i have a 1998 trx300ex and im thinking about getting a big bore kit for it. im wondering what else would need to be done if i get the kit. a couple people say ill have to change a bunch of things such as the air filter etc. im not too smart when it comes to motors but i want to learn. so if anyone knows what else ill need please post it. thanks.

 

about 10 years ago, Dirtwheels Magazine did an article on modding a 300EX. They had Mickey Dunlap from 4-stroke tech do a build up, including a big-bore and more, and it could outrun a stock 400ex with no problem. Here is another members contribution from a few years ago

03-21-2006, 03:42 AM
CoyoteRiverRaptor
Range Rover
Garage is empty, add now Join Date: Sep 2003
Posts: 189

Top Ten 300EX MODS-from Mickey Dunlap, Aug 96 Dirt Wheels

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Here is the old Mickey Dunlap top 10 list from the August 1996 Dirt Wheels magazine. Wouldn't you know it, I found it on my hard drive right after I finished retyping it? This is old so adjust the $$ over time.

HONDA 300EX MODS

This information was taken from the Dirt Wheels – August 1996 Magazine.

With a proven record for reliability, good handling and spunky though under stressed performance the Honda 300EX is enjoying phenomenal success. This electric start 282cc 4 stroke thumper has become a leader in the ranks of sport quads. Second in sales only to Yamaha’s BANSHEE, it has propelled itself into this vaunted position because it satisfies a wide range of needs inside a modest package. Can it be improved? Of course it can. It is under stressed engine-wise and it is relatively simple and inexpensive to extract more power and better performance from this vehicle. Here are 10 of the hottest tips that Four-Stroke Tech’s Mickey Dunlap has to offer for extracting the maximum in performance from this versatile off-road ATV.

PREPARE TO ROOST!!

1. I’d recommend installing a pipe first. The stock 300ex uses a very small diameter head pipe. We offer a 1.5 inch head pipe with megaphone exhaust that picks the power up considerably.

2. If you want to realize the full potential of any pipe mod on the 300ex then you need to install a black box mod with it. This will allow the engine to utilize the extra rpm that the pipe will give you.

3. You can get better airflow from the air box to carb by installing an intake tube from the old 350X. We have bought up quite a few of them and sell them for $45. The really do help overcome the restrictive stock 300ex carb air boots.

4. The stock carb (29mm Keihin) can certainly stand improvement. We have a 32mm carb kit we sell with throttle cable for $210. Combined with the pipe and black box mods you will see quite a bit of improvement in the EXs bottom-end, midrange and top-end performance.

5. After I had tried the pipe, black box, airbox and carb mods, I’d opt to install a higher reving cam for more midrange to top-end performance. Expect to pay from $120 and up for a reputable cam for the EX.

6. For more low-end torque I’d go with a higher compression piston kit. They sell for anywhere from $85 to $125 and are a good investment to make the Honda engine pick up power on the bottom-end.

7. My best power for the money would be a big-bore (330cc) kit for the Honda 300ex. We sell our 330 kit for $220 and it is a good value for the buck.

8. Once you start getting more power to the engine, you will need to do something about the increase in heat. We sell a low-priced oil cooler for the EX that lowers the temperature by 40 degrees for only $85.

9. Drill a 2-inch hole in the airbox lid, install a K&N filter and you will see a substantial gain in horsepower. On a stock motor we have found that we use about a #130 main jet at 1000 feet. A modified engine will generally run about a #140 main.

10. If you do modify the engine you will definitely need to add heavy-duty clutch springs to handle the load. We sell a heavy duty clutch kit for the EX for $17. You should also gear up a modified engine with a 14-tooth countershaft sprocket.

Hope this helps

CRR

 

not that it was your question- but I'd probably just sell the 300ex and buy a 400ex- you can always put the 16 or 40 bore over kit in the 400- and it's a bigger bike chassis wise. You could spend $$ modding your 300 to go like a 400, but why? just buy the 400.

don't know where you live- but here's a nice 400ex for sale by one of the site mod's

2003 Honda EX400

I'd make a real low-ball offer

 

i was thinking about gettin a 400 but around here the cheapest 400 ive seen that doesnt need anything is like 2000 nd up. and i dont think ill get that much for my 300 and i currently dont have a job and i dont have that much extra cash

 

Is your 300 totally stock now? if so, you will need an exhaust system, intake mods ( drill out air box lid, or take off and run Outerwears to keep crud out) along with the machining work to install the big bore kit, for the 330, you need to not only bore out and sleeve the jug, but machine your cases to allow for the bigger piston, this all adds up, you will have at least 1000 if not more tied up in getting the big bore set up. Then there is the down side, yes it will rip compared to a stock 300, but most of the famous Honda reliablility will be gone, and you will still have a smaller framed quad, so handling all the extra power will be harder, you may need to look at suspension mods, and wider a arms and axle to make it easier to handle, more money. I agree with beergut, find some part time job, or do odd jobs for cash, clean up the 300 real good and sell it and get a 400ex or a good used Z400/LTZ400/DVX400 (all the same quad, just different plastic) and you will have all the hp of a modded 300ex without modding, and be able to put in realatively inexpensive mods like a slip on, air filter and jetting for a nice power boost, without hurting engine reliability. Just ride and enjoy the 300 for now, and save up for next year's riding season.

 

100% agree with 95wolv-
I spent an easy $1000+ boring my 400ex to a 440 and buying the parts to make it run right- the machine work alone cost me a few hundred $$. You'll need to upgrade the entire top end- cam, fuel and exhaust.
Sell the 300 for what you can get and start making some low-ball offers on local 400's. You don't really want to start tearing into your 300 with a lack of funds.