# Engine runs but surges.



## Bobby-Diesel (Jul 23, 2008)

Hi

I have a 12hp rider mower that surges/hunts. While it hunts I can hear a knocking or tapping sound. I can stop the hunting and noise by controlling the governor by hand. Can I rule out the carbuter, because the engine runs fine when I hold the governor? I’m also troubled by the knocking sound. Can this be an internal governor problem?


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## justin3 (Apr 10, 2007)

What model engine do you have? If its a tecumseh then most likely that taping/knocking sound is a loose connecting rod or other internal engine damage. If your sure your carb is ok then next check for air leaks and valve lash clearence.


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## Bobby-Diesel (Jul 23, 2008)

Its a B & S 281707 . I did install a new connecting rod but the noise goes away when i control the governor by hand. Hunting appears to be related to the knocking sound.


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## justin3 (Apr 10, 2007)

What was your reason for changing to connecting rod, sounds to me like you have too much play in between the connecting rod and crankshaft, this will cause a knocking sound when engine is accelerating or decelerating but not when the engine is at steady rpm. Another cause could be improperly torqued connecting rod bolts. 
That explains your knocking sounds, when the engine is hunting, do you see any fuel being spat back through the carburator? That is a sign of improper intake valve lash. Here is a simular engine that I had the same problem with a couple years ago.


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## paulr44 (Oct 14, 2008)

Rod knocks quiet up under load, piston slap does not - it can get worse. Rod knocks that aren't that bad, will often be most noticable at idle. Sometimes you figure out a problem and it makes little sense, but I've seen partially sheared flywheel keys and slightly loose flywheels create some interesting symptoms, so that's one thing I wouldn't rule out. If the flywheel is shifting, it can knock as it hammers the key, or from altered timing resulting in a pre-ignition. Neither is very likely, but like I said I've seen some things that defy common diagnostic methods and known symptoms. Sometimes a knocking sound is a result of poor valve sealing qualities, the last poster asked is there fuel spitting out the carb - a very valid point.


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## 30yearTech (Sep 14, 2006)

Your engine should have counter weights in it (Syncro Balance Engine). Worn bearings on the counter weights, and or a worn counter weight link bar can cause a knocking sound very similar to a rod knock. If the link bar fails, you will think a broken rod is nothing compared to what those balance weights can do to an engine when they come loose.


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## Bobby-Diesel (Jul 23, 2008)

Replacement was due to a seized conn rod bearing. The knocking sound does not increase or decrease with load. I’m able to cut the grass with no problems other than the knocking sound. I will check the carb for fuel spitting. Will improper valve clearance and fuel spiting cause knocking?


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## 30yearTech (Sep 14, 2006)

Bobby-Diesel said:


> Will improper valve clearance and fuel spiting cause knocking?


No, but it can cause the engine to surge, if your getting spit back through the carburetor, this would indicated insufficient clearance on the intake valve.


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## paulr44 (Oct 14, 2008)

30yearTech said:


> Your engine should have counter weights in it (Syncro Balance Engine). Worn bearings on the counter weights, and or a worn counter weight link bar can cause a knocking sound very similar to a rod knock. If the link bar fails, you will think a broken rod is nothing compared to what those balance weights can do to an engine when they come loose.


Bobby, 30year probably has nailed your noise - knowing you replaced the rod due to seizure tells us that the rod wasn't the only area starved for lubrication, and it's easy to just fix the obvious. The top main bearing probably took a hit, too.


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## Bobby-Diesel (Jul 23, 2008)

paulr44 said:


> Bobby, 30year probably has nailed your noise - knowing you replaced the rod due to seizure tells us that the rod wasn't the only area starved for lubrication, and it's easy to just fix the obvious. The top main bearing probably took a hit, too.


Paul, you're right, I had a lubrication problem and replace the conn rod and the main bearing.

