Engine surging

bobjfly

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B&S 28M707 starting running erratic and surging all of a sudden then cut off. Found fuel in the oil, so changed the oil and I changed the carburetor to a new one and all new gaskets from one end of carburetor to where everything mounts to engine, including breather gasket. Engine surging and also, engine will now crank without choking, which is unusual, and when you move throttle to full choke, engine does not change at all, where it used to try to stall out as with most any small engine when choked. I checked that the butterfly is actually closing to the choked position and it is. Also put new governor link spring on. Sprayed carb cleaner all around carburetor and manifold but never saw any different smoke color or engine change sound at all. Anyone have any ideas?
 

bertsmobile1

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Bob

Lets start with some common language so eery one knows what is happening
Crank = rotate and nothing more
Fire = goes bang
Run = goes bang several times in a row .

Am I correct in assuming that your engine starts & runs without any choke ?
What carb did you fit & where did you get it from ?
There are a lot of carbs on the web that were taken out of the scrap metal bin then sold as "fits a" or "oem" .

Surging is nearly always becuse the carb can not supply sufficient fuel to maintain a constant speed.
That comes from 2 sources
1) Blockage some where from the fuel tank to the inlet valve ( includes vacuum lock in tank )
2) Air entering after the fuel has been added leaning out the mix .

Fuel in the oil comes from a faulty float valve usually when the engine was standing doing nothing during the week.
 

Gumby83

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If it’s got a fuel line shut off, one thing to check is the flow rate in different positions. I used to use a generator for a portable welding set up that would do what you describe (although if I remember correctly, choking it would actually smooth out the surging). I ended up finding that the fuel flow was at its highest when the shut off valve was halfway between on and off, rather than fully on. I don’t know if the cause was due to some debris that got caught in the valve or some other internal failure, but once I started setting the valve to the halfway position, I didn’t have any problems.
 

bobjfly

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Bob

Lets start with some common language so eery one knows what is happening
Crank = rotate and nothing more
Fire = goes bang
Run = goes bang several times in a row .

Am I correct in assuming that your engine starts & runs without any choke ?
What carb did you fit & where did you get it from ?
There are a lot of carbs on the web that were taken out of the scrap metal bin then sold as "fits a" or "oem" .

Surging is nearly always becuse the carb can not supply sufficient fuel to maintain a constant speed.
That comes from 2 sources
1) Blockage some where from the fuel tank to the inlet valve ( includes vacuum lock in tank )
2) Air entering after the fuel has been added leaning out the mix .

Fuel in the oil comes from a faulty float valve usually when the engine was standing doing nothing during the week.
Thank you for your reply.
Correct that engine starts and runs without any choke.
Bought a Harbot carb, ASIN #B0779BKDPY Link to what I bought Amazon.com : Harbot Carburetor : Garden & Outdoor
I was going to put a kit in old carb, but new carb was cheaper, which is why I replaced old one suspecting faulty float in it. I will check fuel flow to carb, but not sure of rate. Line, filter, and shut off valve all new, came with carb.
 

Scrubcadet10

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Do you know if the original carb was a Nikki carb or Walbro? parts list shows 2 carbs used on this engine. it may not matter though.
 

StarTech

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There is a lot than one version of this engine. I pickup one at random and it had three different Walbro Carbs and one Nikki. Some are gravity fed and others are Pump fed.

It does make a difference as to which is needed. BTW the after market purchased is the Nikki version.

1607280657398.png
Here is the ones just for the 28M707-0137-01

1607280790141.png
 

bobjfly

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There is a lot than one version of this engine. I pickup one at random and it had three different Walbro Carbs and one Nikki. Some are gravity fed and others are Pump fed.

It does make a difference as to which is needed. BTW the after market purchased is the Nikki version.

View attachment 55046
Here is the ones just for the 28M707-0137-01

View attachment 55047
My original engine type was 0112-01.
 

bobjfly

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Original carb had Walbro LMT 5-4993 on it, and the one I bought says it is a replacement for that model which is the 498027 part number. The only real difference I saw in it was it did not have the float bowl screw in it.
 

Gumby83

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I will check fuel flow to carb, but not sure of rate.

I don’t know what the exact flow rate should be - I just meant to check if the flow was stronger at a different position than fully on.

What I experienced with the generator was the flow rate at 1/2 open was greater than the flow rate at full open - it was just a visual check. With the petcock valve at full open, fuel was a steady trickle but not enough to keep up with the demand, so the carburetor bowl wasn’t staying full. At 1/2 open, fuel was steady and strong and flowing much faster
 
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