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technofarmer

1992 Yamaha Big Bear 350 - Engine Rebuild

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Hey there, i am new to this forum and am wanting to rebuild the engine on my old 92 yamaha big bear. Its a 350cc with the five speed clutchless manual, in other words nothing special. Oh, and it has that full time four wheel drive (actually 3wd) that is heavily overrated.

I was wanting to know, is there anything about that engine i need to know before i attempt to rebuild it? Is it finicky in any way? Any certain parts to use and not to use?

This four wheeler has had a very rough life, im almost tempted to save all the good and rebuildable parts and haul off the rest.

Any advice and info will be greatly appreciated, thanks

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they are bullet proof mostly like the honda 350's. known problems with them were stators going and crank bearings, but i'm not sure about the bearings from that year. it's a good motor and good bike IMO, but i'm partial to them LOL

this was posted in another forum >>>

They were pretty much a bullet proof bike when properly maintained. I don't recall any big bears except the special editions being sold with out front and rear racks. They were likely removed at one point in time to remove the front plastics and the rack was just never re-installed. It would not be hard at all to have one made up, they were only light steel and attach/bolt on at 4 points.

The front limited slip differential left a bit to be desired but was not really an issue unless hoggin' through snow or mud. A simple and cheap fix to help it hook up a little better is to change the differential and transfer case fluids and use Amsoil 2000 75W-90 gear lube. It contains friction modifiers (not to be confused with energy saving anti-friction modifiers) to help reduce limited slip differential clutch slip and put the power to the wheels. The other great thing about this oil is, it never needs to be changed EVER. Just drain it, filter it through a coffee filter and re-use it.

Lubricate the cables using a good cable spray and a cable luber tool. Don't forget the shift cable and shifter lock out cable that attaches to the rear brake.

Keep the valves in spec, check them once a year or at every oil change, its easy and cheap insurance.

Use a good quality oil. Don't use an overly heavy oil. Again I use amsoil 0W-40 for all season use and change it once a year. Anything heavier then 10W-40 will just shorten the life of the engines top end components.

These air cooled bikes run super HOT thanks to Yamahas designs and there lean jetting. Keep the oil cooler clean and free of mud. Chang the spark plug fairly often or switch to an iridium plug and even though it's a low compression engine use a mid octane fuel 89-92. This will help lower the combustion temps and eliminate any detonation or pre-ignition.

The Big Bears equipped with CV carbs were horrible for sticky slide valves do to a very tight fit and that can cause a bit of low end hesitation. In colder weather the carbs would freeze up easily and the slide would stick open or shut. The simple fix is to remove slide every few months and spray a shot of cable spray into the slide bore and the slide.

If it does not have the CV carb and still has the low end hesitation or backfires through the carb in tight woods riding etc adjust the float level a 1mm little lower (higher fuel level) turn out the mixture screw another 1/4 ~1/2 turn and again thank Yamaha for using such lean jetting.

The only other beef I have with these bikes is the less then water proof front drum brakes on bikes so equipped. Take some time and pop off the drums, lubricate the auto adjusters with an anti-seize compound or good quality silicone brake grease (sil-glyde) and make sure both pistons in the wheel cylinders are free, the top ones like to seize up. If they are sized up or sticky you can often use a C clamp to hold the one then apply some brake pressure and pop the other free. Then lubricate them with wd-40 and work them back and forth until they move nice and freely. Replacing these parts is costly so a bit of preventative maintenance will save you a lot of headache and cash

The rear brake is a little better, the caliper is adjustable from the outside by adjusting the large bolt. Just loosen the lock nut and turn in the bolt for more adjustment.

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I have a 1998 Yamaha Big Bear 350 4x2 that I recently purchased. These quads have great solid motors. I drive mine thru dirt, sand, mud, and water with good performance. It's not the fastest utility quad or the strongest by any means, but it provides decent power. My only complaint is downshifting into FIRST gear or shifting into REVERSE from neutral is sometimes a HARD shift. Do you have this problem with yours? Usually I have to rock quad back and forth to finally engage into reverse. And it seems worse when parked on an incline. I've turned the clutch adjuster on the side of the atv near the foot peg. It's a small nut u loosen and you can turn the adjuster cw or ccw but I've only turned it small partial turns for fear I'll make my problem worse.

