OutdoorKing Repair Forum Forums Restoring, Customising and Modifying Tech Talk Scott Bonnar 45 Mower: help with engine service

Hello everyone- this is my first post here- I just got myself a nice Scott Bonnar model 45 (?) I think... mower. I love it!

Came out of an estate and had no idea how long since it was last used or if it worked. Had petrol and oil in it. I got a new spark plug, and tried to start it to no avail after many pulls. I then put a half teaspoon of petrol into the spark plug hole- put the plug back in: hey Presto the mower sprang to life after a few pulls with a decent bang and splutter.

Have used it to mow down my back yard golf course project on four occasions: starting has not been easy- requiring many pulls. There doesn't appear to be any primer that I can see? After that it runs fine and mows nice but does blow a little more smoke than I like. Actually I have another Scott Bonnar cylinder mower that has been converted to us an electric motor. A while back the coupler that connects the motor to the drive shaft failed and stopped gripping. I never got around to figuring out a suitable replacement part and neglected it. Then I grabbed this mower for a steal at $85- I couldn't resist. I thought it would be nice to do away with the extension cord (and worrying about running it over) but now I am wondering if the fumes and noise are worth it... so first I want to tune this thing up- the vibrations were pretty bad in the handles: not sure if they are meant to that bad?

I am looking at grabbing a new muffler: will this help at all with vibrations? I checked the air filter and it is good- there is a lot of crap around the carb, etc.

Basically I know nothing about this machine and would like to give the engine a service before I use it much more. I note it has two oil plugs- there is oil in there but after a few uses it has gone down a bit. What oil am I supposed to use and how can I change over the old oil? Is it different oil for each hole? I know nothing...

I would really appreciate the help.

If I can get the engine starting and running smooth I will do the body up.

I hope you can identify my machine from these photos- I realise I should have photographed the little identity plate. I can take a pic of that as well if it helps.

Thanks in advance.

Hi jackg, please post a picture of the model indentification plate, and the long series of engine identification numbers that I hope will be stamped on the cooling air cowl (the tin cover that extends from the cylinder head down around the recoil starter). With that information Deejay can tell you something about your mower, and the rest of us can discuss how to tidy up your engine.

Meanwhile I'm trying to figure out what that engine is: it looks something like a Briggs and Stratton, but doesn't have the right stickers on it, and has an air cleaner and some other details that are a bit Honda-like. It may turn out to be a fairly recent replacement of an original engine, which might or might not be an unlicensed Chinese copy of a Briggs. If so it probably won't have the stamped numbers on the air cowl.

So, we need some detail from you, and hopefully other members will join in, especially if they recognise that anonymous engine.

Grumpy & jackg,

The engine is a Briggs and to my knowledge it is made in the USA not China. It could be one of the later model 45 Scott bonnar which did come out with that engine but I am sure Darryl will confirm that once he reads this post.

Thanks Bruce, I noticed that the mower's reel appears never to have been resharpened, and the paint looks both original and in excellent condition, which all fits for it being a very late-production example. I've never seen a Briggs without a Briggs sticker on it before, and usually a model number/model name as well. jackg, if you give us the full string of cowl plate numbers (model, type and code) everything should become clear, and as long as it is a US-made Briggs, there should be no problem with sorting out the problem, whatever it is.

Hi jackg, and a warm welcome to the forum. It's great to have another Scott Bonnar owner on board.
I would have to agree with you, this machine was a steal at $85.00...Good score there mate!

Both Bruce and grumpy are spot on, it is indeed a very late Model 45, so late in fact, that I suspect it was assembled by Rover...the clue being the solid light green colour and the black Briggs and Stratton, this colour scheme was used by Rover instead of the olive green hammertone, of the earlier Scotties. I have to agree with grumpy though, I have not seen a Briggs without its stickers, they may have come off if the machine was pressure washed at some stage...I have seen that happen.

