OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions on Other Engines Scott Bonnar Greenkeeper Lawn Mower

Hi Guys,

I recently got a Scott Bonnar Greenkeeper mower to refurbish and move onto a new home. It has a standard Chonda engine which I am pretty sure is a copy of the GXV140.

The mower is in excellent condition and had very little work. Despite my best efforts to date engine the starts popping out the carb with a slight surge once it is warmed up. The mower starts very easily and runs excellently until it gets hot.

To date I have done the following:

1) Cleaned the carb thoroughly including the main jet and emulsifier with the correct carb cleaning tools. I get nice easy one pull starts.
2) Adjusted the valves � they virtually had no clearance whatsoever. Intake - .15mm Exhaust - .20mm
3) Cleaned the magneto as it had some surface rust. Also re-gapped the magneto as the gap was huge between the flywheel.
4) New NGK Spark plug
5) Compression registers at about 120 odd psi
6) Top and bottom crankcase seals in excellent condition
7) All governor linkages are functioning correctly. The choke is also set up perfectly with nice easy one pull starts.
8) Air filter has been washed and re-oiled.

My thinking is the magneto might be giving up as the mower only starts to surge/ pop/ backfire when it gets hot.

I�d love some other suggestions as this one has me a little frustrated. I�ll put some photos up after work.

Hi Pete, it could be a timing issue, you won know for certain though unless you split the crankcase. The surging could be a bad gasket on the engine side of the carby,(vacuum leak), but that wont explain the popping and backfire.
Now while I'm typing and thinking about it, check the muffler in case it's blocked.

Thanks for the suggestions.
I am little perplexed about how it could be a timing issue if the mower runs ok until it gets hot but I am probably missing something. If the timing was off would the mower not keep backfiring on a regular basis from cold to full operating temperature?
The carby gaskets are all in very good condition so that shouldn�t be an issue.
Your comment about the muffler got me thinking so I swapped it with one from another Chonda that had a busted conrod. I ran the mower until hot and no backfiring. It very occasionally has the slightest surge but nothing to worry about.
The mufflers were exactly the same but the one that come off the engine was quite heavy which lead me to believe it may have in fact been blocked with carbon. I will give the engine another run and a good test to make sure the problem has gone completely. I am not 100% convinced just yet.
Do you have a good method for testing potentially blocked mufflers. I have had this happen a couple of times with 2 strokes but not with 4 strokes. My method was just to swap one from another engine and give it a try.
I am thinking the previous owner may have had in running on choke for an extended period because the quite severe carby blockage simply because it wouldn�t run without an enriched mixture hence the potential blockage.
Jaffa

I generally just blow through them to check them. There should be no restriction in air exiting the muffler.
Give the mower a test run and let us know how it goes.

Sounds like you may have found the source of the problem.

Mechanical timing is one thing that stays fairly fix but electronic timing can vary form cold to hot to too hot.

One comment here related to ignition coil packs. I have had the electronic control versions do some very unexpected firing related issues. I have seen them to fail completely, fail shortly after start-up, fail only after running 30 minutes, then engine was shut down and restart attempted 10 minutes. I even have a couple chainsaws that would randomly blow the air filters completely off due to random backfires through the carburetors.

I even had one Kawasaki engine nearly set my pants leg on fire because the ignitor was randomly misfiring the coil pack and would ignite the fuel mix on the exhaust stroke. Of course this one was a 4 cycle. The first time it did it while I working on the problem I thought someone discharged a shotgun behind me, not a good feeling at all.

So yes the ignition timing can be off only when hot if the coil pack is borderline bad. Even a partially sheared flywheel key can cause the same problem depending how much timing advance is built-in the coil's electronics as heat can causing the advance to shift slightly too much.

Just a couple of suggestions, I'm assuming it's a standard newer rotary mower with a vertical engine.
1) Is the cooling working ok? If it's not a rotary mower and the engine's mounted sidewards (if this is at all viable!) then perhaps the carby/inlet could be in a place where it's getting over hot?

2) Pre-ignition from carbon or dust buildup in the combustion chamber? Just for a diversion, read my little story here...

3) running too lean will make carby air-cooled aero engines pop and backfire (and run too hot).

4) carby icing but that's not likely to be an issue here?

Image of the mower for reference.

Hi All,

Well this is a rather interesting mower and throws up a whole bunch of questions as to how the name "Scott Bonnar" has come about being on a relatively new mower.

