Hard to reach 100kph /62mph
 

I recently bought a 2nd hand RX3 (carb type) here in the Philippines.



I'm satisfied with it for city driving however when I do get on the highway, i find it hard to reach 100kph /62mph. I have to push the throttle all the way just to reach. What should I adjust?

Odo reading is around 20,000 already. I'm not sure if the 1st owner already had its valve clearance adjusted.



https://i.ibb.co/VN6j27d/243176020-2...46797373-n.jpg

thank you in advance!

Valve adjustment wouldn't . What sprockets are installed? The previous owner may have installed a larger rear or smaller front sprocket to gain low end torque.

stock sprockets are installed.

stock 32mm carb is also installed. maybe it has something to do with the carb?

The stock set up on US RX3's is a 14T in the front and a 44T in the rear, IIRC. If the previous owner went down one in the front, your top speed sounds just about right. I run a 13T in the front and 65-70 MPH is about the absolute maximum I can achieve at WOT, depending upon the terrain and headwinds. We also have 17" rear wheels, I don't know if you have them also, or if you are running the original 15" rear wheel; if you have the 15" wheel, then that would also account for your low top speed.

yes, mine also uses those stock sprocket sizes. 14t and 44t.
I've also upgraded it to 17" for the rear and retained the 18" on the front.

I really noticed it when I rode together with other rx3 users here, I can't keep up with them.

A few other things to check would be a clogged air or fuel filter, a restricted exhaust, and engine cylinder compression; also take a good look at the condition of your spark plug when you remove it to do a compression check - it should be tan or tannish in color, not white, and definitely not black and sooty, which would indicate you are running too rich. If the plug is black and sooty, and your air filter is not clogged, then you will need to install different jets in the carburator. If you need to swap out the carb jets, I would use your friends bikes as a reference point as to which size jets you need.

Those sprocket sizes are likely your limiting factor. When I first got my Hawk, it had 14/55 sprockets on it. It wouldn't go any faster than 45mph, but it would climb a tree straight up! I changed it to 17/39, and with the other standard farkles (rejet carb, pod filter, aftermarket exhaust, etc) it made the bike much more useable on the street. I'd still make sure everything else is taken care of, valve lash being top of that list, but the sprockets are the likely culprit to not being able to keep up.

still using the stock sprockets 14/44.

does it have to do with the carb setting or jets? haven't had the time to send the bike to a mechanic to check on it.

These bikes come from the factory super (and I mean SUPER) lean. Most people find going up two or three jet sizes is just about right. I shimmed my needle with a thin washer, too. Look up Motocheez on YouTube. He has a lot of really good info on getting these bikes assembled, and how to do the necessary upgrades on them.

Always basics first.
 

Ralph, does the engine reach red-line in fourth and fifth gear? Obviously it doesn't in sixth or you would be near 80 mph.

But always start with basic maintenance steps.
1. Valve clearance.
2. Test compression.
3. Ignition system:
a. Spark plug condition.
b. Ignition timing (should NOT really be a problem).
c. Spark quality (fat blue spark).
4. Air filter clean, no obstructions (mice like to nest in them).
5. Fuel system.
a. Filter clean, good gas flow.
b. Carburetor clean, adjusted, throttle opens fully.

Those steps, in order. Applies to any four stroke engine.
Good luck and happy hunting.

on sixth gear, yes it nears 80. but i'm afraid that i might blow the engine since it seems that it red lines pretty fast.

An acquaintance called me one day after learning I had a China bike to ask me to diagnose the problem he was having on his bike -- same as yours, the bike would barely go 100 kph. I told him I was no mechnic but would come look it over. I did, jumped on the bike and found it would easily hit 120 kph with me on it. As I rode my whole 200 lbs. back in to the guy's driveway, I saw his 280-pound self waiting for me, and I had my diagnosis and treatment plan already. Diagnosis -- trying to move excess weight around with a tiny 18-hp motor. Treatment -- lose weight. Trust me, no amount of carb jetting or sprocket changing is gonna fix it if that's your problem.

It sounds like it's just geared really low if it's nearing redline at 80 km/h, either that or the clutch is slipping. Have you checked the sprockets to see what size they are? It could be just geared really low, or like I said, it could be that the clutch is slipping, I would check both.

I'll post a video tomorrow just so you can see the actual problem hehe

@oldenslow, weight is not the problem since i'm thin (60kg).

@franque, the sprockets are still stock. I'll change the rear to 42T from 44T (stock) when my order arrives.

actually this is not of a big deal when I drive it daily on the city, but during weekends, I tend to go highway and that is where it sucks. I'm stuck running at around 75-80kph

Could be as simple as regular maintenance is needed as others have said. Put a fuel system cleaner in the tank too.