Rough Idle - Dirty Carb or Something Worse?

[Tiger Small Engine]

Doesn't it seem unlikely that the original carb was dirty, and the brand new one is also dirty? Seems like in this scenario where I've gone through two carbs, the air leak would be the more likely problem, no?

It seems very likely that OEM carburetor is dirty, and that the aftermarket carburetor is not dead on with air/fuel mixture, so not running properly.

 

[Rivets]

Once again you don’t want to hear what Tiger and I are saying. In my couple of years servicing small engines I’ve seen more than one carb that looks clean, but has a blocked passageway. If you are depending just on your eyes to diagnose a dirty carb, there is not much help we can offer. Second, you would not be the first person to get a bad carb off the internet. Finally, if you understand how carbs work on today’s small engines, you would know that the RPMs being set too low will result in surging especially if you have a blocked passageway. In my opinion the ball is now in your court. You can try to solve the problem by having the carb ultrasonicly cleaned, which may or may not work or return the new carb and get one from a good, known source. Either way you need to get a tachometer and check your No Load high end speed against engine specs.

 

[slomo]

Sounds to me, from your video, that the fast idle or max revs is too low. Crank her up to 3400 or 3600 revs and make another video. Bet she smooths out as she will be on the main jet then.

The second video, where you appear for the second lap, the engine was running longer and seemed to be a little higher rev and cleaner firing. Sounded almost normal.

There is more to cleaning a carb than spraying the outside down. Have you cleaned the emulsion tube or does your carb have one? You have either too much fuel or not enough. Seeing as there is no black smoke puffing out the muffler leads to a lack of fuel. What causes a lack of fuel?

1.Trash in the fuel tank.
2.Internally deteriorated fuel lines.
3.Fuel filter if yours has one.
4.Lack of fuel reaching the carb inlet. 1, 2, 3 and 4 here.
5.Carb float height setting.
6.Carb needle/seat issue causing fuel starvation.
7.Fuel tank vent issues.

Place a piece of cardboard over the air inlet area on the carb. This will cause a choked condition. OR activate the choke slightly. See if the engine smooths out or gets worse.

Is your plug black or blistered white?

Might have a cam that is losing it's max lift.

Betting the combustion chamber is loaded with carbon. It's a Briggs so that goes without saying. Chunk of carbon can cause a valve to hang open slightly. This is not directly your main issue but should be looked at.

 

[Etbrown44]

Slomo gave you 7 things to check. Others have given more. All good.

Doubt it's the carb, since you have tried 2. However For $10 you could always try a 3rd in a different brand to be sure. My experience with installing over 150 of the the chinese carbs has been pretty good. You can always get a bad one, but where does anyone think a Briggs carb is made anyway?

After the carb and Slomos 7 items, I'd try an inexpensive rubber tip compression tester. It will usually tell you if its valves.

 

[My Hoe]

Ok well i disagree with some of what you say, but this part I don't understand at all:

I don't know what you mean here or what you are suggesting I do?

EDIT: First, I apologize that I wrote my earlier comment w/o watching your entire video. As a consequence of that, I missed the fact that in addition to SURGING, your mower actually is MISFIRING as well, albeit intermittently.

However, like others have said, you're not listening to advice, so here's some more you can not listen to.

"Lawnmower idle" is NOT "full RPM" or whatever you called it. Idle is much lower than full speed, which is probably somewhere between 3,330-3,600RPM. You need a TACHOMETER. Here is a write-up on a cheap, but VERY reliable one:"
Farmshow--Vibratach

Vibratach Tool Designed To Read The RPM's On Engines

Precise Engine Repair, 151 Roberts Rd., Yakima, Wash. 98908; www.perr.com: This company offers the Vibratach, a tool designed to read the rpm's on all engines and other rotating equipment. &nbsp...

www.farmshow.com

I mentioned that your engine is surging (or, what I call "hunting"--for a steady speed, either under load or unloaded, when it should be at a [relatively] steady speed regardless of load. That's the governor's job, to keep the the RPMs as stable as possible, under the varying loads placed upon the engine for a given speed).

Surging can be caused by a bad fuel float valve seat, a leaking fuel float and/or dirt in the carburetor--or a combination of all three. But since you've swapped carbs, this is now a head-scratcher.

Surging can also be caused by a malfunctioning governor--but that is less likely than the chance that you're having an improper fuel/air ratio due to an obstruction in the fuel system.

Use the CHOKE As a Diagnositic Tool:
If it slows down, then your mixture is already too rich, as your hand is acting like a choke and richening the mixture further. As another person mentioned, remove the air filter and slowly, progressively cover the carburetor's air intake (with a LEATHER GLOVED HAND, because FIRE)l and note whether the engine speeds up or slows down. If it slows down, then your mixture is already too rich, the engine will slow down. But if the engine speeds up, then your mixture is too lean (which would again point to a FUEL OBSTRUCTION SOMEWHERE).

