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I have a 12S907 single cylinder on a pressure washer. It starts fine and runs well at Idle and under a load until the engine heats up. Runs fine for the first 5 minutes the starts to surge under a load. Tried a new carb, no help, cleaned the old carb and same problem. Is it possible that the head gasket could start sucking air after the head heats up? I'm thinking of pulling the head off next and cleaning the valves, lapping and adjusting them. I'm at a lose as to why it runs fine until it gets hot.

Possible obstruction in fuel line or fuel outlet of fuel tank causing a restriction that doesn't supply enough flow to replace what is being used causing it to run out of fuel in the carb.

Does it have The plastic airfilter base? Sometimes those can warp where they meet the carb and allow excess air in, since most of the time surging is caused by a lean condition (too much air, or as Illengine stated, not enough fuel)

I would check your valve adjustment.
See if that clears it up..

Now that spring is here I got back to working on this. Adjusted the valves. Took the new carburetor apart and checked all the passages using carburetor cleaning wires and compressed air. This has the ready start carburetor which looks pretty simple. Not a whole lot of passages to clean on this one. Didn't help so I took the original OEM carburetor apart, cleaned it with 2+2 carb cleaner then ran it through an ultrasonic cleaner twice. Still surging. It doesn't surge when cold. When the choke fully opens it surges. I can hold the choke half closed and it will run fine even when under a load. I really don't think I still have a carb issue. So here's a stupid question. This engine was originally on a Weed Eater rear engine mower that I picked up cheap hoping to flip it and make a couple of bucks off of it. Wound up that the transaxle was bad and wasn't worth fixing. So I found a pressure washer frame for $10, put this engine on it and found a new pump that would fit it. Thinking back, it didn't surge this bad when it was on the mower. Started happening as soon as I converted it to a pressure washer. I see some posts that claim running a mower with no blade will cause surging but I would think this pump would have sufficient load. Also some people claim a leak between the air filter housing and carb will cause it. I was testing it with the blower housing and air filter assembly off because I didn"t want to button everything up until I have it running right. Sorry for the long post but I want to give as much info as possible.

Did you replace the float needle and seat when you cleaned the carb? Did you check to see if the float level was correct when you cleaned the carb. Surging when the engines choke is opened is most often caused by a lean running condition. Over time the float seat will swell, causing the needle to close to soon, which results in a lean fuel mixture. I recommend you get a new kit, part number 398188, which contains a new needle and seat. Sometimes you must also replace the float. Here is the procedure which I follow.

Needle and seat replacement.
Remove the carb, and then remove the float bowl. Check the float bowl jet (which is the bowl screw) and make sure the jets both horizontal and vertical are clean and open. Tip the carb upside down and remove the float pin and float with needle attached. Look in the float needle passage and you should see the red float seat at the bottom of the passage. This is where a #5 crotchet hook would come in handy as you need to remove this seat. If you have no hook, but compressed air, you can blow through the fuel inlet and try to pop the seat out. Put your thumb over the passage to prevent the seat from flying who knows where. No air or hook try bending a stiff paper clip to dig the seat out.
I would either give the carb a good 24 hour soaking or have it ultrasonically cleaned at this time.
With the seat out clean the passage way with carb cleaner. Now you must find a drill bit slightly smaller than the passage way, to be used to press in the new seat. Apply a very, very small amount of a very light lube to the new seat. 3-1 oil or lighter, to help seat it better. Carefully insert the new seat in the passage way with the rings on the seat down toward the carb body. Slowly and carefully force the seat down with the back end of the drill bit. Once it is seated, check to see that it did not flip and the rings are up. Next check to make sure that the float does not have any liquid in it. If it does, replace. If everything looks correct, attach the new needle to the float and install with the float pin centered. It everything is correct, the float should seat level to the carb body, when looking at it upside down. If everything looks good reattach the float bowl, making sure that both the bowl gasket and the nut gasket seal properly. Reinstall on the engine and test unit. Remember to have patience and take your time. Good luck, but I don't think you'll need it.
PS: On the side of some Tecumseh carbs you will find a plastic cover. Under this cover will be an idle jet. Remove it and check to see that the jet is open both horizontally and vertically. You should be able to push the old float needle wire through the vertical opening.

It maybe the fuel bolt that has the main jet has a wax build up causing a lean condition. Plus have had to ever so slightly enlarge the jet.

Also make the has the Champion QC12YC (Briggs PN 692720) spark plug installed and it is gapped at .020".

I ordered a set of wire drills. Since I have 2 carburetors I'll try going up a size on the main jet. If I mess one up I'll still have another.

I'm at wits end! Drilled out the main jet slightly larger, no help. Put on a brand new OEM Briggs carburetor, no help. Checked the gas tank and hoses for obstructions, good fuel flow. Pulled off the flywheel, key is good and governor linkage is not binding. When I first start it it runs fine at idle and under load until the choke opens then it starts surging. If I hold one end of the governor spring it calms down. This motor was originally on a Weed Eater One that didn't run when I got it. The governor spring was missing so I ordered a Stens 490-406 which is supposed to replace the Briggs # 796484. When I look online at pictures of the OEM part, it looks different ( heavier) than the one I have. Would a weak spring cause a surging problem?

Carbs used on mowers and pressure washers are not always the same, as they are run under different loads. Drilling out the jet is probably your problem, as now you are putting more fuel into the engine with the same amount of air, running rich. If you installed a new carb but are using the wrong governor spring that may also contribute to the problem. As I am at a loss as to tell you how to proceed would you please post a few things so I can get a better picture of what you have in front of you. Please post the model, type and code numbers for the engine you are trying to get running. Please post the part number of the new carb you installed. Have you installed new carb and manifold gaskets?

