New around here, SV735 fast idle surging issue. Already tried all the obvious things

TT_Vert · Oct 11, 2019

Hey guys, this is my last ditch effort at figuring this issue out. My Cub Cadet w/ a Kohler SV735 idles at slow speed fine but on high speed it surges. I can definitely see the governor moving back and forth causing the surging which I've read is generally a lean condition on these.

I've done the following:
1. Adjusted the governor to ensure the shaft is up against it at the correct linkage position,
2. Disassembled the carb, removed the main jet and ran small bristle brushes through all passages, ran carb cleaner through them all and also compressed air. The carb looked brand new inside.
3. Removed float and ensued needle sealed properly. Float level seems fine but i"m basing it off of auto carb float levels.
4. I've not yet checked for vacuum leaks at the intake and carb base but I'd assume if there was a leak it'd rear its head at low speed idle as well.
5. Removed filter to ensure it wasn't restricting flow even though it's fairly new.
6. Removed gas gap to make sure there was no issue w/ fuel flow or vacuum cavitation.
7. Ensured fuel filter was flowing well.

Thanks for any input you can offer.

Dave

ILENGINE · Oct 11, 2019

Sounds like a partial blockage of the idle circuit in the carb. You are getting enough full to idle fine but at higher rpm's you are not getting enough at no load full throttle through the idle circuit.

TT_Vert · Oct 11, 2019

ILENGINE said:
Sounds like a partial blockage of the idle circuit in the carb. You are getting enough full to idle fine but at higher rpm's you are not getting enough at no load full throttle through the idle circuit.

After taking this carb apart and seeing how clean it is cannot see how anything is blocked. I also didn't pass carb cleaner and compressed air through each orifice. How can I get to the idle circuit? There's only one jet in this thing which is clear and there are a couple other things that are plugged.

Also now I've noticed that is not slow idling properly unless I have the choke partially closed just like with fast idle to get our to stop surging.

Dave

bertsmobile1 · Oct 11, 2019

Air , like any other fluid takes the path of least resistance.
An air leak through a bad manifold gasket is DRIVEN by the venturi effect of the air rushing into the engine.
Thus at low revs there is little pressure to suck air through the difficult high resistance path via a leaking gasket or manifold.

The first rule in mechanical diagnosis is nothing gets RULED OUT .
Potential problem points are TESTED OUT

Start the engine then put your finger on the governor arm and hold it steady.
Slowly push it forward to increase the speed.
If you can hear the engine sputtering and missing then you are not getting enough fuel and if the starvation is big enough, it will cause the engine to shut down all together.

Every hole in a carburettor will lead some where so every hole has to have cleaner sprayed through it and the outlet checked
I like to blow backwards through them from the engine side so you blow debris out.
There are two circuits in the carb and both have their own air passage & fuel passage then on top of this there is an air inlet into the float bowl.
All of them have to be clean.
Go to http://outdoorpowerinfo.com/repairs/ and follow their cleaning proceedures.

TT_Vert · Oct 11, 2019

bertsmobile1 said:
Air , like any other fluid takes the path of least resistance.
An air leak through a bad manifold gasket is DRIVEN by the venturi effect of the air rushing into the engine.
Thus at low revs there is little pressure to suck air through the difficult high resistance path via a leaking gasket or manifold.

The first rule in mechanical diagnosis is nothing gets RULED OUT .
Potential problem points are TESTED OUT

Start the engine then put your finger on the governor arm and hold it steady.
Slowly push it forward to increase the speed.
If you can hear the engine sputtering and missing then you are not getting enough fuel and if the starvation is big enough, it will cause the engine to shut down all together.

Every hole in a carburettor will lead some where so every hole has to have cleaner sprayed through it and the outlet checked
I like to blow backwards through them from the engine side so you blow debris out.
There are two circuits in the carb and both have their own air passage & fuel passage then on top of this there is an air inlet into the float bowl.
All of them have to be clean.
Go to http://outdoorpowerinfo.com/repairs/ and follow their cleaning proceedures.

