CT90 Lifan 140 needs more top speed

[FDChappy]

Hello I built a CT90 with a Lifan 140 from DrATV. The engine is bone stock except for a kevlar clutch upgrade. It is still using the carb that came with it which is a Sheng Wey pd17 also PZ-26. The main jet was a bit rich so it is now running a 93 instead of the 94.5 that came with it. The plug is a nice brown. Exhaust is an EVO BigGun oversized. Gearing is 17t front with the stock 41t rear.

The bike runs pretty well but goes flat at higher RPM's current top speed is just under 50 MPH/ 80 KHP. This is primarily a commuter (rural dirt roads) with occasional trail riding so I want to keep as much of the plentiful bottom end torque this engine currently has.

So where should I start spending my $ and what should I buy???

 

[hugemoth]

I have a similar bike, CT90, Lifan 140, same carb. I'm running a foam Unifilter mounted directly to the carb, 94.5 jet, cheap pit bike exhaust, 15/40 gearing. Plug color is perfect. It'll easily cruise at a GPS checked speed of 55 with my 6'2" 250 lb self riding it. My smaller and lighter daughter has had it up to 65 mph with a 16/40 gearing.

I also got the engine from DRATV and found the intake manifold was the wrong one for the engine, 26mm on carb end but 22mm on head end. Ran like crap until I got a correct manifold with 26mm opening on both ends. Wonder if you might have the same problem?

Q

 

[FDChappy]

Thanks for that info!!! I'll take a look see. I'd be a happy camper with 55/60 mph. I'm 5'11" and 220.

 

[FDChappy]

Ok just checked, 26.11mm opening in head and 22.23 opening in manifold. What did you use for a replacement manifold?

 

[hugemoth]

There is your problem. I found the correct one on eBay about 1.5 years ago for around $6 but don't remember the seller.

Q

 

[FDChappy]

I ordered a manifold with a Mikuni VM22 (26mm) carburator attached for $49 on ebay.

 

[hugemoth]

That ought to do it. The Sheng Wey carbs work fine though so you'll have a spare if you ever need it. Be sure to post an update once you get the new one installed and tested.

Q

I ordered a manifold with a Mikuni VM22 (26mm) carburator attached for $49 on ebay.

 

[FDChappy]

Ok got a 26mm intake manifold and port matched it to the head and heat isolator on the carb. Put it all together with a new Mikuni carb and the results are.... the exact same top speed. So that seems to eliminate the intake as the bottle neck. The new carb seems jetted just about right.

So perhaps the exhaust is the problem. The system I have has a rather small 22mm pipe from the head about 12" where it transitions to a 25mm pipe. Thoughts and comments welcome.

 

[hugemoth]

Looks like your intake was only one of 2 bottle necks. 22mm is definitely too small for the exhaust. Look for a 28mm.

 

[FDChappy]

Interesting report. Since changing to the VM26 carby my mileage has increased from a mere 80 to 93mpg! This carby runs a whole lot smoother too. It is now a bit cold blooded, but the choke system is nice and includes a high idle. Just had to richen the idle mixture setting about 1/2 turn. No more flat spots in the throttle response and revs to high rpm's without missing.

To resolve the exhaust problem I'm going to build my own. I've searched everywhere and have been unable to find anyone who can bend 28mm or 1.125" tubing in the approx. 2.25" radius needed for the "J" bend at the head. But I did find a place that makes exhaust parts for large displacement model airplane engines and ordered the bend and some pipe from them. Here's a link if anyone else is interested:
https://www.krumscheid-metallwaren.de/neu/shop.php?lang=EN

 

[tim_]

why not buy a stainless steel header pipe that already has the j bend in it and cut and well the rest so it suits your bike? stainless looks better then chrome dipped and you can get a "fish mouth" exhaust to suit... http://www.firepowerminis.net/index.php?dispatch=products.view&product_id=30371 .... its a fmf pipe too so more horse power and your supporting a mini riders member buy from this guy and he is in the states.. so no so pricey for shipping..

 

[FDChappy]

Well that pipe is $179 + s&h. The pieces I bought will cost about $15 and they are 304ss too. They are also flanged so I'm not having to do butt welds on tube. I plan to use my existing EVO muffler and heat shield.