High RPMs low speed.

Rollingstones · Nov 22, 2014

My bike has started having very high RPMs and I am a bit confused as to what may be going on and how to attempt to resolve the issue...

I attempted to search for the topic but have not been able to figure anything out.

As I hit about 17-18 mph my RPMs will be within the 6000-6800 range. Could this mean that I am too lean? I performed a plug chop a couple of days before the issue started and it seemed to be ok.

66/80 cc China girl, speed demon carb, ADA airfilter on stock carb. Needle is at the second slot from top to bottom.

Rollingstones · Nov 22, 2014

44 tooth rear sprocket as well. Fastest I have been able to go on flat is 24mph... I feel I should be getting more then this.

maniac57 · Nov 22, 2014

Is the clutch slipping? With a 44 you should be going faster than 20mph at 7k...
But your overall speed sounds about right for a stock chinadoll.
Is the carb properly jetted? Does it pull strong all the way up or does it start four stroking at top rpm?
Read up on jetting.

Rollingstones · Nov 22, 2014

maniac57 said:
Is the clutch slipping? With a 44 you should be going faster than 20mph at 7k...
But your overall speed sounds about right for a stock chinadoll.
Is the carb properly jetted? Does it pull strong all the way up or does it start four stroking at top rpm?
Read up on jetting.

It may be. My pads are pretty shot. I purchased new ones and a 68 jet last night but haven't had time to install them yet. I'm really hoping itll fix it. I'm not even sure if I need to jet up or down but from research here it seems that the usual recommendation is to jet down so I'm giving it a go.

maniac57 · Nov 22, 2014

Every stock chinadoll I ever saw was blubbery rich from the factory. They don't care HOW it runs as long as it makes the warranty period so they jet them super rich to make them more reliable.
Once the engine is fully broken-in, try dropping a size or two on the main. I drill my own so I don't know the size. All the china jets are different anyway. You buy 5 jets marked "65" and get 4 different hole sizes so I solder and drill my own to suit. Most seem to like at least two sizes smaller.
Check your plug often while tuning and stay a bit on the rich side for safety and engine life.

Slogger · Nov 23, 2014

Hi Stones, I just checked my handy dandy gearing charts and I see 4500 RPM at 19.3 mph if you have a 44 tooth on a 26" wheel.

My stock china girl with an offset (longer) manifold and a NT carb needed a 66 jet (975 feet above sea level). The SHA clone I put on also needed a 66 to run right.
It makes sense, the same size carb/manifold/engine needs the same amount of gas.
A 68 or 70 probably won't be too rich and will leave you a safety margin against a leaned out engine seize up. Make sure you get the occasional 4 stroking on trailing throttle. Think of it as oiling time.
Good luck to you. Hope you get it straightened out.

Rollingstones · Nov 23, 2014

Slogger said:
Hi Stones, I just checked my handy dandy gearing charts and I see 4500 RPM at 19.3 mph if you have a 44 tooth on a 26" wheel.

My stock china girl with an offset (longer) manifold and a NT carb needed a 66 jet (975 feet above sea level). The SHA clone I put on also needed a 66 to run right.
It makes sense, the same size carb/manifold/engine needs the same amount of gas.
A 68 or 70 probably won't be too rich and will leave you a safety margin against a leaned out engine seize up. Make sure you get the occasional 4 stroking on trailing throttle. Think of it as oiling time.
Good luck to you. Hope you get it straightened out.

Much appreciated!

I rode around a little bit today after upgrading the pads and it seems that my main issue really is going downhill... while going uphill its actually pretty powerful and sounds great. perhaps this could be happening because I have a 44t on a 24" wheel?

When going downhill even slightly if I don't pull my clutch lever the engine sounds as if metal rotating on metal. I assume this is the wheels going way to fast for the engine itself...

I bought a 36 tooth and I'm debating whether to install it or just come to terms with the fact that perhaps this is just how its going to ride.

Any advice is appreciated guys, thank you so much.

Slogger · Nov 23, 2014

A 24" w/44 tooth at 4500 gets ya 17.8 mph. That sounds a little busy. I think the 36 tooth would be a good idea for street use with those 24" hoops. At 5500 RPM you'd be going 26.6 mph, which is pretty nice cruising.

Going downhill with the clutch engaged puts some reverse torque through the driveline.
The bottom of the drive chain is tight instead of the top run, the tensioner is loaded, the clutch basket gear is now driving the drive gear.
I'd grease the gears, tighten the drive gear screw, take up any slack in the drive chain (it could be drooping and rubbing something) and check the tensioner bushing or bearing, for starters.
If it keeps doing it, go downhill with the clutch in, save gas and let the engine cool a bit.

