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post #1 of 8 Old 10-11-2019, 11:48 AM Thread Starter
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SV735 high idle surge

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 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.
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.


Dave
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post #2 of 8 Old 10-11-2019, 04:39 PM
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Re: SV735 high idle surge

Does it stop surging if you close the choke a bit.
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post #3 of 8 Old 10-11-2019, 05:01 PM Thread Starter
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Re: SV735 high idle surge

yes it does so it's got to be a rich condition I would assume. Also now low idle doesn't cooperating unless I close the choke a bit. I couldn't edit my original post to reflect this. Also using propane around the intake to head and the base of carb made no difference.

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post #4 of 8 Old 10-11-2019, 07:22 PM
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Re: SV735 high idle surge

Actually you have a lean condition. Likely in the idle circuit if carb has one. Did you spray cleaner and air in the idle circuit? It's separate from main jet. You could try adding a heavy dose of seafoam to the fuel. May smoke and surge even more for awhile. Run a bit then let sit awhile. Repeat. If it runs fine under load you may wish to ignore idke surge. No reason to idle(slow speed) an air cooled engine.
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post #5 of 8 Old 10-11-2019, 07:37 PM Thread Starter
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Re: SV735 high idle surge

Quote:
Originally Posted by 38racing View Post
Actually you have a lean condition. Likely in the idle circuit if carb has one. Did you spray cleaner and air in the idle circuit? It's separate from main jet. You could try adding a heavy dose of seafoam to the fuel. May smoke and surge even more for awhile. Run a bit then let sit awhile. Repeat. If it runs fine under load you may wish to ignore idke surge. No reason to idle(slow speed) an air cooled engine.
Thanks I'm bass ackwards here. I disassembled the entire carb, took the main jet out and cleaned any passage I could see w/ carb cleaner, tiny bottle brushes, and air afterwards. THere are a few ports capped but I don't know that where the idle circuit is exactly. It runs perfect under idle but now that I'd disassembled it it now needs to be choked to bit to idle as it's idling far too low otherwise and usually dies if i bring it to idle too quickly.

Dave
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post #6 of 8 Old 10-13-2019, 12:35 PM
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Re: SV735 high idle surge

Inspect very closely the link between the carburetor throttle and the governor arm. If the ends are worn into egg-shaped holes, you may just have too much play. The
'slop' in the control linkage allows things(governor arm) to move without any response, triggering a larger motion, which is responded to, leading to unstable speed (rpm) control.
If you do have enlarged holes, you may be able to find a small spring that reaches to both controls at opposite ends. Attach the spring such that is just holds both throttle and governor tight to the link between them. That way, the slop is removed, and the governor again becomes responsive as the throttle plate moves when the governor indicates such.
Many linkages have the described springs, wrapped around the governor link, and hooked onto the control/controlled arms. It just holds them tight, but does not pull in either direction.
If you do not have looseness, then look into the venturi of the carburetor, on the 'engine' side of the throttle plate. Look at the side of the venturi wall for one or more TINY little openings. They are part of the idle system, but contribute to 'response' when the rpms are changed by governor motion. As the throttle opens, the air gets going into the engine RIGHT NOW. The fuel, being more dense, requires more time to get into motion. (like accelerating a semi vs a bicycle...) That results in a lean condition, more air and less fuel. These ports leak a bit of fuel to help that transition even though they are more or less part of the idle circuit. You may be able to place a carb cleaner spray 'straw' against the port(s) and push any blockage out ... to where? Dunno, but it sometimes works.
tom

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post #7 of 8 Old 10-13-2019, 02:00 PM Thread Starter
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Re: SV735 high idle surge

Quote:
Originally Posted by tomw0 View Post
Inspect very closely the link between the carburetor throttle and the governor arm. If the ends are worn into egg-shaped holes, you may just have too much play. The
'slop' in the control linkage allows things(governor arm) to move without any response, triggering a larger motion, which is responded to, leading to unstable speed (rpm) control.
If you do have enlarged holes, you may be able to find a small spring that reaches to both controls at opposite ends. Attach the spring such that is just holds both throttle and governor tight to the link between them. That way, the slop is removed, and the governor again becomes responsive as the throttle plate moves when the governor indicates such.
Many linkages have the described springs, wrapped around the governor link, and hooked onto the control/controlled arms. It just holds them tight, but does not pull in either direction.
If you do not have looseness, then look into the venturi of the carburetor, on the 'engine' side of the throttle plate. Look at the side of the venturi wall for one or more TINY little openings. They are part of the idle system, but contribute to 'response' when the rpms are changed by governor motion. As the throttle opens, the air gets going into the engine RIGHT NOW. The fuel, being more dense, requires more time to get into motion. (like accelerating a semi vs a bicycle...) That results in a lean condition, more air and less fuel. These ports leak a bit of fuel to help that transition even though they are more or less part of the idle circuit. You may be able to place a carb cleaner spray 'straw' against the port(s) and push any blockage out ... to where? Dunno, but it sometimes works.
tom
Thanks, I should have mentioned that I did look for any linkage slop and there was none. There are also springs that are there to take up any slop in the linkages if it is present. I also tore it down again yesterday morning and chased the idle holes at the top of the carb (3 of them) w/ a stainless wire brush bristle. They all appeared clear but I also used some electronics cleaner (All I had on hand) and saw it coming out just fine when I was forcing it through the idle circuits. I also blew air through all passages again but no love. Ended up buying a cheap amazon carb, tuning and it works great. It was $42 bucks or so so you really can't go wrong. There has to be something in a hidden idle fueling passage that I cannot access or am not aware of. At some point I may soak the OE carb and revisit this but I'm not going to spend more time on this until this cheapy carb starts giving me problems. As it sits after some tuning it's slow idling steady at 1225 RPMS and fast idle at 2950. Pretty happy with it and no more surging at fast idle!

Dave
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post #8 of 8 Old 10-14-2019, 10:37 AM
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Re: SV735 high idle surge

Good result. I had been leery of the low budget replacements, but a $50 carb kit may make me venture into that arena if/when needed. I have heard a lot of good things about how well they work in a good majority of instances reported. There have been some reports of some that were(likely) made late Friday afternoon after a liquid lunch. They were just poor quality, fit and finish at the bottom of the barrel, and worked as well as they looked, which is to say, not very.
The carbs used on many Honda and Chonda engines seem to have similar designs, with the idle circuit under a plastic 'plug' that is retained by the hard idle stop screw. Remove the screw, and you can remove the plug and the jet for relatively easy cleanout. It can be done without removal from the engine/intake. The later models seem to be very good quality, Honda or clone. Of course, Honda engines & carbs are likely now hecho in China, perhaps on the same assembly line during daylight hours, while the line is flip-flopped at night to produce clones.(maybe)
Some producers are proud of their product, and seem to plan to stay in business past the first transaction. Others, not so much.
I think some production for the US market will move 'onshore' as the labor differential eases, and the costs of design, development, production and shipping become more significant in overall cost. Software development by overseas personnel can appear a no-brainer at first glance, but time differences, language difficulties in conveying actual design details, and attitude all play a part. Try to maintain and fix problems in something that was coded by one whose native language differs, and it can become clear in a short while that some things may appear to cost less than they actually do. Soapbox move to the back of the room...
tom

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