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Hi all, new to the forum from N.C. I'm seeking some assistance troubleshooting my four wheeler. It is (I believe) a 2000 Arctic Cat 300 2x4 Red. I recently purchased it for $400 with the description "needs solenoid and carb adjusted or new carb". I gambled on it and here's where I am now:
I put a new battery in it (solenoid was good) and was able to get it started and actually drive it some. The carb apparently had a problem as the drivability was poor. It would never take full throttle, and the performance seemed to constantly shift (sometimes idle fine, sometimes not, would always start, sometimes it would pull better than others).
The bike is in overall fair and complete condition. Although the bike was listed as a 2001, it has the Mikuni carb of the 2000 model, rather than the Keihin carb from the 2001 and up. After reading all I could find on the carb and CV carbs theory of operation in general, I took it apart to see what I could discover.
It appears to be in generally clean functional condition. The diaphragm is soft and not torn. The only thing I saw as a concern was the slide is visibly scuffed and worn on one side. I cleaned it thoroughly and reassembled and performance was then decreased from before. So after researching and reading mixed reviews, I bought a CCC (Cheap Chinese Carburetor) on eBay. As I only have $400 in the bike, I don't see dropping a load of money in factory parts, but maybe I'm wrong. The diaphragm and slide alone from Arctic Cat is like $131.00 by memory. Sooooo.. I bought the $40 ebay carb. A friend purchased one for a Timberwolf and it was perfect out of the box. Not so for the Cat 300. With the new carb (which looks pretty good), it fired right up, but still won't take gas as you increase throttle. In fact, it's not really driveable now. It just won't pull enough. I have read enough to know that the pilot circuit generally handles up to 1/4 throttle, the slide and needle jet/jet needle handle 1/4 to 3/4 throttle, and the main jet handles 3/4 and up to WOT.
My spark plug is always black, no matter how I adjust the carb, by my understanding meaning it's running rich. I have tried adjusting the float level, adjusting the fuel/air screw, and raising/lowering the needle, all with various but unsatisfactory results.
I have checked the petcock and fuel flows freely when open.
I have not changed the spark plug, but was assuming it was decent as the bike will fire up consistently even when cold. The choke (enrichment) system is working.
When the bike was drivable, the mechanical portion of the motor seemed tight and good. all the gears work. I checked valves and they were tight, so I adjusted them to spec. No noticeable change.
I want to conquer this project because I feel it could be a great bike, but I am at the end of my carburetor knowledge. I don't know where to get jets for the CCC.
The one thing I haven't explored is the ignition or whether there's some maladjusted rev limiter or something.
I'm hoping that someone with a passion for these, and an appreciation of keeping older thing running well, and some insight, will be able to help me figure out a next step.
Any help will be greatly appreciated... Thanks!
One last item... the intake boot between the carb and head has a slight crack in it. Very slight, but it was seeping a little fuel out. So in troubleshooting, I wrapped it well with e-tape and there is no visible sign of leakage now. I know this is only a temporary solution, but the cheapest I have found that intake is about $86 and I want to know I can get this running before I buy one. I will definitely replace with with new if I can iron out the other bugs.
Thanks again. And sorry for long post, just wanted to give full picture.
 

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How old is the fuel?
Air cleaner and intake clear(mice nests)
Put in a new plug properly gapped
What voltage do you get at the battery when it is running ?
 

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Discussion Starter #3
Fuel is new. Air cleaner looks decent and air box is clean. I have not yet replaced the plug, but will do that tomorrow if you think that's a possible culprit. I checked the voltage with my meter when I first put the new battery in and cranked it up and I don't remember exactly, but it was charging. I think it was 13.7, but I'm not certain by memory, I can check that tomorrow as well. I will definitely get a plug tomorrow. Thanks for your quick input!
 

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Discussion Starter #4
I got a new NGK plug today and gapped at .028. This seemed to make a slight improvement, but only slight. It still won't rev above about half throttle. The voltage at the battery when idling is 13.93. I'm no carb expert, but it seems to indicate too much fuel above half throttle. I don't know if that's correct or where to get a smaller jet for my CCC to test this theory. Any further advice would be greatly appreciated.
 

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So you have a black spark plug. It's running rich...too much fuel. Try this. Shut off the fuel and start it. Does it run better as the fuel level in the carb drops?

Could be an issue with the choke....which really is just an enriching circuit. If it's staying open you'll have excess fuel.

Start it up and watch the slide on the inside of the carb. It should quickly rise as you increase the throttle.

The fact that you have fuel running out of the intake means you have WAY too much fuel flowing into the intake. It should be fuel vapor at the intake...no liquid fuel.

Are you getting fuel into the oil?

Leaks in the intake usually show signs of lean running. If it's leaking there it usually is adding extra air to the mixture due to the vacuum pulling air in.

You need to verify that your float needle valve is holding a seal. What you are describing sounds a lot like a float needle valve leaking.
 

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Discussion Starter #6
Thanks for the input Torque. I will try shutting off the fuel and seeing if it runs better towards the end of the fuel. That’s a good idea. The choke seems to be working properly. No trouble starting it cold, but then need to turn choke off for it to run after that.
i have not watched the carb slide. The original carb would not run with the air box connected, but the CCC will so I haven’t looked in there. I will verify that tomorrow, but it is all new and the diaphragm looks good.
I will look at the oil tomorrow too. Not sure what it will look like if it’s getting fuel in it, but I will check that it looks normal.
I will also try to verify the needle and seat are working well, again, they are new. I just saw a method for testing that this evening while studying on this problem.
I also found a set of 20 jets on Amazon that have the original 137.5 in the middle so several choices downward. I think they will fit but I will try to verify by their stated dimensions a much a possible tomorrow. Thank you for your ideas. I will report as soon as I can test these tomorrow.
 