There should be a SCREEN on the gas pickup tube inside the gas tank. These can look clean but because their mesh is so fine, they can be clogged and need cleaning or replacing, depending upon design.

As a last resort, I would closely examined the holes that the governor wire hooks into, on the carb linkage, and see if you can tell that it used to be in a different hole by virtue of that hole being more elongated than the other holes. Even if they all look the same, you could still try hooking the governor's control rod into the next hole over from the one it's in, in either direction, and see if the surging goes away. But only do that AFTER you've determined whether or not the engine is reaching its specified maximum RPM. (Yes, You will have to look up what your engine's specified maximum RPM is. Try https://www.manualslib.com/.

And if you do start experimenting with the governor's control rod, CHECK YOUR RPMS as you do so, because if you over-speed the engine, you may get to see what the connecting rod looks like, without even taking the engine apart.

Briggs & Stratton made a good small engine manual, probably back in the '80's. IMO, you need to read something like that, as it's clear, to me, that you need more THEORY AND KNOWLEDGE, to build a basic understanding of how small engines actually work. They now offer manuals for more modern, OHV engines, as well (not sure which style engine you have but try to be Brigg's GENERAL small engine manual and the specific manual for your engine, as well (see the FREE manuals library, linked above). You need to get a basic repair manual and READ it--you will find that many of the questions you're asking are answered by such a manual. Good luck.

 

[slomo]

Watched the video again. Looks like that plastic Briggs carb. Verify choke is opening or not. Wire the sucker wide open with a bread tie. Test it out a few laps.

Top me, longer it runs the more it smooths out. Like the choke is lazily opening. End of video sounded okay. Again revs sound low.

 

[l008com]

The mic on my camera doesn't capture the sound perfectly. So it does improve once it warms up, it never gets good. Its always misfiring/hunting. And with the new carb, it also has an awful startup. I didn't see a filter in the fuel tank and now I think its full, or close to it. I'll see if I can run it out of gas today (while mowing) and take a closer look in there. I'll also try covering the air intake and see how that goes. I did check the choke earlier in this post and did confirm it is opening up fully and that does not have any effect on the misfiring.

I'll post a followup tonight and see what I find. if I'm really feeling motivated, I might also try cleaning the original carb and see how that goes.

 

[l008com]

Whaaaaaat a shitshow today turned in to.

The first thing I did was start up the machine and experiment with blocking the air intake by different amounts. It didn't seem to have any affect on how the machine ran. MAYBE a tiny bit smoother with it half covered, but it still misfired. No meaningful difference I'd say.

Then I decided I'd just clean the old carb real quick and put it back on just to see. Even though that doesn't seem like the problem. So long story short, when you spray carb cleaner in one of the holes that the what plastic triple stack thing comes out of, it dislodges the plastic ring in the middle of the carb. So on the old carb, it jammed the choke closed so I couldn't open it up. I didn't realize this until after I put it back on and ran it for a while. It did a whole series of crazy things, really high rpms, then abruptly going to really low RPMs. It was all over the place.

At that point, I figured what the hell, and went to clean the new carb too just for the sake of completeness. After messing around with that for about 2 hours, with jet or whatever its called not being able to be reinserted, I realized what had happened, and took both carbs apart again and properly reseated that ring and snapped that removable part back in. Now I remember why I always opt for the $10 carb replacement over messing with these things. Not worth the trouble at all.

I have footage of it going crazy, I think at once point it was maybe running on access carb cleaner. But in the end, with the original carb reassembled properly, it has a slightly more stable startup, but it still misfires a lot. I don't think it's misfiring any more than it was, but the misfires don't really pick up on camera. I think it does have a more stable startup now, although the really sketchy startup I think only happened with the new carb. I dunno I'm having trouble keeping track of whats what at this point.

Also gas poured out of the fuel line, which I previously replaced. So it's not a rotten fuel line issue or a clogged fuel line issue.
And when I was reassmebling the bowl on the new carb for what felt like the 5th time, one of those screws into the plastic stripped. So I didn't put it back on to re-test.

But this sure does feel like it is not a carb issue at all. Maybe there was a slight dirty carb issue, but the main problem seems to be something else. But what else? Weak coil? Carbon on the valves? If there is carbon on the valves, will it work itself out if I just keep using the machine?

Edit: Also regarding double checking the holes that the rods go in to, there are only two rods and only two holes. Its all plastic and there are no "optional" holes for adjusting the rods. And the two rods are different thicknesses that match the holes, so you really can't go wrong as far as the rods go. There were not other adjustments I was able to see anywhere on the carb.

 

[l008com]

I'm watching valve rocker adjustment videos on youtube right now. Looks easy enough, I guess that will be the next thing I try. I do own feeler gauges, but I have no glue where they are, I'll have to search for them first.

 

[Etbrown44]

You really need an inexpensive compression gauge, and that will tell you what you need to know about the valves.