B&S HP470 Lawn mower runs Rich/Lumpy

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  #1  
Old 09-05-07, 07:15 AM
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B&S HP470 Lawn mower runs Rich/Lumpy

Hi,
My Briggs & Stratton 4hp mower is barely 2 years old and has just started giving me grief. It starts as well as it ever has but it has recently started hunting rather than running with enough revs to cut grass. It seems to run better immediately after starting and then deteriorates.

This mower, by design, has no independent throttle control or any obvious mixture or tickover adjustment screws on the carb. Oil was changed earlier in the year and it has been used lots since then.

I have checked the spark plug which was very sooty presumably meaning it is running too rich. I have cleaned the air filter which now looks like new. I have checked inside the carb and it looks clean as a whistle and I can see fuel squirt into the carb bowl when I use the primer bulb. The plug gets sooty quite quickly.

I can rev the engine by tweaking on the levers behind the carb but it runs with a pulse like someone revving a motorcycle. It just wants to return to a tickover which is slow and lumpy.

The exhaust gas was looking a bit black earlier but is not something you immediately notice. The oil level is correct and is in good condition.
I do tip the mower to wash underneath but this is something I have always done since new so I don't think it is that. I have left the mower 'ticking over' for 10 minutes or so but it continues to run slow and rough and would stall if I try to cut grass with it.

Appreciate your thoughts?

David
 
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  #2  
Old 09-05-07, 08:05 AM
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My thoughts are it is running rich. I would first change the spark plug and air filter and go from there. Looking being clean and being clean aren't the same.
 
  #3  
Old 09-05-07, 06:22 PM
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Based on your descriptions I'd say you have a diaphragm type carburetor and that the flapper integral to such diaphragm is in poor condition and in need of replacing. Briggs part number 495770 is the diaphragm & gasket and should get you back up and running well. If this is not a diaphragm type carb then post back with the model, type and code numbers off the engine shroud.
 
  #4  
Old 09-06-07, 01:14 AM
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Ok, much more inclined to believe this is carburetor related. (I know the air filter is fine and the symptoms are much the same if I run momentarily without the air filter.)

The model is 24-3781-00. The Type is ES464. The serial is 6676775 and it also has the year (2005) stamped on it.

If you are able to confirm this is a diaphragm model and that part number you mentioned, it would be greatly appreciated. Is this a relatively straightforward replacement task without special tools etc?

Thanks,

David
 
  #5  
Old 09-06-07, 02:25 AM
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The numbers you posted do not look like Briggs engine numbers, they look like mower manufacturers numbers. Engine numbers will be on the engine. What country are you from?
 
  #6  
Old 09-06-07, 05:38 AM
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Sorry about that, you are right. I couldn't see anything on the engine itself and didn't realize the base would have a serial etc. I have had another look and found the following on the engine case above the spark plug:

Model: 106902 (the 6 is not very well stamped, so it could be an 8 or 0)
Type: 0141 B1
Ser: 5012552

I'm from the UK. Hopefully those numbers will mean something to you. Thanks for your time, its appreciated.

David
 
  #7  
Old 09-06-07, 09:55 PM
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That's a diaphragm type carburetor. A new diaphragm should fix you up.
 
  #8  
Old 09-07-07, 02:22 AM
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My query search indicates that if the "6" is a number (0 or 8) as you believe, it would have to be a "0" and would then be an old school diaphragm type carburetor. If, on the other hand, this is a letter, as I believe it will be (but not an "O" it should then be a new school diaphragm type carb. Use some Scotchbrite pad, or similar, to clean up the model number and have a closer look for us if you would please and post back. Or, provide us with a link to a pic of the engine and carburetor setup.
 
  #9  
Old 09-10-07, 09:01 AM
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I have exactly the same symptoms with my Mountfield, any updates?
 
  #10  
Old 09-10-07, 12:40 PM
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Bytor, what is the make and ID numbers off the engine on your machine? If you would please, post this information in a new thread so that we're not jumping from one persons problem to another. It may get confusing otherwise.
 
  #11  
Old 09-10-07, 01:55 PM
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I'll have a look tomorrow and post it up on a new thread, sorry didn't mean to hijack this thread it's just exactly the same symptoms
 
  #12  
Old 09-12-07, 08:23 AM
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Just to follow up on my original problem, I fitted a new diapragm and gasket today, as recommended, and it appears to have solved the problem. Incidentally, the old diaphragm was a bit stretched but no obvious splits.

I was (still am) a bit worried about the way the carb fits onto the inlet manifold. There is one rubber ring and a plastic looking ring that sits directly behind it but the plastic ring doesn't seem to do much at all, just a loose fit on the manifold. Is this a future problem in the making?

Thanks for all the help on this guys, greatly appreciated. I just noticed the request for more info, do you still want that?

David
 
  #13  
Old 09-12-07, 09:28 AM
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Unless the O-ring is very loose you won't have a problem, it's amazing they run when other engines are so finicky with air leaks, just keep the 2 tank bolts tight. Have a good one. Geo
 
  #14  
Old 09-12-07, 05:01 PM
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Plastic ring snaps into carb to retain o-ring. It should not just be loose on the manifold.
 
  #15  
Old 10-31-08, 08:53 AM
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I have exactly the same symptoms with a 2 year old Mountfield.

It's pretty poor if there's a diaphragm problem at this age.
 
  #16  
Old 10-31-08, 05:14 PM
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Briggs Carb
You need to install a new diaphram kit, it is Briggs part # 795083 or old # 495770 also Lowe's has a kit #5083 which has the same parts.

Here we go;
Remove air cleaner, there are 2 bolts that hold things together, one on the front of the tank 3/8" and one into the block 1/2", remove these bolts, now "slowly" slide the carb/tank off the intake tube and breather tube, now tilt the tank in to release the throttle linkage and waalaa the carb/tank is removed. Remove the 5 screws from the carb/tank remove carb(don't loose the spring) now spray all holes, cracks and crevases in both the carb and tank surface with brake parts cleaner, remove the main screen(looks like a thimble) with a small screwdriver pry out the main jet(carefully) and clean it, reinstall the jet, it can be difficult some times to get it to snap back in place(I use the rounded end of a screwdriver handle). Remove and clean the fuel pickup stem. Clean any junk/rust from the tank. Install the diaphram on the tank then the gasket(no goo or sealer) now carefully replace the carb(the spring will try to misalign the diaphram), tighten all screws a little at a time so as not to crimp the diaphram until they are all tight. Install the carb/tank in reverse order and you are done. If I missed something one of the real mechanics will correct me. Have a good one. Geo
 
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