Briggs Twin Troubleshooting

15 years ago

Hi all. Have been sifting through old posts looking for my answer, but still have some more questions before I decide to run my mower over with my old tractor.

I have a 1984 Briggs twin - 400707. Got it a few years ago from a neighbor who had let it sit for a long time. Cleaned up the carb, new plugs, fresh oil and I was on my way. I cut about two acres a week with it. It started acting funny and would only run with the choke pulled out a bit. No big deal I thought, so I redid the carb, new throttle shaft bushing and all.

Put back together, ran like a dream for about thirty minutes and started hunting and surging and barely idling. Push back to carport and started tinkering. Found a loose head bolt. Ha! Took both heads off, cleaned all carbon from piston top and valves and heads. Also found four pecan shells lodged in the cooling fins. Put back together and again ran like a damn champ for about fifteen minutes!

So, I'm guessing its either the valves or coil. Before I start throwing new parts into the thing, I'd like some advice on logical troubleshooting steps. What to check first? Compression first? Spark first? Could the heads just be warped from the loose bolt and when its heats up it allows too much air in?

There is also monster sized hole in muffler now due to rust. Intend on replacing, but thought I should mention in case back pressure is an issue?

I have never adjusted the valves on this motor because I understand its fairly involved and actually have to grind material off if clearnace is too tight and pretty well screwed if too much clearance?

Thanks in advance for any advice.

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Comments (7)

larryf
15 years ago
"It started acting funny and would only run with the choke pulled out a bit"

"ran like a dream for about thirty minutes and started hunting and surging and barely idling"
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You suspect valves or the coil, but the problems you described are usually indication of a lean fuel air mixture.

If the fuel tank is located above the engine, pull the fuel line off to see if a steady stream of fuel is delivered. Let the fuel run into a jar. Examine the fuel sample for signs of water. Check the hose for cracking. Clip off the ends of the hose to eliminate cracking. If fuel flow is weak, there may be small stones or grass in the fuel line. Replace the fuel filter. The fuel pump is vacuum operated and sometimes the vacuum hose is loose and vacuum pulses do not reach the fuel pump. I usually remove the fuel tank for cleaning and renew all that old hardened fuel hose.

If the tank is remounted, the same inspection applies. If there are cracks in the fuel line, air can be drawn into the fuel system. Sometimes an old fuel cap does not vent and fuel is not delivered to the engine.

This engine is equipped with a high speed mixture adjustment as well as the idle mixture adjustment. As you sit in the driver's seat, the high speed adjustment is located on the left side of the carburetor. The idle adjustment is located on the lower forward side of the carburetor. For high speed surging, open the main mixture adjustment about 1/8 or 1/4 turn to see what happens. You may need a service manual to follow the exact adjustment procedure.
enikolich
Original Author
15 years ago
Thanks so much for the response. I seem to be running in curcles with this thing. My first gut feeling when it happened was lean mixture or poor fuel delivery. Has new fuel line and filter. Gas tank cleaned out thoroughly. Even added a fule shut off valve since I had it all apart.

Last night got back at it with a cold beer and more calm. Get 90 compression on both cylinders. Don't have official Briggs spark tester, but have a cheap one I've used for years. Set the gap really wide and blue spark was able to jump it.

I suspect something in carburetor. Air fuel screw does nothing and could only keep running with choke on. I suspect something is lodged in there that no amount of soaking or blowing with compressed air will get out.
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This almost sounds like gas varnish on the needle seat/valve. It would be hard to see, and barely detectable to the touch. I ran into this once before on an old IH carb, and it baffled the stuffings out of me until I turned the carb top upside down and the needle stuck in the seat. Good cleaning with carb cleaner took care of that part.
walt2002
15 years ago
A 1984 should have a high and low speed adjustment both. Sounds like crap in the carb bowl. The high speed adjustment needle and housing is located on the lower right side of the carb. It is hex headed and can be removed to drain the carb bowl.

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Walt Conner
pk74
15 years ago
So, eh, if you give up on it - let me know how much you want for the coil, if thats my problem on my own 400707 thread... :(
enikolich
Original Author
15 years ago
Ok. First of all , thanks to everyone who responed. Much appreciated! Secondly, I read through dozens of threads and see very few who bothered to follow up and let the group know the result. So, this is a good one, but damn I feel stupid.

Always thought it was a fuel problem. Cleaned that carb like crazy. Cleaned gas tank. New fuel lines. New filter. Kept going in circles like dog chasing its tail.

Last night messed with carb again. While taking off the vacuum line (pulse line), I happened to notice that the fitting on the motor where it attaches was loose. I got about four full turns on the fitting. Put everyting back together.

The thing has never ran better since I have owned it!!! I don't regret all the work I put into it because its like a new engine. Rebuilt carb, head gaskets, fuel lines, clean tank, filter, fuel shut off valve, intake manifold gaskets, governor springs... You name it! I have done everything I could think of to get this thing back in decent running order.

And all along, the solution was right in front of my eyes! Simple tightening of a vacuum fitting solved my problem! I started it just to test it and ended up cutting nearly two acres last night until it got to dark.

Oh wellm enough of my babbling. But thanks again to everyone for responding and taking the time to answer!
bill_kapaun
15 years ago
We love success stories!

Briggs Twin Troubleshooting

Comments (7)

larryf

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Briggs Twin Troubleshooting

Comments (7)

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