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BrianP

Is There a Carb Doctor in the House - Updated!

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BrianP
When we last left our hero, he’d finally gotten a good hot, (and most importantly reliable) spark on his Simplicity 3410. Visions of making short work of his mowing chores danced in his head. Ah if only real life were that simple. You see, when I parked the machine a year or so ago, I’d been in a hurry to finish my mowing chores. So, I neglected to drain the carb or even use any StaBil fuel stabilizer. I think you can see where this is headed. The problem: The tractor runs, but not on every power stroke. It surges and hunts, almost smoothing out but then backfires and continues surgin. The upper/lower carb-body gasket is weeping, which leads me to believe there is a float problem afoot. I tried adjusting the main needle valve and the idle valve screw. Neither had any effect whatsoever. I chugged, sputtered and back-fired my way back into my shed and shut her down. “Okay,” I thought, “I’ll pull the carb, clean her out real good and I’ll be back in action.” My 3410 is equipped with the heavy-duty air cleaner which means it’s got one of those foam pre-filter which slides over the paper air filter element. Oh well, in for a penny as they say. So, I brushed the loose dirt off the foam rubber and washed it in a pan filled with hot water and some Dawn dish washing liquid I pirated from the kitchen. Great stuff, cuts grease like nothing else, even from your hands. Anyway, I rinsed the pre-cleaner thoroughly with hot water and it practically looked new. I set it in the sun to dry and returned to the tractor. I pulled the carb off, finagled the choke and governor rods loose and presto I had a handful of carburetor. Mighty dirty and greasy carburetor to be perfectly honest. So I grabbed a can of brake cleaner and blasted it clean before disassembly. For the uninitiated, once the carb is sitting on your bench you need to remove the main metering needle valve and seat/jet assembly first. If you don’t the two halves of the carb body will not separate.


Removing the needle-valve is easy, but removal of the seat/jet takes real finesse. I’ve modified a screwdriver by filing the sides of it, making it narrow enough to not booger up the delicate aluminum threads of the carb body.


The difference is more apparent here where you can compare it to a standard screwdriver.


As you can see here, it's a close fit, but it works.


Since I’d rebuilt this carb about two years ago, (replacing everything possible), I wasn’t sure what I’d find. Upon close inspection the main metering valve looked good, (not grooved from over tightening), and all the orifices in the tube were clear. Next, I checked the float valve seat, (replaced during the rebuild) and there was no foreign matter in there or the carb bowl. I was clearly pushing the limits of my digital camera trying to get a shot of the valve seat.


Leaving no stone unturned, I even shook the float but heard no sloshing of fuel indicative of a bad float. I measured it diagonally and it was level as required.


Even though the needle valve itself looked okay,


I rooted around my parts bin, and came up with a previously unused needle float-valve and carb-body gasket and with nothing else needing attention, I buttoned it up and went out to my shed to re-install it, hoping I’d somehow disloged whatever foreign matter had been giving me all the grief. I was in high spirits as I re-attached the choke cable, the rod from the governor, and re-attached the carburetor. Next, I installed the paper element (and freshly re-oiled foam pre-cleaner) wing-nutted the lid back on and re-installed the fuel line. Whew! Ah, the moment of truth. I hopped into the saddle, flipped on the power switch for my coil, pulled out the choke and she fired right up. Unfortunately nothing had changed. The 3410 still ran just as rough as before, surging and hunting all over the place. Insert expletive of choice here. I retreated to my basement workshop, to re-group around an ice-cold brewski and poured over my repair manuals. Trudged back out to the shed, re-re-adjusted the metering screws on the carb and tried once more. Nada! It was getting late, so I closed up my shed and headed for the house. At this point I must confess I’m stumped! So, now it’s time to pick the brains of the more expert members of this club. What have I missed? Am I on the right track with my suspicion that the carb is the culprit? Keep in mind the only problem when I parked this thing in my shed over a year ago was the spark problem. AND NOW, THE SAGA CONTINUES . . . A rough week at work and evening thunderstorms pretty much put the kibosh on attempting to resolve my carburetor predicament until yesterday. To recap, I’ve been trying to get my 3410 back on it’s tires for a couple of weeks now. My initial problem had been intermittent spark, (which I cured by re-wiring the 12-volt supply lead to the coil with internal resistor), which only has a couple weeks worth of use on it. I’d replaced the coil previously, thinking it to be the culprit, when in fact a botched wiring job on my part created the problem. Oh well, I’ve got all kinds of spark now. However, after sitting for a year the carburetor decided it wanted to get into the act. My current problem is this thing is still hunting or surging in between periods of running like a brand-new tractor. I know an intermittent problem is the trickiest to try and eliminate, but I think I am getting closer. My first attempt amounted to little more than removal of the carb, disassembly, cleaning and inspection. I was thinking there must be crud in the carburetor. The result of this was…absolutely nothing. So I tried again with similar results. It acted for all the world like a tractor running too rich. I pulled the sparkplug which was caked with black soot, confirming my suspicions Now into my fourth wave of assault, I’ve installed brand new points, a new automotive rated condenser, a new Champion J-8 plug (gapped at 0.30 per my book) and, believe it or not rebuilt the blasted carburetor a fourth time!


