

Are we still looking at weak mixture or could the spark and/or coil have a bearing?

 

[Burnerman]

I would almost guarantee that you have a partial fuel blockage in the carb here......basically, the engine tries to maintain speed, but the restricted fuel supply won't let it, so the governor opens the throttle, the engine speeds up and then the governor shuts it down again - to repeat the process!
Ultrasonic cleaning cant be done with the carb in one piece - thats crazy. All of the jets, float, needle etc need to be removed and then the remainder of the carb passages cleaned - best with carb cleaner and compressed air, I find.
As for the plug, set it to around 25 thou and then forget about it. The ignitor coil might be a bit grumpy but I'd doubt it at this stage....your starting symptoms occur because you can spin the engine faster on the pull start than the electric one can.
John

 

[Jez650]

My hearty thanks once again, John. I will check the gap and try stripping the carb and cleaning it myself as best I can. I will of course let you know how it goes.

 

[Jez650]

Today I stripped the carb as best I could. Basically this means removed float bowl and float inc. needle. Then I cleaned out all the jets as best I could without compressed air. I did use a very thin strand of copper wire bearing in mind that the jets are prob brass or ally.

I did not touch the plug which is still at 0.020" gap.

I put the carb back on and ran it. The rattling noise has stopped, so I guess it's not pinking any more. It doesn't hunt either. However, it is not easy to tell this as the governor seems to be holding the revs pretty high at idle. It still threatens to stall when engaging the blade.

I guess one of the jets could still be blocked?

BTW the guy at the garden center said that if cleaning the carb doesn't cure it then he suggested replacing the carb. Seems a bit drastic to me but is it likely to be worn irreparably? My problems seemed to start when I got the mower out of the shed for the first time after not draining the fuel for the winter.

I think my next step will be to remove the carb again, remove the float bowl and get it ultrasonically cleaned one more time.

 

[Burnerman]

Tick over speed is usually set with a spring loaded screw on the top of the carb, rather than the governor.
Is the engine running flat out when you engage the blade?
John

 

[Jez650]

I can see the throttle stop screw. The arm is being held away from the screw by the governor. If I gently push the governor rod the revs drop but increase again when I let go.

Yes the revs are high when engaging the blade.

 

[Burnerman]

Ok, and thanks for the reply!
When you had the carb off, did you see a drilling to the bottom side of the main choke, air filter end?
That's the pilot mixture air bleed, and connects to the pilot jet screw, at the engine side of the carb......it must be clean. If you saw the pilot mixture screw, it should be one turn out from fully in.
John

 

[Jez650]

Wow. Thanks for fast replies, John.

If I remember correctly there are two or three 'jet' holes on the air filter side. One is inside the choke flap. To be honest I have my doubts about this one being clear as my tiny copper wire didn't seem to go to the full depth. This jet hole goes in and then 90 deg down to the float bowl.

Are you saying the pilot jet goes straight through from filter to inlet and has an adjusting screw? I didn't see any screw but there does seem to be a blanking plug within the float chamber. I did not disturb this.

If this doesn't make sense it is because there are quite a few jet channels and I can't remember which goes where off the top of my head.

 

[Burnerman]

It is always an issue when you try to check fuel passages without compressed air......fuse wire is just fine, but of course the passage could change direction at any time!
I'm surprised there is no pilot jet adjustment screw......maybe that's some sort of anti pollution system, but a few photos would help here.
Here's something to try......turn the slow running screw so it contacts the lever from the governor.
Keep that lever in contact with the screw with a finger, and slowly undo that screw. If the engine doesn't want to run slowly, but would rather stall would indicate a pilot jet problem.
John