Surging Briggs 6.5 pushmower

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  #1  
Old 07-23-20, 07:51 PM
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Surging Briggs 6.5 pushmower

I can't seem to completely get rid of a problem with varying (surging) running speed on my Craftsman 917-series push mower.
The carb has been cleaned spotless twice. There's no filter to be restricted. Fuel line is ok. Governor spring is undamaged. I can bend the tab that the spring attaches to and set the rpm to anywhere from about 2400 to 4200 or even higher. The higher the rpm, the worse the surging. It's set to 2900 right now, which I've read is the correct speed.

I can put my finger on the governor linkage and stop the rpm cycling (which I believe proves the carb is fine), but the surging resumes when I take my finger away.
It's like the governor linkage needs a shock absorber!
I've watched several YouTube videos about surging and have tried everything suggested (almost every one focused on a plugged fuel jet).
Help!





 
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Old 07-23-20, 08:16 PM
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Go to a welding supply store or the local home improvement box store and buy a set of torch tip cleaners. They are like 4 bucks. Take out the bolt that holds the bowl on the carburetor and there is one hole that goes through the side, and another that goes down the center. Find a tip cleaner that will go through the hole down the center snugly and work it through there until it loosens up. This is the high speed jet, it needs to be opened up just a hair.
 
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Old 07-25-20, 12:14 PM
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Thanks Cheese.
I'm not too sure what you're suggesting.
I've already cleared all the jets and orifices. Twice. The carb is spotless and the engine runs strong & steady at whatever rpm I want by fingering the throttle. This is why I thought the problem must be with the governor.
Are you suggesting I ream the main jet larger?
 
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Old 07-25-20, 05:34 PM
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I'm suggesting that there is a varnish or something in that hole that is. If not, force that tip cleaner in there anyway and open it up. Either that, or the o-ring behind the carb is missing or the carb is loose.
 
  #5  
Old 07-25-20, 06:43 PM
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Craftsman Surge

Does the surge go away when the mower is under a load? That is when cutting heavy grass?
 
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Old 07-25-20, 08:45 PM
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I have a set of needle drills. What size have you found works best for that main jet?
 
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Old 07-25-20, 11:48 PM
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I never had luck drilling them, just barely enlarging them with the tip cleaners. I can't suggest a size.
 
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Old 07-26-20, 08:01 AM
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Before you start enlarging jets and stuff, with the engine running try spraying some brake parts cleaner behind the carb and along the mating surfaces if the engine tempo changes you have an air leak and can take the appropriate action. Have a good one. Geo
 
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Old 07-26-20, 12:55 PM
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Craftsman engine Surge

I agree with Geo in regard to enlargement of jets and drilling etc. In my reply to your post (#5) I asked about surging under load or not. The reason I asked is that I had a Susuki two cycle engine in a Toro that would surge when not under load and then would eventually just stop running. I opened up the jet as cheese suggested and I got the snowblower to run well under load without eventually stalling, but the surge continued when at idle. It still does that, but for my son in law, not me. (I have a much bigger Toro now) Everyone I talked to about this particular engine told me it was the nature of the beast. Just leave it alone. Since it did the job for me, and now my son in law. That's what I did. I told a guy I had a Kawasaki 900. He said" my god! the brakes on those weren't worth a damn" I told him, "I didn't buy it to stop"
 

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Old 07-27-20, 06:09 AM
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I've done most of these suggestions twice already but doing it a 3rd isn't going to hurt.
But i have a question for you guys focusing on the carb:
If this surge is due to running too lean, why does the engine run so smooth & strong at even higher rpms with my finger on the throttle shaft opening it up even more? Wouldn't a carb problem show up then too?
That's the aspect of it that I'm hung up on.
 
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Old 07-27-20, 10:14 AM
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A lean mixture burns more quickly that a mixture of correct proportions - gas to air. It's more of a flash rather than a slow powerful burn. Without a load on the engine a good comparison would be like you running down a steep hill. You'd overspeed and draw back repeatedly. = surging.

But running up hill you would be pushing a load constantly so no overspeed and no surging. That isn't to say the overly lean mixture is good. There would be less energy there to handle the work load, a tendency to preignite = overheating and detonation damage.
 
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Old 07-27-20, 02:21 PM
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Surging

Marbobj. That is the BEST explanation of Engine Surge I have ever heard! From that, logic would indicate that the air fuel mix in a no load situation is extremely critical. So with everything else being tight and adjusted correctly. The ONLY variable that could be changed is the orifice diameter in the jet, or jets that control the idle speed. Do you agree?
 
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Old 07-27-20, 03:47 PM
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Without the mixture screw to make an adjustment, you'd have to change the jet or enlarge it a little. You have to be careful with boring a jet out, though. Putting material back is a little tough. I would put cleaning it at the head of the list.
 
