Briggs and Stratton engine running slow and "lumpy"

[steverob]

I have a Mountfield mower (UK brand) with a small B&S Quattro motor - about 12 years old. It starts OK and then runs with a stutter, or "lumpy" as we say over here. I have:
a) Replaced the plug
b) Cleaned the air filter and re-oiled
c) Cleaned the carb and replaced the diaphragm and gasket (dang - I thought this would be the issue but apparently not)
d) Emptied the tank and replaced with fresh fuel
e) Topped up the oil

So what's left? Maybe the valves are sticky? If so, is there any way to fix them? If the remedy is to take the head off, any suggestions about where to find instructions for that?

 

[Fish]

Have you tried a new plug? Try that and put up your engine's model numbers. If it is just running too slow, you might just need to readjust the governor spring tension, as where it mounts it toward the front on a lot of those models, and can get bent back when mowing around/under bushes or shrubs. If I knew your model number, that would confirm that and I could easily tell you how to fix it.

 

[pugaltitude]

I have a Mountfield mower (UK brand) with a small B&S Quattro motor - about 12 years old. It starts OK and then runs with a stutter, or "lumpy" as we say over here. I have:
a) Replaced the plug
b) Cleaned the air filter and re-oiled
c) Cleaned the carb and replaced the diaphragm and gasket (dang - I thought this would be the issue but apparently not)
d) Emptied the tank and replaced with fresh fuel
e) Topped up the oil

So what's left? Maybe the valves are sticky? If so, is there any way to fix them? If the remedy is to take the head off, any suggestions about where to find instructions for that?

9 out of 10 its the diaphragm that causes your problem.
tighten the screws on the carb diaganolly and not in a circle.
also dont tighten too hard.
Is the diaphragm and gasket the right way.
Diaphragm on tank and gasket next to carb.
The little o ring on the carb can cause it also.
Is the manifold cracked that the carb mounts on?
You need 2 governor springs.
If the small spring is missing then it can cause your problem.

 

[steverob]

Have you tried a new plug? Try that and put up your engine's model numbers. If it is just running too slow, you might just need to readjust the governor spring tension, as where it mounts it toward the front on a lot of those models, and can get bent back when mowing around/under bushes or shrubs. If I knew your model number, that would confirm that and I could easily tell you how to fix it.

Yes - I replaced the plug. I think I'll try the spring tensioner next. This has a fixed speed governor with a double spring arrangement but I'll remove a coil of the main spring and stretch it a bit to increase the tension slightly. FWIW the markings on the engine are: 10D902, 01414B1; 02103152. Thanks for the help

 

[steverob]

9 out of 10 its the diaphragm that causes your problem.

- That's what I thought but...

tighten the screws on the carb diaganolly and not in a circle.

- Yes - done that

also dont tighten too hard.

- Tight but not jammed hard

Is the diaphragm and gasket the right way.

- Yes!

Diaphragm on tank and gasket next to carb.

- Yes.

The little o ring on the carb can cause it also.

- O ring and retainer in good shape - cleaned and re-inserted

Is the manifold cracked that the carb mounts on?

- Nope - all good

You need 2 governor springs.
If the small spring is missing then it can cause your problem.

Yes- have two springs.

I made a kink in the main spring straight bit to shorten it, so now the motor runs faster. So it's better but not fixed - still stuttering but more usable now.
Thank you for all the advice. Still wondering if it's a sticky valve...