Briggs 16HP Popping and Poor Performance under Load

I have a Briggs 303447-0376-01 that I recently rebuilt. Everything was done properly except for removal and cleaning of the main jet under the carb drain plug. The previous owner completely stripped this out and I was not able to remove.

The engine was custom mounted onto a mud-motor (it drives a 3/4" drive shaft and a 10" propeller) for a aluminum boat. Out of the water it runs perfect, but when the prop is submerged it will not provide any torque or get to high rpm's.

I ran the boat for a few hours at idle speed to make our hunting trip. On the way back the last day the motor started popping, similar to a popcorn machine. It would pop for 5 or 10 minutes and then run fine for 5 minutes and repeat. It also started to produce some torque and hit mid RPM's with the throttle set at 3/4 speed. Once we got it on the trailer it will run from low to high RPM's perfectly but still pops a bit.

Is this due to me not cleaning out the main jet? I have the air/fuel screw 1 & 1/2 turns out from a soft seat and have tried multiple turns each way to no avail...

I have also ordered a smaller propeller but the one I have is rated for 10-18 HP engines. I think the issue is in the motor/carb somewhere.

Any help would be greatly appreciated....

 

Probably in the carburetor.

It depends on the main jet being not only cleaned out, but serviceable. When you say stripped out, is the jet still intact or has it been overtorqued and fitting poorly?

Unless it's in good shape and the mounting threads for it are in good shape, it would be a good idea to think about changing the carb if that's a motor you use a lot.

 

Does applying partial choke help? It does sound like a carb problem. This carb isn't cheap. You might try adding a good bit of seafoam to the gas, crank it up and let it run long enough to get the seafoam gas mix into the carb, then shut it down and let it sit for a day, start it back up, rev it and choke it hard and do the same thing a few times until it clears up or becomes evident that it's not going to work. In that case, you may still be able to clean the jet without removing it.

When i get a carb like yours that should be fine other than the fact that I can't remove the jet, I mix a diluted bucket of pool acid from the pool supply store and water. I open the carb up and soak the piece with the jets in the solution for a few SECONDS. Be careful, as this will boil, fumes will come up and hurt your eyes, face, and lungs if you breathe it. Drop it in water and swish it around to get the acid out and see how much cleaning the acid has done. If needed, do it again, but be aware that more than a few seconds at a time can remove too much metal from the aluminum carb and ruin it. I use this as a last resort in cases where the replacement cost is high.

 

Pool acid ? Like muriatic acid

 

Yes, muriatic / hydrochloric acid.

 

As far as going to a smaller prop as a fix, I wouldn't do that just yet. What you have may work fine with the engine running as it should. The popping isn't an over loading of the engine.

 

I replaced the plugs and re-gapped the valves. Still having same problem. I have uploaded a video from this evening, it was dark but you can hear it running. The engine only pops like that when under a load, you can hear it really rev up when I pull half of the prop above the water and then pop like crazy when it gets fully sumberged....


It appears that only one of the pistons is creating the popping noise. One of the plugs was very black and sooty also where the other one wasnt.


What is wrong here... Is it the rings?

Mud Motor Popping - Briggs 303447-0376-01 - YouTube

 

That sounds like a loose valve seat. I think you're going to need to pull the head and peen it.

 

What does pull the head and permanent it mean. Head is currently off...

 

Pull the head means remove it, peen the valve seat means peen it back in place. Take the valves out and check the valve seats. I bet one is loose and will pop out or move, wiggle, spin in place. That one needs to be peened. Use a center punch and with the seat properly in place, punch the aluminum around the valve seat to make small indentations all the way around the seat. Do not hit the seat itself, just the aluminum surrounding it. This swells the metal around the seat (peening it), locking it back in place.

 

What I usually do is when you have the loose seat determined, take it all the way out, clean under it, then stick it back in and put the valve and spring back in it to hold the seat tight. Then peen enough to hold it tight. Then take the valve back out, finish the peen and check the seat.

Some have used JB weld on them, but I only tried that a couple of times and couldn't get it to hold. The peen always worked the best.

Since you have the head off, also, check the valves to make sure they're both opening/closing fully.

 

The seats are not loose. I took all of the valves out and nothing is loose.

I think the problem is coming from the engine only running off of one cylinder. This is generating the popping and poor performance producing any torque. I checked the spark at both of the cylinders yesterday and they seem to be good....

 

Yes, it is skipping on one cylinder and firing out the intake or exhaust. That's the popping sound. Typical of a valve problem. Maybe one of your valves is burned or sticking. It sounds very much like a valve problem in your video.

 

I remember back years ago the small block Chevs would wear the lobes from the cam & they would Pop under load. Is it possible the cam is worn & not opening a valve enough?? It sounds like you've got everything covered.. Just a shot in the dark,,, Roger

 

Check the insulator on the spark plug, make sure it isn't broken so that it can slide down and cover the gap at the electrode.