Outdoor Power Equipment and Small Engines - Briggs & Stratton 18.5HP runs rough, dark exhaust.

Mower: Craftsman 18.5 HP OHV, 46”, electric start, automatic tans. Mower Engine: Briggs and Stratton, twin cylinder, OHV, Model number 407777, Type 0121 E1, 1/C Platinum, InTek, purchased Nov, 1999.


This is an off-shoot from a previous thread: Briggs & Stratton 18.5HP wont start. That problem was solved; now another has raised its ugly head:

After solving the carburetor problem, I ran the mower around the yard a short while. The engine appeared to run well, full RPM range, ran it forward and in reverse, etc., then I engaged the blades. That abruptly sent the engine into labored stress, almost killing the engine, spitting out gray smoke, and running rough. Immediately I disengaged the blades, but the engine now continues to run rough, hardly above an idle. Throttling up stalls the engine. I thought there might be a problem with the mowing deck and belt/electric clutch assembly loading up the engine (the clutch maybe stuck “on”). But I am able to turn the blades by hand, and the belt slips around the engine shaft. I also tried disconnecting the clutch wires, but engine still runs the same.

At the end of the previous thread, cheese suggested I check the oil level, smell for gas. Oil level not high (its in fact at the “add oil level,” and there is no smell of gas.

There is an oddity about the throttle linkage. The “wire” used as linkage seems to be homemade, it does not seem as it would be from the factory. There is what I assume to be an idle adjustment set screw, which the metal “flange” attached to the shaft that goes through the two throttle butterflies does not reach by a long shot when the throttle lever on the dash is pushed down to the idle position. Activating the throttle lever moves what I assume to be the governor mechanism, but the actual throttle linkage is activated only partially. While the engine appeared to operate normally, I did not have the opportunity to view how this linkage worked, though there seemed to be normal engine speeds. I can only suggest that putting a load to the engine may have activated the governor and put it into an incompatible state in view of what appears to be a non-standard throttle linkage situation. Would this be possible?

(While troubleshooting the carburetor problem, I tried making several other wire lengths, but the longest wire crafted kept the throttle at the idle position, not moving at all while the governor mechanism move in cadence with the dash throttle movement.)

My update:
31Mar09

Minor correction to my post: moving the throttle up and down does nothing in terms of engine speed; pulling out the choke kills the engine.

One of my friends suggested pulling off the spark plug wires one at a time to find a bad spark plug or wire. If the engine died with either one off, that would be the defective side. I removed both wires one at a time and the engine continued to run sputtering and at an idle, i.e., no change.

I’m wondering if there still is a problem with the carb; but it ran so good before I engaged the blades?

I think you have a problem in the carb too. There were more than one carb setup on these, but many of them have brass jets that are pushed into the plastic nozzles. If these were to pop out (which often happens during carb work), the engine would flood with gas and cause the problems you describe. Also, if the needle suddenly decided to not seat, it would flood the engine in similar fashion. In any event, I believe the engine is flooding with too much gas.

Solved the rough running–idle speed only–gray smoky exhaust problem. It wasn’t the carburetor! I went over that so many times I could disassemble/reassemble it in my sleep and could not see a thing wrong with it. It took a lot of time and thinking about what was happening. The big question was: what did engaging the mower deck blades have to do with how the engine ran, since it ran so good after I overhauled the carburetor? It came down to the governor. Looking at the front of the engine, unloosening the governor crank set clamp and turning the shaft clockwise solved the problem. The engine began running normally! When I initially engaged the blades, for some reason that load on the engine shifted the governor setting. Why? I kept playing with this adjustment until the engine ran really well making sure the clamp is tight. (Turning the governor crank counterclockwise induces the problem that was plaguing me.)

Even now, when I engage the blades, the load still effects engine function! The deck rocks, vibrates, and will kill the engine if left engaged. I release the blades, and the engine recovers and runs well. Apparently there is a lot of drag in the system. Maybe the mandrels need lubrication, etc. Or, maybe I do not have the engine rpms up high enough, or there is another problem. Any thoughts on all this stuff?

Even now, when I engage the blades, the load still effects engine function! The deck rocks, vibrates, and will kill the engine if left engaged. I release the blades, and the engine recovers and runs well. Apparently there is a lot of drag in the system. Maybe the mandrels need lubrication, etc. Or, maybe I do not have the engine rpms up high enough, or there is another problem. Any thoughts on all this stuff?

Just to be sure you are not chasing a ghost I would check the belt routing. Make sure it has not jumped a guide. Look for any wear on the belt.
You will notice some lug on the engine when you engage the blades but it should recover to a nice whiiiiirrrrrrrr.

A governor should never need adjusting during the normal life of an engine. I think you're compensating for other problems. The governor should not be able to flood the engine. It can't flood the engine no matter how it's adjusted. Something else is flooding it. Did you check the brass jets I mentioned pressed into the plastic part that drops off the carb?

Yes I checked for loose and separated parts; all parts are attached. There are two, small brass perforated tubes that jut down into what B&S call a fuel transfer tube (a black plastic part that fits tightly with an o-ring into the solenoid). They are tightly seated into the metal carburetor body; they lead via passageways to another plastic part that spreads laterally via black plastic tubes (in opposite directions) into each of the carburetor throats. All gaskets in place. Everything seems to be in order.

Response to Jbrokeit:
The belt routing seems to be fine, no wear on the belt.

Now that the engine has been started and run several times, I do notice that it is not stable at times, it falters, and takes time to “settle down” after an RPM change. Cheese must be right, there still is some other problem that I masked with the governor adjustment. It also explains why the engine has no power when the blades are engaged.

I am unable to continue much longer this troubleshooting quest so may take it to a reputable repair center. Thanks for all your suggestions.

You don't have the same engine I have ( I have the B&S 42A707.) I'm wondering though if you have a similar carb. My carb has a thick plastic piece that seperates the carb from the manifold. If you have this, check that it is not cracked. Also, check that it is not warped, allowing air to flow in which could draw more fuel from the pump. I know it's a longshot, but all I can do is suggest that. Otherwise, I'm not sure what your problem could be if you did a complete teardown, clean and rebuild of your carb.
Hope this helps!!!

Good suggestion. I know those carbs can be difficult. I have run across at least 3 of those carbs that I could not get to run right with any amount of cleaning.