# B&S 16hp twin Model 42B707 Fuel Problem



## sirwhinesalot (Jun 1, 2011)

Recently acquired this mower. Engine will idle all day long, when throttle is opened up it will start to flood out. heavy black smoke from exhaust. when mower is engaged it starts to bog down as in getting to much fuel. Carb float is good, no pin holes, float seat doesn't look bad, everything seems clean. I tried to adjust the needle valve, it leaned it out and made it surge so it seems to be adjusted right. Dont understand how it gets to much fuel under load.


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## 30yearTech (Sep 14, 2006)

Is the carburetor the original one? Damage to the main jet or the incorrect one could cause this. When there is no load on the engine, the majority of fuel is metered through the idle circuit. When a load is applied or the throttle is opened then fuel starts entering through the main jet. Must be feeding too much fuel via the main jet, this is where I would look for a problem.


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## sirwhinesalot (Jun 1, 2011)

I am under the impression that its the original carburetor, the person I bought it from said it ran fine then started to act up. He had no knowledge or where to begin to repair it. Not very handy he said. Also very little maintenance has been done, oil is black and thin. The air filter was bad. No grease or lubrication on the front steering axles, and so on. I took the top off the carburetor, and removed the bolt at the bottom and removed that jet to clean it, no dirt or debris was inside. This is the first time I have encountered a carburetor with a diaphragm on the front. Could the governor be affecting this? It looks like it has adjustments on it. I do not know if he touched it. Where is a good site to look for setting it.


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## 30yearTech (Sep 14, 2006)

This link should provide you with a service manual that covers your engine. The diaphragm on the front of the carburetor is a fuel pump. Usually these engines only have adjustments to the low speed circuit and a fixed high speed jet. If it's old enough, it could have an adjustable high speed or load circuit, and then an adjustment may be all that is needed.


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## sirwhinesalot (Jun 1, 2011)

It is the newer style, it does not have the high speed adjustment. Do these symptoms point to a carb issue?


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## 30yearTech (Sep 14, 2006)

double post


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## 30yearTech (Sep 14, 2006)

sirwhinesalot said:


> It is the newer style, it does not have the high speed adjustment. Do these symptoms point to a carb issue?


Yes, it certainly sounds like a carburetor issue to me. Just for grins, I would also check the compression on each cylinder.


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## sirwhinesalot (Jun 1, 2011)

After I rebuilt the carburetor now, I still have the same problem, It is still getting to much gas a high speed. But will idle and run at half throttle. Any other suggestions before I put new head gaskets in?


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## justin3 (Apr 10, 2007)

Does your machine have a fuel pump on it? I have come across this issue before, I replaced the fuel pump because it was the only option I could think of. The pump works off engine vacuum but my guess is that it was building too much fuel pressure.


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## 30yearTech (Sep 14, 2006)

The other thing to check is the valve lash, particularly on the intake valves. Insufficient clearance on one or both of the intake valves could also cause a similar issue. I doubt that the fuel pump on your carburetor is causing this issue.

Justin, these type of fuel pumps operate off of crankcase pulse, not vacuum.


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## justin3 (Apr 10, 2007)

Ooops 30year is right, crankcase pulse, this is what happens when you go back between cars and lawnmowers you mix some things up.


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## sirwhinesalot (Jun 1, 2011)

Okay thanks guys thats something I will check, in the coming days. I have been extremely busy this last month and coming month, my time is limited to work on this thing.
Yes Justin I put a whole gasket kit in the carb and fuel pump. This thing will Idle and run half throttle all day, i let my step daughter ride it around the yard for a while yesterday, when I got on it and cranked up the throttle it started to flood out and lose power, backed it down and it purred. With all the help and advise, im sure I will find the problem.
Thanks to all!!!!


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## chuck_thehammer (Oct 24, 2007)

are you sure that when you go to max throttle that the chock is not closing?
it sounds like the chock is closing when you go to max throttle.


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## sirwhinesalot (Jun 1, 2011)

The choke is on a manual cable. it cant close without me doing it


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## sirwhinesalot (Jun 1, 2011)

Gentlemen I believe I found the problem. I removed the flywheel to check the key way,which was good. I then tackled the head gaskets, As I removed the right side head I find the valve seat dangling around the intake valve, It has left a few dig marks in the case. the valve is mushroomed to the head side. I cant believe this thing still ran without blowing, Now the task of trying to repair the damage or replacing the engine is next. I will keep you posted.


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