31 hp Briggs missing

[tacoma200]

My Briggs 31 is running a bit rough under no load conditions. As far as I can tell while mowing it seems to have plenty of power. When I start it up it is missing on one cylinder. I took the right plug wire off and no change at low rpm's, took the left pug wire off it dies. At higher rpm's you can tell you have removed the right plug wire, it runs rougher with the right wire on (pop's,misses, runs rougher). Used a screw driver on the right side and it does spark to ground so I'm getting fire to both sides (not sure about quality). It misses on high and low rpms but like I said after you engage the blades and start mowing it seems to run on both cylinders because it has plenty of power and I can't hear a miss while under load mowing. I replaced the plugs when it started missing and the right one is clean while the left one has a little black dry residue.

So I know I have a dead miss on the right side at least part of the time. I took the valve cover off the right side and valves seem to be working (I'm no Expert). Does not smoke, use oil, or have any engine noise. Compression was 150 psi on both sides. I have looked up engine problems on the web but most engines seem to have problems under load while mine runs better under load, plenty of power. It may be missing under load but you can't tell it, plenty of power.

Could it be a carb problem, does it have two jets or one feeding both sides? Or would the carb problem affect both sides? These engines do have adjustable valves. Most shops are backed up a bit so I'm trying to narrow the problem down till I can get it in. Any help would be appreciated.

[topsites]

No, the carb itself should deliver straight into the intake manifold, from there it goes to whichever side, but it doesn't feed...
Rather, the engine sucks the mixture in.

However, there usually exist two jets on some to most carburetors, a low speed and a high speed.
So it could be the low speed jet, perhaps the crankcase breather tube, among some possibilities.
On some carburetors these jets can be adjusted, on others it can't.

[djagusch]

Since it started when you changed the plugs, flip flop them to see if it goes to the other side. Might be a bad new plug. Also make sure the spark plug cap "snaps" into place.

After that I would try leaning out the carb a bit because the black soot indicates that and the other plug is clean (does it smell like gas?).

[tacoma200]

Since it started when you changed the plugs, flip flop them to see if it goes to the other side. Might be a bad new plug. Also make sure the spark plug cap "snaps" into place.

After that I would try leaning out the carb a bit because the black soot indicates that and the other plug is clean (does it smell like gas?).

I replaced the plugs after it started missing. Then when I rechecked them the right side (side that misses was clean) and the left side had just a small amount of dry black residue. Even though it is getting a spark to the right side I'm not sure if is enough voltage? I just checked it with a screw driver inserted into the plug wire and held it close to ground and it was arching good? So yes I guess well have to start Monday to narrow it down and check the carb. No the right plug was dry and clean, no gas smell. I'm not the best mechanic just though maybe it was something I could figure out on my own.

What does a coil act like when going bad, dealer wants to check that, but it is getting at least some fire to both sides.

[viper00085]

setting the valve lash would be a good idea, have seen a lot of the bigger vanguards have loose valves right from the factory over the last 2 years. .004-.006" lash when cold.

other than that, pull the choke out slowly while running, maybe 1/4-1/2 way out and see if the engine smooths out. if it does that pretty much tells you it a lean condition (not enough fuel flow)causing the miss. Might need carb cleaning, fuel filter, maybe a intake gasket leak somewhere.

from your descripiton of it runs better under a load, thats usually a lean condition in the carb. when those blades are on, then engine internal manifold pressures change from the load and usually cause it smooth out and run bit richer so it masks the issues.

[tacoma200]

setting the valve lash would be a good idea, have seen a lot of the bigger vanguards have loose valves right from the factory over the last 2 years. .004-.006" lash when cold.

other than that, pull the choke out slowly while running, maybe 1/4-1/2 way out and see if the engine smooths out. if it does that pretty much tells you it a lean condition (not enough fuel flow)causing the miss. Might need carb cleaning, fuel filter, maybe a intake gasket leak somewhere.

from your description of it runs better under a load, thats usually a lean condition in the carb. when those blades are on, then engine internal manifold pressures change from the load and usually cause it smooth out and run bit richer so it masks the issues.

Thanks, it is due a valve adjustment. I will have all you suggested checked. On another note the amp meter has been showing very low but the battery has plenty of juice when I had it tested at the auto parts store (13 volts and plenty of cranking amps). I didn't think the problems were related but it doesn't hurt to throw any additional info out there. I'm going to run it today and start again Monday with the help of the dealer.

[tacoma200]

New findings: It will run on the right cylinder after it warms but it is rough. The rpms/governor surge up and down on the right cylinder only. It runs smooth on just the left cylinder.

1. Runs good under load while mowing
2. Pull the right wire off while it idles no change (running on left cylinder only smooth)
3. Pull the left wire off while it idles and it still runs but surges/rough (Running on right cylinder only rough,surges).
4. So both sides getting fire but right side is apparently starved of fuel? at idle it seems. But as Viper pointed out when under load it seems to be getting a richer fuel ratio and smooths out. I hope I'm not missing the boat here.

Glad I tried pulling wires after it warmed, that changed the symtoms a bit.

[tacoma200]

Well got it back from the dealer today, the mechanic was stumped for a while till he found a wire that had been chewed on by a mouse over the Winter jumping to ground. I never thought to check the wiring harness. It seems to be running OK. Bill $177.00 for mouse damage. I'll kill that little rat (mouse) if I can catch him.