Engine is 12S907-1411-B1. The carb is 796608 and is OEM. Even has the B&S logo. I'm really leaning towards the governor spring. Mine doesn't look as heavy as the picture of the OEM one and if I just hold half of the coils it stops surging. bending the bracket to change tension doesn't help. As I stated earlier, the spring was missing when I bought the mower and I never could get it running right. Since the OEM spring is cheap, I'm going to try it anyway.

I just reread the entire thread and noticed something I missed earlier. If I read right you are trying to install a mower engine on a pressure washer. As I stated earlier these run under different loads and may need different carbs. Engines on pressure washers run at a heavier load and require more low end torque because you are dealing with water pressure, not blades of grass. On top f that if the mower engine was on a WeedEater mower, it is most likely the cheapest engine out there. That spring you are talking about (governor spring) may not be the same on both engines.

The Weed Eater One was a little rear engine riding mower. I would think that turning a blade and being a rider would be a heavier load than a pressure washer pump. My other pressure washer has a smaller motor than this. What has me stumped is the fact that it will run fine under a load for the first 5 minutes before starting to surge. This wouldn't be the first time that I put money into a project only to wind up scraping it.

Three things cause surging on today’s small engines, rich mixture, lean mixture and improper governor setting. Wish I was standing next to you to see what’s happening. Only have two more things I can suggest sitting here. One, adjust the governor, you want to set the governed top speed according to the tag on the washer. I’ve attached a manual for instructions. Two, next time it starts surging take a can of carb cleaner and spray around the gaskets between the carb and engine block, looking for an air leak after the parts heat up. https://docs.google.com/open?id=0B6NaqjIxWV1ybEN0LWo3azl6OUU

Thanks Rivets, I actually have that manual. There is only one governor spring and doesn't have multiple holes for adjusting. If I increase spring tension rpm will increase and will decrease if I decrease tension but the surging doesn't stop. I'm still leaning towards a stiffer spring.

Well even though the new OEM spring was a little shorter than the one I had, I didn't cure the problem. It still takes about 5 minutes for the surging to start but now I just noticed if I place my hand in front of the carburetor I can feel air and a gas mist blowing out. I've adjusteed the valves. Should I pull the head and check the valve seats? Checked the compression and it's only 45 psi

Did you reset the governor according to the manual? I would try that first, plus adjust your high and idle speeds.

This motor uses a Ready Start carburetor. There is no idle speed, it runs at high speed as soon as it's started. The only adjustment for the governor is to change the spring tension by bending the tab on the bracket. Since it has to get up to temperature before it starts surging is it possible that as the intake valve expands I'm losing my valve clearance. I don't think I should feel anything blowing out of the carburetor while its running.

You can adjust the governor and the high speed, if you take the time to read the manual. The governor arm may have moved on the governor shaft and the high speed is adjusted by changing spring tension. The carb jets correspond with governored top speed and if it is set too high, surging can occur. Need a tach to adjust properly.

kh0432 said:

Well even though the new OEM spring was a little shorter than the one I had, I didn't cure the problem. It still takes about 5 minutes for the surging to start but now I just noticed if I place my hand in front of the carburetor I can feel air and a gas mist blowing out. I've adjusteed the valves. Should I pull the head and check the valve seats? Checked the compression and it's only 45 psi

Click to expand...

If you only have 45lbs of compression, seems that it would be hard for the engine to keep up with itself.

If you haven't already, after it runs a while, kill it and check the oil fill tube for smoke. That's usually a good sign of a bad head gasket. I think I understood you right, when you said you could feel air and gas blowing back out of your carb. IIRC, that's another sign of a bad head gasket or PVC valve is bad.

Duh, ???????

Went back to check the valve clearance and realized I had the intake and exhaust adjusted wrong (got them mixed up). The exhaust was too tight and the intake too loose. Readjusted and same problem but it didn't seem like as much blow by coming out of the carburetor. Pulled the head off. Cylinder is like new with no scoring or ridge at the top. Pulled the valves and both seats are nice and shiny with no signs of burning or a seating problem. The thing that has me stumped is the fact that when it starts surging I can hold the governor spring and it will continue to run strong. It doesn't act like it's running out of gas. Even when the choke is fully open it will continue to run fine for a few minutes even under load before the surging starts. My other problem is I'm too hard headed to admit defeat. At least now that Maryland is on a shelter in place order I have plenty of time to play with it. I appreciate everyones help although I don't know what's meant by "Duh, ???????"

Duh means, there is no PCV valve on a small engine and a blown head gasket will not cause surging. You will have a crankcase heather, but I’ve never had one cause surging.

Thanks for clarifying your response. The head gasket was fine and I cleaned and lapped the valves anyway. The inside of this motor looks like it was hardly used. Wouldn’t you think if it were a fuel problem it would act up before running for 5 minutes? I even pulled off the screen in the bottom of the tank, stuck a rod through the nipple to make sure it was open and used a new 1/4” hose connected to the carburetor with no filter or shut off valve to make sure I had good fuel flow. If the governor was out of wack it shouldn’t run as long as it does before acting up. Also if the washer pump is changing the load the engine was designed for it shouldn’t run as strong as it does. This is driving me crazy?

W C Fields once said " If at first you don't succeed, try again. After that, quit. There's no sense in being a damn fool about it" I give up on this pressure washer. Time to scrap it out.?

Well I couldn't give up and finally fixed it. Never got the first engine to work right so I picked up a free mower (deck was rusted out) with a 125K02 engine. Cleaned the carb, got it running and it was fine on the mower. Took the engine off and had to shorten the crankshaft to bolt on the pump but got everything to fit. Started it up and it was surging just like before. Found a box of assorted springs in my basement and found one a little stiffer than the one on the governor. Put it on and now it works great under load and at idle. One thing I learned from this is that it's not worth it to make your own pressure washer‼