Getting Your Benelli TNT 135 Throttle Body Sorted
The stock throttle body on a Benelli TNT 135 is the single most overlooked component when people try to eke out performance from this little two-stroke bike. I had a customer bring his in last winter with a bog that only happened between second and third gear. We spent three hours chasing vacuum leaks and spark plug issues before I finally got behind the carb and saw what was going on. Turned out the venturi was caked with varnish from three years of ethanol-blended fuel, and the plastic housing had warped just enough to throw off the air-fuel mixture at partial throttle. Let me walk you through what's actually going on under the hood and how to fix it without spending money on parts that won't make a difference. The Benelli TNT 135 uses a 19mm diameter throttle body with a slide carburetor. Most people think the bore size is the problem but it's not. The real issue is the calibration. Stock jetting runs rich at idle and lean in the middle throttle range, which is Benelli's usual approach for emissions compliance. You can correct this without a full rejet kit if you know what you're doing.
Before you start tearing anything apart, you need to understand the components. The main jet is located inside the carburetor body and controls fuel delivery at wide open throttle. The needle jet and clip-controlled needle govern the mid-range. The pilot jet handles idle and first few millimeters of throttle opening. On these Benellis, the main jet is typically a 95 and the pilot is a 45, but I've seen variations depending on production year and market. Here's something most guides won't tell you. Cleaning the throttle body doesn't require removing the carburetor from the bike if you have access through the intake manifold. I stopped taking carbs off completely about five years ago. It saves roughly forty minutes of work per service and eliminates the risk of damaging the manifold gasket, which happens more often than you'd expect. I use a simple spray carb cleaner like Gunk or CRC, let it soak for ten minutes, then blow it out with compressed air. The important part is making sure you get into the small passages, especially the emulsion tube holes around the main jet body. These clog silently and cause exactly the kind of hesitation you feel at 4000 to 5000 RPM. When I rebuilt my own TNT 135 last spring, I discovered the throttle shaft had about 0.3mm of play. That's within factory spec but it was enough to cause a slight richness fluctuation on my particular bike. The fix was a shaft seal kit from an aftermarket supplier, specifically the YHM kit. It cost about eighteen dollars and took twenty minutes. Stock replacement would have required ordering from Benelli directly and waiting three weeks, plus another thirty dollars in parts. The aftermarket seals fit fine and haven't leaked in over two thousand kilometers.
If you're looking to upgrade the throttle body for actual power gains, you're probably better off with a pod filter and proper jetting than a larger bore. A 21mm or 23mm conversion will improve top-end flow but you'll need tojet the entire carburetor and potentially modify the intake tract. I did this on a friend's bike and we lost about eight percent of low-end torque, which made the bike feel sluggish in city riding. For a 135cc two-stroke, the stock 19mm is actually adequate if it's clean and properly calibrated. The gains from a bigger bore only become noticeable above 8000 RPM, and you need to be on the pipe for that power anyway. One edge case that trips people up is the air box. The TNT 135 has a stock air filter housing that creates some restriction. Removing it for a direct intake setup will make the bike run leaner across the entire range. I tested this by installing an open filter and running without rejetting. After about five hundred kilometers, the spark plug was white and the clutch was slipping from overheating. I immediately went back to the stock intake and dropped the main jet by two sizes, going from 95 to 90. That brought the mixture back into the safe zone. If you do remove the air box, budget at least two hours for jetting adjustment and plug reading. Another thing nobody mentions is the condition of the reed valves. A worn reed pack will make your throttle body tuning efforts pointless because the engine can't breathe properly regardless of what's happening at the carb. Check your reeds every twelve thousand kilometers or if you notice a loss of bottom-end response. Replacement is straightforward, about an hour of work, and the parts run about twenty-five dollars. I learned this the hard way after spending a full weekend trying to tune the carb on a bike that had a cracked reed valve.
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The throttle cable adjustment also matters more than most riders think. Too much free play causes delayed throttle response, and too little play can keep the carb partially open at idle, running the engine hot and wasting fuel. The correct specification is two to three millimeters of free play at the throttle grip. Use a feeler gauge to check it, not your fingers. The stock cable tends to stretch after about six months of regular use, so plan to adjust it seasonally. If you want a downloadable reference, there are some good PDFs floating around in the Benelli owner forums that map out jet sizes for various modifications. Just be aware that those charts are generic and may not match your specific production year. Always verify by reading the spark plug after any change. The plug should be a light tan color after a ten-minute ride at varied throttle positions. Dark and sooty means rich. White and blistered means lean. Neither extreme is acceptable for long-term reliability. I don't recommend this approach if you live at altitude above two thousand meters without adjusting your jetting. The air is thinner up there and the stock carburetor will run significantly lean. I've lost two pistons to lean conditions at elevation and it's an expensive lesson to learn twice. If you're in the mountains, drop the main jet by at least three sizes from stock and test with the plug read every five hundred kilometers until you find the sweet spot.
The bottom line is that your Benelli TNT 135 throttle body is perfectly capable of delivering solid performance if you keep it clean, properly jetted, and paired with components that aren't worn out. Most of the problems people attribute to the throttle body are actually caused by clogged passages, incorrect jetting for the conditions, or unrelated engine wear. Start with the simplest checks first and work your way down the list. You'll save time, money, and frustration.
