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Collection, Preservation and Display of Old Lawn Mowers

Hayter jubilee 43 with QANTUM ENGINE


AQUIRED THIS MOWER LAST YEAR HAS NEW PLUG COIL AND CARB IS CLEAN WITH NEW GASKETS AS REQUIRED FIRED IT UP AND IT IS POPPING /BANGING THROUGH EXHAUST WITH THE ENGINE REVS RISING UP AND DOWN.tHE ONLY THING i CAN THINK OF IS THE PLUG GAP IS TOO SMALL AS WHEN IT ARRIVED IT RAN FOR A WHILE  IE CUT THE LAWN BEFORE IT GAVE UP AND i FOUND THE PLUG GAP WAS SLIGHTLY UNDER 20 THOU THAT WAS WHEN THE COIL GAVE OUT AND WAS REPLACED.COULD SOMEONE GIVE ME A CLUE WHAT I AM OVERLOOKING PLEASE? MANY THANKS IN ADVANCE. one other thing reset spark plug ran for about a minute then stopped Tank was full and the machine has only ran for a small amount of time and now nearly empty ,I wonder if the float has stuck  allowing an overflow of fuel at the engine hence back fire in exhaust.?? any imput will be greatfully received 


wristpin Fri, 02/02/2018

The plug the plug gap should be 30 thou. Ideally use a resistor plug and a non suppressed plug cap. Is your new coil actually new or second hand new.? The coil armature to flywheel air gap should be postcard width/thickness. Genuine coils are usually marked  “engine side” , if fitted the wrong way round the timing is all adrift. With no plug fitted and with the bend of the lead a quarter of an inch from the head , the spark should consistently jump a quarter of an inch. If not,  disconnect the kill wire and try again. If that improves things , suspect the kill wire or switch. 

Can’t remember, does the Quantum have the long plastic crossover manifold? If so, look for a crack in the pipe or blown gasket where it meets the block. Otherwise, another really good carb clean.



hortimech Fri, 02/02/2018

It sounds about right for a Quantum, engine hunting and spitting through the valves. You can try to mitigate the problem by resetting the coil clearance, cleaning the carb and reseating the valves, but from experience, they all did it. If I remember correctly, B&S issued a service bulletin about this and a different governor spring.

Gerry Norris Sat, 03/02/2018

did wonder where all the fuel went it did not leak out of carb float bowl  as I said tank was full then the machine ran for that few minutes and the tank was virtually empty  Reason on suspected carb was plug is fouling a lot on fuel. The Carb slides on to a small inlet tube on the side of the engine, bolted to a support bracket which holds the carb operating spring / arm etc

Supergrass Sat, 03/02/2018

That is allot of fuel.An over rich mixture would make black smoke slow heavy running and then bog down and stop,like the the choke is left on,+ the plug will be black/sooty.Dont know the engine layout but try running without the air filler,check any jets are inplace  and as you say the float /needle are working properly.One other thought .have a look at the engine oil level/quality,some carbs have a vacuum or breather  pipe to the crank case.

Gerry Norris Sat, 03/02/2018

took air filter of primed carb and then fuel flooded out of air  filter   inlet so would think  float problem ??

wristpin Sat, 03/02/2018

For it to empty a tank of fuel in five minutes it's not what one would normally call a running issue. The fuel has to be going somewhere  - either in the sump or on the floor, but it should be obvious. There was a service issue with leaking fuel tanks on Quantums and a lot of new stock unsold machines were being reworked either at OEMs or dealers and those that were already with owners were being replaced FOC, even well out of the normal warranty period. 

Gerry Norris Sun, 04/02/2018

given I have seen fuel flood out of air intake after priming could the float be sticking?

wristpin Sun, 04/02/2018

Maybe but that’s easily spotted and rectified. However,  with the loss of a tank full in five minutes it would have been an obvious flood!

hillsider Sun, 04/02/2018

I tend to agree with Wristipin about your fuel problem. I don't think the engine would run at all with that amount of fuel passing through it, at the very least you would see clouds of black smoke but more than likely the excess fuel would form a hydraulic lock in the cylinder as you tried to start it.

