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I inherited a 2001 Yamaha Big Bear 400 (carbureted) that was used on the family farm. Like most farm machines it has been worked pretty hard over its life. It is down on power but only at higher speeds. It starts fine, revs fine in neutral, and takes off at lower rpm like normal. It kind of tops out at higher rpm. It doesn’t stutter or fall on its face, it just doesn’t respond to the throttle. I’m not a mechanic and the machine is old enough I don’t want to dump a ton of money in it.

I have:
Replaced the air filter
Seafoamed the fuel and intake
Rebuilt the petcock (leaked)
Checked the plug
Verified good fuel and fuel flow

None of this has fixed the problem.

I have considered:
Rebuilding carb
Worn cam
Worn rings/compression loss

Any ideas?

Last edited by K1500; 12/22/20.
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At first I was thinking it sounded like a fuel flow issue, but I think that would be come with some engine stuttering. It is likely a carb issue if the machine has been sitting idle for any length of time. If you are comfortable doing it, I would open up the carb and give it a good cleaning and inspection. Use lots of carb cleaner, soak the jets, and use firm toothbrush to get all of the shellac off. Blow carb cleaner and air through all of orifices in the carb body. The wire inside of a bread twist tie is smll enough to fit through most jets, and soft enough to not damage the brass.

Also check that the air box, air intake runners, muffler, and spark arrestor screen are unobstructed. Here in Michigan, it is common to find mouse nests, and chipmunk nut caches in airboxes and tail pipes.

What does the spark plug look like? The color and condition of the plug can help to diagnose the issue. https://ngksparkplugs.com/en/resources/read-spark-plug

Good luck!

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I actually ordered a carb rebuild kit. I toyed with the idea of just replacing the carb with one of the $50 knockoff carbs from Amazon, but decided against it for now. I can’t remember what the plug looked like, it’s been a while since I took it out. I was leaning toward the carb myself. I have a jet scrubber and brake cleaner. Worst case, I screw it up bad and order one of the aforementioned $50 jobs.

Airbox and intake are clean. I put a new fuel filter on, but I haven’t checked the muffler and spark arrestor.

Last edited by K1500; 12/23/20.
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Check all of your rubber carb hoses for leaks. Especially the large air intake hose.

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Do you mean the boot that connects the carb to the air box?

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Yes. Every time my Kodiak 400 acts up, My Mechanic mentions the rubber hoses and especially that boot.

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Last October I had to choke the heck out of it to get it started. Turned out to be a cracked rubber hose.

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I’ll add that to the list of stuff to check. Thanks.

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i'd replace the plug. might not do anything but it is a cheap try.


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K1500, How did you make out?

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May seem silly, but check the exhaust system.
Reduced flow there makes a difference all the way back!


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Thanks for the check. I haven’t had a chance to drag it out yet as I got busy with work and other chores. Maybe a spring project, and I’ve got a pretty good list to start on. I think I’ll check all the hoses and replace the plug. If that doesn’t work I’ll break down and rebuild the carb. It does need quite a bit of choke to get going when cold.

The exhaust *seems* to flow but I’m not sure how I would tell otherwise. It is super rusty after 20 years of use and abuse. I’m not even sure how well it would go when taking it off.

Last edited by K1500; 01/17/21.
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I would mess with taking the exhaust off, only because it was already falling off.


Some spelling errors can be corrected by a vowel movement.
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Well, it took longer than I intended to get around to this project. I just replaced the spark plug (no change) and put on a new Caltric carburetor from Amazon. I also put on a new fuel line and filter. At $39 for the whole carb, I figured it was worth a shot. First cold start was slow, but once it warmed up it ran fine. It had way more power than I remember, really quick now. I may have to fiddle with some carb adjustments, but based on a 3 minute post-carb replacement ride, I’m calling it mostly fixed.

Last edited by K1500; 11/13/21.
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Get a can of amsoil power foam and warm it up good then spray a bunch of it in the carb. Then let it set for a half hour. It will smoke like hell until you blow it out good. It will remove build up thru the whole system. Stuff can used on any gas engine, 2 or 4 stroke

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I Seafoamed the heck out of the old carb to no avail. The new Chinese carb has it running well. I think I have the cold start thing figured out. First, I replaced the Chinese choke with the factory choke. The Chinese one did not have an O ring on it. That didn’t seem to make much difference in cold starts.

Second, I had to remove the tamper proof plug to expose then pilot screw for proper air/fuel adjustment. That helped.

Finally, I repaired some wiring to restore the carb heater circuit. It seems to start easier and warm up quicker. Pretty much back to running like it should.

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One of the most overlooked systems on sleds and atvs is the belt/clutches. These are wear items and rely on springs to work correctly. The belt width, spring tension and weight shifting csn reduce efficiency by a bunch...20 plus percent. Lots of guys chase engine mods when clutch tuning can do so much more.


Originally Posted by BrentD

I would not buy something that runs on any kind of primer given the possibility of primer shortages and even regulations. In fact, why not buy a flintlock? Really. Rocks aren't going away anytime soon.
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Also I’ve had the vent hoses clog up that go to the carb and fuel tank. If they are gravity feed fuel system they won’t allow fuel to flow when clog

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A compression Gauge will tell you if the rings or valves are worn ....Most parts stores have loaners or will rent you one.....


“When you eliminate the impossible, whatever remains, however improbable, must be the truth.”
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So I have done the following:

New air filter
New fuel lines and filter
Oil and filter change
New carburetor
New spark plug
New battery
Adjust valve lash
Adjust clutch
Replace gear oil
Clean muffler
Fresh ethanol free gas with stabilizer
Seafoamed the engine and original carburetor

It still doesn’t want to run when cold but runs great once warmed up. I was hoping valve lash adjustment would fix it up, but it didn’t really help.

I need to check compression. Beyond that, I will tinker with carb adjustments a bit more. If none of that works I will rebuild the original Japanese carb and ditch the Chinese Caltric replacement. They get mixed reviews but I figured for the price they were worth a try. The thing runs and starts like new when warm but will fire and die several times when cold. It takes about 6-8 cold starts/die before it builds enough heat to idle. I’m still open to ideas.

I went for a 3 hour ride with all the above changes and the plug looked good. Certainly not too rich. I was going to fatten it up a bit but running the cold bike on the choke doesn’t seem to help, so I didn’t think it was an overly lean condition. I’ll play with it before I rebuild the old carb.

Last edited by K1500; 12/08/21.
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