I did check for vacuum leaks but was unable to edit my original post to reflect this. I introduced propane around both intake to head gaskets as well as the base of the carb w/o any change in performance so I think I can safely rule out any sort of vacuum leak. I can rev the engine smoothly so it appears AFR is close. I did blow out each passage (With carb cleaner and air) that was not caped w/ a plug. I don't have an exploded diagram of the carb to get any more idea of what these plugged passages do. I know the main jet that comes up from the bowl exists and that is it. The entire carb looked new before I did a thing to it. The tractor is only a few years old and I drain the fuel out before each winter. Up until last year it was always stored in a heated garage.

But again I have updated that after reassembly it will have a tendency to want to die at slow speed idle unless I have the choke closed some. SO now, slow speed idle is very low and wants to die unless I slightly choke it and high speed surges unless I slightly choke it. Makes me think it's actually rich.

Dave

ILENGINE · Oct 11, 2019

If it smooths out with partial choke then you have a carb problem. Adding choke proves the engine is running lean not rich. Lean on the idle circuit causes a surge because the engine without enough fuel on the idle side will start the slow and as it slows it tries to open the throttle which in turn then opens the high speed jet and the engine speed picks back up, and then starts to slow back down and is running lean on the idle circuit and the cycle repeats.

bertsmobile1 · Oct 11, 2019

And propane gas around the manifold is dangerous , stupid and ineffectual
Dangerous :- the gas can ignite
Stupid :- because of the above & potential for serious injury
Ineffectual:- because of the dilution effect
Saturating the manifold with WD 40 is the very best way to do it from a trigger pack, not a spray can because the propellants are also volatile.
WD 40 will not damage the engine but will create a lot of dense white smoke so even if you do not hear a change in engine speed you will see the smoke.

TT_Vert · Oct 11, 2019

bertsmobile1 said:
And propane gas around the manifold is dangerous , stupid and ineffectual
Dangerous :- the gas can ignite
Stupid :- because of the above & potential for serious injury
Ineffectual:- because of the dilution effect
Saturating the manifold with WD 40 is the very best way to do it from a trigger pack, not a spray can because the propellants are also volatile.
WD 40 will not damage the engine but will create a lot of dense white smoke so even if you do not hear a change in engine speed you will see the smoke.

I think you're being a bit dramatic here. IN 30+ years I've never seen anyone have any issue w/ using propane to find a vacuum leak in any IC engine. People also use oxy/acetylene torches and still have all of their limbs. You do not ignite the propane externally. many people swear by propane as it works and is cheap/easy to obtain. Heck, even popular mechanics suggests doing it and I doubt they would in this litigious society if there even a hint of danger in doing it. I do agree with you in this situation is the fan blows the propane around diluting it to the point of not being very effective and you'll see the byproduct of burning WD40. I'm generally not a fan of spraying liquids in my engine bay to find leaks so I generally don't do it.

ILENGINE said:
If it smooths out with partial choke then you have a carb problem. Adding choke proves the engine is running lean not rich. Lean on the idle circuit causes a surge because the engine without enough fuel on the idle side will start the slow and as it slows it tries to open the throttle which in turn then opens the high speed jet and the engine speed picks back up, and then starts to slow back down and is running lean on the idle circuit and the cycle repeats.

