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Ransome/MAG mower issue


Ace
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This is the beauty of these inexpensive and quick to use red neon plug caps that flash as HT passed thru.

 

I use them for checking HT in real world engine running actual conditions.

 

The ignition system may work for starting but what happens when engine is working hard meaning the ignition system has to work hard .

 

Bike, self contained CDI ignition system, non battery dependant, a modern take on a magneto if you will.

 

Great starter and runs well for a few miles, then seems to have either a fuel problem or a coil breaking down at about 5 k revs, surging, just will not rev over 5 k revs.

 

Struggle bike home, fit red neons, start and rev...flashes disappear when misfire starts, revs drop, flashes reappear, engine revs to 5k, misfire...repeat.

 

So loss of ignition at revs.

 

Dmm shows acv output of generator to CDI fall with revs ? And at about 5 k revs, acv output goes below about 28v AC.

 

So, Geny coils breaking down under load and heat.

 

Rate of change, acv rises, insulation break down....

 

Or clean carbs many many many many times as it's an ignition fault.

 

These neons save your hair and stop you getting dizzy...well worth the money.

 

A good bit of kit and very easy to use.  I also use them on petrol cars.....if needed.

Edited by Martin Harris - Moderator
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Vaporisation is a thought. If the petrol is a winter grade and has high ethanol content, it may have pushed the simple technology carburation too far. 
 

You could try some fresh premium E5 (or E0 if you can find it) fuel from a “named” garage rather than supermarket cheap stuff if that is what you’re using. 
 

Easy to prove if you can pour some cold water onto the carb without shock cooling the engine.  The fact that it revs on choke tends to discredit the ignition theories.  

Edited by Martin Harris - Moderator
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Thanks, lots to go at.

Neon/compression testers later this week.

Will also try cold water on the carb body for vapor lock

Higher octane petrol. Although runs fine for first 20 min.

One test/change at a time.

Just to clarify, once fault occurs it still starts and ticks over but won't rev even with partial choke and full choke kills it.

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Well yesterday didn't go exactly as planed ?

The compression tester wasn't available but we still went armed with the new neon HT checker, 5l of water/towel and fresh 98 octane fuel.  Plan was to run until fault arose, check HT with neon, if ok then cool carb with towel soaked in cold water. If still no joy replace fuel with fresh 98 oct. So only checking or changing 1 thing at a time.

Started 2nd pull ? worked fine for 20 minutes and were just waiting for the fault to develop - then bang and the cutting cylinder stopped rotating. Removing the side cover revealed that the drive chain had snapped - long string of expletive's.

The bolt holding the sprocket had come undone allowing it to come off which in turn meant the chain then got wedged on the drive roller mechanism and something had to let go.  You can see the very bottom link is splayed open.

Finished off with our self-propelled rotary and will now need to source a new chain and are still no closer to identifying the fault.1570470422_20210422_1758371.thumb.jpg.77477369d0c52ab04aa797fd0c83a506.jpg

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True Rich. However once we identified the broken chain it was driven off the patch and back to its container storage a couple of hundred meters away still without the fault raising its head and anyone  considered it could be still driven without actually cutting the grass. ?

Mind set was - broken - fix it then continue another day and get on with some flying.

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Brian, doubt icing as carb/silencer are on the same side and with the motor running for 20 min+ B4 fault carb is warm but not hot. Could try the wet towel heated on silencer then wrapped around carb if cold towel to test vapor locking failed.

Will look at bearingboys if we struggle locally ? 

Edited by Ace
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Hi, must say this; I used a number of sit on mowers some years ago.I had problems similar, was the safety switches that exist to make sure that you are seated, and other things re grass box etc. they may be simple but can malfunction. Does not seem applicable in this case but always a possibilty. Most faults usually came down to simple things

Bas

 

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  • 2 years later...

