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TheFieldJournal

1989 Yamaha Big Bear Carb, Intake Boot, & Vin ?

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Hey Guys, 

I picked up my first ever ATV on friday. It's an 89 Big Bear. It has some cosmetic issues, but seems pretty decent mechanically. I did have trouble getting the idle tuned to the book spec of 1,300 - 1,500 RPM. I took the carb apart and cleaned it out. Somebody must have lost the spring for the pilot screw, as it's no longer in the carb. Also, I have a dual cable carb, I noticed all the IPL's show the single cable carb, but I see others on youtube had dual cable carbs. When I would adjust my idle speed screw, nothing would happen. I bottomed it out and it didn't seem to make much if any difference. When i pulled the carb apart I saw the screw doesn't even touch the linkage, it's too short. Do the two different versions have different length carb screws? I'm wondering if somebody lost the ordinal, replaced it with the IPL number for a single cable carb which is shorter, and gave up on getting the ATV to idle. If you hold the throttle just a bit to get it to the RPM range listed above, it "idles" great. Any ideas? Or any links to the dual cable IPL?

Have you guys have good luck with the aftermarket intake boots? Or is this a buy once, cry once deal? I'm guessing the OEM units last longer, but there is a huge price difference. If you have had good luck with aftermarket, where did you get it? My current boot doesn't seem to have any air leaks, but it's encased in JB weld... 

I'm getting a 9th digit check sum error on the ATV when I put the VIN in. Has anybody seen this before on a Yamaha? The VIN  number looks original as well as the paint around it. 

 

Thanks, 

 

Scott

 

 

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I figured out the VIN, I took a close look at it from different angles and one of the values was a C not a 0 as I thought. The checksum checks out now as valid, but the decoders are confused as they think it's a 2019 instead of a 1989. 

Have you used that carb on a big bear? The reviews look ok. The only issue I see is that's a single throttle cable carb, where my machine is a dual cable unit. 

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You can probably order a rebuild kit and get that spring for the fuel/air screw with the kit. 

Is there a name on the carburetor?

The idle screw may be wrong. Look up the blown up carb view in the manual and see what you find.  If you can post a few pics of the carb.  

You can also pick up the oem throttle cable and double check the length against the old one.  

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It's a Mikuni carb, I didn't see a model number on it. But it's the early dual cable type. I zoomed in on my service manual and it does show the connection for the second cable on the top of the carb. But the fuel bowl gasket isn't the same shape as mine which is weird. 

I found that a ball point pen spring cut down is the perfect diameter for the pilot screw, but the tension seems to be a bit low. I think I'm going to order a kit and soak the carb in chem dip and rebuild it. Then I'll check the float heights, main needle height, and set everything else per the book. 

For whatever reason the butterfly is now contacting the idle speed screw, it must have been hung up in the bore just a bit. But adjusting it has no effect on idle speed. The ATV is running super rich. The plug is black with carbon and the threads are wet with gas. 

Adjusting the pilot screw has almost zero effect on the idle as well unless I fully unthread it and it starts sucking enough air to lean it out. I'm thinking a port may be blocked in that circuit keeping air from the carb which may be part of the issue. I originally thought that screw adjusted fuel flow to the carb in to lean it out, out to richen it up. But it adjusts air to it, so it's actually the opposite. In to make it rich, out to lean it out. I haven't messed with a slide carb before. 

I went to check the mid drive oil level tonight and when I pulled the plug oil gushed out of there. Somebody must have tipped the quad on it's side and filled that location full. It does seem to leak out of that area by the drive shaft outputs, I'm not sure if they filled it up to keep it from running low due to leaks, or it being filled up is what's causing the leaks. 

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A full rebuild kit soaking and blowing out the ports would be a big help.  That fuel air screw is prob beyond shot.  I would go this route before replacing the carb with an aftermarket. Mikuni is a good carburetor and can be rebuilt. 

The mid level oil if over filled will cause problems with oil leakage.  

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