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Old 07-27-20, 04:25 PM
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I agree completely with cleaning coming first.Since my experience with the Susuki powered Toro improved with the orifice getting bigger in the jet. I should have kept going, until it was still larger, but I had other priorities. If I was the guy who originally posted with the Craftsman surge program, I would get a couple of bowl jets and drill them out a bit at a time until it ran the way he wanted it too, what do you think?
 
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Old 07-27-20, 07:42 PM
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The governor prevents the kind of overspeed situation you describe.
The only way the motor "knows" there's a load on it is it slows down. The air over the governor paddle decreases & it opens the butterfly more to compensate, letting in more air AND fuel in the same ratio.
If surging is due to fuel starvation as Cheese suggests, then it should occur at all speeds & loads.
"Better" carburetors have several jets/fuel circuits and transfer ports that change the A/F ratio depending on butterfly position. As far as I can see this crude carb has none of that refinement.

Raining today--couldn't work on it or test.
 
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Old 07-27-20, 07:56 PM
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You still haven't answered my question. Does the surging go away when the mower is under load?
 
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Old 07-28-20, 04:42 AM
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An engine built for a specific application and sold accordingly will have a narrow range of predictable behaviors. But even at that you will have a series of compromises with the engine in the application. It doesn't just come from the carburetor, but from fixed ignition, materials used to make the machine and its life expectancy. The main qualifier on all this, though, is what you can sell it for and to whom.

Engineers design and build these things, but the consumer actually tells them what to make with what they buy and how they use it.
 
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Old 07-28-20, 09:35 AM
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You still haven't answered my question. Does the surging go away when the mower is under load?
As I said it was raining yesterday. Most of my grass is straw right now but I'll find some thick grass somewhere to test for you.
I CAN say that it used to surge in thick or thin, and would get worse over several minutes until it would just stop running. It was embarrassing to use.
It's much better now that I've brought it home where I can work on it easier.
I've replaced it with a Tecumseh-powered Craftsman I got from a neighbor. I usually prefer Briggs over Tecumseh but at least this replacement has an actual choke.

Have you heard B&S has filed Chapter 11?
 
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Old 07-28-20, 10:40 AM
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Craftsman engine Surge

From my previous post about the Suzuki :Powered Toro Snowthower. I would be willing to bet that from YOUR description, it is doing exactly what mine was at the time. The fix became a ream job with a torch cleaner as Cheese suggested. It stopped the stalling, and the surging under load, but the irritating surge while at rest remained. If I had kept it, I would have made an adjustable screw type jet for it under the float bowl so I could tune it like the OLD days. Remember that the Cheese man is ALWAYS right.....
 
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Old 07-28-20, 08:33 PM
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At this point (after cleanings) the surging is subtle at rest and sometimes it settles down and runs steady after a minute. It's hard to be sure but I believe it stops surging now once cutting grass of any length.
I've brought home a set of torch tip cleaners. No time today but it should only take 10 minutes to give the jet a scrub/ream.
Stay tuned...


 
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Old 07-29-20, 10:12 AM
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Craftsman engine Surge

Its only a Craftsman, When the local Toro Dealer has a sale, buy one and give that one away!
 
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Old 07-29-20, 07:40 PM
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I accept gifts...
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  #23  
Old 08-03-20, 09:59 AM
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To bring this to a close--
I removed the carb for a 3rd time to clean and/or ream the main jet. The torch cleaning wires did nothing special because as it turns out they aren't textured at the ends. They do nothing to a blind shallow hole like the main jet.
SO...foraging around in my shop I found a spool of florist wire that was just a bit too fat to feed into the jet. I cut off a piece & held it to a spinning sanding disk to thin it enough AND add some roughness to the stiff steel wire. I used that to scrub the brass jet. I can't say for certain that I enlarged the jet but I can certainly say there's no chance any varnish survived the assault.

I once again sprayed out & ran a wire through every other passage in the carb, then reassembled & installed it.

I ran it for a few minutes that day & again for a half hour the next day.

The surge is still there when not mowing. It's smooth while mowing. I think. Mowing grass is itself somewhat variable so it's hard to be 100% sure the governor isn't pulsing.

I'm done. It isn't perfect and perfection maybe isn't possible "by feel" (reaming to an unknown size).
It's good enough to not be embarrassing to use.

Lesson learned: Don't ever get a gas-powered tool that has NO adjustments. It's NOT an evolutionary improvement!


 
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Old 08-03-20, 11:06 AM
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I don't think you have much of a choice if buying new. Pretty much all normal walk behinds have no mixture adjustments anymore. Glad you got it working better. A little more on the jet will probably stop the surging but if it's good enough it's good enough. On the torch tip cleaners, you just clip the non textured bit off the end for a shallow hole like that.
 
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