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Collection, Preservation and Display of Old Lawn Mowers

Another Engine "Hunting" problem (Balmoral 17s)

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Atco Balmoral 17s

Tecumseh 5148 Carb

Hi all, I know there's many topics on here with similar issues but I can't find one with exactly the same issues so I'll list the symptoms:

Cold engine starts 1st time on 3/4 choke and once warm the choke can be reduced to zero and all seems ok for about 15 mins of mowing.

After approx 15 mins of use on full throttle the engine starts to "hunt" with the revs cycling between tickover and full chat every few seconds with the governor arm swinging back n forth like a slow metronome.

This is temporarily cured by applying 1/4 choke and the hunting stops but starts again after another few mins and is once again cured by moving to 1/2 choke, another few mins go by ok and then the hunting starts again and I'm moving to 3/4 choke, eventually ending up with full choke to stop the hunting and inevitably after another few mins the hunting is back even on full choke.

The hunting only ever occurs above about 1/4 throttle and the machine idles perfectly at ALL times, it's only when the throttle is increased the issue starts.

I've stripped the carb right down and cleaned everything with a proper airline so many times I've lost count believing it must be fuel starvation caused by a blockage.

The flow of fuel to the carb from the tank is good and strong and I'm using fresh petrol and the air filter is clean, in fact I get exactly the same results running with no air filter attached.

The float in the carb appears to be functioning correctly

What I don't understand is:

A) Why I get approx 15 mins of everything working fine with zero choke as the engine is basically warm after about 2 mins of mowing. (I assume this proves there's no clogging of the main jet)

B) When the problem starts and I'm increasing the choke to full on what's now a hot engine it's not actually "choking" the engine to a crawl but instead making it run smoother.

C) All of the above :)

Regards, Andy

Forums

wristpin Tue, 04/04/2017

First stop, when it starts hunting unscrew the fuel tank cap a couple of turns and see if it has any effect.

AndyS Wed, 05/04/2017

Thanks for the advice but sadly I've already tried that without any improvement.

However I have made progress.

Whilst the engine is hunting I manually operated the throttle directly on the carb and the engine maintained high and steady revs proving there was no starvation of fuel. Next I moved the position of the governor to throttle cable spring, moving it up a few holes on the governor arm adding a little more resistance to it's "swing". This immediately stopped the hunting but resulted in the engine running too fast in the throttle=hare position so I brought the spring down a hole and the speed was reduced a little although it's still a little too fast for my liking. Brought the spring down a further position and the hunting returned so for now am resigned to the non-hunting, little too rapid position.

The downside to all of the above is now the tickover is a bit too quick and I can't adjust that out as the "thingy" on top of the carb isn't fully returning to quite make contact with the throttle-stop screw which is a shame as I'm a bit of a perfectionist and like the tickover to sound a bit like a Harley Davidson on idle if you know what I mean.

I still can't figure out though why I used to get 15 mins of trouble free running before the hunting started, am wondering if a new governor spring might be a good idea?...... Young's modulus over time etc etc.

Plus.... I can't help wondering if I've not actually got to the cause of the original problem but moreover stuck a plaster on it and performed a "workaround".

Would still be keen to hear other peoples views and ideas.

Regards, Andy

wristpin Wed, 05/04/2017

This is grasping at straws time !

Despite what you say everything is pointing to a degree of fuel starvation that builds up over time, but..........

Start  with a full tank and as soon as it starts hunting top the tank up - does it make a difference? If it does and you've not already done it, pull the pipe off the carb and starting with a half full tank and the tank cap in place , let the fuel run into a container   observe whether the flow diminishes right to the point that the tank is empty.  

I know that there is no immediately obvious logic to this but how good are the valves, both in seating and clearances? A lot of time can be wasted fiddling with ignition and carburation  when the root cause lies deeper. Apparent good compression does not always signify good valves.

Re the Harley tick over, are you old enough to remember the Ford Thames Trader lorry? 

 

AndyS Wed, 05/04/2017

smiley Just about remember the Thames, although I'd have been young at the time.

Am certain it's nothing to do with the tank and flow from the tank or head of fuel, the fuel flows freely to the last drop and I've eliminated and fuel filters. As a last resort I've even dropped the float bowl whilst "hunting" to check there's plenty there (whilst wearing full face mask and welding gauntlets).

Honest answer is I havent a clue re the state of the valves and seals as I've never had the head off this engine but it might come to that.

Did wonder yesterday if maybe there was a crack or similar in the inlet manifold or gaskets, opening up only when very hot but after inspection there's no evidence.

Haven't got the time today to tinker but I'll have it on the bench again soon.

This is the tickover I like https://www.youtube.com/watch?v=h4T8Q874RNM although the video has nothing to do with me, just plucked from Youtube.

You should start a 4-stroke lawnmower engine LOW tickover competition on here.