Thanks, I was bass ackwards on this for some reason, brain fart. I'm not sure how to proceed w/ this as I'm certain every passage on this guy is clear. I've also confirmed good fuel into the pump and the float is not adjustable as the needle slides into a channel on the plastic float..

bertsmobile1 · Oct 11, 2019

Using propane seems to be a USA specific idea and while you are right, the chances of a Whoof is low that still does not make it a good idea.
IT also does not work particularly well in any situation other than on a V8 where you have a dam for the gas/air mixture to accumulate and strong induction.
Nothing wrong with putting a liquid on a mower engine, or any engine for that matter just so long as you avoid the electronics & exhaust.
Motorcycle engine happily run in the rain & there is not too much difference between a mower & a motorcycle.
if you are a wash after use type person then you would be doing it regularly.
You sound like you know your way around engines so the fact that being slightly choked makes the problem go away should say too lean in the back of your brain.
EPA requirements have mower engines running way too lean for their own good to start with so it take almost nothing in the way of obstructions to cause them to surge .
Go to the site I linked to and see if you can find your carb.
They have gone to the effort of cutting a lot of them up so you can see where the various passages go.

On the very left field side, if the shut down plunger has a rubber cap on it these can come loose & partially block off the feed to the fuel jets.
Usually it becomes a problem at WFO where the venturi effect sucks the rubber cap into the opening.

It would also help if you mention which carb you have on the engine.

And for your benefit, Kohler have all of their repair manuals on line as a free download so it would be worthwhile getting your hands on one.
We are all here to either help you or get help.
It is not face book when 2/3 of the posters are there trying to inflate their position by tearing others down.

TT_Vert · Oct 11, 2019

bertsmobile1 said:
Using propane seems to be a USA specific idea and while you are right, the chances of a Whoof is low that still does not make it a good idea.
IT also does not work particularly well in any situation other than on a V8 where you have a dam for the gas/air mixture to accumulate and strong induction.
Nothing wrong with putting a liquid on a mower engine, or any engine for that matter just so long as you avoid the electronics & exhaust.
Motorcycle engine happily run in the rain & there is not too much difference between a mower & a motorcycle.
if you are a wash after use type person then you would be doing it regularly.
You sound like you know your way around engines so the fact that being slightly choked makes the problem go away should say too lean in the back of your brain.
EPA requirements have mower engines running way too lean for their own good to start with so it take almost nothing in the way of obstructions to cause them to surge .
Go to the site I linked to and see if you can find your carb.
They have gone to the effort of cutting a lot of them up so you can see where the various passages go.

On the very left field side, if the shut down plunger has a rubber cap on it these can come loose & partially block off the feed to the fuel jets.
Usually it becomes a problem at WFO where the venturi effect sucks the rubber cap into the opening.

It would also help if you mention which carb you have on the engine.

And for your benefit, Kohler have all of their repair manuals on line as a free download so it would be worthwhile getting your hands on one.
We are all here to either help you or get help.
It is not face book when 2/3 of the posters are there trying to inflate their position by tearing others down.

I appreciate the help for sure.

Agreed, most modern connectors are moisture resistant and not an issue. I'm more opposed to it on my cars as I don't want to have to clean any residue off my engines or risk having it discolor aluminum or composite materials as it burns into them. I'd mention what carb I have if I knew exactly LOL. I'll have to take off the airbox again to take a look. I see no PN stamped into it and the service manual I found for my engine shows a different image of the carb. They are similar but mine does not have the flat top piece secured w/ fasteners (See attached pic). KH-32-853-28-S is the PN for the Kohler SV735 according to what I've located. Every place I google SV735 carb I find this one I'm showing.

Tomorrow I may pull the shutoff solenoid again (had no cap, just a needle that closed off the jet) and see if it is in fact not retracting enough as that makes a bit of sense. It seemed to be free when I tore it down but I don't know if I'm hearing the solenoid engage when i'm keyon but I don't know that I paid any attention to that to be honest.

BTW, none of those kohler carbs are exactly what I have but the closest is the walbro LMK carb. It mentions a low idle fuel adjusting needle but unless it's under one of the welch plugs I don't have this either. Would you mind telling me when you find the kohler manuals? I've not found any manual for the 3285325-S. I found thisbut my Kohler carb isn't in there.

Dave

New around here, SV735 fast idle surging issue. Already tried all the obvious things

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