Surging engines - causes? remedies? tips or tricks

[Jeff-C175 7,202]

The widder lady next door asked if I could fix her DuroMax 4400 generator.  Why sure I can... 

 

BADLY abused, oil like mud, carb bowl off and laying on the ground, air filter housing missing.  Her nephew 'played around' with it she said.

 

OK, new carb ordered... cheapo from internet of course.  Got it installed no problem yesterday.  Started up first pull.  GREAT!  and it ran GREAT for like 2 hours.  Load, no load, all good.

 

Shut it off and she came over a few hours later so I wheeled it out and started it up first pull.

 

VROOM... sag ... VROOM ... sag ... VROOM ... sag ... you know the drill.

 

Half choke it runs FINE!

 

OK, so what the hell?  Why did it run FINE for the 2 hours I had it going and now surges like crazy?  NOTHING CHANGED!  Gas is fresh and clean, filters clean, what happened?

 

I'm also asking this in general because my snow thrower does the same thing.  It has from nearly it's born date.  It's not a problem for me to run it at half choke and it's been fine that way for 10 years or so, but a generator MUST NOT! run like that.  My neighbor has a pressure washer that does the same thing.

 

I don't believe these are governor adjustment issues.

 

Is it just that with all the EPA emission regs that the engines are running so lean they need a little choke to richen the mixture a bit?

 

[OutdoorEnvy 1,522]

I think it's probably a little piece of debris that is partially blocking one of the jets.  They have made the jet holes smaller and smaller for the emissions crap.  It doesn't take but the smallest little something to restrict the gas flow enough that it won't run fully open.  I would see if you just pull the bowl off and shoot some carb cleaner or air into the main jet and see if that clears it.  That is usually what I find to cause it.  I think when it's running it's moving around so much that the vibrations keep it loose or something.  Then once it's off and sits it settles and lodges itself somewhere.   It could be other things but that is the most common issue when it runs on choke or partial choke, it just isn't getting the full gas flow to keep up with the air flow at wide open. 

 

I've done what you did and had that happen even when you think you've cleaned the tank etc, it would be something got in or something was in the new carb already that was from assembly...who knows...

 

 

[stevasaurus 22,765]

Best trick I learned from here BTW ...clogged hole in the top needle valve.  This is easy to check and to clean.  Take the top idle screw and gently bottom it out...count the turns from where it is until the bottom.  Back out the screw.  Notice about 3 small holes in the shaft near the bottom.  Take a small wire and some carb clean and poke out any gunk in the holes.  Put screw back in and gently bottom out and then back out the number of turns to it's original location.  See if that fixes it.  I know it's a new carb, but you may have had junk in the line or in the pump.  

 

Outdoor Envy posted at the same time.  He is saying the same thing. I am taking out the screw.

[Jeff-C175 7,202]

3 minutes ago, OutdoorEnvy said:

probably a little piece of debris

 

I would tend to kinda sorta agree but for one fact...  I drained the tank of the old gas, flushed it with some new gas with the tank fitting removed.  NEW filter sieve in the tank, NEW fuel hose, NEW fuel filter, NEW carburetor.  So that tends to steer me away from debris, although there could have been something inside the carb, I didn't remove the bowl before installing.

 

6 minutes ago, OutdoorEnvy said:

something was in the new carb already

 

Takes ten minutes to pull the carb.  I guess I gotta do what I gotta do!

 

[Jeff-C175 7,202]

6 minutes ago, stevasaurus said:

junk in the line or in the pump

 

See above... also, gravity feed, no pump.

 

OH... idle circuit on generator is never used by the way.  Fixed speed, well above idle...  

 

But yeah, I'm gonna pull the carb again and see.

 

Edited April 26, 2021 by Jeff-C175

[Jeff-C175 7,202]

Vacuum leak?  crappy gaskets?  maybe I'll put a little silly cone on them when I reassemble.

 

BUT my snow thrower has been running this way for a decade (Briggs Snow engine).  And neighbors pressure washer, Briggs 8 HP.  These can't ALL be 'partially clogged' jets?

[stevasaurus 22,765]

Check out that screw for clean holes in the shaft.  

[Jeff-C175 7,202]

5 minutes ago, stevasaurus said:

Check out that screw for clean holes in the shaft. 

 

Just a wee bit too cryptic for me to understand Steve?

 

What screw?  what holes?

 

WAIT... sorry Steve, I missed your first reply.  This might hold true for the Briggs carbs, but not for this cheapo Chinese junk carb

 

 

Edited April 26, 2021 by Jeff-C175

[OutdoorEnvy 1,522]

2 minutes ago, Jeff-C175 said:

Vacuum leak?  crappy gaskets?  maybe I'll put a little silly cone on them when I reassemble.

 

BUT my snow thrower has been running this way for a decade (Briggs Snow engine).  And neighbors pressure washer, Briggs 8 HP.  These can't ALL be 'partially clogged' jets?

