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?

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!

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.

Check all of your rubber carb hoses for leaks. Especially the large air intake hose.

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

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

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

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

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

K1500, How did you make out?

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

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.

I would mess with taking the exhaust off, only because it was already falling off.

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.

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

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.

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.

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

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

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.

I contacted my son , who works as a mechanic and a place that sells motorcycles, snowmobiles, and quads. And probably won’t have an answer until tonight.

I appreciate it. I messed with the pilot screw a bit today. Making it more rich *may* have made it start easier, but it still ran poorly when cold. When warm it was too rich and kind of fell on its face. Leaning it back out made it run better. I tried a cold compression test but the meter didn’t have have the appropriate adapter. Just holding it on there have a poor seal but still yielded over 90 PSI. I’m guessing compression is probably ok.

So you have a $50.00 carb from Amazon, probably made in China, and you have obvious air/fuel ratio issues. What does that tell you?

That’s the conclusion I’ve arrived at too…time to rebuild the original carb. It’s just weird it only does it when dead cold.

I had a 2000 Big Bear. Finally had to replace the carb in 2018 or so. Ran fine after that. If you are having cold start run problems, you have a choke problem. The old Big Bear is still running fine over at a neighbor’s place. Only problem with that is if it ever acts up, he brings it and parts to me to fix. Luckily, those old Big Bears are almost indestructible.

I'm no mechanic, but have a 2000 Kodaik 400, that just underwent major electrical overhaul. CDI box, rectifier, evrything short of harness and stator. Unlike yours, mine was sputtering at high end, or would die unexpectedly. Lucky a work buddy figured it out, he likes tinkering, he has race machines and such.

Have an 03 Big Bear. It's been almost trouble free. Had to rebuild the carb and change out the fuel valve.

I’m moving slow over the holidays, but I have the factory carb on the bench and see it has a torn diagram. That would explain the poor high end. I’m going to rebuild it and swap it back in. I’ll post up when I do.