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I can always get it into gear eventually but sometimes first gear or neutral is a PITA. The acceleration is pretty good but Yamaha put a super short granny gear 1st gear in these things. If I gun it from a stand still I basically have to shift into 2nd almost immediately. The rest of the gears are a bit longer but first whines out fast. I recently gave this thing it's FIRST oil change and I noticed a small amount of metal shavings in oil filter cover area. When I removed cover to change oil filter I noticed the shavings. I regret not changing the oil immediately after purchase (I bought it in Jan 2012 and previous own claimed he JUST changed the oil). I have been driving it for months off and on and it drives fine. Could this just be from a worn out part in the gear box or maybe a bigger problem? I am a very careful driver but I did let my inexperienced son drive it a couple times under my supervision and his remedy for hard shifts was to stand on the foot lever and cram it into gear. I ease it into 1st gear by rocking the atv when it gives me trouble. BTW he no longer rides it. LOL. I bought him an old 1992 Polaris Trail Boss 250 with PVT transmission. No shifting. Just gas and go. Any ideas or concerns about the metal shavings? I plan on riding it some more and changing the oil a little early just to see if it's still creating shavings or it was just a one time thing. This quad runs like a champ.

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i had a 2003 350 Honda that had the bull low like the bears too. they are a work bike more than a speed machine for sure. if your bike is running good and has good compression, i wouldn't worry about the small pieces of metal myself. the bike is 20+ years old and expected to have some wear.

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Very True. Thanks for the advice bro!

Sometimes I forget how old this beast is. Always fires up immediately and never leaves me stranded on trails. I do hate engaging the REVERSE tho with that stubborn shift lever and rear brake on. I think my next quad will be a 2 UP automatic. Maybe a Polaris Sportsman or Can Am Outlander. My wife sometimes rides double with me, so I bought a Kimpex Flexi Trunk rear seat box. Seat is comfortable as a Lazy Boy but makes the front end too light from all the person weight on rear. Dangerous to ride double going up steep inclines. Quad wants to hang wheelies on hill climbs. I mainly bought box seat for storage. Do you have any recommendations for a Mid to Large size quad that's automatic? I've heard complaints about PVT or CVT atvs require more revs to make them move compared to auto clutch in my Yama Big Bear. I know my sons' Polaris Trail Boss 250 2 stroke is like this. It always sounds like it's in FIRST gear at any speed. I thought something was wrong with it when I bought it until I researched the Polaris PVT system.

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i think most or all of the big brand bikes are pretty solid now. the atv market is very competitive, and no brand can afford a big mistake. i would take any brand out there right now without batting an eye. it will come down to your preference or past experience that will leave you reaching for the check book. i have always liked hondas and will to the end. yamaha is a strong second for me. but that's just me.

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I dont know for what reasons, but mine has never had that problem. Its usually been easy to shift and its never refused to shift since ive had it. My dad has a Suzuki 230, that is nearly impossible to downshift and ive tried to fix it twice. He also tried to fix it twice.

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Well I did mess with the clutch adjustment on my 98 yamaha big bear and I THINK it helped some. I don't know if it really made a difference or I'm just getting better at finagling the quad into gear. (ex. Rocking atv bk and forth) Since it only gives me a hard shift downshifting into 1st gear or neutral or REVERSE I can still ride it ok. Just annoying every time we stop for a break and start moving again. I'm going riding this wkd, so I may turn that clutch adjustment another partial turn and see if it helps. On another note, I was pondering replacing replacing my foot shifter with a Heel Toe shifter lever. Depending what boots I'm wearing sometimes the toe up shift is a PITA. Never used a Heel/Toe shifter, but it's a fairly cheap upgrade. I was also pondering getting a bigger foot board. the Big Bear only protects your heels with a guard, but toe area totally open. I like how my Polaris Trail Boss and my friend's Honda Rancher have HUGE foot boards to stand on. No risk of your foot slipping off peg. I found a couple after market boards online but wasn't sure if they would accommodate the space needed for the foot shifter and brake lever.

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Question; Does my '98 Yamaha Big Bear 350 4x2 supposed to have a fan behind the oil cooler? I see threaded mounting holes and space for a fan, but my quad has no fan. I wasn't sure if these were just mounting holes for something on a different model yamaha like the 4x4 version? My Big Bear has several nuts welded to the frame at different locations suggesting something may have been bolted to it at some point or maybe bolt on accessories. I know the bottom of my frame has several which are to hold a skid plate on that's missing from my Big Bear too. I can see if I can snap a photo of oil cooler area for clarity. Any help is much appreciated.:wink:

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Think I'll research a fan that fits and hook her up. I was pondering installing a snorkel on Big Bear. I don't plan on swimming with her, but for the occasional small ponds I cross on trails after a nasty rainfall I'd feel better. I've driven her thru water up to my fenders and that's about as deep as I care to go. My BB is only a 4x2 so I'm cautious which paths I take when it's really wet out. Local trails turn into mud holes after it rains here in FL. U never know how deep the water is until your quad nose drops and swallows u in. LOL. Any recommendations on Snorkel kits? I found Submarine Snorkels offers a kit that fits my quad, but most places only offer kits for the Larger and Newer atvs.

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