Ask The Editors: Do I need a Bigger Carb For My Big-Bore?
Dear ATVC:
I’m building up my son’s Honda TRX90 with the following engine mods: Ported head, TBQ Race cam from Beatrice cycle and a 114cc cylinder kit With Flat top piston, stepped header pipe to stock muffler. Here’s the question; Do I run the stock carb and jet it up (I think it comes stock with a 19mm bore) or run a larger Honda TRX125 carb (which is 22mm bore) and go from there?
Db z28
Sounds like this little TRX is going to be a screamer. The answer to your question depends on what you’re trying to accomplish and whether or not you have the larger TRX125 carb lying around. Contrary to common misconception, bigger carbs aren’t always better nor are they always necessary to make the most out of a big-bore. The reason for this is simple: a bigger bore cylinder and piston will be drawing in more fuel but also proportionally more air. Additionally, the way a carburetor works (the venturi effect) the greater the pressure difference inside the cylinder, the more fuel is drawn through the jets so while logic would suggest you want to go bigger, often times you actually have to jet down an overbore to eliminate bogging/ flooding down low.
Like always the case with a carburetor, properly setting it up will totally outshine its sheer size. In short you should certainly be able to rejet the stock carb to accommodate your modifications.
In the event that the quad begins stumbling in the top end and want to compensate with a larger carb, we’ve talked to several tuners who swear by the Mikuni VM 26mm Series 606 Performance Carb for the Honda TRX 90s. Of course these 90s are also running intake kits as well as full aftermarket exhaust systems so if you go that route, be sure to pick up a variety of spare main jet sizes (stock is # 195 but you’d probably want to pick up a 155, 165 & 175 to fine tune).
We’ve also heard that the big bore kit responds very well to CR8HSA spark plugs.
