Cub Cadet ZT1 engine surging problem (FR691V motor)

[Cajun power]

Here is the issue. (before commenting/reply, please read the items I have replaced and checked already.)
Issue: FR691V V twin motor starts up fine. When it starts, it begins to surge. RPM goes up and down. Governor rotates up and down. This surging happened no matter if at idle or at full throttle.
Here are the things I have replaced and checked:

1. I replaced the carburetor...I verified the choke spring is set properly also and governor to throttle and throttle cable...with correct spring and tension. all linkages are correct...still surges.
2. I replaced both spark plugs...and verified the gap...still surges.
3. I replaced the fuel, fuel filter and oil and oil filter. 10w-30 Mobil 1 full synthetic at correct level. still surges
4. I replaced both intake manifold gaskets and correct torque...there is no crack in the aluminum intake manifold..I cleaned it up and visually inspected it. Both..still surges.
5. I checked the valve lash and it's spot on at .005 inches for both intake and exhaust valves. I replaced the valve cover gaskets. still surges.
6. I replaced all fuel lines and replaced the fuel vapor return line to the from the air filter breather tube and I also replaced the vacuum line from the engine case to the fuel pump. still surges.
7. I replaced the fuel pump...still surges.

Now here is the very interesting part: I decided to pull spark plug wires from one coil...and this smooths out...no surging. I did this to the left spark plug wire first. Then I plugged it back in...surging starts happening again. So I go to the right side and remove the spark plug wire...and surging stops! And when plugged back in...surge begins again? whoa! What's that about?
What does this mean?...what would cause the surging to happen but stops when EITHER of the two spark plug wires is removed while the engine is running? My understanding is that if the surging is due to a bad coil or a bad coil plug wire or bad spark plug. .then the surging would stop when you remove ONE of the coil plug wires. But what I found is the surging stops when I remove either one! That has to mean something ? What is that?

Here is what I have NOT done and are next on the list of things to check:
check for correct and steady consistent voltage to the anti backfire fuel solenoid....maybe there is wiring damage that intermittently is causing the solenoid to pull back and then thrust back up from a weak voltage or damaged wiring...or maybe there is a ground problem ? My understanding is that the chassis and engine back to the battery is a negative ground circuit design. right?
I have not inspected the valves...maybe the valves are not opening and closing properly? Maybe the valves seats are damaged or carbon buildup?
Maybe one or both cylinder head gaskets have failed?

Here is the issue. before commenting/reply, please read the items I have replaced and checked already.
Issue: FR691V V twin motor starts up fine. When it starts, it begins to surge. RPM goes up and down. Governor rotates up and down. This surging happened no matter if at idle or at full throttle.
Here are the things I have replaced and checked:

1. I replaced the carburetor...I verified the choke spring is set properly also and governor to throttle and throttle cable...with correct spring and tension. all linkages are correct...still surges.
2. I replaced both spark plugs...and verified the gap...still surges.
3. I replaced the fuel, fuel filter and oil and oil filter. 10w-30 Mobil 1 full synthetic at correct level. still surges
4. I replaced both intake manifold gaskets and correct torque...there is no crack in the aluminum intake manifold..I cleaned it up and visually inspected it. Both..still surges.
5. I checked the valve lash and it's spot on at .005 inches for both intake and exhaust valves. I replaced the valve cover gaskets. still surges.
6. I replaced all fuel lines and replaced the fuel vapor return line to the from the air filter breather tube and I also replaced the vacuum line from the engine case to the fuel pump. still surges.
7. I replaced the fuel pump...still surges.

Now here is the very interesting part: I decided to pull spark plug wires from one coil...and this smooths out...no surging. I did this to the left spark plug wire first. Then I plugged it back in...surging starts happening again. So I go to the right side and remove the spark plug wire...and surging stops!
What does this mean...what would cause the surging to happen but stops when either one of the spark plug wires is removed while the engine is running? My understanding is that if the surging is due to a bad coil or a bad coil plug wire or bad spark plug..then the surging would stop when you remove ONE of the coil plug wires. But what I found is that the surging stops when I remove either one! That has to mean something ? What is that?

