(Slow?) Top Speed around 26-30 mph 80cc SBP Shift Kit

S

SomeGuy

New Member
So I have a Schwinn Ranger 26" mountain bike with an 80cc motor and the HD SBP Shift Kit installed. I thought my engine was doing quite well before I had the shift kit - 44 tooth sprocket I was going around 28 MPH top speed. I already had about 200 miles and was definitely way past the break in point, so I decided to order the shift kit.

However, since the first test ride with the kit (about 150 miles ago and many attempts at tuning everything,) I am able to top out in 4th, 5th, 6th and 7th gear all around 26-30 mph (varies as I ride down roads full throttling.) The difference between 4th and 7th (of course) is the extreme difference in RPMs. When I'm in 7th, it is a very low RPM, but it just stays steady at its pace. So it's like I'm hitting a brick wall in top speed on a level road.
For going downhill it's not much of a different story. I have noticed that when I am using the muffler that came with the kit, I will hit about 30-31 mph (so its not much of any advantage) but when I use my "banana" expansion chamber (no muffler) I have hit 35.7 mph going downhill in 7th (but going 29 mph before starting the downhill). In addition, the expansion chamber gives it better acceleration and generally leans towards a slightly higher top speed still up to 30 mph.

But after seeing so many videos about the shift kit (including SBP's promotional video) my engine seems comparably weak both in top speed and acceleration. Others seem to have the ability to cruise at 35+ no problem. I don't see my setup as much different, and I am not a particularly heavy rider (190 lb) so I just want to know what I can do to boost this setup.

So my engine setup is basically standard (NT carb, standard intake, standard muffler), and I am using a 32:1 fuel mixture. The fuel economy is probably around 80-100 mpg. However, I had rebuilt my engine after it had been on a different bike frame (it was truly a dud before though, only could go 15-20 mph with standard chain setup and it leaked gas throughout every gasket.) But after about 400 miles of riding it originally, I decided to pull it off the frame only to find out the jug was chipped in multiple places along the chrome liner.

My engine rebuild consisted of:

- New Jug
- New Piston + rings + wrist pins + piston rod (got the ring gap about perfect)
- All new engine gaskets
- New Shaft seals
- New magneto, coil, spark plug

But the end result: an engine that can idle well, doesn't leak fuel, doesn't have intake/carb leaks, doesn't really 4 stroke, has proper ring gap, spark plug gap, head torquing, 32:1 fuel, doesn't have much drag in clutch shaft area, pedaling is easy, bike wheels don't rub brakes, and bike wheels spin well.



I just really wanna use my bike to its greatest potential, and I don't think my engine is pulling it.

If any of you have questions, thoughts, or suggestions, please post!

I'd like to know what my options are, what things I should consider checking, your similar experiences, and any kind of insight. Thanks for reading!! :)

SomeGuy
 
often a bike with shift kit is a bit slower than one without - but the big thing is weight, with the rider being the greatest part of the total - 150lb riders go 3 to 5 mph faster than 200lb riders, and 250lb riders need to pedal a bit to get uphill
 
crassius said:
often a bike with shift kit is a bit slower than one without - but the big thing is weight, with the rider being the greatest part of the total - 150lb riders go 3 to 5 mph faster than 200lb riders, and 250lb riders need to pedal a bit to get uphill
Click to expand...

I know that a jackshaft puts some extra stress on the engine, but that doesn't negate the gearing advantage it receives. The 44 tooth sprocket compared to the high gearing ability of the shift kit is like 4th gear vs 7th. The shift kits can increase top speed because it can return to powerband as you switch to higher gears. Attempting to run at 30 mph+ with a 44 tooth sprocket is a screaming engine way past its powerband. In my case, however, it seems there has been no powerband for my engine to climb at higher gearing unlike many users of the shift kit as we've all seen on youtube. So that brings me to my question: Why cant my engine pull it, and what can I do to boost it?

But today, I may be on track to answering my own question:
So I decided to try the stock muffler without the end on it. It made my acceleration much faster, and increased top speed to about 31 on flat land in 7th and feels much more steady at that speed.

While I find it much more fun now, it still isn't comparable to many of the others with shift kits out there. So now I wonder if I would need to invest in performance upgrades (proper expansion chamber, boost bottle) at this point to get it to run at a higher speed.

