Z335e engine surge

[Jonnie_S]

B&S 40N877 engine is surging most noticably at idle and low speed, but even at high speed it does not sound right. Sounds a bit rough.
Mower has less than 30 hours on it. 2 years old.
New fuel filter, air filter, spark plugs (old ones looked fine)
Fuel pump seems to work fine. I see fuel trickling into the filter. Crank the engine with the pump output line disconnected and gas spurts out pretty vigorously.
Just for kicks I gravity fed fuel from a container - no difference.
Opened up the carburetor - it looked perfectly clean. Blew out the orifices and re-assembled. This carb has a single emulsion tube, not the 2-tube version I've seen in several youtube videos.
Governor linkage and springs all good.
Fresh fuel (although it is 10% ethanol).

Read lots of posts here and in other forums about engine surging and I'm running out of things to check.
The mower runs ok at high speed and is perfectly usable, but just doesn't sound like its running smoothly. At its lowest speed it lopes badly, almost stalls.
Its still under warranty but a hassle to bring it to a JD service center. Plus, if they determine it just needed a "tune-up", i.e. normal maintenance, that's not covered.

Any ideas?

-Jon

 

[PTmowerMech]

If yours has the Nikki style carb on it, I hate those things. Surging is usually caused by the pilot jet being clogged. But if it runs better at higher RPM's, you might try choking it a little at low idle. If that clears it up, then the pilot jet is probably clogged a little. And needs to be cleaned out.
If it's still under warranty, I wouldn't tell them you took the carb off.

 

[Jonnie_S]

If yours has the Nikki style carb on it, I hate those things. Surging is usually caused by the pilot jet being clogged. But if it runs better at higher RPM's, you might try choking it a little at low idle. If that clears it up, then the pilot jet is probably clogged a little. And needs to be cleaned out.
If it's still under warranty, I wouldn't tell them you took the carb off.

At low idle, if I block off most of the air intake, the idle smoothes out completely, so most likely a fuel problem.
I'll open up the carb again.
-Thanks

 

[Jonnie_S]

Second time's a charm. Not a speck of dirt in the carb, but stuck a thin wire through every orifice and flushed out and blew everything. Re-assembled and its running perfectly smooth at low and high speeds.
I guess there was something in there somewhere.

Thanks PT for keeping me focused on the carb. Can you identify the carb I have (see pics)? It looks like there's a B&S logo, but I can't find info on what make/model it is.

 

[shadetree#1]

That 593197 is the carb part number and it's a Nikki type.
Them carbs will have the symptoms you describe.

Here is some links that have some repair/re-build info about Nikki's and info about low priced re-build kits/parts. You got lucky that your twin engines does not use the dual throat Nikki, they can be a pain. some of the links below are for the dual throat but the info is good for applying to the single throat when you are working on them. Them little o-rings around the jet/jets are critical and MUST FIT SNUG as well as the o-ring rubber gasket ABOVE the emulsion tube. If it leaks the carb will flood even though the needle and seat is good.

Small engines repairs links

Nikki Carbs, etc

https://outdoorpowerinfo.com/repairs/



12/2022

Nikki dual throat carb repair/rebuilding links

Nikki dual throat carb tips and re-building LINK.

Guy asks about jets sizing and re-building a dual throat Nikki carb

https://www.lawnmowerforum.com/threads/nikki-dual-carb-main-jets.29187/



You tube video part 1: (newer style aluminum float bowl Nikki) USE KIT 54832 @ $7

part 2 (older model with steel float bowl) USE KIT 54833 or 54834 @ $9 each depending on the style of Nikki. See part 2 of the video to determine which kit needed.

Search on-line for

VITON o-rings for Briggs & Stratton Nikki carburetor main jets rebuild kit

VITON o-rings for Briggs & Stratton Nikki carburetor main jets rebuild kit for sale

 

[Jonnie_S]

Thanks for the info shadetree!

I've seen lots of generic or no-brand replacement carbs on ebay/amazon/repair-parts sites in the $50-$75 range. Are any of these likely to be better than the Nikki, in the case this carb makes a habit of failing from microscopic-sized particles?