Here is what I have not done and are next on the list of things to check:
correct and steady consistent voltage to the anti backfire fuel solenoid....maybe there is wiring damage that intermittently is causing the solenoid to pull back and then thrust back up?
I have not inspected the valves...maybe the valves are not opening and closing properly?
Maybe one or both cylinder head gaskets have failed?
maybe the voltage regulator is shot? (remember that surging stops when one or the other spark plug wires is removed? hmmm)
maybe the case main seal/gasket is shot?
maybe the governor is broken or out of adjustment?
bent or broken valve rods?
Maybe there is a cylinder crack somewhere?
maybe piston or piston rings are damaged/cracked?
I am at a loss.
I am looking for some good advice on what might cause this surging to happen...but goes away when I pull one OR the other spark plug wires. I would expect that if one of the coils or cylinders are bad, the surging would stop only when pulling one of the spark plugs. but the surging stops when I pull one or the other! weird ? What am I missing?
That seems unusual.

maybe the case main seal/gasket is shot?
maybe the governor is broken or out of adjustment..?
bent or broken valve rods?
Maybe there is a cylinder crack somewhere?
maybe piston or piston rings are damaged/cracked?
wiring issue or voltage problem with the seat safety switch...maybe intermittent wiring damage or connector problem causes engine to stop/start intermittently?
wiring issue or voltage problem with steering arm safety switches...maybe intermittent wiring damage or connector problem caused by one or both steering arm safety switch circuits..causing engine to stop/start intermittently..?
I am at a loss.
I am looking for some good advice on what might cause this surging to happen...but goes away when I pull one OR the other spark plug wires. I would expect that if one of the cylinders is bad, the surging would stop only when pulling one of the spark plugs. but the surge stops when I pull one OR the other! weird ? what am I missing?
That seems unusual.
Any ideas?
thanks for any feedback and advice

 

[ILENGINE]

I am leaning toward a bad talk back diode inside of one of the ignition modules. try disconnecting the kill wires and keep in mide that they key won't shut the engine off other than maybe the fuel solenoid preventing fuel flow into the carb. If the surging stops with the module kill wires disconnected then I would be replacing both modules.

 

[Cajun power]

ahhh.. I did not think about that. I will look at the wiring circuit for the talk back diode. Maybe both coil and connected to a single ground point? And if one of the coils is bad, then this causes surging but not when one of the coil plug wires is removed. that kind of makes sense. Like a dead fault condition when one of the coil internals is bad...but disconnecting one the coils from the spark plug "solves" the intermittent dead short condition. I like this.

I will remove one of the spade connectors one at a time, coil 1 to cylinder 2 and see what happens. Then reconnect and remove spade connector kill wire from coil 2 to cylinder 2 and observe. (Noting: if this isn't the problem and the problem is actually a fuel solenoid circuit issue, then I can kill the engine simply by reconnecting the kill wires with an insulated needle nose ...so maybe this will be a good test to check two difference possible causes! )

I will report back here tomorrow evening after I go through this test.

THANK YOU!

 

[Bertrrr]

This is interesting , I've run across something similar to this with a Whole different type of engine - Long story short I was the 5th mechanic to deal with it - ended up being the wrong type of spark plug- thing is it was running side by side with an identical engine and that engine didn't do this ( same plugs in both ) - changing to a different plug design corrected the problem - Sounds like you've tried everything else - don't rule it out,

 

[Cajun power]

I have new plugs inbound. I have the NGK OEM spec plugs right now, but it's a cheap and easy troubleshooting no doubt about it. Even if this isn't the problem I can always use a set of spares in the toolbox. Thanks!

 

[boatmoter]

I see you adjusted the governor to throttle, but what about the governor arm to governor shaft?
and you may have received a defective carb,it happens. when the surging stops when you pull a plug wire off,this puts a heavier load on the engine so the surging will stop. I would also make sure it is not starving for gas

 

[bertsmobile1]

The diodes that you were alerted to are not a service replacement part
They are embedded into the coils along with the Hall Effect trigger chip.
So do as advised, remove both kill wires from the coils and get back to us.
In nearly every case surging is coused by an air : fuel ratio that is too lean for the engine to maintain a constant speed.
Engine slows down so governor relaxes, throttle opens up engine gets a big gulp of air which sucks harder on the main jet causing what should be a richer fuel mix into the engine ( this is why it accelerates quickly ) .
In the case of surging, the sudden glup brings the air : fuel ratio back to normal so the engine picks up speed.When it stops accelerating , the velocity of the air passing the main jet slows down so it no longer is sucking extra fuel
With no extra fuel the engine is then back into a too lean situation so it slows down which causes the governor to relax which allows the throttle plate to open up wide & thus the cycle continues at infinatum.