Has Rover finally done the unthinkable and sold off the naming rights to a Chinese manufacturer or has the registration of the name expired, thus again some Asian manufacturer has poached the name while it was not copyrighted ????

MTD owned Rover for a while but I'm under the impression that MTD no longer own Rover any more. I stand corrected on that matter as there is no mention of Rover on MTD's website, but I have been told by an Adelaide based Mower shop that MTD still are the supplier of Rover parts to the industry, so from that I'm somewhat confused.

Regardless of all that it's sad that the name "Scott Bonnar" has come to this and to make matters worse they couldn't even capitalise the letter "B" in Bonnar.

Disgraceful................................

Cheers
BB

******UPDATE

This mower in question was in fact made by Rover in China using the SB name, and MTD still own Rover. If one wants Rover parts you have to go to the Rover site as MTD are a brand of their own sporting the MTD logo on their own products.

Guys thanks for the suggestions.
I swapped the muffler with a known good one and it still had the same problem. It runs great until it gets hot then is starts to cough and backfire. It won�t stall but it sounds pretty poor.
AVB you got me thinking about the coil so I swapped one from another Chonda. Again it ran good whilst cold but the difference this time was it kept running nicely. Exactly what you would expect from a well-tuned overhead valve engine � no missed shots. I gave it a good 5 minute or so test.
I am not yet 100% convinced but things are starting to look up with this unit.
At this stage pending no other suggestions I plan on giving the mower a good couple of runs before I pass judgement on the repair.
Any other suggestions or thoughts are welcome.
I have marked this coil in my spares box as being potentially faulty.
BB � It�s definitely a Chonda. Rover make a similar version just with a different cowl. As for the history comments that sure isnt my area of expertise so I�ll leave that for the other guys.

Yes, Rover did make this model Scott Bonnar in China a few years ago now. Surprisingly enough this was the budget end of the range and retailed for about $329 when new. The Rover branded mowers were slightly dearer with different options available unlike the Scotty which was only one model.
A shame really, such a well noted and prestigious brand being cheapened to the point of extinction.

Hello All,

it would appear that the Scott Bonnar brand signifies entry level machines.
Here is a ride-on I saw at the local Mitre-10:



How the mighty have fallen.
------------------------------
Jack

I have seen something very similar here in the US at a local Wal-mart under the Murray brand. Almost laughed myself silly as how small it was when a 6 ft 230 lb man was sitting on it.

To me they look like the toy ATVs I have been working on lately; just too small for adults to ride in my opinion.

Hello Jaffa and other members
When I first started reading this thread, what popped into my mind. Was something expanding when hot.
Maybe there is some jamming / sticking / friction in the valve train when up to operating temp.
#Lifter
#Valve stem

Not being oiled.

Try another head from a wreck, check lifters for any restriction.

What do you reckon?

Thanks for the comments and suggestions.
I gave the mower another run today and it performed very well after replacing the ignition coil off a wrecked engine.
I bought the mower up to operating temperature 3 or 4 times today and no faults were present at all. The mower performed well hot and cold both at idle and full speed. I let the mower idle for 5 or so minutes today and it didn�t miss a beat.
I am confident this machine is right to go now so thanks for the assistance.
I must say the mower is very easy to start both cold and hot with about half a pull of the starter rope.
The only thing I could fault about the mower is the base seems very thin and only tack welded underneath so I might be prone to cracking as it gets on in age. I�ll make sure it goes to someone with a small to medium lawn so it gets some good service.
Cheers
Jaffa

It's a bit warm, even in the ACT today, so I reckon you've got a well deserved result Jaffa, due to your good diagnosis and persistence.

Thanks for the base thickness info, I was thinking of asking 'cos it even somehow looks thin (to me) in the photo.

It sure was a hot weekend. 37 degrees plus each day and the mower performed spot on.

Theo was definitely right about this mower being on the budget end but I am sure if it goes to a house with a small to medium lawn they should get some good service from it.

Good thread - the moderators can probably close it now however I'll leave it to their discretion.

Hey you guys should've been in Adelaide where the mercury nearly hit 44 on Saturday.
Unbelievable 4 days there.

On a positive point Santa Ana couch loves it, unlike myself.

It's been a good thread with a positive outcome, which will be just left open as there's no need to lock it.

Cheers,
BB