Any advice, comments or suggestions?

Thanks
SomeGuy
 
You might try a adding a header to the pipe, adding length to the pipe will give you more low end power and move the power band lower on the throttle range. This might help you when you shift into a higher gear. Other things you can do to get more out of the engine are: (1) upgrade the carb (2) get a high compression head (3) reduce your squish gap (4) upgrade intake (5) port engine.
 
I've never heard of a boost bottle doing anything good for a motor without reed valves.

Since changing exhaust flow changed the way it ran, that could mean a clogged exhaust or a carb that got a better mixture at the increased flow rate - two options here would be get carb mixture correct for exhaust you have or get a better exhaust, like an expansion chamber type.
 
A properly tuned pipe is the single best power gain you can make with a bolt-on.
Find a real Japanese factory pipe (RM/YZ/KTM/CR) from a 65 or 80cc dirtbike and you'll see a big improvement.
I have one from a 1979 RM125. WAY too big really, but totally wakes up a chinadoll, even bone stock.
Remember you have more drag from the added reduction, so porting and a pipe are what you should be doing.
 
Thanks for the tips guys!

I have done over 100 miles since my last reply, with similar performance until yesterday. Unfortunately my motor locked up. Apparently some junk got in and the jug chrome plating chipped away a lot, and so I pulled the motor. (Lesson learned, get a good air filter and dont seal up an intake with junk and disintegrating gaskets) So nevermind that dud of an engine I have had since the start, I ordered a new 80cc engine for 80 bucks.

And with your suggestions maniac57, I decided to look up some of said exhaust pipes, and I ended up ordering a 2001 CR80 exhaust pipe. Do you have any suggestions for how I should get it mounted? I'm thinking of chopping the mounting end of my fake expansion chamber and getting it connected to the end of the exhaust (probably weld it). I have until the end of the week before the stuff arrives, but I'm just trying to figure out my options before it arrives.

Thanks!

SomeGuy

:-||
 
On my pipe, I used the stock RM flange and welded an adapter to the chinadoll port to the stock flange. This allows pipe to stay untouched and useable on the original bike, plus I get spring mounting for easy top end work.
I dislike altering Japanese specs. Hard to beat them for power, so I change as little as possible. Basically just made a cone from chinadoll port size to RM pipe size.
Mine has second pipe hanger placed perfectly for seatpost tube behind undertank mount, so it fits like it was made for a bicycle.
 
Well my new engine and the expansion chamber have arrived. I'll keep your tips in mind as I rebuild my bike and let you know how it goes ;)

SomeGuy
 
Hey guys!

Just thought I'd give you an update. So I had gotten the new engine installed after making sure it was put together properly (greased it, faced piston correct way, tested ring gap, replaced several and tightened engine bolts, torqued engine head to about 130 lbs each nut.) And from there, I tested with standard muffler and 16:1 break-in fuel. The main difference was that this engine didn't have any significant gear noises like the last one, and so it was extremely quiet but I was able to pull 28 MPH top.


After that, I worked on getting the CR80 exhaust pipe custom fit. Basically took my small "banana" expansion chamber, cut both that and the dirtbike exhaust so it could be welded together:


And sealed and painted it:


And finally, here is basically how it looks now:


So by this point, I've run it about 75 miles (still using 16:1 till I hit 100 miles) but its quicker than its ever been, and it easily gets up to 30, often can go up 32 mph, but lately it has been starting to go up to 34 mph. Earlier I was heading down a long road and when I had a slight backwind I was able to hit 37.0 MPH on flat land in 6th gear. I would have gone to 7th cuz it would have totally stuck onto the powerband at that speed in that gear but I caught up to traffic LOL.

But anyway, I feel like this is on its way to only getting better as I dial it in. It often 4 strokes and I keep tweaking the idle and I had dropped the needle a notch (closest to the top) but it seems to get better after it warms up for a few minutes of riding. I figure after break-in, that will also help with performance (and maybe I'll be able to cruise in 8th gear lol).

Again, thanks for the tips guys! I'll keep you updated if things start to take another turn.

.shft.

SomeGuy
 
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