I bought a '91 TRX 300 Fourtrax and I have a few problems. When I shut it off I hear something winding down after the motor has stopped, sounds like it could be the starter, is that possible and/or normal? Is it supposed to stay engaged or could it be stuck?

It also has a rattle on accel, like maybe a loose chain, but have not taken it apart yet to see. The noise could be related to my first problem, hard to tell.

Lastly, the cylinder has 421cm3 cast on it. Could someone have replaced the top end with a 450 setup, it is a 300 bottom end for sure. I can't find anyone who sells a 421cc kit for the 300 but the factory one's are all liquid cooled arent they?

I'm just not sure what I have here and would like some insight or direction prior to tearing the motor down completely. Drivability is great, the bike starts instantly, just afraid to push it too much til I resolve this noise concern. I'm an auto mechanic but new to atv's and any info would be greatly appreciated.

 

The wind down you mention is the starter over running clutch, and thats fairly normal on those units.
I doubt that it has a 421 cylider on it... Im guessing those are mold numbers.
Those units are all air cooled the 300s were never water pumpers.

 

Thanks for the info on the starter, that makes sense. I was just not sure how the starter on this model engaged. I knew that the 300s were always aircooled and I find it hard to believe that someone put a 421 head on this but it does say "421cm3". I could see if only the numbers were there but the "cm3" is what leads me to believe that is what it is. Would it somehow be possible to put such a large cylinder on that bottom end if you changed the head, rocker cover and everything up top? would such a large cylinder even bolt on to a 300 bottom end.
Thanks for your help already and any other help you or anyone else may be able to provide. I'm knew to the ATV scene and just want to fix this bike and enjoy it.

 

The side of the cylinder should say "281cm" if I remember right .. The cylinder (approx. 1 1/2") drops inside the crankcases , I'm sure for a 421 cyl to fit the crankcases would have to be machined out some around the rod... Does it have the stock 32mm carb , even with larger jets I don't believe the engine would get enough air thru a 32mm carb ...

 

Actually the bore on that machine is 74mm if memory serves me right which would make it about 3"across the cylinder not 1 1/2". If it had a 421 on with the same stroke as stock then it would need a bore of about 90 mm which is 16mm more then stock, Im not sure the cases would hold that much more bore without having to stroke it, to decrease the bore size.

 

I would say it is possible that they may have changed the cylinder, depending on how bad they wanted to do it. Measure the piston and stroke and you will know for sure. Bore and stroke specs can be found in the manual if you have it, if you don't have the manual you can find out from the dealer's service manual for that machine.

 

Thanks for all of your info. I did order a honda factory service manual, just waiting for it to arrive. I'm not sure what carb it has on it, I haven't measured it and i'm not sure how else to tell. Is there a part number or model number i can look up? I assume the carb has been changed accordingly because the bike starts instantly upon pushing the button and idles great as well, no choke or anything. Due to the rattling noise, I have yet to run it very hard for fear of damaging something so I can't really say how well it runs under hard load. It sounds like possible cam chain noise, maybe tensioner is worn out. I suppose the only way to know what i have for sure is to tear the entire engine down and measure the bore and stroke. I would imagine if it has been upped to a 421 than it should be a pretty good running bike, considering the weight and size vs power output?

 

Can you put up some close up pics of the engine?
Maybe of the carb also.

 

No I mean the cylinder drops down into the crankcases approx. 1 1/2" ..

 

I'm trying to get some pics of the engine and carb but they keep coming out blurry and you can't read the numbers on them. It says KEIHIN on the right side of the carb on a piece of black plastic and the number stamped on the left looks like 90CWECF? I can't see really well, could be way off on those numbers and will pull the fuel tank off to gain better access. Should the size and/or model of the carb be obvious just by looks or should it have the size stamped on it somewhere?