I checked the carb for spitting and found no problems. I think the surge is causing the knock. I backed off the spark plug wire to the point where it barely touched the spark plug. The engine surge was greater than ever and knocking got louder. I started to think maybe I have an intermittent armature problem and the knocking is detonation. Is this possible?

I don’t believe I have internal problem because the engine runs fine when I control the governor by hand.

Thanks for the help
Bobby


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## paulr44 (Oct 14, 2008)

Bobby,
Without the luxury of knowing just where the knock is eminating from, or being able to hear it which can tell a lot, this is surely a tough one. Can you verify please that the ignition coil is on and gapped correctly? Ignition timing is not something you can usually attribute a surging condition to, but certainly a knock. But, if it was off, and creating detonation, then holding the throttle steady shouldn't make the knock go away. Just for sh*ts and giggles, if the coil is on upside-down, timing could be off a fair amount. Many solid-state units will work in either direction, but as you can imagine no one puts them on upside-down to see what will happen unless they have a serious curiousity problem. You may have to tear the engine down again and inspect anything and everything, even perhaps having a bent flywheel key or something you'd least expect. Knocks and surges don't usually correspond to each other. Sometimes bad valves create a knock-like sound, and usually result in spitback - something often overlooked. Justin pointed this out, and poorly sealing valves can create a host of symptoms and operational conditions, some not always making sense. Holding a length of fuel line to your ear and placing it on the block in various location may help you locate the knock, unless you have a mechanic's stethoscope. If it didn't knock before, I'd offer that you either missed something, as 30year pointed out with the counter-balancer, or you mis-assembled something. When the engine seized, if it was a violent stop you'd be surprised what can happen and yet be hard to pick-up on. Example: I've seen gear teeth crack, and unless you look very, very closely you'd miss it - again with the only the obvious as I said before.


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## Bobby-Diesel (Jul 23, 2008)

I decided to break down the engine, but before I start to disassemble I like to give the carburetor a complete look over. While disassembling the carb(large one piece flo-jet) I noticed something moving between the throttle plate and choke plate. I believe it’s the venturi. Should this be loose? I have the main jet needle and emulsion tube off. Looks like the emulsion tube traps it in place.


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## 30yearTech (Sep 14, 2006)

Bobby-Diesel said:


> I decided to break down the engine, but before I start to disassemble I like to give the carburetor a complete look over. While disassembling the carb(large one piece flo-jet) I noticed something moving between the throttle plate and choke plate. I believe it’s the venturi. Should this be loose? I have the main jet needle and emulsion tube off. Looks like the emulsion tube traps it in place.


You are correct, it's the venturi and nozzle does hold it in place, it is removable.


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## pyro_maniac69 (Aug 12, 2007)

before you completly tear into it, take a screwdriver and start the engine, put the tip of the screwdriver against the block of the motor, and put the other end up to your hear, you will be able to tell that wait if it is from the inside......it sounds kind've cool too

I call that the Beethoven Method


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## Bobby-Diesel (Jul 23, 2008)

Pyro,

I used the screwdriver method, to no avail. All you hear is engine noise and the knock is silent. I have no choice but to tear down. I'm getting to be an expert at it. It shoudnt take long.


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## paulr44 (Oct 14, 2008)

30yearTech said:


> You are correct, it's the venturi and nozzle does hold it in place, it is removable.


Lord oh my, I haven't seen a removable venturi since, uh, the 80's!?


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## 30yearTech (Sep 14, 2006)

paulr44 said:


> Lord oh my, I haven't seen a removable venturi since, uh, the 80's!?


Yep, it's been awhile since Briggs made an engine with the Flo-Jet carburetor. Can't say I miss them that much either!


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## Bobby-Diesel (Jul 23, 2008)

I partially disassembled the engine. Internally there are no evidence of loose parts or rub marks. The flywheel and key are in good condition. The one thing I noticed was the conn rod side movement (between rod & crankshaft web) is about .030. The manual does not give any tolerance for this area. Is this an average float?

Crankshaft endplay is .015 within specs. Should I decrease it?