Hope it's nothing too tough to fix.

maniac57 · Nov 23, 2014

I like a 36 or 38 on My Pig. It's a good way to gain a bit of speed and lower engine rpm at your normal cruising rpm.
I seldom have to pedal assist and only on the steepest hills. Memphis is fairly flat so it's not an issue anywhere except downtown, and I can usually get a run at even the steepest hills.

crassius · Nov 23, 2014

going downhill, I always lock the clutch out - good chance to listen to chain PLUS I don't want to turn the engine that fast with no oil coming in the carb

2door · Nov 23, 2014

crassius said:
going downhill, I always lock the clutch out - good chance to listen to chain PLUS I don't want to turn the engine that fast with no oil coming in the carb

Ditto:
Downhill = clutch disengaged. With practice you'll find the right engine speed with the throttle to re-engage the clutch when the road flattens out to make a smooth transition from coasting to using engine power again. While coasting you'll hear chain noise. That's normal. The better the chain/sprocket alignment, the quieter it will be. If too loose or too tight the chain will produce some "metal to metal" noises.

Tom

Rollingstones · Nov 23, 2014

I have been having a hard time finding the right time to re-engage the throttle after going down hill but my chain alignment is actually on point and the noise is minimal.

Tightened my motor mount bolts and it actually helped with noise and vibration an incredible amount. Both became almost nonexistent.

Still am far from getting 27mph at 5500rpm but perhaps I just need to stick that 36 toother on there and see where that takes me. Perhaps I won't, I'm getting the itch to start a new build so I might look for a new home for the current one as it is extremely reliable and though not perfect it is an extremely fun ride.

The clutch pads were installed today and tomorrow I am going to jet my carb down to a 68 and see how she reacts. I am also going to invest on new bolts for the motor mounts and double them up.

Guys the help has been unbelievable, I cannot express my gratitude enough.

Rudz · Nov 24, 2014

I generally always pull the clutch in on downhills, I didn't know if it was common or not, I just always did it. I do like listening to the bike with no clutch engaged to hear any issues

Rollingstones · Nov 24, 2014

So I re-jetted the carb and it seems to be four stroking at low speeds. The 68 jet I put in looked to have a bigger hole then the unmarked stock jet...

I'm not sure what to do so I'm just going to putt the old jet back on :-||

Going up hill it feels so smooth and perfect with this thing, its just when I get going fast on a straight or downhill that I really have issues. I can go up a hill 18-20 no prob.

MotorBicycleRacing · Nov 24, 2014

Rollingstones said:
So I re-jetted the carb and it seems to be four stroking at low speeds. The 68 jet I put in looked to have a bigger hole then the unmarked stock jet...

I'm not sure what to do so I'm just going to putt the old jet back

Going up hill it feels so smooth and perfect with this thing, its just when I get going fast on a straight or downhill that I really have issues. I can go up a hill 18-20 no prob.

You can't tell what size the jets are by looking at them.

I measure the jets by using the wire gauge drills as go, no go gauges.

What clip is your needle on?
the second from the top would be good.

It might need a 66

#BitSize Inches Millimeters Jet Size

#80 0.0135 0.3429 34.29
#79 0.0145 0.3683 36.83
1/64" 0.0156 0.3969 39.69

#78 0.016 0.4064 40.64
#77 0.018 0.4572 45.72
0.5mm 0.0197 0.5 50

#76 0.02 0.508 50.8
#75 0.021 0.5334 53.34
#74 0.022 0.5715 57.15

#73 0.024 0.6096 60.96
#72 0.025 0.635 63.5
#71 0.026 0.6604 66.04

#70 0.028 0.7112 71.12
#69 0.0292 0.74168 74.168
0.75mm 0.0295 0.75 75

Rollingstones · Nov 24, 2014

Hey MBR!

Yea since I don't have the drills and haven't been able to find them anywhere I don't have a way to measure it.

I may just have to come grab a smaller one from you sometime in the near future.

Kioshk · Nov 24, 2014

I'm with Crassius, the only time I clutch-in without power is when starting, an when I need some clutch-breaking.

In any case, I suspect clutch-slip. I've used brake-cleaning fluid in the past to combat this. Make sure your clutch plate and main-spring (the one in-between the clutch-bearings) are adjusted properly. Only THEN consider brake-cleaner if you think the pads are oil-contaminated.

EDIT: and reduce that sprocket-size!

MotorBicycleRacing · Nov 24, 2014

he installed a brand new squeaky clean set of pads that he bought from me..........

Even filed the edges so the pads are a floating fit in the clutch so they grip front and back as opposed to being jammed into the housing like they come from the factory.

Yes, install that 36 tooth sprocket.

Will122391 · Nov 28, 2014

Rollingstones said:
I have been having a hard time finding the right time to re-engage the throttle after going down hill but my chain alignment is actually on point and the noise is minimal.

To smoothly reenagage at high speed you have to throttle it up to the revs it would running if it were in gear, then release the clutch/put it in gear. This is known as rev matching. As far as your top speed, it sounds completely normal. My bike always registers 25mph on speed traps on campus.

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High RPMs low speed.

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maniac57

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