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Why does my Briggs and Stratton Quantum 6.0 engine keep stopping?

This is a discussion on Why does my Briggs and Stratton Quantum 6.0 engine keep stopping? within the Small Engines forums, part of the Tech Support Forum category. I have a B&S Quantum 6.0 motor 125H02-0122-B1 02041B56. Did some research at our local library and believe from the


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Old 03-26-2009, 01:06 PM   #1
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I have a B&S Quantum 6.0 motor 125H02-0122-B1 02041B56. Did some research at our local library and believe from the numbers that this engine was manufactured in 2002. It's fitted to a Grillo HWT 550 line trimmer (strimmer) which I might say is a wonderful machine for taming grass.

The problem is that the engine won't keep going. It starts fine, the same way it always has, but stops after a couple of minutes running. It won't restart immediately but will do so after just a few minutes, but stops again after another couple of minutes. If you restart it sooner rather than later it seems that it also stops sooner. That's my impression anyway. I haven't actually timed it. Sometimes when you restart it quite soon it will give one backfire. Compression is fine when you pull the cord. When it first started doing this the engine cutout quite quickly, but now it sometimes cuts out more slowly, even occasionally recovering some revs but never continues running for long. I've tried pressing the prime button during this time but that doesn't help.

I have an ordinary motor mower with a Quantum 5.0 engine which uses the same spark plug so changed them over but that didn't help. I then decided to clean the carburetor. On the way to it I found that the air cleaner was pretty blocked so replaced the folded paper cleaner and cleaned the gauze cleaner. Also cleaned the carburetor but it didn't fix the problem. When I went to the small engine shop to buy the new air cleaner the man told me that the problem would be gunk in the carburetor so came home and cleaned it again but still no luck. A mechanic friend of mine also reckoned it would be the carburetor so I took the carburetor off the Quantum 5.0 and put that on it. They appear to be identical, but that didn't fix the problem either.

I've also changed from 95 to 91 octane fuel but that hasn't made any difference.

Any suggestions would be greatly appreciated. I've Google searched but haven't found this exact problem anywhere.

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Old 03-26-2009, 07:40 PM   #2
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Hi and welcome to the forum

I know this sounds stupid, but I have seen this before.........

Check the vent hole on the gas cap. Next time is dies/runs bad loosen the cap and see it that helps, if so, the vent is blocked.

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Old 03-30-2009, 10:00 PM   #3
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Many thanks BG for your suggestion. Tried running it with the fuel cap loose and even used fuel cap from my other mower but unfortunately this didn't fix the problem. Any other suggestions very welcome. Thanks again, StewartD.
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Old 03-31-2009, 06:06 AM   #4
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Why are using such a high octane gas? Probably should be just be regular I think. Would that make difference? I have no clue, but I would try.

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Old 03-31-2009, 02:08 PM   #5
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If you are totally confident that the carb and all the jets are clean - then that will rule out fuel starvation and all that is left is a rogue ignition part.

Whilst they usually fail and stay failed - you might have a dodgy coil or electronic ignition part that gets antsy when it gets warm / hot and recovers once cold.

BUT before I ran off and bought new ignition parts - I would just make sure that fuel is flowing properly and freely thru the tank to carb line and that the carb is absolutely clean - If so - then the new ignition parts would be my next step

A test you might try is to start the engine and as soon as it quits - take the plug out and check if you still have a good spark - if not, that should confirm that the ignition is shot. Wait till the thing cools down - test the ignition again - if you now have spark - you can be pretty sure that you need new ignition / magneto parts

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