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

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??

ScottyB
if it's still got the original motor on it, I'd say then it's a 1990.
I believe the 9010030- on the motor pull-start cowelling (see his posted pics) the 90 under the code is the year.