I have an old Briggs and Stratton 3.5hp push behind lawnmower and the idle revvs up and down constantly. Never idles normal. The spring on the shaft idle looks good. Any help on what this might be would be greatly appreciated.
First you want to make sure all the phase alignment coils are peaked and the transratiometric detector is properly balanced. Then inject a circular wave about 1A P-P into TP31C and listen for feedback around the modulator. If any is found, replace F4.
Eric Law
on
Spongehead--
Recommend cleaning the air intake sponge first then let us know if the problem still exists. I assume you have the 99 dollar Walmart mower with no variable throttle.
Tut
I agree that cleaning the air intake and replacing the filter may help. I'd also suggest making sure the carburetor intake is clean and the mixture adjustment is set properly.
Also make sure all the linkage spring are in place too.
If I had to guess, I'd say that the mixture may be too rich (or lean) and the engine's governor is keeping it alive.
Good luck!
Bob
Hi...
Happens that I have one; as well as still have the manual... hard to believe 'cause it's about 15 years old :)
Anyway, still runs perfectly, so I'd be happy to take a couple of pics of the springs and carb linkage etc, and mail them to you if you like?
Could also scan the applicable pages of the manual if you think it would be helpful.
Take care.
Ken
Carb KIT needed....
I would guess that the carby needs cleaning, or the fuel filter is clogged
David
Bob Shuman wrote:
Depending on the model, there may be an idle adjustment, but as others have said, the carb either may need cleaning or a refurb.
--- sam | Sci.Electronics.Repair FAQ Mirror:
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The speed control servo on these does not have much phase margin, and may even have a pole in the right half plane. Fuel mixture errors can easily add enough loop delay to push the system into oscillation. Accumulation of gum and varnish and the tendency of the carburetors's soft metal to flow with age and develop leaky gaskets all contribute to long-term performance drift.
In layman's terms; adjust, repair, or replace the carburetor as needed. A professional who works on these often enough to be familiar should be able to diagnose your specific needs in very short order, but will probably expect to be paid for his expertise.
"cnctut" bravely wrote to "All" (11 May 05 10:52:14) --- on the heady topic of "Re: Lawnmower idle question"
From the small motor faq:
" Air vane governor. The air flow provided by the flywheel/blower passes by a plate which is coupled to the carburetor throttle plate acting against a spring force.
Flyweight governor. A spinning gear assembly with a pair of weights is driven by the camshaft. As the centrifugal force of the weights increases, they move a sleeve which presses against a lever whose shaft passes through the crankcase. This lever then operates the carburetor throttle plate against a spring force. With both types, increasing the spring force will increase the engine speed. "
Hunting is often caused by a defect in the governor system. Check the blade (plastic?) of the governor paddle is free to move and that the cam to which it connects has the proper spring tension. As the motor speeds up the flywheel impellor blows air against the blade and cuts the trottle down. When the motor slows down the blade drops back and the spring returns the throttle higher.
Often grass goop gets into the governor paddle axle and may stick or seize. You might need to remove the pull cord/motor cover and perhaps the carb/fuel-tank to get at it. Thankfully you won't need a puller to get the flywheel off. If you ever do, inspect the key pin in the shaft, as it must be snug, not worn. This could also cause hunting.
OTOH, if instead of the fan your motor has a governor linkage coming out the bottom of the motor, then the linkage may have slipped. Hunting means in this case it may be opening the throttle too much. There is also a spring arrangement in this type to the trottle cam.
A*s*i*m*o*v
cn> From: "cnctut" cn> Xref: aeinews sci.electronics.repair:47853
cn> spongehead wrote: > I have an old Briggs and Stratton 3.5hp push behind lawnmower and the > idle revvs up and down constantly. Never idles normal. The spring on > the shaft idle looks good. Any help on what this might be would be > greatly appreciated.
cn> Spongehead--
cn> Recommend cleaning the air intake sponge first then let us know if the cn> problem still exists. I assume you have the 99 dollar Walmart mower cn> with no variable throttle.
cn> Tut
... The first rule of intelligent tinkering is to save all the parts.
"EL" bravely wrote to "All" (11 May 05 13:52:03) --- on the heady topic of "Re: Lawnmower idle question"
Eric,
Don't quit your day job to become a stand up comedian.
A*s*i*m*o*v
EL> From: "EL" EL>
EL> First you want to make sure all the phase alignment coils are peaked EL> and the transratiometric detector is properly balanced. Then inject a EL> circular wave about 1A P-P into TP31C and listen for feedback around EL> the modulator. If any is found, replace F4.
... Don't mess with a man that has a rubber chicken.....
Thanks for all the replies, I will be taking the carb off for a good cleaning and replace the air filter. I'll let you know how it goes. Thanks again!!
Change the needle valve.. Get a Carb KIT.
yikes
Unless he tightened the needle valve with a pair of Vice-Grips, needle valves don't go bad, but may get dirty.
Clean the carb first.
--- sam | Sci.Electronics.Repair FAQ Mirror:
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Important: Anything sent to the email address in the message header above is ignored unless my full name is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs.
spongehead--
Take off the air cleaner and run the engine--if the problem corrects itself--clean or replace the old foam filter. Make sure the carb is securely mounted to the engine.
Does your mower have an adjustable throttle control on the handle or not?
Yeah, it has a lever which is never used because its stuck, but can adjust it right on the carb itself.
I replaced the air filter and even let it run without it, but still revvs up and down. Next will remove the carb and I actually have another carb from my older lawnmower which I'll swap.
Asimov
Not sure what you meant by "XX bravely wrote" but--Tut "bravely writes" more again.
Briggs makes a 3.5 HP engine matted to a 20 Murray mower sold by Walmart for $99 US. My experience is that if the air cleaner (which uses a sponge filter) is dirty--which happens frequently--the engine rpm will search for neutral idle. Perhaps because the mixture is over rich and the engine Air Fuel ratio changes on the "fish hook" pattern as engine speed changes--or perhaps because manifold vacuum fluxuates excessively causing irratic flapper valve overshoot action--not sure. Cleaning the sponge is quick, easy, cheap--and requires no $$.
The 3.5 Briggs engine is also sold in a throttle free configuration--meaning no throttle control for the operator. In this case, a small thin bar toward the front of the engine holds the idle springs at the proper tension thus preventing the engine from unneccessary RPM searching. The bars location is just at the proper height to be bent rearward when the mower is pushed under low brush--this reduces the idle spring tension and the mower will RPM search.
Tuts suggestions/comments.
The answer may not be here--but a great starting place--short of carb teardown or flywheel removal--wouldn't you agree?
Tut
One note though: He has an idle speed problem where the governor is likely not involved unless it is way misadjuated.
--- sam | Sci.Electronics.Repair FAQ Mirror:
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Important: Anything sent to the email address in the message header above is ignored unless my full name is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs.
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