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The Home Smithy
The Home Smithy, Small Engine Tech
Category: Small Engine
Satisfied Customers: 9622
Experience:  Small engine tech
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I have a 25 h.p. OHV V twin briggs and sratton on ...

Customer Question

I have a 25 h.p. OHV V twin briggs and sratton on a john deere tractor. THe motor starts fine but it seems like its missing. at any throttle position it sounds like its popping and backfiring. Under load when the blades are engaged it seems to go away. I put new plugs in checking the gap and a new fuel filter does the same thing.
Submitted: 9 years ago.
Category: Small Engine
Expert:  The Home Smithy replied 9 years ago.

Hi rapalla5;

This sounds like you may have a valve spring that has broken.

What happens is the valve does not get noisy in most cases because only a small portion of the valve spring is broken off and it is still applying pressure to the valve when the cam passes its open duration period. As the motor fires a small portion of the flame (explosion) escapes the valve past it and the seat.

Eventually this will cause a point of the seat and valve surface to wear and the motor will get to the point where it will not operate.

The reason it seems to stop doing this under a load is the pressure exerted by the load tends to increase the pressure of the explosion to a point where the explosion itself applies enough pressure to the valves face that it seals better and nothing escapes the combustion chamber.

The other thing is that you may not hear it due to the noise of the blades when they are engaged. I doubt that this is the case though.

Do a compression test on the cylinders. This will show if this is the problem. Ant discrepency in the height of the valve adjusters will be a definate sign of a busted spring.

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The Home Smithy, Small Engine Tech
Category: Small Engine
Satisfied Customers: 9622
Experience: Small engine tech
The Home Smithy and other Small Engine Specialists are ready to help you
Customer: replied 9 years ago.
i checked the springs and they fine any other ideas i accepted the answer. could it be the valve lash adjustmnet and can i do this.
Expert:  The Home Smithy replied 9 years ago.

Ok that is good. The valve lash could very well be too tight and cause the same problem. As far as you doing this yourself it is not rocket science, but considering that you have to ask if you can I must conclude that you do not have the necessary skills to do this. It requires a certain "feel" for the feeler guage. If you have done tappett adjustments before then you will know what I mean.

What you might try is asking around with your friends and neighbors and see if any of them know a good handyperson that might be able to stop by and do this for you.

If that does not yield any results the next best thing is a trip to a local working mans watering hole. You would be suprised what you can get done for the price of a 12 pack of suds. Just ask the bartender if they know a good mechanic or handyperson. They will point you in the direction of one if there is one there.

Thank you for the accept. If you have any other concerns please fel free to post them. I will be out for the rest of the evening but will be here tomorrow morning.

Best regards; The Home Smithy

Expert:  roger replied 9 years ago.
Hello,Set your valves at .004 with both valves closed on one cylinder then rotate 180 and do the other side.Loosen lock nut with ten mm wrench while holding center torx screw.But briggs uses nikki japan made carbs with no adjustment.The problem is they will get small sediment in bowl and make one cylinder run lean thus making it pop.It has to be removed and cleaned to make run properly.If your engine turns over easily then your valves are set.If they were not the engine would grunt while turning over like a weak battery.The cams have compression release on them.Roger