OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines Another unit with a B&S 3.75 HP

Hi,

On a separate topic....
I've also taken-up a Rover fitted with a B&S 3.75 HP engine, model 098902. Anyway, it seems to turn-over well, has fuel and good spark, compression of 95+psi whilst pulling the cord, however won't fire. The spark seems rather wet, I'm wondering whether this unit has adjustable valves etc.. which may be causing the problem.... what should normal compression on these 4 strokes normally be?

First, B&S do not specify a compression pressure, and recommend a compression test that does not involve a pressure gauge because of their EzySpin feature which keeps one of the valves off its seat during the first half of the compression stroke. The correct procedure is to disconnect the spark plug and turn the blade plate backwards until you find compression, then flip the plate backwards to see if it bounces off the back side of the compression. If it does, you are good to go. It does not sound as if you have a compression problem, but you should check it by their method. Just be careful that the plug lead does not fall back onto the plug - it needs to be secured to keep this from happening.
If the spark plug is wet with petrol (not oil) it sounds as if you have an ignition problem, but that does not match your report of a good spark. One possibility is that when you are trying to start it, the ignition kill device is operating - it is connected to the speed control.

Thanks grumpy,

Does the ignition kill device comprise of a separate small lever connected to the throttle via a small spring, this device seems loose and I'm not entirely sure what is should be doing, however it seems to remain in the same spot no matter what I do with the ignition ie: spring not really working.....Only curiousity is that wouldn't the ignition kill device prevent a spark?...
PS. I will try each of the other suggestions, just not sure if it will be this weekend...

The ignition kill device is a small metallic tab that sticks through the throttle plate with a wire on the bottom that runs back to the ignition coil. On the stop position the ignition coil is earthed out via this device to stop the engine from running.

However over time, these parts wear, insulation breaks down, etc and sometimes you can have it short out when you dont want to.

I have even come across spark plugs that show a good spark when outside the cylinder but loose spark when under compression.

I keep an old spark plug with a gap of about 1.5 mm just for testing spark. Sparking across a 0.6 mm gap - especially if the spark is red - does not tell you there is enough spark to run an engine. The old Champion spark plug tester that petrol stations used to have long ago tested plugs under continuous sparking in a chamber with a glass window - you could vary the air pressure in the chamber up to 80 psi, and watch the spark. Often at maximum pressure it started to run down the outside of the insulator instead of across the gap. If you increased the plug gap, it always ran down the insulator.

Hi Guys,

Took a while to get back to this one, I gave the unit a check, tested the spark plug with a large gap (nom. 1.5mm+) and managed to get a yellow spark which was good, cleaned out the air filter and replaced the fuel in the tank, this seemed to do the trick, she runs well enough. I'll need to do blades, oil etc... but otherwise usable. The unit is also missing a catcher so I'll be chasing around for one of these.

PS. Grumpy, I think I know why you prefer B&S, cheers