 

[shadetree#1]

You asked:
. Are any of these likely to be better than the Nikki, in the case this carb makes a habit of failing from microscopic-sized particles?

No.

If you can remove/install a carb and take one apart and put it back together you should take a shot at yours first. Get a can of carb spray cleaner with the little plastic straw nozzle and a kit that has the bowl gasket and the emulsion tube gasket and if the jet is loose in the emulsion tube (not snug replace that jet o-ring) If the bowl is fairly clean inside the carb most likely just needs spray cleaning and gaskets.
I've only had about a 50/50 when doing a clone Nikki replacement.
I have to be careful and not break the jet when removing the Idle plastic limiter (if it has one) I use a Dremel tool and cut the plastic then slot the shaft for adjustment. If you try to pull the plastic off the jets shaft will usually break. I make a note of the number of turns CW the idle jet is set then remove and spray through the jet hole. The idle jet sometimes only has a good response when going in at about 1 /1/2 turns and no change as it comes out further CCW.
Sometimes has to be re-tweaked when the engine is cold.
Install a Briggs $5 red fuel filter just before the carb. Do not try to use a Red $1 clone filter.
Some of them replacement Clone Nikki's do not even have the correct size jet/jets and they will flood or hunt and surge.

 

[StarTech]

Actually those after market can be jetted wrong and have other defects. I had a customer that decided to buy a mower from another shop as he insisted on a Craftsman mower. When he got it home the first he did was to call me out. He was told it had a Kohler engine which of course it didn't. It was lean burn surging so I he him to have the other shop repair it. Well all they did was to swap out a perfectly good carburetor that just needed cleaning with an after market that was surging even worst. When I got my hands on it I found the none replaceable idle jet was the wrong size and resized it.

But I only had one Nikki twin barrel to a grain of sand in one of idle jets. And it was because the customer flipped his fuel filter and filled carburetor with trash. About drove nuts as I couldn't see the grain of sand as it was clear. Only when I probed with my micro drill bits that I found it.

Normally you don't jets getting solid material clogs; unless, someone is not using the correct fuel filter, not using a filter at all, or had did as above.

 

[Jonnie_S]

Sounds like I'll stick with what I've got since at least I know its jetted properly for the engine.
And I'll get a high quality fuel filter.

Lots of great info -- thanks to all.

 

[PTmowerMech]

Actually those after market can be jetted wrong and have other defects. I had a customer that decided to buy a mower from another shop as he insisted on a Craftsman mower. When he got it home the first he did was to call me out. He was told it had a Kohler engine which of course it didn't. It was lean burn surging so I he him to have the other shop repair it. Well all they did was to swap out a perfectly good carburetor that just needed cleaning with an after market that was surging even worst. When I got my hands on it I found the none replaceable idle jet was the wrong size and resized it.

But I only had one Nikki twin barrel to a grain of sand in one of idle jets. And it was because the customer flipped his fuel filter and filled carburetor with trash. About drove nuts as I couldn't see the grain of sand as it was clear. Only when I probed with my micro drill bits that I found it.

Normally you don't jets getting solid material clogs; unless, someone is not using the correct fuel filter, not using a filter at all, or had did as above.

Those cheap aftermarket carbs fits several different HP's. But the jet's only actually fit one. I'll only buy an aftermarket carb for equipment that's on it's last leg. Customer just trying to finish out the season, or finish up his mowing til he can find a newer model.
I've got a box of aftermarket carbs that have anything from 5 minutes on them to 5 months.

This craftsman blower I inherited from a customer a few months ago, the high speed jet had to be completely closed off and everything ran from the low side. And it still strokes a little at WOT. I bet it doesn't have 2 hours on it. So I got an aftermarket carb for it, and had to set it the same exact way. I started the low side at 3/4 turns and couldn't get it to idle (either of them) until I was about 2.5 rounds out.
I'm thinking craftsman is using aftermarket carbs. LMAO