OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines Briggs engine running roughly

My Victa 4012TX's engine, a 12.5 horse Briggs is being a pain.
It runs fine at low to medium revs but really rough at high revs.
It sometimes backfires when I shut it down.
I've cleaned the carby with "carby cleaner", replaced the fuel filter (with a bigger one) and fuel lines but no matter how I adjust the mixture I can't get it to run properly at working revs.
It will potter about the yard beautifully but not under load or quickly.
Should I be looking at electrics? Points or condenser perhaps.
Any help will be appreciated.
Best regards.
Rob.

We need to know some details of your engine, Rob. If you post the long string of numbers stamped on the cooling air cowl, we can get most of what we need, but a picture of the carburetor and linkages would also help clarify things.

You probably know that the Flo-Jet carburetors tend to have two mixture adjustments, one for idle and one for high speed. If you have only been adjusting the idle mixture you would be likely to find it made no appreciable difference to the way it runs at high speed or under load. A picture of your carburetor will tell us which type it is, and we can then talk about potential problem areas.

I am not ruling out ignition difficulties, but let's begin with the mixture. Engines with weak ignition usually show it first in hard starting.

Thanks very much for your response grumpy.
The engine details are:
Model...289707
Type....0114-02
Code....960514ZB

I've been using the mixture screw center left in the photo. It originally had a black plastic cover on it.
Thanks again.
Rob.

Your engine was made on 14 May 1996. Being a more modern engine than I expected, it is subject to emissions controls and does not have a Flo-Jet carburetor. Here is the illustrated parts list:
http://bsintek.basco.com/BriggsDocumentDisplay/Z6hrABVJ1DajI.pdf

The carburetor appears to be a Walbro. The mixture screw you have been adjusting, Item 94, is the idle mixture adjustment. The main jet, Item 142, is not adjustable. If your problem is lean mixture, it seems it will be due to an induction leak or dirt or water in the carburetor rather than maladjustment. Because there is no main jet mixture adjustment, it makes sense to check other items before attributing the rough running to a carburetor fault. The first question is what you mean by "rough running". Mixture seldom becomes rich due to a fault except in the case of float bowl flooding, and this usually causes hard starting and idle problems, not problems at high speed only. Lean mixture, on the other hand, can easily occur at high speed only, but it is usually indicated by hunting, or cyclic variation in engine speed, when it is a mild case. In more severe cases there will be pronounced hesitation when you advance the speed control. There may also be misfiring, and in extreme cases, spitting through the carburetor (sometimes misdiagnosed as backfiring, which is an explosion in the muffler).

Your engine has Magnetron electronic ignition - there are no points to require adjustment. It is possible, but fairly unlikely, that the Magnetron is in the process of failing and is causing erratic ignition at high speed. A fouled or faulty spark plug might also cause high speed ignition failure, though they more often cause hard starting.

The next thing we need is a more specific description of the rough running you mentioned.

Thank you for your response.
Your description of the effects of running lean fits this problem exactly.
With the engine running smoothly at low revs I increase the throttle and adjust the mixture screw but it gets to the point where adjusting the screw has no effect and it begins "hunting"...more throttle and it misfires badly and spits back through the carby.
Occasionally when I shut it down it will backfire loudly (which frightens the bejeezes out of the dog).
This engine sat for 4 years or more with fuel in the system.
I have attempted to clean the carby using Nulon Throttle Body & Carby Cleaner.
There was a build up of gum on the needle and seat, the float and most other visible parts of the carby which I have cleaned off.
I have been unable to remove the main jet, it appears to be stuck in place so I have sprayed the cleaning product through it (several times) and blown it out with compressed air.
The problem persists.
Is there a way of removing the main jet without damaging it beyond repair. I'll need to get it out if I have to replace it.
Best regards.
Rob Patterson.

Thanks Rob, we are further ahead - we have confirmed that the problem is lean mixture, probably due to gum in the Walbro carburetor. (We also need to consider the possibility that the gasket between the carburetor and inlet port is damaged, creating a vacuum leak.) If you look at the illustrated parts list, you will see that Item No. 127 is a welch plug that is driven in below the main jet, and holds it in place. To remove the main jet it is necessary to prise out the welch plug, which will be trashed by that process and you would have to get another one to keep your main jet from dropping down out of position. Before we attack the carburetor though, I have to ask whether your carburetor has an anti-afterfire solenoid screwed to the bottom of the float bowl? I can't tell from your photo, the view doesn't go down that far. The version of the carburetor without the solenoid just has a metal screw at the bottom center of the float bowl, holding it on. The version with the solenoid has a black cylindrical object there instead of the metal screw. A wire attaches to the black object. If you have a solenoid, that may well be the source of the problem since they are prone to giving trouble.

Here is the version without the solenoid:


Here is the one with the solenoid:


Those pics are cut from a set of internet instructions for cleaning an early version of your carburetor, before the welch plug was added as an anti-tampering measure:
http://outdoorpowerinfo.com/repairs/briggs_walbro_lmt_carb.asp

Check whether you actually have a welch plug. If you do not, just follow those instructions for cleaning. If you do, we can talk about a partial cleaning that should do the job without requiring a new welch plug.