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Older Briggs & Stratton 3.5 Barely Starting/Running


DPDISXR4Ti's Avatar
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08-03-15, 07:49 PM   #1  
Older Briggs & Stratton 3.5 Barely Starting/Running

I've just assembled a push lawn mower using an older (early 80's, maybe 70's) B&S 3.5 that was given to me ~10 years ago (and has been sitting all that time). This is the older motor that has the oil-fill down low (no dip-stick).

I've done a cursory cleaning of the carb and cleaned out the gas tank. Compression is 60 PSI. I have spark. Flywheel is timed correctly.

I've gotten it to run twice, once for about 5 seconds, the second for maybe 20 seconds. It ran roughly, and I haven't been able to get it started since.

I'm thinking I have a fueling issue. Would you agree? What would you suggest as next steps?

I do have another running mower with nearly the same engine (slightly newer - has the built-in dipstick) that I could temporarily raid parts from to isolate the problem. I was thinking about swapping the entire carb. The elbow inlet from the crankcase vent looks a little different, but otherwise it looks like the same plastic carb.

Appreciate any input.

 
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08-03-15, 10:33 PM   #2  
Most likely it will need a carburetor diaphragm. If it's that old, it might be an old auto choke carb.


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08-04-15, 05:16 AM   #3  
That 60 PSI seems kind of low. My own experience includes leaving an older Tecumseh engine "sitting" for a few years after I had become discouraged with some aspect of it. I left the Air Filter OFF.

Years later, I got motivated to put it back in running condition (on a Chipper), and discovered that moisture had condensed and pitted the Exhaust Valve, so I needed a small valve job to get that valve to close completely, and return the compression to an acceptable level.

Had I left the Air Filter on the unit, I probably could have avoided that valve work.

What kind of environment was this engine stored in ?

That's my 2.

 
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08-04-15, 05:22 AM   #4  
Can you post the model number or at least a picture of the engine? Hopefully a shot showing the carburetor so we have a idea of what you've got. At that age I assume it's a side valve engine but there are several choices as to the carburetor. Does your carburetor sit directly on top of the fuel tank?

 
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08-04-15, 07:47 AM   #5  
Post the engine model number, it should be stamped in the metal engine shroud just above the spark plug. xxxxxx-xxxx-xx Have a good one. Geo

 
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08-04-15, 07:40 PM   #6  
Sorry for the delayed response. Just now getting a chance to get back on here.

The model/type is 93502 01XX 02 (can't read two of the digits)
Code is 81082108

There's a sticker on the side of the shroud which simply indicates "Model 81"

Indeed the white plastic carb is mounted directly to the rectangular fuel tank with 5 or 6 Phillips-head screws.

The engine was stored indoors in a box for the past ten years. Not ideal, but could have been far worse.

I had the carb off the tank and the diaphragm looked okay, but I can't say for sure that I knew what I was looking for other than an obvious tear.

 
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08-04-15, 11:47 PM   #7  
The diaphragm should be flat, no stretched spots, and soft and flexible. No stiffness. (Along with no tears or holes).


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08-05-15, 05:08 AM   #8  
If it has been sitting for 10 years I'd replace the diaphragm. It may not be stretched because of it's lack of use but it may be stiff or have pin holes.

 
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08-05-15, 06:00 AM   #9  
Totally agree. You won't see anything visibly wrong with the diaphragm, it will just no longer be as pliable as a new one. Rinse the gas tank out while you're at it. I would think the points would need to be checked for corrosion, or the condensor replaced if it has been sitting that long, but if you have a crisp blue hot spark thats amazing.

 
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08-05-15, 07:32 AM   #10  
While I had it apart I thoroughly rinsed the gas tank out and let it dry in the sun before reassembling. Spark is blue so I think I'm okay there.

BTW, here's video found on cleaning the carb. This is the engine I have. Only difference I see is that this one has two dip-tubes into the tank vs. the one I have on this carb. Not sure what that is about.
https://youtu.be/fdFrniPyBwE

 
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08-05-15, 12:08 PM   #11  
An additional important data-point...

I just swapped carb/fuel tank assemblies. The previously running engine totally runs fine with the carb from the non-running engine (the one I'm trouble-shooting here). I did a section of lawn without issue. It then re-started on the first pull. In short, it operates exactly the same with either carb/tank assembly.

As long as I had it off, I tried installing the carb/tank from the running engine, and it still doesn't start.

To my surprise, we do not have a fueling issue here, at least not as it relates to the carb and/or tank.

Hmmmmm....

 
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08-05-15, 03:47 PM   #12  
I'd probably start by checking to see if the coil has been updated to a magnetron electronic spark system or not.


