11HP B&S surges on run, smooth on choke, tried everything - I think?.

[svlawnmower]

My problem:
My engine surges on run, dies on 1/2 choke, runs smooth on full choke and I cannot figure out how to adjust the fuel mixture or whether I missed something. What appears to be a mixture screw does not stay locked in position (see brass screw in pic) and adjusting it didn't seem to make a difference anyhow.

What I have done:
I have a B&S 11HP 256412-1153-E1 engine on a Homelite Generator. I have new fuel lines, the fuel filter is clear, fuel comes out the carburetor feed, the fuel pump internally is in good shape, I cleaned up the piston and valves of carbon build up, outright replaced the carburetor with a new one (thinking maybe it just needed more than cleaning), the spare plug is new, the coil is new and the gap is correct.

I'm not sure what else to try. Now the fuel is new but I added a little Seafoam because of the carbon internally to help clean up the traces. I could use some suggestions or thoughts. If you look at the pic, I'm not sure if I'm to adjust the brass screw for mixture or how to do it without it vibrating closed --- and as I previously mentioned it didn't seem to make a difference when I tried to adjust it.

 

[StarTech]

That is a fix idle mixture screw meant to be fully screwed in. It can not be adjusted except with micro drill bits. And using the micro the adjustment only get richer and can never be leaned so you got to know what you are doing or it another carburetor.

Research shows that you have Chinese clone carbs which is the wrong carburetor for you engine.

Here is what the Briggs 695501 looks like on their website and it has an adjustable idle mixture.

and here what the Chinese clone looks like.

 

[Rivets]

The picture of the carb you posted, is That your actual carb or one that you think looks like your carb? The reason I ask is the brass screw you’ve circled, in my opinion, is not an idle adjusting screw, but an idle jet. Remove the screw and tell us if it comes to a point or has a hole in the side and bottom. When I look up your engine, it calls for an adjustable carb, Briggs part number 695501. Your symptoms tell me that you are not getting enough fuel in the float bowl, more that likely caused by a float needle seat which has swelled, shutting off the fuel too soon.

 

[svlawnmower]

Startech & Rivets,

Thanks for the replies and I did order the correct part number and that is the picture of the one that I actually received. I guess I really wouldn't have known that it was Chinese. But when I received it I did think that was unusual and apparently I got burned on that since I didn't look at the pictures thoroughly.

The screw does come to a point but it's not like the normal mixture screw that you would see. It doesn't have any hole in it either like you would normally see.

I'm not sure either of your thoughts but I wonder if I could just use the gaskets and parts on this Chinese carburetor and just rebuild? With the old carburetor I was getting the same symptoms as with the new. I have cleaned and rebuilt many B&S carburetors and I never ever had to replace a carburetor. The old carburetor just wouldn't allow me any more than a quarter turn adjustment on mixture.

Any thoughts on that or should I just purchase a legit replacement?

 

[slomo]

My engine surges on run, dies on 1/2 choke, runs smooth on full choke

You have a fuel delivery issue. Most likely the carb.

Clean fuel tank
Flush clean fuel into a glass jar towards carb. Verify good fuel flow AT carb.
Old fuel lines could collapse or disintegrate internally passing trash to the carb.

Cleaning the cooling fins and top of engine block are a yearly maintenance item for ALL MOWER ENGINES. Neglect this and you are looking at engine damage $$$.

Great carb cleaning tip from Bertsmobile1

"Before I got the ultrasonic cleaner we used to boil carbs using a big pot & a dishwasher tablet
The ones with the ball in the middle seem to work better
A nice rolling boil works best
Give it an hour or so then boil some more water and take it out of the boiling soapy water then strait into the boiling rinse water
Give it a 5 minute rinse hen take out & blow dry.
Do not allow the carb to sit in the wash water unless it is boiling or it will oxadise.
Some like to do a pre-rinse in boiling water with some vinegar in it to neutralise the alkaline soap first."

slomo

 

[StarTech]

Heck I use 12.5ph non residual soap here in my USC. A couple tablespoons to 9-10 liters. And I have been doing this since 2012. The only time I prewash a unit is get the exterior dirt to a minimum. And I only a rinse afterward if I going to store the carburetor. I also been know to use Dawn Ultra dish soap. A few of the heavy corroded carburetors does get a sulfuric acid bath (all plastic must be removed) with a rinse cycle in the USC but most of us just use white vinegar to remove the corrosion.

 

[svlawnmower]

Thanks for the additional responses. I don't argue that it still might be the carburetor but when you put a brand new one on it and it does the same thing I'm not sure that it's the carburetor. However I was taking apart fuel pump and I was looking at a diagram and there are parts missing. Two tiny Springs that are not there. So I'm curious whether that's enough to keep the fuel pump from working properly? Anyhow, I ordered the kit.

 

[StarTech]

Well I just repair earlier this year a mower where another shop replaced a carburetor was surging with a cheap Chinese carburetor. I they did this as I am the one that sent the mower back in the first place to have the carburetor cleaned.They instead just stuck on a cheap Chinese which worst than the original carburetor and told the customer that this is the way these engines run. Well It ended in my shop where I had to resize the idle mixture jet.

 

[bertsmobile1]

Thanks for the additional responses. I don't argue that it still might be the carburetor but when you put a brand new one on it and it does the same thing I'm not sure that it's the carburetor. However I was taking apart fuel pump and I was looking at a diagram and there are parts missing. Two tiny Springs that are not there. So I'm curious whether that's enough to keep the fuel pump from working properly? Anyhow, I ordered the kit.

Never assume anything is in good working order strait out of the box, particularly after market parts.
I don't know about the other techs here, but I keep around a 1/2 dozen different good carbs on hand for the purpose of diagnosis .
When you read a lot of service manuals they often use the phrase "replace with a known good part" . The important bit is KNOWN GOOD .
Now I try to only use branded after market parts from reliable wholesalers that warrant the parts they sell .
But this does not mean that every part I get from them is good , just that it has been through some sort of QC at the factory so should be good & if not it will be replaced .

Lots of what is on line are the parts that failed the QC at the factory which may or may not have been repaired before being sold for less than the scrap metal price then on sold to you.
I have had a customer who bought 3 carbs from 3 different vendors on ebay & got a carb with the same fault every time, which was a venturi tube with no holes in it at all.

There was another batch of Nikki copies that were all missing the welch plug which goes in the hole that the idle volume screw used to be prior to having adjustable idle mixtures banned.
That was excellent for me as I buy rebuild kits that come with 2 plugs and rarely need to pull the old one so end up with a lot to spare .
Probably saw at least 10 of these over a couple of years .

If you hang around here a bit you will see most of us advising owners to fix their old carb, even when the rebuild kit costs more than a complete new knock off.
The sole exception are cube carbs where they often do suffer from unrepairable problems but for all mower carbs it is very rare to find a body that is beyond repair unless it is 50 years old and the holes for the butterfly shafts are flogged out really oval & leaking badly .

 

[StarTech]

Very true.

But have you installed bushing to repair where the shaft have the carburetor hole ovaled.? Or seen a choke shaft worn completely in two. I have repaired a few Nikki twin barrel where I had to install bushings because that was all was wrong other than the worn shaft. When the Carburetor is $300 and the bushings are very low cost that is the way to go.