Now to the chassis...it is a 17" cut, 6 blade cylinder reel which, as grumpy has said, appears never to have been spin- ground, without a pic of the underside I cannot tell the condition of the bottom knife-blade.
Some observations; you are missing a left-hand grass box (catcher) support rubber (opposite the height adjuster)which helps stop grass box vibration. Also the grass deflector needs to be aligned correctly.(just undo the 2 black knobs on the top of the front rail and push the deflector up and in, until it is square with the rail. This controls how the grass is directed into the grass box).

The vibration through the handlebars would be caused by the engine running roughly and once that is sorted, should be rectified. The machine will come up a treat once cleaned, and with the engine running sweetly, should give you hours of mowing pleasure.

Once again, jackg,

[quote=jackg]

I am looking at grabbing a new muffler: will this help at all with vibrations? I checked the air filter and it is good- there is a lot of crap around the carb, etc.

Basically I know nothing about this machine and would like to give the engine a service before I use it much more. I note it has two oil plugs- there is oil in there but after a few uses it has gone down a bit. What oil am I supposed to use and how can I change over the old oil? Is it different oil for each hole? I know nothing... [end quote]

jackg, the muffler has no connection with smooth running or mechanical vibration unless it is blocked up (usually, either by a wasp nest or accumulated carbon).

The two oil plugs are just because those horizontal crankshaft engines are used in a variety of applications, and Briggs may choose to sell them with a "universal" timing cover that has both the front and rear oil filler plugs rather than just one or the other. The correct oil is 30 weight, to an automotive specification (perhaps SG - there are several of them of different ages). If you use a standard automotive oil (such as 10W30) the mower may burn more oil than you would wish, and might even carbon up its muffler eventually, causing it to run poorly. To change the oil you remove the drain plug, which is a square-headed metal plug sticking out from the front edge of the engine's base, above the mower's reel. Before draining, warm the oil by running the mower (preferably, mowing the lawn, but otherwise just run it at moderate speed for something like 10 minutes). Don't forget to put the drain plug back in, then put the mower on level ground and fill it with oil through either the front or back filler plug, as far as the top of the thread (in other words, put in as much oil as you can before it runs back out).

Is the smoke you have seen blue or black? Blue smoke is oil smoke, black smoke is petrol smoke. The two colours have entirely different causes. Either one can be fixed, though.

And please post that long string of numbers from the air cowl.

Hellot here all,

wow- this is an active forum- thanks for all the replies and tips.

Deejay I think you are right about it being a Rover version. Rover is right there on the identity plate. Also well spotted on the missing rubber bumper- it fell off just after I started the machine- it had split in two.


I looked for the strip of numbers everywhere but cannot see it? I have included an image of what I think is the air cowl- is there something I am missing?

As to the smoke: it is whitish and not too bad- perhaps a normal amount for a mower of this type?- my $100 chinese whipper snipper makes more. It's just with the mower you are walking right behind it- and I was hoping a new muffler may decrease the fumes... but I don't think it indicates anything majorly wrong.

I am keen now to restore this machine: and really like the look of that green hammertone on some of the mowers on this forum...I love hammertone paint.


My electric converted Bonnar looks bad in this photo with grass everywhere- but it is actually very clean underneath- and the blades seem to be excellent. I am wondering if I could use the chain cover from the side on my 45- they look to be the same except the older one has an orange cap in the center- for oiling the chain?. The one on the 45 has a few dents and some rust as you can see. But perhaps I should keep the electric one original? Is it worth anything like that without a motor and clutch?


Here are the pics of the bottom of the machine and the identity plate (last image).

here is a close up of the cylinder and bottom blade- looking good?

jackg, this diagram shows where the numbers can usually be found on that type of engine:



The lower position (that is, on the right side and therefore out of view in your picture) is far more common than the top position.

