Motorized Bike Lurching a Bit

[angus]

Hey everyone!

I just got my bike up and running! I'm very pleased (new build)! 48cc 2 stroke on an old cruiser frame - a 1985 Murray.

As I ran it up and down the road, I noticed that it lurches a bit. The engine runs smoothly without a load (clutch disengaged); however, under load (clutch engaged), it tends to lurch or buck. I'm pretty sure that it is the drive train. In fact, I tightened the chain a wee bit which helped, but it continues to do it. Any thoughts given my very brief description?

By the way, I used Norman's approach to sealing the intake manifold and it works like a charm! Thanks Norm!

angus

 

[deacon]

I have that lurch from time to time and have never found a real answer.

I hope someone can help you with it. My chain was too tight and I think that might be it, but you said tightening yours seemed to help. I thought mine might be the sprocket not centered but it doesnt seem to happen often enough to be that. I mean if the sprocket is the problem it is always off center if thats it. It only lurches now and then.

 

[angus]

I have that lurch from time to time and have never found a real answer.

I hope someone can help you with it. My chain was too tight and I think that might be it, but you said tightening yours seemed to help. I thought mine might be the sprocket not centered but it doesnt seem to happen often enough to be that. I mean if the sprocket is the problem it is always off center if thats it. It only lurches now and then.

The chain tightening definitely smoothed out the power transfer to the wheel. I aligned my sprocket with the drive sprocket during the build; it also appears to be true - in the same plane as the drive sprocket. I'm hoping that as I get better at adjusting the throttle, the lurching will diminish.

Funniest thing: I was taking great care during the initial start-up but forgot to turn on the switch (I installed an on/off switch on the black wire to the CDI). I pedaled like crazy... fuel on and showing flow... no luck with a start!!! Took me several attempts to figure out that the switch was in the off position. I have to laugh at myself... being so excited by the start-up and forgetting to turn it on! Feeling like a kid.

 

[Norman]

I know the feeling you are talking about very well. Is the jerking at low rpms? if it is then try ot run it a little faster I know sometimes I get mine to jerk when I goof on start up up to the point of knocking me off the seat. I have to look around to make sure no one is watching when that happens as I feel dumb.
norman

 

[deacon]

I set mine up the same way with the on off and have done that more than once since. The dumbest one for me was I set everything up and then ran up and down my drive. nothing, checked all the wiring then the plug and finally it downed on me. I had the throttle shut down to idle. I opened the trottle and it fired right up. one of those Duh moments.

 

[Norman]

We all have our midol moments.
norman

 

[angus]

I know the feeling you are talking about very well. Is the jerking at low rpms? if it is then try ot run it a little faster I know sometimes I get mine to jerk when I goof on start up up to the point of knocking me off the seat. I have to look around to make sure no one is watching when that happens as I feel dumb.
norman

Now that you mention it, it is at lower rpms that I see the lurch. I'll try giving it more throttle tomorrow (...I was trying to take it easy today for the initial run...). I'll let you know how it comes out. Thanks guys!

angus

 

[RedB66]

I'd like to jump in on this thread. My lurching is at high rpms. As I'm motoring along the lurch feels like I'm running out of gas. It is very suttle and quick. As I'm riding I can hear and feel a momentary drop in the rpm's. Sometimes it will happen 4-5 times in a mile.

 

[Bikeguy Joe]

I'd like to jump in on this thread. My lurching is at high rpms. As I'm motoring along the lurch feels like I'm running out of gas. It is very suttle and quick. As I'm riding I can hear and feel a momentary drop in the rpm's. Sometimes it will happen 4-5 times in a mile.

Carb float height?

 

[Bikeguy Joe]

On my kits, I noticed a "lurching" on the run down. I always pulled the clutch in when I was off the throttle, whether on a downhill grade, or when coming to a stop.

 

[Ilikeabikea]

Mine lurches a little from time to time. I discovered when it was not getting my fuel valve fully turned on. I have learned exactly where the valve goes now and have had no more problems. Thats probably not what ya'lls problems are but you never know..........................

 

[Pablo]

These engines are not exactly torque monsters, so when not at the top of high rpm torque curve they will buck a bit. Always happens when breaking them in, but will get better with time.

It's also a matter of plug selection (NGK B5HS and do the wire upgrade), fuel control, fuel oil mixture (IE tuning)

Not sure what part of the country you are from, but when cold the problem is exacerbated. Mine takes farting forever to get warm when ambient is at 50°F or below.