I truly believe I could now remove and install my carb blindfolded. (Perhaps this could be a contest at a future Spring-Fling gathering?). Where was I? Ah yes, after my fourth carb re-build (I’m nothing if not persistent), the periods of smooth-running were longer, but still not constant. No amount of adjusting the main venturi screw made any difference whatsoever. Standing there in the 90 degree heat, sweat pouring down my neck, (and running into my eyes), I decided to see what would happen if the engine were under a load. I lowered the deck, tightened the belt-tensioning lever and engaged the PTO. Amazingly, the engine smoothed out. Hmm, let’s rev her up a bit and see what happens. Still okay, perhaps just a wee bit more…nope, too much, (back to taa…taa…taa,taa…POW!...taa), so I backed off a bit and she smoothed out once again. I usually run at about ¾ throttle when mowing my little suburban plot which, (South Carolina being in a drought), pretty much amounts to leveling off the weeds. So, I hopped on put it into first, and went for broke. I was cutting grass! Whooee! I cut for about an hour with no problems, (my confidence building with each successive pass), when she started surging again. I pulled out the choke and after a second or two, she smoothed out and I cut for another 30 minutes before using the old choke trick. As my job drew to a close the surging was happening more and more frequently. Realizing a spark plug can be a key to engine diagnosis, I’ve included the following comparison shot. The original spark plug I tried (on the left) and my most recent plug on the right.


Drawing on my old hot-rodding experience I’m leaning towards a point-gap problem giving me my high-speed misfire. Particularly since I don’t set the gap with a feeler gauge, but by eye, when I see the best spark. But at the same time, the plug tells me I have a too-rich condition. If anyone out there has an idea or solution I haven’t tried yet, I’m all ears. I personally think I’ve got a float problem, but at this point I think I’ll let it “wear in” a bit and see what happens. If I have to pull that carb a 5th time, I think the temptation to use a sledge hammer on it might just be too irresistible to ignore . . . just kidding folks. Despite all the problems I’m still extremely pleased with how this machine manicures my lawn. You should see the way it makes mincemeat of my pesky Magnolia tree leaves. Runs rings around my father-in-law's Murray that I used last week, what a turkey!

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johnsup
Brian, How does the cross(where the throttle linkage hooks up to the carb.) shaft feel if you wiggle it....If it feels loose then it needs to be rebushed....Otherwise the carb. will get too much air....And surge....I have chucked a few because I couldn't get the bushing out...I am not factory trained....Since I am not there to look at it , I am grasping straws...Good Luck, John

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Roy
One possibility is a governor problem. Check the governor lever arm adjustment and make sure the springs and linkage are OK and not binding. UCD posted a Briggs troubleshooting chart posted in Commonly Ask Questions that should help. http://www.simpletractors.com/club2/topic.asp?TOPIC_ID=35948

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andrewk
To remove the brass seat, I believe you are supposed to tap it with a 1/4"-20 tap, and then install the new one with a briggs special tool. If you installed that seat with that pointed tool you made, then there is a good chance the seating area for the needle is distorted. How did you get the seat out? It's been a long day, but I am nearly certain that is a solid brass seat that is supposed to use the aforementioned procedure to remove/reinstall- Otherwise, look for a vacuum leak- One thing- did you clean the idle circuit? I can't tell from the pics if you had removed the low speed mixture adjustment screw or not- Andrew

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acken
Are you certain you still don't have a spark problem? All too often ignition, and fuel symtoms can act very similar. I have seen worn cam lobes which affect the amount of point plunger travel, points may not be opening/closing all the way. Very well written account of carb repair.

