Odd Carb Surging

Besides my new Husqvarna (which I don't like at all), I have an old MTD with a single cylinder Briggs engine. It had been causing me difficulty over the last year or so requiring constant carb cleaning, mostly due to rust in the float bowl, lack of use (so kept gumming up) and failure of the fuel filter. Eventually, it needed a more thorough cleaning and that meant removing the emulsion tube. It wasn't coming out despite the fact i could move it the tiniest bit and kept trying to wiggle it with copious amounts of penetrating oil.

Knowing I would undoubtedly break it and the float bowl was rusted and the fact that it was quite old, I decided to buy a new carburetor. Some of this decision was based on the fact I could purchase an entire carb for less than the cost of a new emulsion tube and float bowl for my old one.

But the old adage of "you get what you pay for" came true. When it arrived I found it to be the Nikki equivalent of the old Walbro.

I installed it without trouble, but it would surge... at all speeds. There's a lot of info on Nikki 6 carb surging and the answer appears to be drilling out the pilot jet for more idle (idle meaning any speed, but no load) fuel flow. I will admit this makes sense as the surging either disappears, or mostly disappears when the lawn tractor is under load, such as going up hill or with the blades engaged and cutting (where it runs mostly on the main jet)

However, when I first start the engine, it runs fine for about 20 seconds or so before it starts to surge or hunt. This indicates to me it's too rich; i.e. cold engine likes more fuel, but as it heats up it doesn't.

At first a little application of the choke merely caused it to die, but if I carefully advanced the choke very slowly (at the carb vs. the lever) it did smooth out immediately before further application over-choked it and it would die. So maybe it is lean at no-load.

I checked the governor and it's adjusted properly.

Removing the intake tube and running it shows a bit of flame coming back out the carb every time it surges. There's also a black puff of smoke from the exhaust at every surge. I havn't checked the valve lash yet, however considering it does run OK when I first start it or when under load, I think they're OK.

Basically, I don't want to go drilling out the pilot jet in case it's already running too rich vs. too lean.

I was thinking of bending a very small piece of wire into a V and inserting it the pilot jet to lean things out and see if that makes it better or worse. That's reversible, but drilling it out isn't.

And this brings up another point. Darn Nikki carb... Every time I take it apart, the multi o-ring bowl gasket has expanded due to the gasoline and I have to boil it back down to size to re-install it. What a pain.

Other than taking a sledgehammer to the carb and then finding another Walbro, is there any other advice on this whole situation?

Thank-you.

John

post model xxxxxx, type xxxx, trim xx numbers from B&S engine, thanks

Nothing odd about it. I hear it all the time.
99.9% of surging is caused by a lean condition whether it be EPA induced or a blockage.

Russ2251 said:

Nothing odd about it. I hear it all the time.
99.9% of surging is caused by a lean condition whether it be EPA induced or a blockage.

Click to expand...

What I found odd was that it runs fine for about 20 to 30 seconds before it starts surging.

Another, issue, that I neglected to mention, is that the idle mixture screw has no effect. All the way... still runs and surges. Many, many turns out (past where it would have any restricting effect) and it still runs the same.

I may go with the wire idea to see if it gets worse. Then I'll feel better about enlarging the pilot jet.

Another option might be to remove the mixture screw and insert the end of a syringe with gas in it. A gentle small squirt to see if the surging stops momentarily. Might not work as it would be gas only, no air, so no emulsified mixture. However it might spare me another session hovering over the pot of boiling water to re-shrink the gasket once again.

The fact that it smoothed out with application of choke answers the question.

Start the engine gt off the mower and hold the throttle/governor shaft to over ride the governor.
Note how the engine runs.
If it is too rich it will be 8 stroking and blowing black smoke out the muffler
If it is too lean it will just miss and the possibly backfire through the carb because the incoming charge is ignited by the over hot cylinder.
A lean miss sounds high and sharp a rich miss sounds deep & hollow.

When you have finished pull the dip stick
If you get smoke coming out then it is saying
Blown head gasket kimmosade

As for valve lash, it is always best to do the mechanical bits first.
A broken or cracked timing key can also cause this to happen.

I did try holding the governor lever from moving, but it fought pretty hard to keep moving. All I achieved was to minimize the surging a little.

I do get a puff of black smoke at every surge.

The surge isn't a sharp change.

However, seeing how the consensus seems to be that it's running lean, I'll try cleaning it again before I attempt any modifications. It's a new carb and perhaps there's some debris in there from manufacturing.

Thank-you for your replies.

Make sure the float level is set slightly high, to provide extra fuel in the float bowl.

Finally, almost a year later, the mower is fixed. The surging was from running too rich and any amount of choke stalled it. Here's what I found and how I fixed it.

First of all, the new Nikki carb I bought turned out not to be a NIkki,, but a cheap knock-off. That odd rubber gasket swelled up so it wouldn't fit back in and I had to put it in boiling water for 5 minutes to shrink it back (boils out the gas from the rubber). Also, after just a year sitting, the electric shut-off solenoid was firmly stuck.

A friend, who collects old small engine stuff gave me another carb like it, but it was a genuine Nikki. I noticed on this one, the main jet looked a little smaller that on my new one. As the top half of the carb was probably gummed up, I used the top half of my new one and the plastic lower part from the old one. This helped quite a bit, but it now ran rich without surging.

Next step was looking at the float. Both the NIkki and the Nikki copy allowed the float to swing way up before the neelde would shut off the gas. In fact, with regards to the "clone", the float would hit the underside of the pastic piece before the needle would shut.

I wound up using the float and needle from the original Walbro and now it works A-1. The Walbro needle is a bit longer and it isn't a spring loaded one like with the Nikki and its clone. Turning the carb upside down allowed the float to move to a horizontal, or level position, which is how I think it ought to work.

Oh... I also used the old Walbro gas shut-off solenoid without a problem.

Thought I'd follow up as sometimes people find threads like this when they have a similar problem, but it's of no help if the OP never comes back with an answer.

Sometimes you do what you have to do. May not do perfect, I would suggest going this route but if it solves the problem go for It. Thank you for your input.