# 3.5 B&S classic has me going nuts



## bw1 (May 25, 2010)

Hello All,

engine 95902 3107 01 92011652

problem: will not fire. Also for no reason when I used it last week I suddenly lost throttle control. extremely slow throttle response going to absolutely no or extremely delayed response.

What I have done.
new plug, compression tested at 75 first pull cold. Second pull shot it up to 105 and it stayed there for the next few pulls.
rebuild carb, tried new carb. vane governor working fine. new spring for gov anyway with nothing. no binding. flywheel key fine. starter clutch fine. Timing set according to repair manual, first time @ .008 second time at the usual .010 and nothing. spark seems to be yellowish per 7 yr old daughter cant tell for sure as I have no one to look while I pull rope. spark might be a little weak perhaps but more than strong enough to ignite. (my humble opinion). new fuel, tank drained cleaned and washed out. still zippo. I do see gas shooting into carb and engine with both carbs. 

I checked and rechecked and then I rechecked some more and dang if I know what problem is. Guess it might be time to take her out back and drop a few rounds in her. 

Everything points to gov with sudden loss of throttle control followed by very delayed throttle response but everything seems fine. only have vane which moves freely, new spring does nothing and the little bar thingy is not bent, broke or and seems to be operating correctly.

Anyone have a clue? Im not the best out there but I am a stickler for following manuals, using oem parts and paying attention while having a blast but boys this one has me stumped. 

thanks all

brad


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## bw1 (May 25, 2010)

*forgot to add*

pulled valves and no problems. lapped and reinstalled with new head gasket and this did nothing either

bummer

brad


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## geogrubb (Jul 28, 2006)

Did you run under something and bent the front loop the big spring attaches to, that happens a lot. Have a good one. Geo


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## bw1 (May 25, 2010)

thank you for your reply but no luck on bend in that part. I guess she is to old. If I knew for sure it was rings or something cheap like that I would redo it because I have the time and rings are not more that $15 or so for the self doer.

brad



geogrubb said:


> Did you run under something and bent the front loop the big spring attaches to, that happens a lot. Have a good one. Geo


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## rrrhre2s (Sep 18, 2010)

Let's put on our thinking caps:

For any internal combustion engine to run...

1. needs the proper *fuel*, that is of good quality.
2. needs *air* to mix with the proper fuel to allow combustion.
3. needs good *compression*, for the type of engine.
4. needs a good *spark* at the proper time (not usually
applicable to diesel engines).
5. needs an unobstructed *exhaust*.

For a typical lawn mower engine:

1. uses gasoline, is the gasoline fresh and uncontaminated ?
a. no water !
b. not older than 20 days!

2. Is the air cleaner clean and unobstructed ?

3. most lawn mower engines have 6:1 to 8:1 compression ratio
a. 6:1 will run with 50 to 90 psi compression.
b. 8:1 will run with 75 to 125 psi compression.
c. you cannot usually get a proper compression reading at the slow cranking
speeds because of the compression release. (makes for easier cranking)

4. most lawn mower engines use a magneto ignition
a. A small dynamo with a secondary winding that produces a high
voltage spark, it is self generating no battery is required.
b. older systems used breaker points which were prone to failure due to
corrosion or wear. (went the way of the dinosaur in the 1980's)
c. newer systems use a solid state switch, still prone to failure, substitution
is the best way to test. (barrow one from another engine that runs)
d. will the spark jump a .166 gap ?
e. check that flywheel key, timing may be off!

5. not seen much with 4 cycle lawn mower engines, but a plugged or 
restricted exhaust will occur more often with 2 cycle engines.
(think older lawnboy 2 cycle )

Now rethink where is the failure which system ?

Fuel ?
Air ?
Compression ?
Spark ?
Exhaust ?

Have a good one!

rrrhre2s


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## rrrhre2s (Sep 18, 2010)

Have you tried a known good spark plug ?

Don't overlook the simple things !

Blue sky's ahead !!

rrrhre2s


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## Lawnmowertech (Nov 12, 2008)

have you taken the carb off the tank yet and clean out the tank inlet where the screen on the carb sits in normally with these symptoms there is trash built up in the screen area on the tank usually a pick of some kind can be used to clean it re do a new diaphragm on the carb it should work after all this is complete


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## bw1 (May 25, 2010)

Lawnmowertech said:


> have you taken the carb off the tank yet and clean out the tank inlet where the screen on the carb sits in normally with these symptoms there is trash built up in the screen area on the tank usually a pick of some kind can be used to clean it re do a new diaphragm on the carb it should work after all this is complete


Yep did all that including trying new carb. At this point I have completely torn down the engine and started making measurements per the repair manual. When I'm done I going to throw it back together and give her a whirl. I am actually in a small engine class at the local community college and this will be my project. Lucky the instructor lets me take tools home between classes so that helps. 
Still having fun and this with this one Im learning alot so I just need to be patient and I'm sure I will get it fixed.

brad


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## rrrhre2s (Sep 18, 2010)

For educational purposes, or your own equipment time only 
matters to you.
If you are repairing this for someone else and they are 
paying for the time, you will soon be very hungry. 
Diagnosis before disassembly will save time 
because you are not chasing your own tail 
finding and possibly creating new problems.

Remember:

*F*uel ? 
*A*ir ?
*C*ompression ?
*S*park ?
*E*xhaust ?

Fuel without air (oxygen) will not burn.
Without compression spark will produce no power.
If exhaust is plugged causes back pressure and poor or no running engine.

Small engines can be fun that's why they can be a good hobby, but when
you get frustrated you need to step back and think !!

May the bird of paradise *NOT* fly up your nose !!
That would be painful !!

rrrhre2s


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