Motorized Bicycle power output going on and off (Noise)

Sharif

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Joined
May 28, 2019
Messages
114
Was going off to work today all happy after I sorted few more things on my bike today all for another issue to arise, but this time worried it may be something serious.

Some background info first.
  • Engine is on roughly 300 KMs (+-50KM) .
  • Stock kit NT Carb, no aftermarket parts yet, apart from changing out the magneto that got rusted out by water 2 weeks back.
  • Exhaust never cleaned.
  • 66cc/80cc engine, model ZB-80JET-SR, recently put together 2 months back.
Day Prior,
  • Clutch wouldn't randomly engage, had to quickly tighten it as I needed to get back from the store and need the bike to head to work an hour later.) Bike been hesitating to idle since then from a cold start (Choke 50%), but idles normally once I turn choke off and engine is warm.
  • Ran out of fuel 10m from my home and just turned engine off after I noticed I was losing power
Things I did prior to the ride today,
  • Replaced Fuel lines mainly because it already started to harden, discolour and more importantly the hose wasn't a straight path downwards to the carb.
  • Seal fuel inlet port threads with teflon tape (Found out later fuel was actually just coming out air filter, but more than before strangely)
  • Refuel with 24:1 ratio mix of fuel (Previously had 20:1) used 160 mL of Engine Oil and 4L of Petrol this time.
  • Loosen the clutch one notch as I tighten a bit too much the day prior, noticed the bike would creep a bit with clutch completely disengaged.
Symptoms,
  • Going to work full throttle, maxing out at 41 kmph, all of a sudden the power just drops to the point I feel like I applied the brakes when I didn't and goes back up again (Plus new rattling noise of sorts, not sure where it's coming from, but I checked the bolts with my finger and they were snug).
  • Happened few times on the way to work (4.5KM ride) ended up peddling half way being too worried (Went back home with the engine on uphill, happened again, but this time just twice and avoided going above 60% Throttle)
  • New noise that goes away after turning engine off and just coasting down hill (Noise still there when clutch disengaged and engine on)
I tried being descriptive as possible and please ask away for any questions, any help would be appreciated, also I am new to all this so please go easy on me.
 


Sharif

Member
Joined
May 28, 2019
Messages
114
Little update,

Noise was just one of the engine mount rattling. Forgot to check that. (one bolt snapped, yet to take out)

Power output still feels strange and drops when going over bumps
 

Poor boy

New Member
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Jul 11, 2019
Messages
12
If it drops when going over bumps it is probably a fueling issue. Check the carb and clean the exhaust (it never hurts).
 

gary55

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Joined
Nov 27, 2012
Messages
3,958
Little update,

Noise was just one of the engine mount rattling. Forgot to check that. (one bolt snapped, yet to take out)

Power output still feels strange and drops when going over bumps
When you get the mount fixed it will start running right again. The reason it is speeding up then dying down by it's self with no change in throttle position is due to excessive vibration caused by poor engine mounting.

What happens is with the loose mount once the engine gets to a certain rpm it starts vibrating. With proper mounting it is not violent enough to affect anything. With loose mounting it is violent enough to cause the fuel in the carb bowl to foam.

The bubbles in the carb bowl along with the needle rapidly vibrating interrupts the fuel from properly feeding and the engine dies down. Once the engine has died down below the point of vibration the fuel stops foaming and flows correctly speeding up till it starts vibrating again and then slows down again. It will repeat this slow down speed up slow down cycle with no change of throttle position till you get the engine properly secured.
 

gary55

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Nov 27, 2012
Messages
3,958
One more thing. You need to get the mount fixed as soon as posible. With one stud broke all the stress that should be getting shared by 2 is now on 1. It will soon break as well being under double the stress it should be. Then if it's the front mount the engine will twist out of your frame form the force caused by the chain. Probably hit you in the leg destroy your chain maybe your wheel and who knows what else.
 
Last edited:

Sharif

Member
Joined
May 28, 2019
Messages
114
When you get the mount fixed it will start running right again. The reason it is speeding up then dying down by it's self with no change in throttle position is due to excessive vibration caused by poor engine mounting.

What happens is with the loose mount once the engine gets to a certain rpm it starts vibrating. With proper mounting it is not violent enough to affect anything. With loose mounting it is violent enough to cause the fuel in the carb bowl to foam.

The bubbles in the carb bowl along with the needle rapidly vibrating interrupts the fuel from properly feeding and the engine dies down. Once the engine has died down below the point of vibration the fuel stops foaming and flows correctly speeding up till it starts vibrating again and then slows down again. It will repeat this slow down speed up slow down cycle with no change of throttle position till you get the engine properly secured.
Long story short,
Aluminium frame, kit parts didn't fit, got a steel rod minus it was made from mild steel. This is actually my second time taking it out and swapping it after it sheared off.

With that said, learned about metal grading now and got my hands on 50mm Class 10.9 bolts and am just currently on my way to Canadian tire to get my hands on a screw extractor.
Last time around a fellow Motorized biker helped me out, don't want to bug him again and getting my hands on one of them myself this time.
I didn't ever think that far, something far as bubbling forming in the carb, this only happens at high rpms, which makes sense.

Thank you for the detailed response!

Edit
Forgot to mention my second bolt sheared off today. Taking the bus
 

Sharif

Member
Joined
May 28, 2019
Messages
114
If it drops when going over bumps it is probably a fueling issue. Check the carb and clean the exhaust (it never hurts).
That's the next thing I'll do after sorting the mount 😁 Already bought carb cleaner in advance expecting it to clog again at some point.
 

gary55

Well-Known Member
Joined
Nov 27, 2012
Messages
3,958
Long story short,
Aluminium frame, kit parts didn't fit, got a steel rod minus it was made from mild steel. This is actually my second time taking it out and swapping it after it sheared off.

With that said, learned about metal grading now and got my hands on 50mm Class 10.9 bolts and am just currently on my way to Canadian tire to get my hands on a screw extractor.
Last time around a fellow Motorized biker helped me out, don't want to bug him again and getting my hands on one of them myself this time.
I didn't ever think that far, something far as bubbling forming in the carb, this only happens at high rpms, which makes sense.

Thank you for the detailed response!

Edit
Forgot to mention my second bolt sheared off today. Taking the bus
Good call on graded bolts or graded thread rod. This is how I do the front mounts. Use a muffler clamp and split some tubing to place between the clamp and frame. This especially important on aluminum frames to protect them from the wear caused by constant vibration and pinch pressure concentrated on a small place.
View media item 61612
 

Sharif

Member
Joined
May 28, 2019
Messages
114
Good call on graded bolts or graded thread rod. This is how I do the front mounts. Use a muffler clamp and split some tubing to place between the clamp and frame. This especially important on aluminum frames to protect them from the wear caused by constant vibration and pinch pressure concentrated on a small place.
View media item 61612
That's pretty good idea, I already have tubing on the upper part of the mount I never got to add to the bottom as the snapping chronicalls started then. Plus I wish I had space to put on the universal mount, but my kit was missing bolts for that and even if it did, wouldn't have the space for it.
One of the make shift set screw got out with just 5 minutes of fiddling around with the screw extractor, but the other one is far too gone. Before I bought the extractor I was trying shove a Allen key in there to make DIY extractor, during the whole trial and error process I removed too much of the material and the whole wasn't centered nor was it parallel with the bolt, basically I ruined the bolt and threads around the bolt as well now, only solution in my head being redrill with a 8mm bit and retap it, don't have that tool either of course. Will just cycle to work tomorrow.
 
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