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08-05-15, 07:53 PM   #13  
Indeed I think I have a spark issue after all. I removed the plug and with it grounded against the case I only very rarely get a spark.

How would I identify the updated Magnetron electronic spark system? Is that related to the "System2 Manual Start" identifier I see on the later engines? This engine has a single wire that goes from the coil to under the flywheel near the crankshaft. The "System2" engines don't have that.

 
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08-05-15, 08:01 PM   #14  
Personally I would pop off the flywheel and check the points, like I mentioned earlier. They are probably corroded after all these years. With points closed, open them slightly and just slide a bit of emery paper or fine sandpaper between them pull it out, do that for each side a couple times and see if it helps. Sand the rust off the magnets while you're at it, but don't move the magneto.

If you reassemble and there is still no spark after this, replace points and condenser. Inspect flywheel key for any grooves that could throw the timing off. If, when you pop the flywheel off, you find the key is sheared, that's your problem.

 
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08-06-15, 10:20 AM   #15  
Sounds like you are still using the points. I would also suggest simply getting the new coil and be done with the points all together. I will also avoid the need to remove the flywheel.

 
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08-06-15, 10:31 AM   #16  
I agree, messing with the points is "pointless" . I mean, you might get it going again, but the new coils are electronic, no points to clean, change, or adjust or worry about moisture messing them up. Electronic is more reliable, more accurate, and all it takes is to remove your old coil, clip the wire going under the flywheel, and put the new coil on. Most small engine shops will have the coil in stock or an old one on a junk engine.


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08-07-15, 08:56 AM   #17  
I actually already have a another "junk" 3.5 motor that is the more modern System2 ignition type so it sounds like I can raid that for its coil. I also have a junk 6.5HP B&S engine circa 2003. Any idea if that uses the same coil? Thinking as I type, I'm thinking "no" since the flywheel is larger and thus the curve on the coil would be different.

Looks easy... https://youtu.be/0Q_sBKJUGdw

 
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08-07-15, 10:25 AM   #18  
Are you sure the flywheels are different size? I believe they will interchange if they are both in the 3 to 4 hp range.


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08-07-15, 12:20 PM   #19  
If you have a good spark and ur old carb worked on the newer mower it might be the intake that's giving you the problem. Causing too much air and not enough gas. Gasket might be bad.

Just a thought !!

 
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08-07-15, 12:47 PM   #20  
Flywheels between the 3.5 and 6.5 are indeed different sizes. The 3.5 measure 5.75" and the 6.5 measure 6.5"

Update to follow once I try a new, later Magnetron coil.

 
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08-07-15, 01:23 PM   #21  
I couldn't find you model # listed as 93502 01XX .

I put a zero in front of the # and it still didn't pop up.

I wish I still had my old manuals from years ago those huge red binders have more info than the PC kinds !!!

 
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08-07-15, 01:50 PM   #22  
"XX" is the two numbers I couldn't read. Try inserting some random numbers???

Scratch that... A little phosphoric acid brought them back to life as 68

So the full Model/type# is 93502 0168 02

 
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08-07-15, 02:59 PM   #23  
It came up.

Wow I forgot how many parts on the older style motors !!!

If you have your engine apart go ahead and take the intake off and check it for a rust / corrosion hole then change the gasket. A couple bucks.

If you can get a coil cheap thats awesome also.

Hope this helps !!

 
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08-07-15, 05:32 PM   #24  
The coils on the 3.5s should be the same, but no, the coil on the 6.5 won't work because the flywheel is different. I saw the 3.5 on your post first and missed mention of the 6.5.


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08-07-15, 07:07 PM   #25  
It runs! I clipped the wire to the points, dropped the newer style coil in, and it started on the first pull! Of course, since I hadn't yet installed a kill switch, I couldn't get it to stop! <LOL> Some quick work with the insulated pliers had the lead pulled off the plug to solve that in the short term.

I thought I might be able to use the wire that goes to the throttle as a kill switch (the other wire I clipped that previously went to the points), but that must be permanently grounded at the throttle. I hooked it up to the coil and no spark, regardless of throttle position. I'll explore that further or maybe just go simple and hook up a separate switch.

Thanks for all the help. I now have a second lawn mower that didn't cost me a dime!

 
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08-08-15, 07:26 AM   #26  
That sounds like it may well have been the original problem. If you have a wire going to the throttle, it should only be grounded when in stop or off position.
You could use a separate switch for the kill wire from the coil. Wire it so the coil is not grounded to run, and connects to ground to kill the engine. Switch can be momentary or two position...either way.

 
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08-10-15, 07:02 PM   #27  
I wired up a momentary switch for the kill circuit using a trunk-release button I had grabbed at the junkyard one day. Good idea - works great!

 
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