Briggs & Stratton 3HP 125cc Model 80202 side valve

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Re: Briggs & Stratton 3HP 125cc Model 80202 side valve

Postby Plugsnpoints » Mon Jan 14, 2019 10:19 am

That makes sense! The pipe from the valve chest operates the diaphragm on the carb! It works by the difference in pressure and 'flaps' the rubber diaphragm on the inlet side and 'pumps' the petrol at the correct time. If you make sure the pipe isn't sucking in air it will improve it. This is crucial.


As I said in an earlier post, the diaphragm needs to put under a little tension as you tighten the screws between the carb halves. I take it the carb is spotless inside and out now?


Andy :)

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Re: Briggs & Stratton 3HP 125cc Model 80202 side valve

Postby 56G80S » Mon Jan 14, 2019 5:35 pm

Been at work all day today. "screws between the carb halves"

Oh dear, I think I'll start with gettng an exploded diagram.

Or I may just settle for explosion.

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Re: Briggs & Stratton 3HP 125cc Model 80202 side valve

Postby Cjay59 » Mon Jan 14, 2019 6:10 pm

I would start by checking if it has a fuel filter, that could be dirty and blocking the fuel line, then on to a dirty air filter, then air leaks, then take the carb apart, all the Briggs & Stratton stuff is fairly bullet proof from my experience.

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Re: Briggs & Stratton 3HP 125cc Model 80202 side valve

Postby 56G80S » Sun Feb 10, 2019 8:46 pm

Thanks for the useful advice.

All running fine after head off, remove valves, lap in, replace, dismantle carb, thoroughly clean, seal the joins in the tube to the diaphragm, replace diaphragm, replace other seals, redo the linkages that were connected incorrectly then clean and reset plug gap.

The linkages still aren't right but I can get it to start. idle and go smoothly up to working speed. I thought my torque spanner went down to 11 ft lbs but doesn't so did the head old fashioned; it's hardly F1.

Thanks again.

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Re: Briggs & Stratton 3HP 125cc Model 80202 side valve

Postby 56G80S » Fri Mar 29, 2019 8:13 pm

Re-animating this thread.

After a test "rotovate" on land near the lock-up (with owners encouragement). Went to the allotment.

Start up, get going. Ground a bit hard and stop to take of the pair of outermost tines. No restart.

Cleaned up plug, going again. However, the machine would only idle, move the lever towards the position that previously gave full speed and the air vane controller operated and the engine cut out.

When arm got tired of the recoil starter pack up and return to the lock up.

Later, getting dog cage out and final (ha ha ) fettling of brackets for the panniers, this bear of little brain distracted to the rotovator. Starts. Push the lever. Full throttle no problem.

I can only think of three things:

1) Condensor - cream crackered and fails when hot although no wish to break the engine down again;
2) Tubes driving the diaphragm leaking - might wire them;
3) Diaphragm - I couldn't work out how to "tension" it and maybe it's slack and gets slacker when everything warms up;
4) Rent one for £42 a dayand sort out later as despite the diagrams on the net, the wires and springs I have don't seem to match up with what I've got.

The irony is that it was just about coping with both sets of tines on quite hard ground.

Johnny B

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Re: Briggs & Stratton 3HP 125cc Model 80202 side valve

Postby Plugsnpoints » Sat Mar 30, 2019 12:42 pm

Good afternoon. The old B & S engines can be a bit fickle! Before you remove the flywheel to check the points/condenser I would suggest the following-remove the air filter and you will see the rod which is attached to the diaphragm. Release the slotted screws (4 or 5, can't remember) holding the two halves of the carb body together. Gently push a tiny bit of pressure on the rod to tension the diaphragm, then tighten the screws evenly, keeping it under tension.

The other thing to check on these is obviously to make sure the jets and various holes are clear. The inlet pipe on the valve/carb may need to be checked to see if it's sucking air.

Another tip with Briggs engines is to tilt the engine on one end if possible via the handles. This fills the bowl a and to give chance to make sure it has enough fuel to start. Last but not least is to remove the air filter and try and start it when hot. That works with mine, then I just refit it when it fires up.


The old original workshop manual is worth buying as it is very detailed and full of tricks and tips. Last but not least get in touch with Briggsbits as they have everything!

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Re: Briggs & Stratton 3HP 125cc Model 80202 side valve

Postby 56G80S » Sun Mar 31, 2019 7:46 pm

Thanks. Starting is fine. It's the unreliability of full throttle. From memory, when I take off the air filter all I can see is the choke slide which is a tube with a cut out. Pull out for choke on and not see cut out, push in for choke off and see cut out.

I'll try the diaphragm tension (5 screws in this case) using the wire if I find it.

I'll try and post a pic later in the week.

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Re: Briggs & Stratton 3HP 125cc Model 80202 side valve

Postby Plugsnpoints » Sun Mar 31, 2019 8:48 pm

56G80S wrote:Thanks. Starting is fine. It's the unreliability of full throttle. From memory, when I take off the air filter all I can see is the choke slide which is a tube with a cut out. Pull out for choke on and not see cut out, push in for choke off and see cut out.

I'll try the diaphragm tension (5 screws in this case) using the wire if I find it.

I'll try and post a pic later in the week.

Johnny B

I've just remembered, twist the throttle flap which tensions the diaphragm when you tighten the screws!

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