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Discussion Starter · #1 · (Edited)
Hi All!
I have a 28 year old "Franken-Mower" with a Briggs + Stratton 3.5 HP engine
Model 95902 Type 3147 01. It's got a diaphragm type carburetor.
(Franken-Mower = Made out of several grave robbed deceased lawn mowers, but the engine has been mine since new.)

For the last couple of years, it will surge and stall often until the engine warms up for several minutes. This is with no load- I don't move the mower until the engine is warm & stable.

I am stumped for the cause. I also have to leave the throttle on high speed (ca 1800 rpm) while it warms up, otherwise it will stall for sure.

I've treated this engine like a king since it was new, as I do with all of my tools:
The engine is run-until-stall (out of fuel) after each use.
The fuel has stabilizer.
The cord is pulled until resistance is felt before storing (to keep valves closed so no moisture enters the cylinder.)
The gas is always fresh- up to 2 months old at the oldest. Stored in air tight container.

I've tried:
New carburetor diaphragm & gaskets. (Three times)
Carburetor Cleaner (Spray & long soaking)
New Primer button
New Fuel filter (A pick up tube with a screen)
Clean Air Cleaner
New Spark Plug (Twice)

I have even older engine tools- up to 50+ years old- with diaphragm carburetors that have no problems, just this mower engine has the bad attitude.

The part that really has me stumped is that it runs fine once it has warmed up.
That tells me the carburetor is OK and the problem is elsewhere.

Do any of you know something else I can try to get this engine running well again?

Thanks for sharing your knowledge!

Be Sure To Enjoy Today,
Paul
 

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Consider using a tool like maybe a eye dropper and dribble a drop of gasoline around all the connection points of the hoses and carburetor that could be sucking air until the heat has warmed the material and stopped the air intrusion at the wrong location. The engine will immediately change its attitude if some air sucking point is found with the gasoline.
 

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Consider using a tool like maybe a eye dropper and dribble a drop of gasoline around all the connection points of the hoses and carburetor that could be sucking air until the heat has warmed the material and stopped the air intrusion at the wrong location. The engine will immediately change its attitude if some air sucking point is found with the gasoline.
You can do this with a propane torch as well (which I think is easier anyways). Don't light the torch, just let a bit of gas out and aim it at various potential air leaks around the engine and carbs. Make sure it's a pretty slow stream of propane, it doesn't take much... You don't need to blast it at full speed that just creates a combustion hazard and it'll be harder to pinpoint the leak.
 

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Discussion Starter · #4 ·
Thanks Senior Citizen & Joeywhat for the good tips! I appreciate the knowledge sharing. Now I have two new tricks in my arsenal!

I chickened out on using gasoline, since whenever I get near the stuff I end up getting it on my clothes. (Old guy + Shaky hands = Yikes!)

With the propane, at first I thought it found a leak because the engine sped up. Then I realized the carburetor was drawing it into the body's air intake, creating a rich mixture. (Air cleaner was off for access.)
I put a cardboard tube from a paper towel roll over the carburetor air intake to make a "chimney", thus keeping the propane from getting sucked in the air intake by mistake.

I did find that the gasket at the carb-to-tank was leaking. I fixed that by taking the carb off and starting over. I must have wrinkled it when originally installing the gasket. The engine now runs faster when warm, but still surges & stalls when the engine is cold.

Could it be possible that the carburetor jet is partially clogged and letting not enough gasoline through for cold use, but enough when the engine is warm? (Kind of like if it had a manual choke.) Or, jet is worn and too much fuel is entering?

Thanks Again For Helping!
Paul

PS: Your propane idea, Joeywhat, worked great on my truck which has always idled rough. Two service places could not find the problem, but, in a minute, the propane found a tiny crack in a vacuum line. Replacing the line made the truck idle very smoothly.
 

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Discussion Starter · #6 ·
Thanks JoeCaption for linking the video.

Assuming it stops raining (All day for 3 days so far), I'll get back to the project today.
The jet on my carburetor is molded into the plastic body, so cleaning will be done in place.
Hopefully the jet is at such an angle that allows access to clean with a drill bit.

I'll step up from my smallest drill (#80 = 0.0135") & move up until I feel a little resistance.

Thanks Again!
Paul
 
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