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Collection, Preservation and Display of Old Lawn Mowers

Morrison (Briggs & Stratton) engine hunting and carb fuel leak

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Having successfully repaired the rear roller on my Morrison 500 (see another post), I now find myself facing a further and different problem on which I would welcome any thoughts/advice.

The engine for this mower is a 3HP Briggs and Stratton Model 80202 Type 1809-01.  It previously ran reliably and smoothly.  After my repair it exhibited 2 new traits: first it seemed to leak petrol; second, it would no longer run steadily but would tend to "hunt".  It also would suddenly lose power and the engine would stop unless the choke was briefly and quickly applied. 

When I first saw the petrol leak I thought it was from a seam in the tank around the filler cap.  I repaired this but closer study showed subsequently that the leak was actually coming from the carburetor body emerging from the chamber with the sliding choke barrel and dripping down onto the tank below.  While the engine was running the leakage was not particularly noticeable but when the machine stopped fuel would continue to leak out of the carburetor for some time,  Given that the carburetor (a pulsa-jet type) sits above the tank, it was clear the petrol was not driven by gravity but was either still inside the carb or being sucked out of the tank below.

It seems clear this is a carb problem.  The questions is "What has caused this sudden change in behaviour?"  My work on fixing the rear roller did not involve any changes to the carb.  My only explanation is that placing the machine on its side to work on the roller (and doing this with engine and tank attached due to another problem which meant I could not separate them) has caused some dirt in the tank to migrate into the carb.

The carb for my engine is a B&S "pulsa-jet" with 2 plastic tubes - the long one drawing petrol up from the bottom of the tank, the shorter feeding from the upper reservoir built into the fuel tank).  There is one adjustable needle jet on the side of the carb body and a diaphragm pump driven by pressure pulses created by the engine piston.

I have taken the carb off and checked the diaphragm in the pump chamber and given the carb and the needle jet a rudimentary clean.  However on re-assembly the same behaviours remain.  I did notice that the rivet at the base of the sliding choke chamber was loose (see photo with red arrow showing loose rivet).  While this does provide a hole for any fuel in this part of the carb to drip from, I would not have thought that liquid fuel should be present in this part of the carb if everything is working correctly.  [My googling has revealed that I am not the only person with this type of carb to have experienced petrol dripping out of the choke barrel but I never found a concrete explanation for what the cause/solution to this problem is.]

View looking down through air intake of pulsajet carb (loose rivet arrowed)

I did not have everything necessary with me to do a proper clean so my plan when I next have access to this mower is to strip the carb down properly and use carb cleaner and an air blower to clean all the orifices more thoroughly.

I would however welcome any thoughts on what could be the cause of this problem.

Forums

wristpin Tue, 09/05/2017

The hunting is almost certainly due to fuel starvation / weak mixture, so a thorough carb clean starting with the gauze screens on the pick-up pipes should sort that. Another possibility is an air leak between the carburettor flange and the cylinder block caused, usually, by a warped flange . On early versions of that carb set-up, the carb and fuel tank just hung on the manifold flange while later ones had an additional support and were not so prone to coming loose and consequent over tightening. When the carb is off put a straight edge over the flange and file it flat if necessary.

I can't remember seeing the type of fuel leak that you describe but seepage between the carb and tank was not unusual. While you have the carb off the tank check both the tank top and the carb flange with a straight edge. I've seen two gaskets used to eliminate seepage at that joint. Even if both surfaces are true I would use a new gasket as over time the original ones harden and don't seal as well as they might.

May also be worth checking that the "cardboard" sealing disk inside the tank cap is allowing the tank to vent  - in both directions.

Finnkai Tue, 09/05/2017

Thanks Wristpin.  Have ordered new gaskets in preparation.  My engine does have a strut running from the base of the fuel tank to the engine block to reduce the stress on the inlet manifold joint but I note your comment and will replace that gasket and tighten the joint very carefully on re-assembly.  I did make a new petrol cap seal as the original was missing and I originally thought that might have been the cause of the petrol leaking; I was however careful to make a couple of air holes in the cardboard having read other posts on the importance of avoiding an airlock,

Will update with news when I have anything further to report (which will probably not be for a couple of weeks as I will not get to work on that particular mower again until then).  As you know I have 2 of these machines so if all else fails I may try swapping the carb/tank from the other one.

You may be interested that as a result of all my recent work on mowers, I seem to have caught the "bug" and I bought another (i.e. a 3rd) old Morrison (this time the larger 600 machine) at the weekend.  The engine (a larger 4HP B&S) looks OK - it did start and run briefly when I put a little fuel in via the air intake and pulled the starter.  The chassis is a pretty rusty but I think that can be restored; and the chains/sprockets looked at least as good those on my other machines.  And the rear roller looked good too!  I'm not a good judge of cylinder/blade - both are pretty corroded but showed no obvious defects apart from that.  The mower had stood in a shed unused for the last 15 years with the fuel in the tank slowly evaporating, so my first job will be to clean the carburetor/tank which is I am pretty sure likely to be gummed up.  Hopefully I won't be asking too many more questions of you as I go about this latest project! 

wristpin Tue, 09/05/2017

Well, the Morrision theme makes a change from Suffolks and and Atcos. If you wnt to vary your collecting bug but stay with the Antipodean theme, keep a look out for Scott Bonnar mowers. At some point some UK firm must have been importing and distributing them as they turn up fairly regularly . In the 80s we used to see them in regular use and they always struck me as simple well made machines but, of course, they pre-dated all the operator presence safety controls etc that started to clutter up machines from the 90s onward.

Finnkai Sat, 03/06/2017

For the potential benefit of others, I thought it might be worth updating this topic with what eventually proved to be the solution to my B&S carb problem (see above).

I first carried out what I thought was a pretty thorough clean - i.e. removing carb with petrol tank from engine, separating carb from tank, removing jet and dismantling fuel pump chamber then subjecting all to a thorough clean.  This time I used a carb cleaner aerosol and a pressurized air can; I also thoroughly swilled out the tank.  I fitted a new diaphragm for the fuel pump and new gaskets at the joint from carb to tank and from carb to engine intake, tightening them carefully .  Having reassembled everything, I started the mower.  Unfortunately the engine behaviour was unchanged.  Much disappointment.

I then removed the carb/tank and replaced it with the more or less identical component from my other Morrison mower.  When I started it up it ran ok without any of the symptoms described above - i.e. surging, refusing to tick over and leaking petrol from the bore of the carb which dripped down onto the top of the tank from the sliding choke barrel at the rear of the carb.  This helpfully confirmed to me that it was the carb/tank combination that was causing my problems.

Determined not to be defeated, I then took the faulty carb to pieces for the 3rd time and this time used a fine wire to probe all the orifices/jet holes as well as squirting carb cleaner and air through them.  I also re re-checked that the air vent holes in the petrol tank cap and washer.  I then refitted this carb and tank to my mower and this much to my relief it ran properly again - i.e. no more petrol dripping onto the tank, surging and, after a bit of fiddling, a decent slow tick over was achieved.

Morale of the story - cleaning B&S carbs is a serious business requiring much patience and persistence.  Hope this tale gives hope to others with similar problems in the future.