White smoke may be due to a type of combustion failure caused by water being ingested into the engine. Please check your petrol tank to see if there is any water in the bottom of it. Also see if there is water in the engine's sump. This kind of thing can be caused either by leaving it outside in the rain, or pressure-washing.

Your bedknife looks good in that view, but the top surface is the critical part and we can't see that from the bottom.

Hi jackg, re your chain-case cover, the one from the earlier model would have its attachment screws horizontally across the cover, where the Rover-SB has the screws vertically, so it can't be done. Incidentally, the orange plug in your electric cover in for an edger attachment that was an optional extra for the Model 45....They were not a success and very flimsy. Most users got rid of them.

Re your electric, that could be easily be restored as a petrol model, all you would need is the engine shaft, clutch assembly and of course an engine to suit....but being a 14" chassis, I don't think it would be worth the money spent. Of course you could restore it as an electric. Spare parts are available.

Thanks agian guys,

Grumpey: that picture is tiny: but from what I can make out the numbers are not in either of those places on my engine. Indeed I can't see any numbers or names or marks anywhere on the engine at all...

Deejay: The electric machine only needs some type of coupler to attach the shaft of the electric motor to the drive shaft of the mower. There was a sleeve added to the electric mower shaft to make it thick enough for the clamp that is there but the sleeve failed. It keeps slipping. But I will leave that machine for the timne being and focus on my new fossil powered beast.

any one willing to give me an evaluation of the model 45? The styling and plastic parts look like an 80's model to me?

jackg, these are the standard positions for the numbers on horizontal crankshaft engines:


The numbers tell us, among other things, the year and month the engine was manufactured.

hi jackg your 17 inch s/b looks about 92 many of those green rovers had tecumseh 3hp on them not much chassis prep / paint comes away easy your cylinder cutter and bed knife look ok good luck steve....

hi again jackg if want to sell the rear roller of the 14 s/b id be intrested thanks steve

Devisos: sorry I won't break up the electric machine- it is too good as is.

Grumpy thanks for the picture- moine certainly looks like the first engine pictured-

thanks again folks... so far no luck identifying the 45 engine or fiding a service manual. I have checked everywhere and cannot see any numbers at all- in fact no names, or anything on the engine anywhere I can see. Is there a chance I need to remove any of the panels or anything to find the model numbers?


I think it is a briggs and stratton- maybe 3 HP? I am guessing this because of this part on ebay which I have just purchased- it is identical to mine:

ebay muffler listing

I also grabbed those rubber bumbers :-) and whilst there I saw some real nice decals that I guess I am going to need soon enough. Looks like I will have to repaint as well... green hammertone? This thing deserves it.



What steps should I take to get it starting easier? Diagnostic process, etc and basic maintenance?


ebay muffler listing

Here are more details of the engine- and another view of the cylinder:

The missing engine identification numbers are stamped into the tinware in a small font before painting, so you have to look carefully, with oblique light, to see them in many cases. I suspect that the reason your engine has no stickers is that it has been repainted, and the extra paint has completely filled the stamped number-impressions on the right side of the cowl, near the top. One step you might take is to remove the cowl (it is held on by three small hex-head screws) and examine it carefully all over, including on the inside looking for pressure marks from when it was stamped with the long series of numbers.

If you post a picture of the carburetor (under the air filter, above the fuel tank) I'll try to identify what kind it is. As I said before, it sounds rather like it has water in its fuel or crankcase. Have you checked those points?

We can go through a standard first-principles diagnostic procedure if necessary, but that won't help you in future because you still won't have a manual unless you identify the model and type. Also, you are likely to have to disturb things that are better undisturbed.

As a first step, we will need to check spark and compression. That means remove the spark plug, clamp its metal body to an unpainted part of the cyulinder head, attach the plug lead, put the speed control in the start position, pull the starter cord, and watch the gap between the electrodes. You should see a steady series of blue sparks. Next, check the compression. Reinstall the spark plug but don't connect it, put the speed control in the stop position, and turn the clutch in the drive shaft backwards until you reach compression. (Note, turn it backwards, not forwards). When you reach compression, rotate the clutch backwards with a quick flip so it hits compression and bounces off. Observe how vigorously it bounces back. Pull the starter (plug still disconnected) and see how the compression feels. Let us know the outcome from those tests.