 

It could be a vacuum leak.  Same problem would occur with too much air and not enough gas.  That one at this point seems less likely to me since you ran it for 2 hours and it was fine.  The gaskets were doing their job at that time and I don't think they would have went that bad that fast. 

[stevasaurus 22,765]

A picture of the carb would help.  There should be an adjustment fuel mixture screw on top of carb.  Turn it in counting the turns until it bottoms out...may 1 1/2 to 2 1/2 turns.  Take screw out, don't lose the spring, and look and poke the little holes in the shaft.  A paper clip may be too big for the holes, I ended up pulling a wire out of one of my wire brushes.  Spray carb clean down the screw hole and the adjustment screw.  Put screw back in and bottom out, then back out to where the screw was.  

[stevasaurus 22,765]

This is a Carter "N" carb.  In the 1st picture, I am holding the fuel screw and you can see one of the holes I am talking about.  2nd picture shows it in place.

 

 

[Jeff-C175 7,202]

9 minutes ago, stevasaurus said:

adjustment fuel mixture screw on top of carb

 

FIXED main jet.  Only adjustments are idle stop (never used on a genset) and idle mixture.  NO adjustment on main jet.

 

[stevasaurus 22,765]

I did have another situation that was causing surging.  It was a "C" series Wheel Horse with the fuel tank under the seat.  Turned out to be a dead white spider that got stuck in the bottom of the gas tank and was obstructing the flow of gas.  

[Jeff-C175 7,202]

5 minutes ago, stevasaurus said:

Carter "N" carb

 

Worked on a multitude of those over the years... know exactly what you mean.  But this is a cheapie Chinee

[stevasaurus 22,765]

Maybe put some Sea Foam in your gas and run that through.  

[Jeff-C175 7,202]

2 minutes ago, stevasaurus said:

bottom of the gas tank and was obstructing the flow of gas

 

First drained about 8 ounces into glass jar and nada... bottom of tank was clean

 

[squonk 41,164]

Sounds like you typical genset to me.  I've seen these run good when they are new for like a week and then all bets are off. sounds like it's a tad lean still. I would clean the old carb if possible and reinstall that if it has a mixture screw.

 

Used to work on a bunch of those Onan RV Gennys. Darn things would run like a top without the air cleaner installed. Just putting the housing on would send them into a tizzy. Clean and readjust. Clean and readjust. Throw hammers at it and adjust some more.

[Chestnut 315]

I've had and fixed the dirty carb surging issues on several small engines, but the Subaru Robin on my generator didn't change after several cleanings. It actually was the governor on this one.

[Jeff-C175 7,202]

15 minutes ago, squonk said:

clean the old carb if possible and reinstall that if it has a mixture screw.

 

Definitely not possible.  Widder lady had to make some room in the garage after nephew took it apart so she wheeled it out onto her patio and there it sat over the winter with no bowl on the carb, no air filter, etc... I tried cleaning the jet out but it was REALLY REALLY a hot s4i77y mess.  I have a set of 'plug gauges' in 0.001 steps that come in real handy for pushing junk out of tiny holes.  I probably ruined them for serious QA work, but that's not what I need them for anyway!

 

This is the carb.  That 'main jet' is a plastic plug with a metal jet insert pushed into the bottom of it.  I pulled the one out of the old carb quite easily but am hesitant to pull the one out of the new one.  I'm thinking though that I might try it.  In addition to the plug gauges I also have a set of precision drills and if worst come to worst, I may attempt to drill the main jet a thousandth or two bigger.  I really think the problem is that it's just a tad on the lean side.  I have no explanation as to why it ran well for a time after I first installed it.  I'm going to pull that plug jet as a last resort though.

 

[Howie 886]

I believe what you saw is the main jet is actually in the idle circuit. Take the bowl off and up in that 

stem is a screw in jet. Remove it and clean up thru there. Just had one of those apart today and

put a.002 thousands bigger jet in the thing to get it to run without surging. Had it off about three

time already. The one I am working on did not have an adjustable idle mixture screw on it either.

[Jeff-C175 7,202]

10 hours ago, Howie said:

I believe what you saw is the main jet is actually in the idle circuit.

 

I did a lot more studyin' today Howie and I believe you are correct, but that idle circuit does in fact affect the hi-speed to a degree.  I'll get to that in a minute but wanted to ask you where you sourced the .002 larger main jet from?

 

I too had that new carb on and off several times this afternoon / evening while I was analyzin' it by squirting carb cleaner in the various ports to see where it came out, watched a few YT videos etc...

 

There was zero debris inside the bowl or the jets.  Spotless, as I had suspected.

 

Pulled the little plastic plug and measured the jet in that, it's .016 and the main jet in the center tube is .031

 

I got it to stop surging though, here's how, and I LIED when I said earlier that 'nothing had changed' because I had in fact held the throttle against the idle stop and tried adjusting what I thought (and marked in the pic) was the idle jet.  It didn't seem to make ANY difference in the idle at all.  So I mistakenly figured since it was never going to be running at idle that it didn't matter...  well... it DOES matter!  When I backed it out about 1-1/2 turns from where I left it, the engine stopped surging!  It looks like it's 'sweet spot' is about 4-1/2 turns out from bottomed.