SO either not enough fuel is being added by the carburettor or air is entering the engine after the carburettor .

Now back to you.
As you have replaced so many items there is the real possability that one or more of them will be defective or damaged during the replacement.
Very common for defective carburettors to be sold on the sites invented for thieves to sell stolen goods or crooks for selling scrap metal pretending to be good parts .

Two simple tests
1) remove the air box and either apply the choke if you can or partially cover the air intake with some card ( not your hand )
work the governor linkage to open & close the throttle
If the engine stops surging with a partial blocking of the intake then it is definately a lack of fuel situation .
2) Remove the blower housing , start the engine and saturate the region from the throttle plate shaft ( the bit the throttle butterfly sits in ) to the engine with WD 40 from a trigger pack, ( not a spray can ) .
While doing this work the governor rod back & forth to make the engine accelerate hard & back off .
IF you get white smoke from the exhaust then you have an air leak .

Do this and get back with the results

 

[7394]

 

[Cajun power]

I see you adjusted the governor to throttle, but what about the governor arm to governor shaft?
and you may have received a defective carb,it happens. when the surging stops when you pull a plug wire off,this puts a heavier load on the engine so the surging will stop. I would also make sure it is not starving for gas

The governor is not the problem. it's perfectly adjusted (I tried both ways too, just to make sure I had it in full sweep). The carb is not defective. I put the old carb in and it still surges. It's a V twin....I mentioned this weird condition in the first post: when pulling EITHER of the two coils, the engine smooths out. No surging. It does not matter at all which of the coils I pull, the surging stops. BUT when I have them both connected and both are firing, the surging happens. This should have been my first significant clue when I first began troubleshooting. But, I was not paying attention and I ended up replacing just about everything. (carb, fuel pump, fuel filter, all fuel and vacuum lines, new intake gaskets, new exhaust gaskets...new carb gaskets...new breather hose...new coils, new plugs)...I threw parts at it..and I did so knowing that I could end up not solving the surging problem, but also knowing these are spare parts I can keep for maintenance down the road. I cleaned the tank, new fuel...gapped the valve clearance...cleaned up the flywheel and coil mounts of any rust and debris. I did all the things you possibly could except that one thing I should have done in the very beginning.

then I got serious...I did a compression check and leak down.

both head gaskets were blown...both were leaking compression down into the case. The symptom of one coil pulled and no surge is because the gasket problem was very tiny...just enough to cause problems, but only when BOTH cylinders were firing. When only one was firing, the surging goes away because the head gasket leak is not back pressured by the other cylinder...because there is no combustion happening, so there is no backpressure that creates excessive torque load on the shaft!. But when both are firing, and are in different phase...(one in ignition, one in exhaust for example)...causes enough back pressure in the case from both cylinders happening at different cycles then surging happens. The surging happens because the case pressure accumulates enough blow by pressure from the cylinders directly into the case to cause the engine to "feel" it us under a heavier torque load and the governor responds ...with the up and down surge...blown head gaskets...not much...but enough to cause excessive pressure into the case when both are operating...but not enough to case excessive pressure when only one is firing.


If I had started this troubleshoot with a little more attention to the "weird" symptom, and then done a compression and leak down test, I would have figured this out much faster.

sidenote: I did NOT USE the OEM Kawasaki Head Gaskets for the replacement/repair. For one, finding the IMPROVED two part composite gasket (metal PLUS a fiber insert), was too difficult to try and verify with all the garbage that is sold out there in the ether. Even the official Kawasaki website states two different part numbers, one for the original and one for the superceded "improve" composite gasket. But when I called they said I would receive which ever they had in stockl! WTF! And it was very pricey...no way I am going to pay 30 bucks for a simple stamped aluminum gasket. So, What I ended up doing is buying COPPER GASKETS...and used a copper gasket spray dressing. There is no fiber insert on this gasket but it is exactly the correct thickness dimensionally to the OEM gasket, so there is no problem with "squish". They were cheaper by a mile from the IMPROVED Kawasaki composite two element gaskets...18 bucks a piece. Copper should also provide better heat transfer, so there is another benefit by this upgrade.