Re the fuel in the carb after priming that could be as a result of the priming but could also point to a problem with the float valve so further investigation may be required there also.

wristpin Sun, 04/02/2018

could it have flooded through to exhaust ?

Well yes, but you're the man on the spot, would you not have noticed? 

Gerry Norris Mon, 05/02/2018

did not notice anything untoward, BUT when the misfiring happened ththere was a fair amount of Black smoke, will inspect to day and report back with findings 

Gerry Norris Mon, 05/02/2018

right everyone with wifes help got Lawnmower on a bench so I can inspect more easily (I was having to bend down which and as some of you know not easy when you have a double disc prolapse) anyway removed air filter pushed primer bulb and strong jet of fuel coming straight out of centre tube in the carb straight from the float bowl which is then leaking out of air filter on to lawnmower body and out through gap into blade area ,Is this centre tube/jet suppose  to work this way? noticed also slight trace of fuel at rubber pipe from back of air filter body leading to crankcase breather but no evidence of fuel getting past rubber pipe to metal breather as quite a steep angle to climb no fuel trace between rubber pipe and metal tube and oil no evidence of fuel, level  the same in sump nice and thick no trace of fuel.

wristpin Mon, 05/02/2018

That's about how it should be when the primer is operated to give it a rich mixture for starting instead of a manual choke. 

Have you carried out the ignition checks as per my earlier posts? I mentioned the possibility of the coil being fitted the wrong way round but you should also consider a partially sheared flywheel key,  but nothing that's been suggested so far explains a tank of fuel used or lost without trace in five minutes. 

If all that checks out ok try a new plug just in case last year's new one is a dud.



Gerry Norris Tue, 06/02/2018

coil was new new, will go through all your checks you suggested again when weather improve,s but I will try spark plug 1st as it has been knocking around for a while. thankyou once again Wristpin. 

Gerry Norris Thu, 08/02/2018

whilst checking through  engine parts and so forth on the Briggs &S site I came across a chart concerning spark plugs   It would seem some models of Quantum engines ie 625 series 725 and a few others I cannot recall had a plug gap of 20 thou I wonder if this engine is one of  those? will try to check through the ignition today and see what I find.

wristpin Thu, 08/02/2018

Re, plug gap. 30 thou according to my chart but if you will post your Model Type and Code numbers (stamped into the blower housing or muffler shield) it can be pinned down to your exact build.

Gerry Norris Fri, 09/02/2018

Hi Wristpin,

Its 127802-2004-E1-99012057  this what is pressed into the Muffler shield  Much appreciated 

wristpin Fri, 09/02/2018

My book says a 30 thou plug gap.

If it’s of any interest your engine was made on th 20th of January 1999.

Gerry Norris Sat, 10/02/2018

Thankyou wristpin  much appreciated  and yes it is nice to know when the engine began its life, fuel problem was a dodgey fuel line connection.

Gerry Norris Tue, 13/02/2018

Set plug 30 thou wristpin  primed up and it fired and run as well as these do then cut out primed again fired up then cut out. Looks  like I must have not cleaned the carb well enough or missed the main jet in the carb, I think it is a walbro carb although it does have a primer which you push 3 times to prime any idea where I might get a diagram of it -ie where each jet is located? 

wristpin Tue, 13/02/2018

Perhaps the easiest way to find a diagram is to use your model Type and code to look at an illustrated parts diagram but the only one that I've found so far is too indistinct to reproduce well.

Walbro carbs usually have a type number stamped into the edge of the mounting flange - LMT or some such.Worth looking for that as well.

Gerry Norris Thu, 15/02/2018

Decided to check electrics, fitted a spark tester between plug and lead, and I have lost my spark so it could be as you Suggested wristpin Electrical instead of Fuel will now go through everything again.