Exactly what did you change over the winter from last fall ?

 

I changed the oil in the front forks, changed the rear suspension, removed the rust from the inside of the gas tank and re-coated it, redid the paint on the tank, put new upholstry on the seat. I'm trying to remember if I had changed the jet in the carb back to stock or not. I guess that would probably be the most important piece of info...

 

Did your work inside the gas tank partially block the fuel flow? The coating might have partially blocked the fuel outlet?

 

I checked that and also checked fuel flow through the new petcock I put in and everything looks good on those.

 

If you haven't changed gearing and your rpm is the same, your speed won't change regardless of power increase.

My crf230l tops out around 70 at wot. I can't imagine a 120cc having enough umph to propel a bike with gearing capable of 70.

 

If you haven't changed gearing and your room is the same, your speed won't change regardless of power increase.

My crf230l tops out around 70 at wot. I can't imagine a 120cc having enough umph to propel a bike with gearing capable of 70.

Hmm. I guess one possibility would be that my previous speedometer was off and was showing me topping out at about 100-110kmh hour but wasn't 100% accurate. Otherwise the only changes I made that I can think of that would possibly impact top speed would be to the carb. I may have swapped the jet I had in it back to stock over the winter while cleaning the carb. The stock main jet is a 110, and I have a 120 and 130 coming in the mail to see if those give better results. I also adjusted the jet needle and raised it a notch would could impact medium/high end speeds, but it seems unlikely that moving the needle up one notch would reduce top speed by 10mph.

Also, I did verify that my new speedometer (actually an original TC120 speedo) is accurate by checking it against a GPS speedometer app on my phone.

 

Could the clutch be slipping?

 

Now that I'm not 100% sure of. As far as I know it still has the original clutch plates, so I don't know how worn they are. Would it be feasible that the clutch would work fine in 1st and 2nd gear, but only have slipping issues at higher RPM's in 3rd gear?

Is there an eays way to test/check for clutch slippage?

 

The only way you can change too speed would be to be able to increase RPM.

If your tire diameter, front and rear sprockets are the same and you haven't changed transmission final drive, no amount of power increase can overcome a maximum RPM limit. It'd just get you there fast/easier.

You can Google speed calculator to understand top speed better.

 

I just used the top speed calculator here: The Colorado Sportbike Club - Speed Calculator
I filled out my bikes information the best that I could (Diam-18", rear tire size 90/90, primary reduction ratio 3.13, front sprocket teet-14, rear sprocket teeth-41, gear ratio (Used the ratio for third gear)-1.05, at 9000 rpm) and it calculated a speed of 67.8mph, which seems to match up with what I remember the top speed being last summer.

 

9000 rpm seems kinda high for that type engine

 

The engine powerband stops at 7500 and 9000 is too high a number, more like 8000 going downhill. Top speed listed at maybe 65 mph. If third gear was in low range of course it won't run that fast. One has to allow for forgetting the dual range if they can forget whether they jetted or not. Why you write it down somewhere, to stop such issues.

You DON'T jet just to get more power only and why do so many think that? It will only go slower doing it as same with the needle. Unless there is a PROVEN reason to mess with the parts, like a modification of some sort.

'I want to go faster I'll jet it up' was the reason I repaired so many messed up bikes back in the day. 50 years ago and the rules haven't changed.

 

I know my stock 1974 CB 125 would run about a maximum of 55 on flat ground, 60 down hill. That's by the speedometer which I doubt was accurate. My AT is the only one that's been accurate, my CBR was off a bunch, 71 was actually 65. My 230 was 69 actual when speedometer read 72.

 

I always assume the OEM speedos are high by up to 5 mph back then. My Bridgestone 90 and even the Yamaha 80 though would easily hit 55 moh and get on toward 60 flat land. We cruised at 55 mph. The twin 100 could hit 60+ for sure but that was an 8500 rpm range motor. The CB160 could hit 70 all out, those could buzz to 10,000 rpm. All are INDICATED numbers and likely high.

 

Thanks for the input guys. At this point I'm thinking that either:
A. there is no problem, its going as fast as its going to go
B. I was reading the following article: Motorcycle Clutch Slipping: Symptoms and the Causes
It almost seems like I could be having problem number 1:

1. High engine revs after releasing the lever
If you are keen enough when riding, sometimes you will notice your bike does not accelerate even after shifting gears. When shifting gears, you hold and release the gear lever but limited or no power transmission is transferred to the engine. So when this occurs, the bike either accelerates at a slower rate or decelerates. The high engine revs result in high rpm and a slow acceleration rate.


I can tell that its shifting from 2nd gear to 3rd gear, but once I hit third gear the bike doesn't seem to really increase its speed anymore (although the testing I've done has been over shorter distances of about a mile or so, I haven't gotten out on a long flat country road to hit 3rd gear, go WOT, and keep it there for a few miles to see if it does slowly increase speed). It may be that it takes it a good distance for 3rd gear to really get up to speed. Right now when in 2nd gear at high RPM's I'm pretty much hitting 48-50mph, and when I shift to 3rd gear I don't decelerate, but there doesn't really seem to be any acceleration either. I'll get to around 50-52mph in third gear at WOT and that's about it. I did change my transmission oil over the winter. I'll check it on my lunch today and see if maybe I overfilled it and its causing some clutch slipping in third gear.

 

Oh a 2 stroke, it might hit the 9k rpm.

Does it have reed valves, and do they have nice crisp edges? If they are frayed or chipped, it'll give you some issues.

Running a 2 stroke WOT, I've done that for about 10 miles on a cr125. It stalled about 7 miles in to that trip, started, died as soon as I got to my destination at 10 miles and let off the gas. Never ran again until I put a new cylinder, piston and ring on it🤭

 

Now you know what happens when you run one at 9000 rpm unless it is ported to do so. The engine locks up. Or tries to. That Suzuki is either piston port intake or maybe rotary valve and before reeds came out. Could have broken the rotary valve of course. Pieces can break off of them to mess the intake tuning up.

When you overrun a 2 stroke past normal rpm range they overheat, similar to having the wrong expansion chamber on it, which can do the same. The piston then gets bigger than it should and tries to seize up in the cylinder.

How much driver weight are we talking here? Too many potatoes over the winter may be the only difference here. I ask because weirdly the only TC120 I ever met at close range was owned by a friend who happened to weight some 300 lbs in the 10th grade. My Bridgestone 90 would easily drop him like a rock, he could not go any faster than barely 50 mph IIRC. That was one whopping big boy.

The baffle there should still bolt in, pull it and take a torch and burn the carbon out if it has the holes clogged. How much carbon is in the pipe is an indicator of how much may be in exhaust port.