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2019/09/14 15:51:07

fishlaw

18.5HP Briggs and Stratton Intek surging

I have an 18.5HP lawn tractor motor (Model # 31P777) that is surging.... It's been doing it for some time, but getting worse. I recently replaced the head gasket due to oil in the fuel pump-the gasket had clearly failed. Valves adjusted fine, and basically runs as it did before. New plug,new air filters and fuel pump. No imporvement in running, but not blowing smoke and fouling plug. I pulled the carb and cleaned it, , governor spring is in place... If I choke it, it runs fine for a minute, then bogs down. Once choke is off, it runs fine for a few seconds and then surges again.
I am thinking I have a fuel problem-should I just replace the carb? The carb was loaded with crap and possible there is a clogged passage I can't see. (Part is 698445)

12 comments Leave a comment

Eye in the sky

My engine did that wouldn't even run when engaged the blades. Saw a youtube video was one little port poked it with a paper clip has ran fine for last 5 years.

2019/09/14 18:15:25

AVB

First have you done the static governor adjustment. Linkage do wear so slack develops. Also the minimum idle speed might to adjust slight higher.

Although your might running at near full speed if it is off load it is still running off the idle circuits of the carburetor. It when you put the full load of the deck on the engine is when start to the main fuel circuit in the carburetor.

Also be aware that 698445 has been superseded to 594593. But replacing the carburetor have it cleaned via Ultrasonics as trash in the carburetor can intermittently block metering jets.

There a couple things we need to verify that you are looking for the carburetor; the TYPE number plus the date code of the engine. There several different carburetors use on the 31P777 series depending on type and date code.

2019/09/14 18:37:46

fishlaw

I did notice that it seems to run fine if I put my finger on the governor arm. Numbers on motor are 31P777. 0348E1. 040629ZE. The carb part number came out of the manual that came with the tractor.

2019/09/15 09:04:44

Roy

That indicates a goovernor problem, it is too sensitive. As AVB suggested your minimum idle speed may be too low.

2019/09/15 11:38:06

fishlaw

Thanks guys!
I did the governor linkage adjustment (took 15 minutes to find a 3/16 socket) and seems to run fine at high rpm, whether fully loaded or not and then adjusted the low idle and idle mixture and all seems good. My tenant cuts my lawn-will have to hear his assessment, but all seems good.

2019/09/15 17:18:01

fishlaw

So, here is the report from my tenant:

"Tractor started out great, seemed to be running as good as the day it was new. About a ½ hour in it died. Would only run with full choke and without the blades engaged. I did lift the cowl and noticed the fuel filter was empty."

It does have a new Briggs and Stratton fuel pump-but the filter, which has about 10 hours on it, never seems to fill-always has air in it.

Any thoughts? Definitely sounds like a fuel problem, but this is actually new.

2019/09/18 16:02:16

Roy

Check the line at the fuel tank fitting, it may be loose or the line has deteriorated.

2019/09/18 18:30:24

fishlaw

The fuel lines were replaced last year after we were getting gas smell from the tractor-turned out to be a small bit of damage on the tank fill that I was able to repair. Everything except tank was replaced last year, to and including fuel pump.
Fuel filter was full of air.
I checked fuel flow-gas runs easily from tank through filter when I disconnected it and held it below tank level.
Fuel pump definitely pushed fuel when cranking the engine.
Engine stalled when I engaged the deck the first time, but would drive at full speed.
Then I re-engaged the deck and it ran fine at high speed or low speed with deck engaged and cutting.
So now an intermittent fuel problem? Something blocking main nozzle? Air leak in fuel system?

2019/09/20 09:14:49

AVB

Check the anti after fire solenoid especially for a loose tip. I had several this year for some that tips have come off blocking fuel flow. Alternately you temporary install a screw of the appropriate size. I think the Nikki carburetor is using a M8-100 threads. But double check before just screwing a screw but I do know it is metric.

2019/09/20 09:41:43

fishlaw

I actually installed a new fuel solenoid when I had the carb apart last week, so I kind of doubt that is the problem, but will check it. (Had it left over from last year's search for fuel leak) Not sure why if fuel was getting shut off, it would still run when choked.

2019/09/20 09:53:31

AVB

If that is the case check that actually is retracting. I had two lately that were brand new from Briggs and one from aftermarket will not retract unless I bump the plunger.

2019/09/20 11:09:55

fishlaw

Well, it is definitely retracting, since the unit seems to run fine, to a point, but then dies! Since it will run choked, I suspect a carb problem.
There was a lot of rust in the bowl when I took it apart, so thinking i will clean it one more time and this time maybe even replace gaskets... I ordered a new bowl, which apparently comes with a bolt to replace the solenoid.

2019/09/20 11:13:47

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18.5HP Briggs and Stratton Intek surging

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