 

[Norman]

There is 101 ways these bikes will lurch We've covered 5 or 6 reasons. You all have good inputs on reasons why. Running out of fuel or starvation at high rpm will do it and so will too low of rpm cruising. Spark can cause it but I'm still stubborn and running the stock plug. I changed the plug wire out to an industrial wire which for me didn't improve my problem one bit it turned out to be my stock plug getting fowled out. I blast cleaned the plug and I'm still using it with no problems. High rpm and the lurching for me was not having the fuel valve on all the way. I know that's what got Bob as well. A short in the wiring will do it and can be hard to find. Run your hand over the wires while its running if you think it might be the cause. You will know if it is cause you'll get lit up if you've got just one strand of the wire sticking out and touching the frame now and them. I found that out, that thing will bite!
Wet climates will short out the kill switch and wet wiring can cause it as well.
Anyone else got one?
Here's my last if you buy one of these engine kits it can and will do all sorts of hateful thing if you let your guard down. But that is sort of the fun isn't it?!
Norman

 

[deacon]

This lurch thing reminds me of my third x wife. She came home cursing because she had to buy an alternator on the road. the man asked her didn't you hear the bearings grinding. She said I heard a grinding sound but I turned up the radio.

Sounds like thats the best thing to do with the lurch, turn up the radio except the other noises would drown out the radio. At least on my bike they would.

 

[angus]

...didn't you hear the bearings grinding. She said I heard a grinding sound but I turned up the radio...

Deacon - I did indeed turn-up the radio! In fact, I gave it more throttle and the lurching stopped. It appears that the combination of running at (too) low rpms and babying the motor during break-in caused the lurching. The little sucker loves the throttle and has started to purr a wee bit as I have given it more and as it breaks in. I live in a hilly area and am finding that the stock sprocket works pretty well - enough torque to get up the (moderate hills) with enough speed to keep it interesting.

I'm impressed with the little engine. It runs smoothly (so far) and fairly quietly. It sounds like I'm riding my chainsaw! I'm running stock plug wire and plug and built a wiring harness for it; therefore, shorting / grounding potential is minimized (hopefully). The gears make the characteristic whine. I'll try a wee bit of grease as I've seen in some posts. The chain is stretching; gonna have to take out a link soon.

 

[paul]

it will just get better and better. i could not believe when it broke in. you will know it. all of a sudden it goes and it sounds better. first engine i every had that i knew it was broken in. how long it takes i am not sure for every one of these bicycle engines is different and they sure are fun

 

[deacon]

That chain is a thorn in my side. I have a new one that does better as far as stretching but it will still 'move' the rear wheel forward after a while. I run a cruiser frame.

 

[angus]

Re: Chain Tension / Rear Wheel Movement

You know... I've been toying with the idea of designing a clamp that allows 1) positive locking of rear wheel (can't shift out of the drops) and 2) uses a threaded fastener to adjust the location of the wheel in the drops. It would actually have to be a pair of clamps that sit on both sides - both drops. I envision something that would allow for fine tuning chain tension and trueness of the wheel (...how parallel it is to the centerline of the bicycle).

Ever come across something like that? I've seen 'locks' on front wheels but not the adjusting functionality.

 

[Bikeguy Joe]

Deacon - It appears that the combination of running at (too) low rpms and babying the motor during break-in caused the lurching. The little sucker loves the throttle and has started to purr a wee bit as I have given it more and as it breaks in.

Definitely give it the throttle! Don't baby it past the first 5 minutes.

I'm impressed with the little engine. It runs smoothly (so far) and fairly quietly. It sounds like I'm riding my chainsaw! I'm running stock plug wire and plug and built a wiring harness for it; therefore, shorting / grounding potential is minimized (hopefully).

A good plug/wire/cap will do you a world of good...try it, you'll like it.

The gears make the characteristic whine. I'll try a wee bit of grease as I've seen in some posts. The chain is stretching; gonna have to take out a link soon.

Lithium, or white grease is best, don't get crazy with it!

........................................................................................................

 

[Norman]

Most all motorcycles have chain adjusters on the axles the ones that don't use drive shafts. My Harley runs a belt but it is like a chain and it too has adjusters to get the belt to the proper tension which when right seems tight to me..