this is exactly what I bought. And so far, compression and leak down check after replacement has been terrific. No leaks..and compression is 155 at #1 and 153 at #2. (versus below 60 psi on the old blown gaskets)...no surging. Problem solved. It has small leaks in both head gaskets!

as far as why the head gasket failed, both at the same time, is probably due bad metal gasket design PLUS a time or two when my cousin decided it was a good idea to spray down the engine when it was hot.

case closed. head gaskets.

lesson: when dealing with engine performance...do a compression and leak down check. Can save alot of time chasing gremlins

 

[Cajun power]

The governor is not the problem. it's perfectly adjusted (I tried both ways too, just to make sure I had it in full sweep). The carb is not defective. I put the old carb in and it still surges. It's a V twin....I mentioned this weird condition in the first post: when pulling EITHER of the two coils, the engine smooths out. No surging. It does not matter at all which of the coils I pull, the surging stops. BUT when I have them both connected and both are firing, the surging happens. This should have been my first significant clue when I first began troubleshooting. But, I was not paying attention and I ended up replacing just about everything. (carb, fuel pump, fuel filter, all fuel and vacuum lines, new intake gaskets, new exhaust gaskets...new carb gaskets...new breather hose...new coils, new plugs)...I threw parts at it..and I did so knowing that I could end up not solving the surging problem, but also knowing these are spare parts I can keep for maintenance down the road. I cleaned the tank, new fuel...gapped the valve clearance...cleaned up the flywheel and coil mounts of any rust and debris. I did all the things you possibly could except that one thing I should have done in the very beginning.

then I got serious...I did a compression check and leak down.

both head gaskets were blown...both were leaking compression down into the case. The symptom of one coil pulled and no surge is because the gasket problem was very tiny...just enough to cause problems, but only when BOTH cylinders were firing. When only one was firing, the surging goes away because the head gasket leak is not back pressured by the other cylinder...because there is no combustion happening, so there is no backpressure that creates excessive torque load on the shaft!. But when both are firing, and are in different phase...(one in ignition, one in exhaust for example)...causes enough back pressure in the case from both cylinders happening at different cycles then surging happens. The surging happens because the case pressure accumulates enough blow by pressure from the cylinders directly into the case to cause the engine to "feel" it us under a heavier torque load and the governor responds ...with the up and down surge...blown head gaskets...not much...but enough to cause excessive pressure into the case when both are operating...but not enough to case excessive pressure when only one is firing.


If I had started this troubleshoot with a little more attention to the "weird" symptom, and then done a compression and leak down test, I would have figured this out much faster.

sidenote: I did NOT USE the OEM Kawasaki Head Gaskets for the replacement/repair. For one, finding the IMPROVED two part composite gasket (metal PLUS a fiber insert), was too difficult to try and verify with all the garbage that is sold out there in the ether. Even the official Kawasaki website states two different part numbers, one for the original and one for the superceded "improve" composite gasket. But when I called they said I would receive which ever they had in stockl! WTF! And it was very pricey...no way I am going to pay 30 bucks for a simple stamped aluminum gasket. So, What I ended up doing is buying COPPER GASKETS...and used a copper gasket spray dressing. There is no fiber insert on this gasket but it is exactly the correct thickness dimensionally to the OEM gasket, so there is no problem with "squish". They were cheaper by a mile from the IMPROVED Kawasaki composite two element gaskets...18 bucks a piece. Copper should also provide better heat transfer, so there is another benefit by this upgrade.

this is exactly what I bought. And so far, compression and leak down check after replacement has been terrific. No leaks..and compression is 155 at #1 and 153 at #2. (versus below 60 psi on the old blown gaskets)...no surging. Problem solved. It has small leaks in both head gaskets!

as far as why the head gasket failed, both at the same time, is probably due bad metal gasket design PLUS a time or two when my cousin decided it was a good idea to spray down the engine when it was hot.

case closed. head gaskets.

lesson: when dealing with engine performance...do a compression and leak down check. Can save alot of time chasing gremlins

this is head gasket I bought...cheaper than the improved kawasaki superceded gaskets...and better heat transfer...no squish problems because they are exactly the same thickness dimensionally as the original "unimproved" simple stamped aluminum gaskets.

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