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I’m working on a friend‘s rider that has a CV25-69543 engine that is running rich, but still starts and accelerates fine, but it’s obvious that something’s not quite right. The manual choke operation is absolutely perfect, so it’s not a choke issue. Both spark plugs were black, so replaced both plugs with new RC12YC plugs, and after running for awhile, the new plugs were black as well. Air filter and pre-cleaner are both clean. After removing the entire air box, I cleaned all the ports in the front of the carb with brake clean and compressed air, I noticed the breather tube coming off the breather, going to the air box, had a large tear in it. It became obvious that there were no high speed or low speed jets to adjust on this carb once I removed the air box. Would a leaky breather tube cause the engine to run rich? I wanted to check the air gap of the coils, because I was wondering if the spark wasn’t hot enough to burn the fuel/air mix completely, but have not yet done so. The engine does seem to have some excessive vibration while running, but it’s not the same vibration you get with bearing issues, as I have no oil leaks in the upper or lower seals. My thoughts are leaning toward coil air gaps or replacing breather tube (already ordered one). I also considered doing a complete ultrasonic carb clean, in case there were blockages in some of the internal air passages. If it were only one plug being black, it would be easier to try and figure out, but with used and new plugs getting black carbon on them, I’m definitely scratching my head on this one. All advice much appreciated!

On the side of the carb do you see one or two small rubber tubes going to a cover plate held on by 3 screws? That's the accelerator pump cover. Carefully pinch off one of those tubes. I say carefully because there is a metal restrictor in one of them and you don't want to crush it when you pinch off the tube. If when you pinch off the tube the engine stops running rich you have fuel leaking by the diaphram under that cover. If that's the case, replace the diaphram and you should be good.

Not sure what VS is on about as I am yet to see an accelerator pump on a mower carb
But apparently I was wrong & this mower dose have an accelerator pump on it .
If it has the standard Kohler twin barrel carb then the main jets aew in a tune held in place by O rings.
These are prone to deterioriating and then allowing fuel to bypass the main jet hole and go up the sides of the main jet.
This of course makes the engine run quite rich.
The other culprit for sooty plugs is the timing ket to be braoken allowing the engine to go out of time
Usually this will fail in the over advanced state but occasionally the flywheel manages t go backwards and the engine ges retarderd .

Well, we can’t all know everything which is what is nice about forums with a lot of members.

If you do find pinching off that hose stops the rich running here is the part Kohler part number for the mysterious accelerator pump kit.


This kit is a little expensive and before buying the kit some guys will simply put something in the hose to block the fuel from going where it isn’t supposed to go. Obviously not the Kohler recommended fix. This will stop the rich running but could cause some hesitation on throttle up and when the governor kicks in as the engine will no longer be getting that little pulse of fuel. Some guys have blocked the tube off and told me it worked fine.

Bert mentioned the 2-barrel carb and not being familiar with accelerator pumps. And he is correct on the 2-barrel. Those carbs don’t use accelerator pumps as they have 7 progression holes in each barrel that as the throttle plate opens and uncovers each hole a little more fuel is added removing the need for the accelerator pump.

Your engine, as you know has a single barrel carb. Kohler didn’t start using 2-barrel carbs until they introduced the CH and CV750 somewhere in the mid 2000s which at the time was a 30 hp. Then over the next few years they started introducing 2-barrel carbs into their lower hp engines.

For the single barrel carbs found on CV and CH Command twins from the 1990s until the mid 2010s and even some current production the majority of those use an accelerator pump. On the CHs it is a mechanical plunger and on the CVs it is a vacuum diaphragm.

A leaky diaphragm may not be the reason your engine is running rich but it’s a simple thing to eliminate by pinching of the hose and it is a very common issue. Depending on the brand of carb your engine has it will either have one or two little hoses going to that accelerator diaphram cover. I've attached a picture below from an older Kohler CV manual of a Keihin carb.

Thanks guys. I did indeed notice the two small hoses, and after what you told me, I’m leery I might pinch the one with the metal restrictor inside it, not knowing which is which. I guess I could remove them and check them both over to see which is which. So would you recommend pinching it off partially or completely, and will pinching either one of them suffice in testing for a leaky diaphragm? I just don’t want to damage something inadvertently. I appreciate all your help! I’m going to be pretty busy tomorrow, so I will keep you posted with updates as they happen.

Also about learing things.
I like to learn and so thanks for the education, possibly not used on what gets imported down here .

If you squeeze it with your thumb and forefinger your usually can feel where it's just hose. The restrictor might even show up as a bump in the hose and is brass so it won't crush all that easily. If you squeeze the hose with a needle nose slowly you'll know if you hit it. I think squeezing off either hose will work but I haven't actually done it myself in a while. If you squeeze one and nothing changes, squeeze the other one. If still no change then that diaphram is probably fine and something else is causing the rich condition.

OK, thanks! Will do! I too am still learning, and I so appreciate the expertise here on this blog. We’re all in this together, and this is, in my book, a great forum!

bertsmobile1 said:

Also about learing things.
I like to learn and so thanks for the education, possibly not used on what gets imported down here .

Click to expand...

I did spend the better part of 27 years giving factory Kohler training to Kohler dealers but I don't think there was a class I taught that I didn't learn something from a tech in attendance. The troubling thing is that after being retired for just 18 months, stuff that used to just come off the top of my head doesn't come as easily as it used to. Funny how quick you can forget stuff when you don't need it everyday.

Well, I ran the engine some with the air box off to see what I could see. After a couple of minutes, I noticed gas dripping and pooling up in the front of the carburetor. So I pinched off the small hose, and immediately RPM’s went up and the gas dripping and pooling went away. I also pulled the plugs and found less black carbon than before. So I went ahead and ordered the rebuild kit for the accelerator. I believe Vegetive Steam hit the nail on the head. I’ll post back once I get the kit installed. Thanks so much for the tip!