X9 180cc top speed / RPM limited?

[KrekerisLV]

Hello!

I'm pretty sure similar topics have been brought up in the past, so I apologize in advance for that.

Anyway, a short backstory: my X9 180cc developed starting and performance problems a couple of years back, so I took it apart to change piston ring and adjust the valves (compression was bad). When I took the head off and cleaned it, I saw a crack going from one of the valves to spark plug thread. Life got kind of busy and the bike sat like that in the garage for 2 years. Since I had lost a lot of stuff over the years, I simply ordered another 180cc engine from eBay and put it in. I swapped the two large pulley assemblies in variator from the previous engine since they seemed better visually (not an expert). My X9 starts well and runs great in the city (acceleration is okay, reaches 70 km/h fairly fast). Anyway, onto the problem:

I had a chance to take it out for a about 100 km ride on the highway and noticed that the top speed is very low (around 80 km/h at 7000 RPM) and it is reached at around 2/3 of throttle. The RPM doesn't go higher. Opening the throttle more causes the engine to stutter as if it was running low on gas. After the ride, I lifted it on the center stand and with the rear wheel free I opened the throttle fully - same thing, max RPM is around 7000 RPM, and it sounded like it's skipping ignition cycles. Before the previous engine started having problems I could squeeze out around 120-130 km/h.

My initial thought while riding was that the compression is poor, I haven't had a chance to measure it yet. By the way, what would be considered normal compression?
Then I thought about the carburetor - it could be dirty, and I'm pretty sure it's from a 125cc engine.

I dug around the forum a little bit and found some references to rev limiter which seems to cause similar behavior mentioned above.

Could anyone enlighten me about what the problem could be / where to start troubleshooting? It really sucks going 80 km/h on the highway, especially in places where the speed limit is 110 km/h. Thanks!

[Dave Milnes]

Low compression wouldn't stop it revving, but fuel starvation would, although on the centre stand it would tend to behave better than on the road as there is no load on the motor. As it's the same I doubt this is the problem.
If the top speed cut off was smooth, I would suggest the transmission rollers were badly worn, sticking or of incorrect weight, but your description of the misfire would tend to eliminate this too.
The rev limiter is built into the ECU so if you are using the same one as before, then it should still allow higher revs...unless something in the new engine is sending a different signal to the ECU.
The main stator coil that generates battery charge has a tapping that goes to the ECU for spark timing on smaller motors, so maybe the one in your new motor is not the same as in the original, or you have wired something up wrong, incorrect polarity or something. Also make sure the motor is properly grounded to the frame and the battery terminals are clean and tight.

[KrekerisLV]

Hello, Dave, thank you for reply and advice!

I checked the transmission rollers just in case and they are all going smoothly. I also changed the fuel and vacuum tubes since they might've had cracks somewhere from sitting for 2 years.

What I failed to mention in my original post is that the issue with limited revs only occurred when motor and transmission was warmed up. I could go to the city (~15 km) and on the way there everything was okay - could go full throttle with no rev limit, but on the way back I could only go up to 7000 RPM.

I realized that when installing the new engine I didn't clean out carburetor (although I had plugged all openings before storing it). So I cleaned it and now I have a new problem. When accelerating past 7000ish RPM engine suddenly starts to cut out but this time it dies fully instead of 'restarting' when I close throttle. I can make it run with stuttering for some time by rapidly opening and closing throttle (probably using the little spray pump on carburetor). After letting the bike sit for a minute I can start it again. On center stand there is no such issue - I can rev it up to about 10000 RPM (at least that's what the tachometer shows) easily and engine runs fine.

Could it be a bad carburetor or the rotor/stator assembly? I'm a little lost 

[Dave Milnes]

Definitely a carb/fuel problem. Is this carb the type with a rubber diaphragm, in which case try changing this. They do perish and this uses the inlet manifold vacuum to control the amount the metering needs rises so it meters the fuel smoothly to engine demand. When you open the throttle when on the stand and the engine isn't under load it goes to zero vacuum straight away and the needle will lift fully and quickly.
It sound like a thermal issue so it has to be something that changes with temperature. The rubber diaphragm could be perished and when hot is either holed, cracked or just too 'floppy' or something else that meters a richer fuel supply when cold is possibly not turning off and so over fuelling the hot engine.
Has the carb got one of those electric choke devices in which case remove it and test it off the bike to see if it moves. You may have set up the carb to compensate for this not working so it starts and runs well when cold but is way too rich once hot.
Not being able to start until it's stood and evaporated the excess fuel and the plug has dried is a typical symptom of over rich flooding.
Do you still have the carb from the original engine? Try putting that on.

[KrekerisLV]

Hello!

I managed to sort it out. You were right, it was the carburetor. Before that, I cleaned out the fuel pump vacuum mesh as it was quite dirty, but that didn't work. Turns out I had a carburetor from a 180cc engine after all. Luckily, I had a carburetor from a 125cc engine, and I cleaned it and put it on. Now everything works great, and I can squeeze out about 110 km/h, which will be fine for this summer. In winter, I will probably take the engine apart and replace the cylinder gaskets and piston ring as well as adjust the valves.

Thank you, Dave! 

[Dave Milnes]

Glad to hear it's sorted.