Stumbles at full throttle but only with light load.

[spinningwood]

John Deere D110 with 19hp 540cc single cylinder B&G engine. MIA12859 Model Year 2015 Engine Family FBSXS.5402VL

240 hours on the run clock.
Head gasket replaced and valves adjusted at 100 hours.
Starts with no problem.
Idles perfectly.
Runs full throttle with the blades engaged with no problems.

Run it at full throttle without blades engaged, just driving around and the engine starts to stumble badly. No backfires. No smoke. Just periodic pulsing loss of power.

I've changed air filter, plug, fuel filter and oil/filter. Oil level is mid fill. No obstructions (or oil) in the vacuum line from the oil fill tube to the fuel pump.

Fuel pump is working (didn't measure volume pumped, but it was a good flow).

Carburetor solenoid tests good.

The little spring on the governor linkage is intact.

Fuel in tank is fresh (I've run 3 tankfuls of fresh fuel with no change in the situation).

I've now reached the point where I have to ask for help because I don't know what else I can try next to solve the problem. Any and all suggestions welcome. I really want to solve this myself rather than having to take it somewhere for service.

Ed

 

[Fish]

Recheck the valve adjustment.

Just to be sure,

they may be adjusted a little tight.

 

[spinningwood]

Thanks Fish - I'll give the valves a check. Looks like a rainy weekend here so that will give me something to do.

Some additional info. When I first start from a cold engine there is no problem. After it's been running a couple of minutes the stumbling starts at full throttle if there is no load on the engine. I've tried running with the gas tank cap off to see if it might be a venting problem, but that didn't make any difference.

Ed

 

[Fish]

Thanks Fish - I'll give the valves a check. Looks like a rainy weekend here so that will give me something to do.

Some additional info. When I first start from a cold engine there is no problem. After it's been running a couple of minutes the stumbling starts at full throttle if there is no load on the engine. I've tried running with the gas tank cap off to see if it might be a venting problem, but that didn't make any difference.

Ed

Yeah, that would be the case with the valves being too tight.

 

[sgkent]

at or near full load mixture issues will appear. That doesn't mean it is the mixture. First thing I do is think about fuel pump diaphragms, air filters fuel filters etc.. I also would check the valves again because the expand when hotter. Last - if the engine has a catalytic converter a partially plugged one will show up at full load. Good luck.

 

[spinningwood]

Yeah, that would be the case with the valves being too tight.

Thanks for all the replies. As Fish first suggested, it appears it was the valve clearance. The specs I found for a B&G Model 33 R87 were

Intake 0.003" - 0.005" and exhaust 0.005" - 0.007"

I found the intake at 0.004" and opened it to 0.005". I found the exhaust at 0.006" and opened it to 0.007".

On my first test drive the problem appears to have been resolved. Woohoo!!!! I'll give it a real run later today and hope that it is all good.

I suppose I should just declare victory and move on, but I find myself wondering why this issue cropped up. I set the valve clearance at 100 hours about 18 months ago when I had to change a blown head gasket. It ran great for the next 120 hours after that. It started the stumbling at around 220 hours a month ago. Loosening the clearance seems to have solved the problem, but is the fact that they had to be loosened an indication of another problem lurking in the engine somewhere? Or am I just being paranoid and I should find something better to do than worry about problems that might not exist?

All thoughts welcome!

Ed

And again, thanks for helping me solve my problem.

 

[Fish]

I wouldn't worry too much, if it is running fine now.

Just keep the cooling fins clean, as well as the oil.

I always like to try the cheapest things first.....

 

[Fish]

Run it at full throttle too.

 

[spinningwood]

I had more time this afternoon and gave it a workout. The stumbling problem is still there. I had really hoped the valve adjustment had fixed it, but it seems not.

To review:

Engine starts right away.

Engine idles smoothly (whether it is standing still or driving sloooowly). It idles smoothly hot or cold.

Engine runs smooth up to half throttle (whether standing still or driving). It runs smooth up to half throttle hot or cold.

Above half throttle, within a couple of minutes of engine start, the engine starts to stumble. This happens whether it is sitting parked or whether I am driving around. I can stop the stumbling by engaging the mower blades. With the load from the blades engaged it runs as good as new.

Without the load from the engaged blades, I seem to be able to sometimes reduce the frequency of stumbling by slightly nudging the throttle past full into a slight choke position. But this doesn't always work, possibly because I can't reliably hit the throttle position that reduces the stumbling (or maybe it has nothing to do with it).

I spent 10 minutes driving up and down my driveway which is on an incline (5% grade). The stumbling would stop on the downhill part and resume on the uphill and level part.

There is no catalytic converter on the engine. I checked the fuel flow from the fuel pump and it seemed to be adequate, but maybe my visual identification of adequate is not what the engine considers adequate. Should I consider replacing the fuel pump or look elsewhere first?

I'm also going to check the valve clearance yet again. Just want to be sure that nothing changed since I set it this morning.

Ed

 

[Fish]

Now it starts to sound like a layer of dirty water in the bottom of your carb bowl.

Loosen the nut or solenoid on the bottom of the carb, with a small can, try to catch some of the fuel to look at.

Let it dribble a bit to get all of the water out.