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HubbardRA
I am suspecting a bad condenser. I had the same problem the first time I fired up the engine on my 716H. It spit, sputtered, just wouldn't smooth out no matter what I did. I finally called a friend who ran Kohlers in Pro-Stock Garden Tractor Pulling. He asked about my condenser. I said I had not pulled the points cover to check it. He said no, the condenser is on the coil on a Kohler. I said, not on mine. He told me to put one on. It ran fine after that. So replace the condenser and see if that helps.

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MrSteele
Before replacing the condensor, which might be the problem, check your points. If they are burned "lopsided", or on one side more than the other, you are in need of a new condensor. I will agree that a condensor may well be your problem, as they can go bad from sitting as well as from use. If the points are burned evenly across the contacts, it is unlikely that the condensor is the problem. My first thought would be trash or varnish in the low speed assembly.

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BrianP
After being stymied two nights in a row by violent thunderstorms (featuring that Southern specialty, marble-sized hail), I finally got back to my 3410 project. My approach is to address every suggestion made by fellow members, hoping one of them will turn the trick and get me back in action. I started by investigating the spark. I replaced the condenser (which had been an automotive grade one) with another unused one from my parts cabinet.


No change. I attempted to adjust the points (which are brand spanking new by the way)…again, no change. Sigh. A suggestion was made to reverse the polarity of the secondary wiring of the coil. I discounted this, since my 7016 is wired identically (positive to source, negative to points), and works perfectly. Another suggestion was to check the governor. According to all the shop manuals I have as well as my memory of how things looked before, all is well in spring-land.


Another suggestion was made regarding cam-lobe wear. Well, if I had been having these symptoms before I parked the machine this theory might have had some validity. But this thing was running perfectly, (other than my fouled up supply voltage to the coil), when I parked it. So, I can’t point the finger at the plunger or the cam which drives it. If they worked when I parked it, they should work now. Oh and keep in mind I completely rebuilt this engine in 1988, so all the internals date back to then. Okay, so I’ve got a fuel problem…right? Well I delved into that realm next. A suggestion was made that I drain the tank, so I did.


There were no tell-tale bubbles of water in the fuel and the fuel flowed steadily with no bubbles of air or change in speed (surging) of the flow of fuel. This leads me to believe I don’t have a supply issue. I don’t think the fuel filter is clogged either Alright let’s take a deep breath now and not let this blasted thing get under our skin. It is a machine after all, and couldn’t possibly care less how frustrated this whole thing is making me. I regrouped around a slice of pizza and a couple cans of Bud and went back out to the shed. Just for kicks, I fired it up again. No change, but I snapped a couple of quick shots (while it was running), showing where the carb appears to be leaking fuel.






This had not been happening when I parked it. Ah ha! Besides, belching black smoke is a sure sign of running over-rich right? My only reservation is, I’ve already torn it down and replaced the needle valve. It was mentioned the needle valve seat may have been distorted during, um, installation. I did use a self tapping screw to extract the old seat, but I used the old seat to press in the new one as outlined in the manual. Another suggestion was made regarding the main throttle butterfly shaft being worn. Well that might have caused the problem gradually, over time, but the problem I’m having happened overnight. As if you parked a perfectly running machine one Saturday, then the following week the darn thing won’t run. The sky is getting dark again, so I’ve got to get off-line and wait it out. I’ll dig into this further later in the weekend.

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MrSteele
Fuel should not be leaking at the base of the idle mix screw, as that is part of a solid casting. It may be leaking from the bolts that hold the carb to the manifold, though, allowing air to enter the mix at the wrong time during running of the engine. That will surely cause the hunting you describe. Perhaps a new gasket or a thin film of sealer at the connection?

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toomanytractors
Brian; May be a little late on this but I haven,t had much time to be in here the past week or so. Thursday e vening storm dumped a tree top thru the windshield of my little car and damaged the roof so I,ve been busy getting an old pickup ready to drive. You said this tractor is a 3410 so I,m assuming a 10Hp briggs...you also stated you put in a J-8 plug. I,ve been told by my parts suppliers that this should have a J-19LM plug. That,s what I,ve been using and haven,t had any problems. Hope this will help. Regards, Dave

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tec2484
This topic sure hits home tonight. I was trying out my new to me 3410S. and it's running just like Brian's. great when you first start and then starts chuging, (richen the hi speed needle vale runs better for a few then gets chuging again) This sure has gave me a lot to try. I belive the first thing I will do is get new gov springs as mine only has one on it.

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