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If you are getting fuel in the oil it'll be over filled, it'll be thinner, and it'll smell like gas.

Does it use a vacuum actuated fuel petcock at the fuel tank? I don't think it does but if it does, it could have a leak in the petcock diaphragm which would be leaking in excess fuel since the petcock is activated by vacuum from the engine side of the carb.
 

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Discussion Starter #9
On my lunch hour I ran home and did a little testing. I turned off the petcock (btw, there is no vacuum connection. Gravity feed only, and I checked it and it flows well) I tried your suggestion of shutting it off and running it till the bowl fuel ran out. I could never detect a change of performance. It simply died when there was no more fuel. I also removed the air box intake tube and watched for the slide to go up and down when increasing throttle. It did, but only about halfway, because it just rev fast enough to do more. It looks like the vacuum portion is working properly, diaphragm is pulling slide up. BUT, very noticeably, it did not run near as well with the air box disconnected. It was harder to start and performed even more poorly than before. To me, this is opposite of what I'd expect from rich conditions, because to my thinking, no air box means less restriction to airflow and more air moves you in the direction of leaner. But perhaps that's just in the pilot circuit at that point.
You mention the hit and miss of the Chinese carbs. Do you have a lot of experience with them? Do you recommend trying to revive the original Mikuni instead? I think it is a BST 31.
 

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None of them run very good with air box removed
 

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I'd do everything to try to use the original carb. If you can't do anything to get the OE carb working, then it's worth trying a Chinese carb since the OE carbs are so expensive new. That said, the OE carb is usually your best option. The slide should react pretty rapidly when increasing the throttle.

Here's a video showing the slide in action. Go to about 5:25 and watch it.

 

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Discussion Starter #12
Yeah, that looks about how my slide was reacting. However with the rpms not going up, it never opened fully. I took the Chinese carb if and out the original back on today. The original works better, but still not really drivable. Seems to be doing ok on the pilot circuit and a little into the slide. But is definitely rich. I ordered a set of keys today. How we it’s confusing to me what would change over time to make the original 137.5 jet run rich. Will reducing the jet size lean out the midrange too? Also what are the things to check to verify ignition? That seems to be the only thing I haven’t checked. The motor sounds good and it starts and idles well, so I’m assuming there’s nothing wrong with the mechanical side of the motor. Do you think that’s a safe assumption? I don’t know what else to test except for reducing main jet size to see if I can illicit a change in performance.
 

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I would replace the jets with OE size jets. It's possible the original carb just needed cleaning but if the prior owner tried cleaning out the jets and increased the size of the jets, it would run rich. The jets would then need replacement.

Have you tried a new spark plug yet? It's not too common but I've seen spark plugs that would run ok at idle but not at higher RPMs. I think it's probably not your issue but since a plug is cheap, it's worth going ahead and replacing it just to rule that out.
 

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Discussion Starter #14
Well the kit I ordered has the original jet size and sizes above and below so I should be covered. I also think I’m about to buy another used OEM carb so if the problem is in the fuel delivery, I should have enough components to get somewhere. I may end up considering. The cheap Chinese carb as a $40 education. Any thoughts on ignition being the culprit?
 

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If it's running rich that's not an ignition problem. The fact you had some fuel running out of the intake it appears to be an excess fuel problem only.
 

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This makes sense. I really appreciate both of your guys input on this issue. My jets are looking like they'll arrive Friday, so at that point at least I can have something to experiment with. My thoughts are put a smaller jet in and see if that leans it out a bit and hopefully I'll see a change. I will post my findings.
 

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Well, I received my jets today. I went down in size incrementally and each step improved the ability to rev higher. It still starts and idles fine. I went all the way down to the smallest jet (90) and could get many more rpms but the the bike is weak. When I actually tried to drive it, it hardly had any power to pull itself. Also, while I had the rpms up I measured the voltage at the battery which remained a steady 14.2. I was hoping to find a non fuel related problem. After thoroughly testing (to the best of my ability) the OEM carb, I put the Chinese carb on and got similar results. I am frustrated and defeated. I know there has to be an answer, I just don’t know what it is or what the next step is to finding it. I wish I had a good running example so I could swap a known good carb and narrow the problem to the carb. Incidentally the plug is no longer black, so I think I have definitely leaned it out. Just can’t get good top end or consistency or power. Could compression be a factor? I do not have a way to test it currently, but it started well even when cold, so I was ruling compression out. That only leaves spark air or fuel. I just don’t know where to go from here.
 

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Is it too much fuel or not enough air?
Have you checked your air intake tube right to the front?
What do you have your fuel/air screw set at ? Not real familiar with that carb.

Are you trying to rev it up in neutral as a lot of machines have a rev limiter in neutral.
 

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This is a very unscientific way of checking compression but you can pull the plug and hold your finger or thumb over the plug hole and crank it. It should blow your finger off the hole (not off your body!). If you can keep your finger on it, it doesn't have enough. Not a replacement for a compression tester but a quick fuel test.

I still think there is something wrong in the carb. I don't trust the Chinese carb. Some work great and some don't.

Have you verified the float level is within spec? A float level too high can definitely cause rich running.

Put the original jet sizes back in where they belong for further diagnosis. Your diagnosis appears to have confirmed the rich running condition but the problem doesn't appear to be in your jets. The issue may well be in your slide. It's also possible you didn't assemble it correctly causing it to run rich.

Reach in with your finger, lift up the slide, and let it fall. It should fall with some minor resistance and you might hear a slight "sucking" sound. Test that and let us know what you find..
 
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