Engine stumbles when under load.

[mr-moe]

Briggs, single cylinder 31 CID
Model 31A707 0874 G5.

This unit starts right up and runs fine at all throttle positions.
When a large load is applied, ie, blades engaged or heavy grass, the engine stumbles for a few seconds, then catches up and continues.
It never actually stalls, it just stumbles.
It lets out a puff of black smoke when it stumbles, indicating rich, or unburnt fuel.
Carb is clean.
Has great spark.
Have tried 3 different coils, all the same.
Any help, will help.

[KE4AVB]

Well this is a problem I haven't seen in a long time. So I am quite rusty on it. Usually a loose main jet on the Nikki causes this but the does hint a Walbro might be present. But even a worn exhaust camshaft lobe can.

If working with the Nikki always replace the old fuel bowl gasket set. Otherwords don't use old compressed gaskets. The main jet o-ring is Kawasaki PN 92055-7013; Briggs only sells the jet with o-ring.

Now BGS probably has more ideas on this one.

[bgsengine]

KE4 used most of my ideas, typically a stumble as you describe can sometimes be due to fuel puddling (behind the throttle plate where there's little air turbulence to atomize the fuel) and typically only in colder operating temperatures (often can go away after running to full hot) but other than that is usually a fuel mixture issue or idle transition port (In a carbureted car engine, I'd be looking at accelerator pump/ accelerator well) - Anything that can affect fuel atomization/vaporization (air flow issues, poor quality fuel, excess fuel, thin air, disruptions in air flow pattern- That is often an aftermarket gasket that is too "wide" and some gasket material sticks out into the inside of the intake tube or port - can cause areas where air isn't flowing properly and allowing misted/atomized fuel to drop out of suspension causing fuel puddling, which then when throttle opens wide to respond to the load applied, the sudden increased air volume picks up that excess unmetered fuel which causes your over-rich stumble.)

[Arkie]

Is the carb a Nikki or a Walbro?

Is this a new to you engine from a customer that had the complaint or your engine?
(reason I ask is if your mower engine did you just recently notice the issue or??????????)

[mr-moe]

A follow up to my post.
This was a unit that I bought this spring for resale.

After sitting for the last 5 years under the original owner's deck he decided to sell it.
Carb. needed to be cleaned out. This was completed c/w Briggs parts.
Upon reinstalling, my fumbley hands dropped the carb on the floor and broke the plastic choke shaft.
The pricing of a new choke shaft kit from Briggs was about the same price as a replacement carb. from one of those offshore companies.
I opted for a new carb.

That was when my troubles with the stumbly running eng. started.
With the help of your suggestions, I realized that it was a carb fuel management problem.
So I simply took off the choke parts from the newly purchased carb and installed them on the original Nikki carb.
Very carefully not to drop it again, I finished up the installation.
Fired it up and it ran like a Briggs should.
Cut my lawn for an hour, and it never missed a beat.

Thanks for all the suggestions.

[Arkie]

I've got lucky ever once in awhile and used the China Clone carbs for parts and pieces into a Real Nikki and got good results also. (usually the o rings and gaskets.

Noticed that the choke/throttle plate area is one size don't fit all quite often.

And for the 28, 31, and 33 series Briggs carbs I keep couple of the good NLA Walbro's carbs around as test carbs to confirm that the rough running issue is actually the Nikki carb.
I can usually make a Nikki carb eventually get a good run when the Walbro sub confirms it's actually the Nikki carb that is the problem.
If a it's a NEW China clone carb that runs rough it's most generally a waste of time trying to make a silk purse out of a sows ear.