Edited a couple of weeks later:
jackg, I wonder if your air cowl was replaced with a new one, somewhere along the line? That would explain the lack of both model/type/code numbers, and B&S stickers.

Chances are your engine is just about identical to the one discussed in later posts in this thread: a late-manufacture 80202. If so the owner manual and illustrated parts list I have shown for that mower will apply similarly to yours. This means that if you come back to this thread and post more about your smoke problem, we will have a proper background to discuss it.

Hi There

I have exactly the same mower - see photo. It's a Scott Bonnar 45 model# 45058, with 3 hp Briggs & Stratton, 17 inch cut. The model no of the Briggs & Stratton engine is 80202 , code 90100303 which means it was built 3/10/90. I have recently stripped & cleaned the carbie as has not been started for 15 years, replaced the gaskets & diaphragm, cleaned the fuel tank & new oil. It starts fine but may not have put the springs back in the right order - was wondering if you can post a photo of what the carbie & springs should like like (when air filter cover removed). The other issues I have are 1. the drive mechanism is still engaged at times when the knob is in the disengage position (is this a clutch issue?)
2. The drive & clutch chains have around 20mm of play & cannot be adjusted to take up the slack, the cogs look fine. Would you recommend I take a link out of each chain as first option

Furter, can you post the type (second block of numbers, between the model and code blocks) and pictures of the carburetor and linkage please? At present I can't find a specific setup for a horizontal crankshaft 80000 engine in the manual, but may be more successful with more information.

Also, can you post a picture or description of where the numbers are on your engine please? That may help jackg.

If your chains are correctly installed, they are severely worn and need to be replaced. Taking links out just enables you to keep using out-of-pitch chains long enough for them to destroy the sprockets. Why not post a picture of them, so we can try to figure out whether the installation is correct, before you go to the expense of replacing them?

Full numbers from the front of the starter housing are 80202 2324 01 90100303 - see attached photos let me know if they don't come through. Also attached carby in open & closed throttle positions. Chains appear loose - The bolt that goes through the big cog is a touch loose, when I tighten it, the mower does not pull backwards as the left drum appears to lock, it will move forward only. The other issue as outlined in previous post - the drive mechanism is still engaged at times when the knob to engage blades is in the disengage position (is this a clutch issue?)

Furter, you can download the B&S Operator's Manual here:
http://bsintek.basco.com/BriggsDocumentDisplay/default.aspx?filename=flivAJX-nfBhU7y
This is the illustrated parts list:
http://bsintek.basco.com/BriggsDocumentDisplay/default.aspx?filename=Z6ntABVJ1DajI

Here is the illustration of what your carburetor should look like:


Your speed control cable is shown in your first photo at maximum speed setting. When the speed is set for maximum, as shown in that diagram, the governor spring should be moderately stretched, and it should move back to being free (no tension on it) at the idle setting. Your first photo shows the cold start position, which fully closes the choke as well as setting the speed for maximum. Your governor spring is somewhat damaged at the left hand end: it has been over-stretched, and should be replaced, though it will work after a fashion as it is.

To diagnose your issue with the way the engine runs we would need more information about what it is doing at the moment, that you don't like. If it is hunting (surging recurrently in speed, at about 2 second intervals) that is usually caused by lean mixture. It can be cured by turning the mixture screw anticlockwise a modest amount (probably a quarter turn). On the other hand if it is sluggish and/or blowing black smoke, you may be advancing the speed control too far, which closes the choke.

Can you post close-up pictures of the chain sprockets please? In the pictures you have posted they look as if they may be severely worn.