Help with mystery...bike won’t rev past 5000 rpms

#1
Need some help here folks. Ct200ex with lots of mods and torque converter. Bike is fast. Pulls hard. No breaking up. It won’t rev past 5k rpms though.


Build:
196 Hisun
Head shaved .070
Ported/polished
18lb springs
32 degrees timing on 3.1lb flywheel
Billet +.020 rod
Stock piston
Mikuni vm22
Staged Header
Upgraded msd wire
3910x plug
Cl1 cam
Rider+bike=260lbs

Tav 30 series, 6” driver with white springs, 3100 rpm engagement
19” tires.

Gearing- 50/8, 50/9, 72/10

I ran the engine first with 50/8. Driven on middle setting with green spring. Bike hit 41.x mph. Rpms were about 4800. I thought maybe I needed more gear to gain rpms. Switched to 72/10. Bike hit 5100 rpms, still showing 41mph, and did NOT feel as peppy. Switched to 3rd hole on driven...still same. Switched to 50/9 on 3rd hole. Bike felt peppy like it did on 50/8 but still 41mph.

Tried moving main jet up and down. Still 41-42mph.

Bike isles great, mid range is great, top end doesn’t break up and it isn’t excessively popping. Seems good.

What is preventing the bike from going faster? I know the cam is small and I have a lager 265 cam on order, but I still don’t understand why the bike won’t rev higher. I thought the lower 72/10 would make it rev deeper but all that did was shift to OD quicker.

Am I missing something. The fact that I’ve tried huge variations in gearing along with different driven settings leaves me at a loss. All I can fathom is the head work and mikuni are too much for the small cam....but I’m even doubting that.
I’ve heard of people having mystery problems with torque converters limiting speed. I have plenty of power so something is up. Oh, and slide is fully open. No issue there.

Thanks!
 
#3
I’ve tried 135,140,145,150,155...no change in speed or rpm. Honestly I think it’s the torque converters fault. I just don’t know why. I have read stories about built motors being held back by torque converters but never really given an explanation.
 
#5
Check your valve spring installed height. May need a shim or two. What’s your elevation, and average temp? I’m running 22mm mikuni with 275 cam using a 115 main at 5000 elevation in Colorado spark plug is a tannish brown after WOT.

I believe the issue is valve springs or main jet setting.

try 115-130 on the main jet - your main jet may be too rich - check the spark plug after a WOT run, and report back.
 
#7
Valve springs were installed by Dover Powersports. They did all the portwork as well. Valve lash was se at .003 cold.

elevation is sea level.temps have been 60-75 during all runs. . I could try a lower jet but at sea level with my mods I don’t think that’s it...but I can try.
Only mods to driver are white garter springs. I have aluminum weights available but that would shift engagement to 4000 with the white springs.
 
#9
I can do that but can you explain to me what the difference is using the aluminum/stock weights vs stock weights and white springs? If engagement is roughly the same rpm then how does the aluminum change the equation?
 
#10
The weight of the flyweights effect the engagement with the same spring. Thats why i recommended the oem springs. Unless you wanted the higher engagement use the white springs. The heavier the flyweights the harder it will load the motor regardless with what gear ratio you use.
 
#12
Valve springs were installed by Dover Powersports. They did all the portwork as well. Valve lash was se at .003 cold.

elevation is sea level.temps have been 60-75 during all runs. . I could try a lower jet but at sea level with my mods I don’t think that’s it...but I can try.
Only mods to driver are white garter springs. I have aluminum weights available but that would shift engagement to 4000 with the white springs.
What does ur spark plug look like? Best to do a spark plug chop at wide open throttle to see if the main jet is rich or lean.. simplest way to tell, and will save you a lot of head scratching.
 
#13
I’ll have to check the plug. I haven’t messed with it because nothing has indicated either yet as far as feeling goes. The smallest I have is a 130. I can pop it in.
 
#14
I’ll have to check the plug. I haven’t messed with it because nothing has indicated either yet as far as feeling goes. The smallest I have is a 130. I can pop it in.
Cutting out at 5100 rpm would be the indication. If its too rich it'll bog out, and can foul the plug. If your plug is black then your main jet is too rich, and you'll have to go anywhere from 115-130 in my opinion.

Did your engine builder install the carb with the proper jets in it? Or did they leave that for you to do?
 
#16
That’s the thing. It isn’t cutting out. It isn’t breaking up. It simply isn’t going faster. Bike will accelerate from a stop to full speed extremely fast and then remain there. No fluctuation in rpms. It holds steady.

I did the jetting. Genuine mikuni. Genuine jets.
 
#17
Thats why i asked. Didnt see anything mentioned of it breaking up. If it was you would be changing a coil. Try what i mentioned and give us some feed back...
 
#18
Ok. Progress folks. The problem is some kind of inertia issue. Physics related. Anyway, I installed aluminum weights with white springs. Rpms went up. Installed aluminum weight stock springs. Rpms went up. Not fully into overdrive. Put driven back in 2nd hole.

Currently it has aluminum weights and stock springs with 2nd hole driven. Bike hits over 6100 rpms and settles in at about 5800 rpms in my short testing space. Overdrive still not fully engaged. Jetting is a bit off up top.

5800-6100 rpms is the upper range of the cl1 cam so that looks good. Speed increased to 45mph.

Now I know I’m headed in the right direction. The issue was the weights of the driver. Even though stock weight with white springs should have same engagement as aluminum weight/stock springs....the latter had a tremendous impact on the rev range. It also engages later than the stated 3300 rpms. Closer to 3800.

I also switched to a comet belt.

I will still be swapping cams and putting the 7” driven on when they arrive. Now I’m getting excited.
 
#19
Glad to here your making progress! I rev those cl1 cams to 7500 rpm all the time. Your spring pressure will be the limiter. I would use 22lb springs with that 265 cam your putting in. It has about 25 lb more open spring pressure. What gear is installed?
 

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