I recently bought a used 2006 Kodiak 400 4X4. One of the first things I did was take it to a local shop and have them change, oil, spark plug, gear oil, and fix a little leak there was in a seal. No biggie. Got it back and it ran great. Ran it 5-6 hours since then. I'm in the habit of running Sea Foam through the gas at least once in the spring on just about all my motorized toys so I did that too.

But, within the last two or so weeks, it noticed that it would hesitate a lot in the lower range of speed and would run rough in the mid and upper speeds. I took it to a shop and they pulled off the carb at my request and cleaned it. Guy said it was pretty dirty and added a fuel filter to the line to help keep stuff clean. Great or so I thought. As soon as I got it back I noticed it still mildly hesitated like it either wasn't getting gas or air or something in the speed ride above just barely moving. I called the guy and he said there was some sort of flat spot in the carb and that it accelrated fine in mid to upper speeds. I agree, but I'm not always running 30-40 mph! The only way I can describe it is that it runs like I was moving my thumb on and off the throttle slightly as I rode slower. When I give it a lot of gas, it accerates fine and runs great. It's just that slow/mid speed that hesitates.

Any ideas? Another shop said I might need a jet kit. Anyone recommend this to help with this problem? Thanks in advance.

 

Well I am not a Sea Foam expert but I think that is where I would look first. You could try dumping the gas and replacing it with fresh gas to see if that makes a difference. I don't know how "pale oil" and "NAPHTHA" burn compared to gasoline but maybe it is throwing the fuel mixture off just enough at low speeds. Next I would wonder if the Sea Foam maybe did a partial job of cleaning the carb. You said the mechanic told you it was pretty dirty. I can't imagine why a two year old ATV would have a pretty dirty carb. Gasoline has detergent in it and is not likely to cause problems that quickly. Did the bike sit around unused since it was new? Let's assume that it was pretty dirty and for some reason the Sea Foam did not completely clean it out as it claims to do. Could the Sea Foam have loosened some of the crud causing the crud to get stuck in a new place that is causing the problem? Another idea is some kind of air leak. At low speeds a small air leak makes a bigger difference than at high speeds. When you press on the throttle you are causing the carb to let in more air. If you are already getting in a little too much air, adding more at low RPM could cause it to stumble. A shop said it might need a jet kit. I would ask what has changed since it ran right. Changing jets is good if you changed the amount of air allowed into the engine. Nothing you said would cause you to need to change the jets unless they were set wrong to start with. Since it ran right before everything, that doesn't seem likely. Also I would check the idle speed. Does it idle nice. Could the idle be set too slow? Good Luck

 

I second the Traildad's air leak theory.

Since the carb has been cleaned/inspected and it seems to be more prominent at lighter throttle application when vacuum is the highest. It might be leaking air at the mounting flange. The surging that you describe is often a symptom of mixture changes.

The float may also be set a little too low or the jets could still be partially restricted. Bad gas sitting in a carburetor turns into a gummy varnish like substance that likes to restrict small passages. Sometimes it's difficult to remove and soaking the metal carburetor parts in lacquer thinner or carb cleaner helps.

Believe it or not the fuel pump can also be a suspect.

My 600cc sled had a low end bog/hesitation that we suspected was the carbs so we cleaned and adjusted them. This didn't eliminate the problem so we replaced the carb mounting boots because we thought they may have small cracks in them and be leaking air. This also didn't eliminate the problem.

It ended up being the fuel pump. In the midrange the engine is running at it's leanest so there is adequate fuel. When accelerating the demand for a rich mixture is higher and the carb bowls were already low from driving at mid to high speed because the pump was losing to the demand. So it was the worst when running at high speed, slowing down and then re-accelerating.

Just a suggestion.

 

Thanks to both of your replies. OK, so both of you have suggested air leaks. What would be the procedure to start diagnosing that? Is there a way to diagnose the air leak other than starting to replace parts?

To answer some follow-up questions to both of you:
-It idles and starts well. No problems there.
-The bike probably did sit around for maybe up to 4-8 months before I bought it. I bought it through a dealer that buys from National Powersports auctions. That's why I ran sea-foam through it and had the carb cleaned. I'm gonna get educated about how to do that minor maintenance myself in the future.
-Could the SeaFoam loosen something and it get lodged? The Sea Foam was months before this problem occured. I would hope that having a shop clean the carb would clean out any bits loosened. Then again, maybe not.
-I'll try taking off the mounting flange and inspect it. I would hope that the mechanic put it back on correctly after cleaning it. I'll look for any visual punctures, cracks, etc. Any shortcuts on that?
-If not that, I'll try and diagnose the fuel pump.

Thanks for all the helpful info!

 

If you can get it to run in the problem range while on a jack or platform you might be able to test it. If it could be done safely I would try spraying carb cleaner on and around the carb while it was running badly. If there is an vacuum leak it will suck in the carb cleaner. The carb cleaner burns like gas and it will cause the engine to change speed.

Does you quad have a fuel pump? I thought they were just gravity fed.