 

I do believe it's still running lean though.  I used a 1500W heat gun as a load.  When it was running smoothly with no load, I switch on the heat gun and the engine 'bounces' for about 2 seconds.  Brrrm... Brrrrm... still surging when a load is thrown on but stabilizes.

 

Now, I take the load off, still running smoothly, and close down the choke until it JUST starts to sputter, and open the choke back up a hair.  NOW when I apply the sudden load, the engine does NOT surge but immediately adjusts and takes the load.  This is what tells me and makes me think it's still lean.

 

So, back to your source of the main jet.  I'd really like to know where that came from.  I do have the precision drills to open the jet a bit but if that causes a problem I need a 'fall back'.  I can't put back metal I take out of the jet!  

 

[ edit: I just measured the small jet in the black plastic plug (which is rightfully called a PILOT JET) from the OLD carb.  It seems to be .001 bigger.  .017 gauge fits, but .018 does not.  I could easily pop this jet in without taking the carb off, think I'll try that tomorrow and see what affect it has.  I know this is supposed to be the idle circuit but like I said, the supposed idle mixture screw had affect so why not the jet also?  The main jet in both carbs seem to be .031]

Looking very closely at these pilot jets, there is a number on the top.  The new one is marked "3" and the old one "45"  ... gotta hit the calculator and see if that's .3 mm and .45 mm.  hmmm, maybe not.  The one marked 3 is about 16 thou, the one marked 45 is about 17 thou.  The latter seems about right, .45 mm is .0177 but the 3 should only be about .012 and my plug gauges say it's .016.  I'm gonna drill one out and see what happens.

 

 

Edited April 27, 2021 by Jeff-C175

[Howie 886]

I was going to say that on the idle affecting how the engines runs and did not. I had bought

those jets from a go cart company, would have to try to see which one.

On the one here the jet in the carb only a .026 would go in and had one a .028 would go in

and that is what I put in. In my case you can not adjust the idle because they set it and clip

the end off or however they do it.

[Jeff-C175 7,202]

6 minutes ago, Howie said:

jets from a go cart company,

 

I just found some on Amazon for a Predator 212 and they appear to use the same style carburetor.  Need to research that a bit more.  Do you recall if the ones you bought were for the Predator?

 

The first kit I found has .035, .036, .037 main, .024 pilot, and distribution tube.  I wonder what's different on the distribution tube from 'stock'?  

 

 

I'm going to try drilling out the pilot jet from the old carb to .018 first and see what that does.

 

[Jeff-C175 7,202]

On 4/26/2021 at 12:51 PM, Chestnut said:

but the Subaru Robin on my generator didn't change after several cleanings

 

I think I saw a YT video yesterday which said pretty much the exact same thing... was that your video?

 

@stevasaurus @OutdoorEnvy @squonk @Howie

 

I'm going to look into the Snow Thrower with the Briggs this Fall, but wanted to put a wrap on this one for now.

 

Previously I mentioned that the old carb had an .017 Pilot Jet but it was actually .018 after I cleaned it up real good.  The replacement carb was a teensy bit bigger than .016. I took the Pilot Jet out of the old carb, cleaned it real good, stuck it in the new carb, readjusted the idle screw a bit, and everything is now 'hunky dory' enough to call it done. It still 'hunts' just a wee bit with no load.  When I throw on a 1500 W load it will kinda bog for a split second and surge a little... but only once, and not as deeply as it did before, then run smoothly.  I strongly suspect that it is better than it was new, or at least the same.  Can't expect perfection from Chinese Junque.  Just hope that it floats long enough to get the job done.

 

I don't think she's going to use this one so I should probably empty the 1/4 tank of gas before returning it.

 

Should I charge her for the half quart of oil?   Naaahhhhhh... as long as she bakes me one of them deeeelicious Apple Pies that I shouldn't eat, but will.

 

Oh, I should mention this:  When I went to check the oil level before starting it, as soon as I pulled the fill plug oil came pouring out!  Grabbed a drain pan and drained almost a half a quart with the machine setting level.  I thought it might be gazzaline, but no... it was definitely oil.  HOWINDAHELL does one put that much oil in an engine?  Whoever did it would have had to tip it sideways to get it in there.  No wonder the breather was spittin' oil into the air filter!  It was saturated.  Crazy!   Widder Lady doesn't know either.  (at least she SAID she didn't know, but she looked guilty! )

 

 

Edited April 27, 2021 by Jeff-C175

[OutdoorEnvy 1,522]

Glad you got it sorted out.  The aftermarket stuff is always hit or miss.  Put this one in the memory bank though...I know with some new honda engines they surge from day one.  There's a youtube channel called "Steve's small engine saloon" that has a good fix for it on his channel where he drills out the main jet a tad larger than factory.  That same issue seems to show up more lately on all kinds of stuff yours did.  Anyways...thanks for the update and again glad it worked out for you and your neighbor...

Edited April 28, 2021 by OutdoorEnvy