Tractor races, produces less power

fearboy(z6b MA)November 24, 2013

Hi everyone -

I believe this is a question of general small engine mechanics, but here are the particulars of my machine, in case it's helpful. It's an older Craftsman tractor, with a Briggs engine, model 28N707, type & trim 0160-01, code 9406094B.

In response to the amount of oil it was burning, I pulled the engine and fearlessly replaced piston rings. Being mechanically inclined, but also a bit of a moron, I figured this would be doable; and it was, ultimately. After pulling the engine three times, I had it put back together (with no parts left over, even!), the valve timing correct, and, finally, valve clearances set correctly. And it runs! Mostly!

And now, The Problem: it revs much higher than it used to, even at low throttle settings. It drives fine (though faster - which is plenty fun), but with the blades engaged, the engine rpm drops very noticeably.

Intuitively, I believe the carb needs adjustment, because it runs a bit rough. But, again, moron. I seated the adjustment screw, backed out 1.5 turns, and with the engine running, adjusted until I found the spot where it ran smoothest.

Other information: at one point, with the throttle at its lowest setting, the engine rpm started rising on its own; it stayed high until I reached in and closed the throttle at the linkage - it was ok from there on. I should also mention that the idle speed adjustment is set such that the throttle is able to close down correctly, so I believe I have that set correctly.

So. Any guesses what I screwed up? The rising rpm thing makes me wonder if I messed up something with the governor (boy, do I hope that's not true), but the high idle, rough-ish running, and drop in power elude me. Should I just go back out and play around with the carb adjustment some more? Or should I be looking elsewhere?

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Unless you followed the Briggs service manual procedure for STATIC GOVERNOR ADJUSTMENT before you ran the engine, there is little chance of affecting true governor action when the engine is running.
If you omitted the static adjustment, go back and do all that (including carb adjustments) first.

    Bookmark   November 24, 2013 at 1:03PM
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fearboy(z6b MA)

hi mownie -

definitely did not do that - didn't even know it was a thing. google is my friend, though, so now i'll have a project for the long weekend.

i sure hope i'm running out of things to screw up...

    Bookmark   November 24, 2013 at 7:51PM
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Your friend is Briggs service manual # 276781.
There are at least 400 things you have not touched yet :^)

    Bookmark   November 24, 2013 at 10:21PM
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fearboy(z6b MA)

hi mownie -

thanks for the tip on the manual - it didn't happen over the weekend as i'd hoped, but i'm sure i can get there. none of the steps SEEM like they're all that difficult. anything i need to watch out for in the process?

    Bookmark   December 2, 2013 at 5:44PM
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Not really anything peculiar about it, just make sure you hold the governor shaft as prescribed in the text on static adjustment (found in the maintenance section, page 18).
People get mixed up and sometimes hold the governor shaft opposite of what the manual is instructing.
So make sure you really read and understand Step 2 of static adjustment.

    Bookmark   December 3, 2013 at 12:31AM
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fearboy(z6b MA)

I'm resurrecting this thread from last fall, because I finally managed to do the governor adjustments. Mostly.

The engine runs more smoothly now than it ever did before, which is a nice bonus. On the other hand, as soon as it has any load (a hill, for example - or, heaven forbid, engaging the blades), the rpms drop, the engine kinda sputters, but keeps running, and it doesn't recover until both a) the load is removed (disengage the blades, get over the hill, whatever) and b) I lower the throttle a bit. Once that's done, the engine picks up again - until the next hill. Or the next time I need to, say, mow something.

Intuitively, I think this means I didn't get the governor adjustment quite right, but my lack of experience says the thing might just as easily be infested with goats or something. Anyone have any ideas?

    Bookmark   May 28, 2014 at 2:30PM
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Actually, it is probably not getting enough fuel in its diet.
Has the carb ever been cleaned thoroughly?
This complaint is typically cause by a carburetor that has some partial clogging of the main jet. But it can also be caused by a clogged fuel filter, or by using the wrong fuel filter.

    Bookmark   May 29, 2014 at 1:00AM
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fearboy(z6b MA)

Interesting. I did actually rebuild the carb just before I did the ring job, so the main jet is essentially brand new (I'd say it's got about an hour's running time on it since the whole thing got put back together).

I also replaced the fuel filter back in the fall, so I doubt it's dirty - and the new one certainly looks very much like the old one. At the time, I remember being satisfied that they were functionally equivalent, but it's hard to remember why anymore.

I have some sea foam around here somewhere - I'll start with that and see what happens. If nothing else, it'll be nice to try something that doesn't require me to take anything else apart. Am I right to assume that, if this doesn't work, the next stop is removing the thing and giving it a nice soak in a solvent of some sort? Or is there something else I should try before that? (Now that I think of it, I'm pretty sure there's some gumout kicking around, too.)

    Bookmark   May 29, 2014 at 9:44AM
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fearboy - just a simple soaking or running a bit of seafoam thru that carb won't help much if that load jet has trash in it, imho.
You will need to tear it down; probe all orifices with a small dia stiff brass wire, use a magnifying glass to see them and compressed air to blow out any dislodged particles. Then soak, clean, rebuild with new kit and gaskets.

    Bookmark   May 29, 2014 at 11:41AM
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It is also important to make sure the float level is set correctly.
If a float is set too low, the carb will not draw an adequate volume of gas from the bowl for heavy loads.
Too low fuel level in the bowl will act much the same as when the gas tank or float bowl is running out of fuel.

    Bookmark   May 29, 2014 at 11:49AM
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fearboy(z6b MA)

bummer...i was really hoping i was finished taking things apart for a while.

thanks for the suggestions - i'll go over the thing and post whatever i find.

    Bookmark   May 29, 2014 at 1:12PM
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