I’m trying to get to the root of the knocking problem. I’ll tackle the surge problem later.


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## dawgpile (Apr 28, 2008)

Bobby: Have you checked the Crankpin Journal dimension on your crank? The reject size is 1.247". The brand new rod spec is 1.252 for essentially a .005" slop. I know you replaced the rod but if your crankpin journal is worn down, you can get a knock. I have a 7hp Tecumseh in an old JD tractor that has now thrown 3 rods because of excessive slop between crankpin bearing and crankpin journal. In my case, I have about .011" play and I can hear the knock. I've been lucky that every time it goes, no other damage happens. I'm just waiting for it to come out the side of the case!!!
Unfortunately, I'm too cheap to buy a new crank and I keep looking on Ebay with no luck so far.

Also, there is no rod available from Tecumseh with an undersize crankpin bore. However, there is for your engine(490469), if that were to turn out to be the problem. They make one that is .020 under so you could have the crank ground down(assuming it's not worn that bad already!). Just something else to put the mic on to verify where you are at.

Regarding the side to side play, IMHO that doesn't pose a problem. All of the forces generated by the engine are perpendicular to the axis of the crankpin. The rod can essentially 'float' back and forth, albeit only .030 and I don't believe it to be a problem. Nothing is causing it to 'slam' into either side, like what happens when a loose fitting rod slams into the crankpin journal!

Good luck!


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## Bobby-Diesel (Jul 23, 2008)

Dawg,

The crankpin measures 1.2485 and with the new rod I have minimum play. Thanks for the info on the side movement.


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## paulr44 (Oct 14, 2008)

dawgpile said:


> Bobby: Have you checked the Crankpin Journal dimension on your crank? The reject size is 1.247". The brand new rod spec is 1.252 for essentially a .005" slop. I know you replaced the rod but if your crankpin journal is worn down, you can get a knock. I have a 7hp Tecumseh in an old JD tractor that has now thrown 3 rods because of excessive slop between crankpin bearing and crankpin journal. In my case, I have about .011" play and I can hear the knock. I've been lucky that every time it goes, no other damage happens. I'm just waiting for it to come out the side of the case!!!
> Unfortunately, I'm too cheap to buy a new crank and I keep looking on Ebay with no luck so far.
> 
> Also, there is no rod available from Tecumseh with an undersize crankpin bore. However, there is for your engine(490469), if that were to turn out to be the problem. They make one that is .020 under so you could have the crank ground down(assuming it's not worn that bad already!). Just something else to put the mic on to verify where you are at.
> ...


Not always true. Side play in a splash-lubricated engine may be acceptable, but in fact in a pressure-lubricated engine there is a tolerance. It's been 3 decades since I worked in an auto-engine rebuild shop, but I think it was about .005" using a feeler gauge between the side of the rod and the crank. The tolerance is part of the oil pressure system, and would account for a loss in pressure. I've never really looked into this aspect in OPE, as if you have a knock it is normally obvious - we see dusted engines, clogged cooling fins and plain old neglect all the time. Just had 2 kawy's come in with less than 1,000 hrs. that bit the dust - cooling fins were packed.


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## Bobby-Diesel (Jul 23, 2008)

am happy to announce that I solved the knocking sound with the help of many here. The knocking was attributed to an out of round (.007) crankpin. All along I thought I had a good crank because I kept checking the same two points which was the good side. I purchased a used crankshaft on ebay, which worked out great.

I still have a slight hunt, but I’ll worry about that in the spring. Thanks to everyone, especially 30 yeartech,paur44 and Dawpile


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## dart451 (Nov 30, 2008)

I was just reading this post and realized that I have a similar problem with my 13hp B & S. The engine surges and spews gas out of the carb air intake. One of the main causes posted was possibly improper intake valve clearance. I checked the compression on the 13 hp briggs and I'm getting 145lbs of compression. With this high of compression, does it not seem logical that my valves are seating properly? Thanks.


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