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

The engine appears to be running smoothly, but the when turn to full throttle could do with a bit more power -could be the throttle cable needs replacing as well as the governor spring. Will post pics tomorrow of the sprockts & chain

Your speed control lever on the governor body seems to be travelling for its full stroke in your pictures, so I doubt the speed control cable is at fault (though it is a lousy cable and very poorly suited to the B&S cable clamp at the lower end, which is designed for a cable with a steel outer). However the stretched governor spring will certainly reduce the maximum engine speed, and therefore power. If you want full output, you need to replace that spring.

Hi Grumpy - the machine is running OK & yes I will replace the governor spring to improve the max speed. See attached close up of the sprockets, the smaller ones are much sharper on the tips. I am assuming I need to replace all the sprockets & chains (have done this on motorbikes before) - so assuming it not that hard. Where do you suggest I source the parts from & what would you


expect I pay for them . I have the parts list ,Thanks

Both sprockets in the third picture seem to have terminal wear. The one in the first picture looks OK, and the little one in the second picture could probably soldier on for a while - I think I'd replace it when I was replacing the chain, but keep it in the meantime. Is that one of the worn chains?

Deejay and deviosi would be the main guys here on where and how to get parts for Scott Bonnars I think, but some other members are likely to be of help also.

How can I tell the chains are worn ? All I can see is the chains are way too loose & cannot be adjusted any more to tighten

There are two standard failure modes for chains: broken rollers, and worn pins/links. You check for broken rollers by looking, they should be obvious, and are not common with relatively modern chains that have not been abused by impacts. Pin and link wear causes the whole chain to go out of pitch, and ruins the sprockets by overloading the pickup tooth - the next one to take up the load as the chain comes onto it. The easiest way to check for pin and link wear is to remove the joining link, straighten the chain, and turn it on its side. Hold the two ends and measure how much it sags from a straight line at mid-span. If it is more than, say, an inch in two feet, it's time for a new chain. A new chain has barely-measurable sag.

OK thanks I can do that,but the chains are way too loose & I can see what you are saying about being out of pitch. If the sprockets are getting sharp teeth is that time to replace the sprockets.What type of cost would you estimate to purchase new chains & sprockets - would outdoorking be where I would source the items

If the sprockets get sharp-pointed teeth, they have to be replaced. Further use will actually undercut the points until they fall off, leaving stub teeth. At that point the chain will slip. However sometimes the tip of the teeth gets deformed in other ways, especially if you fit a new sprocket with a stretched chain. The small sprocket in your last picture looks as if this may have happened to it.

Deejay is the man with information on where to buy Scott Bonnar parts. He may be away visiting his parents in Brisbane at present, so he may not get back to you right away. If all else fails, send him a PM.

Hi Grumpy

I have mowed today, when I take the spark plug out , there is some oil on the cylinder head. Is this a valve problem & how hard is it to fix. When the engine is cold it wont start without some Aerostart in the carbie. However when hot the engine starts easy. My concern is the oil on the cylinder head -what are your thoughts ?

Where is the oil, Furter? Is it inside the engine, so you can only see it by looking in through the spark plug hole? Is there also oil on the spark plug electrodes?

There is no oil on spark plug electrode, it's dry. When I look through the hole there is about oil on the cylinder head a little runny, more than a smear - obvious this must affect the starting

Sorry I mean oil is on the piston head, looking through the spark plug hole

It sounds as if your engine may be sucking some oil past the rings, though it could also be that blow-by is pressurising the crankcase and blowing oil through the breather into the intake pipe. Either explanation points toward worn-out piston rings. You can make a simple check for this. Pull the starter cord a couple of times with the speed control in the stop position, to see what it feels like. Then remove the spark plug, put a spoonful of engine oil into the cylinder through the plug hole, reinstall the spark plug, leave the speed control in the stop position, and pull the starter cord a couple more times. If you can feel noticeably higher compression the second time, with the oil temporarily sealing the piston rings, it means that your rings are worn or broken and need to be replaced.

If your rings are very bad, compression could be so low that the engine won't start easily, but you may have two unrelated problems. The oil in the combustion chamber on its own is not a problem: what is a problem is if the engine is using a lot of oil, or compression is poor, or there is an objectionable amount of blue exhaust smoke, or the spark plug is getting fouled. If none of those is happening, it is usual to treat the oil in the combustion chamber as a sign of wear but not a cause for immediate action. So, the important thing is whether compression is weak and oil through the plug-hole improves it. If it doesn't, there is probably another cause for your hard starting. We can discuss that after you report on your test with oil in the cylinder.

Performed the test with the oil & there was a slight increase in compression

The test for compression that Briggs and Stratton recommend, is to put the speed control in the stop position, disconnect the spark plug, and rotate the engine backwards until you feel compression. (You would do this by turning the clutch on the engine's output shaft. Check which way it rotates normally by pulling the starter cord, then turn it the other way.) When you find the compression, pull on the outside of the clutch firmly in that backward-rotation direction, and let go so it rotates freely at a moderate speed and hits the compression pressure, backwards. If it bounces off the compression pressure and moves forward slightly, the compression is satisfactory. If it stops soggily when it hits compression, it is unsatisfactory.

Because you only felt a slight improvement from the oil test, your problem may not be lack of compression. B&S engines traditionally run fairly well with not much compression - they are not sensitive to it. Have you checked that the choke closes completely when you put the speed control in the choke position? If it doesn't, there is an adjustment problem.

Hi Furter, and a warm welcome to the forum. It's always great to have another Scott Bonnar owner on board.
I suspect that your machine is also a Rover/Scott Bonnar due to the solid green paint scheme and a fairly late one at that with the Briggs being manufactured in 1990.
They are a strong mower that if well maintained, will give you hours of mowing pleasure. I see that grumpy is giving you excellent advice so that you will soon have your machine in tip-top condition.
Once again,

Hi Deejay

Yes it was manufactured in 1990, original owner had not used for over 10 years, so needs a bit of work, but otherwise working fine.Regards Frank

Hi Frank, that's the really good part of owning a Rover/Scott Bonnar...all parts are readily available and the machine is quite easy to work on. Best wishes for the resto mate..

Hi Fellas, I'm a bit of a Newbie here, but have been working on 45s for the past 9years (my own and neighbours).
My Red Rover 45 developed a real bad vibration in it and ended up replacing both halves of the motor clutch (Aluminium parts).
When I 1st got the mower the previous owner mentioned that it rips the clutch cork off the Cone section regularly.
What I found was that this has caused wear on the tapered surface of the Aluminium Clutch as well as wear in the motor side also.
It's a dream to use again now, after replacing them both for just over $50 all up.
Just thought I'd post another possible cause for Vibration, that maybe others are experiencing too.

Hi Jack, and Hi to all reading in, this is also my first post here, i have just picked up a what seems to be a mid 70's SB 45 17inch, wasnt running, but i pulled the engine right down, degreased it and cleaned all the carbon out of it, the fuel tank had water in it, and used a full can of carby cleaner on the carby, then put the engine back together with new plug and oil, gave it a full tank of juice,a squirt of aero start, 3 pulls on the crank and she fired up, now the only way to stop the motor is to earth the plug on top of the engine.

Your mower looks younger than mine do you know how old it is??