Small Engines (Lawn Mowers, etc.): my mower keeps surging, toro lawnmower, primer style

Question
QUESTION: Dear Eric

Hi, I have a 3.5 hp B&S engine on a Toro lawnmower the engine surges when running I gave the carb a clean out I tried holding the butterfly open and it still surges. I also tried tinkering with the adjustment screw and it still doesn’t help.
The specs on the engine are Model 92908    Type 1457-01     code 74102905

Any suggestions appreciated
Thanks Derek


ANSWER: Surging is generally caused by a dirty carburetor.  However, you engine is really old, a 1974 model.  If you still have an aluminum carburetor, chances are it is warped causing an air leak which is contributing to your surging.

You can remove the tank and carb assembly, dis-assemble and then use a flatfile and filing the tank and carb bottom to help "true" the parts.  You will have to remove th fuel pick-up tube(s) to file the carb flat.  

An alternate is just to replace the tank/carb assembly with a new one.  The carburetor part number is 395957 or they sell a new tank/carb assembly as well.

If the mower engine is in good shape it may be worth replacing.

They also sell a kit that is supposed to help fix the leak; part number 391413.

You can also convert you old carb/tank to the newer primer style: part number:  495912

Several options, you just need to decide what you want to do.  If the mower is in good shape I would opt for the 495912.
Eric

---------- FOLLOW-UP ----------

QUESTION: Thanks !!! Eric

I will definitely try that out and let you know how it goes.
But I was wondering could you help me fix another mower
  I have a 3.5 hp B&S and when I start it revs up to full throttle then cuts out, then catches again and revs up burning black then stops burning black then cuts out again. The engine has good spark, good compression (112 P.S.I)
The engine is a 4.5 hp B&S
Model# 10G 902
Code# 03022855
Type# 0189 B1

Mower
Model# 204210X31A
SKU#101-763
Serial# 7515 306432F323
has good spark, good compression (112 P.S.I)

Any suggestions appreciated.
Thanks  Derek


ANSWER: Burning black indicates too much fuel.  

Parts list:

http://www.briggsandstratton.com/pdf/illustrated_parts_list/100/MS5549.pdf

Carb part number 697415.

Have you tried dis-assembling the carb/tank and cleaning the carburetor?  Use compressed air from an air compressor and blow out the carburetor orfices.  I did find new carbs for less then $20 on the internet.  However, most of these carbs typically run lean rather than rich.

How does is run with the air filter removed?  Is the air filter dirty?
Eric

---------- FOLLOW-UP ----------

QUESTION: thanks Eric
I tried a carb kleen out and it still doesn't help
and the problems I described are with the air filter off.

any other suggestions appreciated
thanks again Derek

Answer
If it stil runs rich, black smoke with the air filter removed, you can try replacing the primer bulb and check valve.  These parts do not cost much but usually they cause a lean mixture.

It may be cheaper and eaiser just to buy a $20.00 carburetor The black smoke indicates too much fuel but there is no adjustment on the carburetor to lean the air/fuel mix.  Did you soak the plastic carb in industrial carb cleaner? Either way I think I would opt for a replacement carb.
Eric