Ace, I have exactly same problem with 7.2 HP MAG engined 30” Ransomes Mastiff. Runs on idle jet only after about 5 or 6 mins, splutters and pops if given more throttle and then finally stops and won’t start again until cold. Tried most things, cleaned carb several times, installed fuel filter, tried Aspen 4stroke , changed plug , cleaned out tank and shut off valve etc Will try the neon spark test. Did you resolve the problem? Thread seems to have terminated. Thanks, P

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Sorry no solution, even after-

We have had the head off, cleaned off the carbon, lapped in the valves and fitted a new head gasket. Valve/follower gap correct as we thought if it was tight the heat expansion may have propped them slightly open when hot. However compression seems ok after its stopped.

Thoughts now are that the coil/mag? or condenser are breaking down when getting hot, although not yet investigated those possibilities.

Edited by Ace
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Thank you very much for responding so quickly, especially after a two year hiatus. Thank you also for mentioning the work tried on valves . I had also thought next job would be to look at valves, expecting reseating etc But as you say, plenty of compression, and so if I can get access , will change coil and condensor . Not sure if it maybe electronic ignition. Will let you know if I manage to sort this frustrating issue, present in various degrees since acquiring the mower, and think the failure on main jet has occurred more quickly over the period. Yesterday’s trial was to use a new NGK BP6ES , clean carb again, no joy at all. But the ancient standby 36” walk behind , same engine but earlier Heikin carb, old stale fuel, rusty plug, started immediately after all year in the barn and no problems at high temp…

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Forgot to mention we also changed the plug and a new pattern carb which needed a different air cleaner - not difference 😥

Runs for around 15 mins then stops. Wait half hour or so then good for another 15 mins. Usually 3 stints does us. Flying in-between so not waisted time.

If we can't sort it a new ride-on will be guaranteed as we can't continue as is.

We have manuals I can share if you don't have them?

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Get yourself one of these…

 

https://simonbbc.com/sparkrite-spark-plug-tester-single/

 

Very cheap and will indicate if the fault is electrical (loss of spark) or something else. When the engine fails, simply plug it on top of the spark plug and connect the ignition lead to it. If the ignition lead is too short (it often is on mowers) then just use a short lead with croc clips or similar but don’t let it touch the engine. You’ll need to be in shade to see it working.

 

If, when your engine won’t start, this still shows that you have a spark, and if you can still feel that it has compression, then really it can only be a fuelling issue.

 

I have an old Hayter mower with a Briggs and Stratton engine which starts and runs fine, but if it is stopped with less than about half a tank of petrol it won’t start again. Top up and away it goes. It’s the fuel pump (part of the carb / tank assembly) that is causing this. I live with it because this is the last season that it is required to work.

 

Brian.

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Have you tried cleaning the main jet? We had a similar problem with a strimmer that started and idled happily but wouldn't pick up to full throttle. One of our club members had a look and fixed it by taking out the main jet and cleaning it. Apparently they can suffer from a build up of a varnish-like coating which obviously limits the fuel flow. The main jet was not easily accessible as it was retained by a non-standard bolt so the head needed a slot to be filed in it it to allow removal with a screwdriver.

 

Worth a look?

 

Incidentally, I had the same problem with the gas burner in my caravan fridge a few years ago.

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Hi Ace, Brian & Nick , very many thanks for your thoughts. I have taken off the carburettor many times, and cleaned it out, including both jets, were dirty to start with but are now really clean ! Plus in-line filter, aspen petrol , new correctly fitting air filter, so I feel prob unlikely on that side. The issue is that she runs fine, full power on main jet, but only for around 6 minutes, was 10mins plus, but then will only idle and eventually fails altogether if too much throttle applied. Needle has also been replaced, as the little neoprene ( ?) point may have deformed slightly. In fact, changing that needle - by the commercial mower supplier, was in response to a fluttery sound , and seemed to cure at the time, but on reflection probably had the problem from day one. Otherwise the machine is in unusually excellent condition

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