50cc 4 stroke: terrible clutch action: any solutions please?

911hillclimber

New Member
Aug 5, 2019
11
11
3
68
Hello from England!
I have just installed a 50cc 4 stroke into a French 1950 motorcycle as per the pictures.
Gone in a treat with some custom engine mounts and sprocket machining due to the old chain specification.

Rode the bike for the first time yesterday and everything works, lovely smooth engine, BUT....

The action of the centrifugal clutch is truly awful.
The engagement is very rough, very noisy and gets worse with engine speed increases. It disengages instantly with engine speed loss.
The vibration is so bad during all this I'm temped to think something is very loose and further use will destroy the clutch.
Gear ratio is 14 teeth front, 46 teeth rear.

Going to take it apart today, but has anyone any tips please as to the cause?

Thank you, Graham.






Clutch apart and nothing seems loose etc, lot of dust and the shoe faces were not smooth. Cup surface as new.



 
Last edited:

FOG

Well-Known Member
Mar 3, 2019
282
609
93
68
I don't know much, still learning, but that clutch bell and reduction box are very familiar. It looks like you have a 4 shoe clutch that I haven't seen before, but I know there are 3 and even 2 shoe variations that fit that bell.

From what I've heard the 2 shoe is a bit radical, but you do have options and they aren't expensive.

Hopefully someone with more knowledge than me chimes in soon.

And that is a beautiful bike!!!
 

911hillclimber

New Member
Aug 5, 2019
11
11
3
68
Thank you for your reply. I'm certainly still learning!
The 4 shoe clutch does seem unusual, but it also looks in great shape and nothing obviously wrong, so checking the rest of the drive system too for run-out and alignment. Slowly I hope to catch this monkey!
The bike has been a pain from the start of the restoration, and it continues to 'resist' me.

I am suspect of the sprocket and the smooth running in the new chain, so will increase the taper on both faces of the teeth to allow the chain link plates to engage smoothly.

The bike was like this when I got it: A LOT of work so far and the cost....!

 

Apache Bronco

New Member
Jul 19, 2019
26
11
3
60
A beautiful build, and then to have this problem. Something is definitely out of tolerance. I would disassemble the clutch and blueprint it. Weigh, measure everything. check for trueness. Look on YouTube to see if there are any videos about clutch performance enhancements. I haven't had to mess with a clutch....yet.
 

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
6,608
416
83
Los Angeles, CA.
That's a really beautiful build!!


I think what you're experiencing is 'clutch chatter'.

That little 49cc engine barely has enough power to take off on it's own when installed on a bicycle; & you really have to pedal your arse off to avoid clutch chatter when starting off.

I would really suggest that you find a larger engine if you aren't going to have pedals to assist in take off.

The only thing I might suggest (if you really want to keep the 49cc) is using a Cheetah clutch & building a custom gear reduction drive for it.
 

911hillclimber

New Member
Aug 5, 2019
11
11
3
68
This has given me a lot to think about.
These engines and indeed bike conversions a re VERY rare in England, the 'craze' has not taken off at all here, but electric assist has.
I have looked through the transmission a bit deeper, removing a lot of old stuff so I can check things easier.
The custom sprocket (14 tooth) was 3mm out of line and I feel the tapers on the teeth were uneven, so I have done a bit of machining and will do a bit of mig welding this afternoon which will bring the rear sprocket (44 tooth) in line to the engine sprocket and with even tooth tapers both sides.
I hope then if this cures the chatter (a 'light' way to describe this vibration) I can progress, but if not then I'll check if I can get a cheetah clutch.
Mini dirt bikes are popular here and the stronger :confused: clutch assembly can be bought cheaply (20$) but I will have sprocket issues I think as the output splined shaft is very short.
If I really have to I will return to the original engine and try to get that going but I singularly failed to do that last year!

Thank you for the help so far everyone.
Graham.
 
  • Like
Reactions: Mark Murphy

911hillclimber

New Member
Aug 5, 2019
11
11
3
68
Checked out a few things on this conversion:
Spent ages getting the front sprocket to run true and to get the alignment right to the rear sprocket.
Feel I have this right now.
The rear wheel is not true though, the rim is not right, wobbles in every plane, so took the wheel to an expert locally today to get it true as possible. It is the original 1950 rim, should be done late next week as he has a lot of wheels to do (his speciality).
This should remove the vibration caused in the drive chain leaving the clutch which I think just chatters rather than engaging smoothly and locking. It seems to start to engage around 3K rpm.

Is there a 'good' cluch I could buy and use that is known to be smoother?

I can't find out much on the internet, but would a pit-bike clutch be better?
 

911hillclimber

New Member
Aug 5, 2019
11
11
3
68
Bought a 3 shoe clutch assembly for all of £5.46 inc post from a UK company.
The original one popped off easy and the new back on likewise. No keyway, just a taper.
Have run the bike up and around outside the house, and the power is awful, the torque moreso!
Took a look at the clutch and the wear is on 3 high spots only, not the entire shoe surfaces, so will run it is (buy wearing the clutch in under engine power until the 3 surfaces are bedded into the clutch drum and try again.

Spanners crossed this will work, or it is back to the original 1950 engine...or electric!

gas station near e
 

911hillclimber

New Member
Aug 5, 2019
11
11
3
68
Seems I'm on my own with this engine, but one last question please:

The new clutch I have fitted still only engages on the drum for about 30% of the surface area (you can see the wear marks).
Should the clutch when the engine is at max rpm slip?
 

Greg58

Well-Known Member
May 1, 2011
4,140
1,032
113
62
Newnan,Georgia U.S.A.
Given the weight of the bike with rider the clutch probably will slip, what size rear sprocket are you using? A large sprocket will reduce the strain on the clutch. But will increase rpm and lower top speed
 

911hillclimber

New Member
Aug 5, 2019
11
11
3
68
Thank you Greg, have taken your advice and because I cannot change the rear sprocket I've ordered 3 different engine sprockets this morning.

These are 10/11/12 teeth so with the rear @ 46 I will have ratios of 4.6 / 4.18 / 3.83
I machine the engine sprockets to suit the 10.5 mm engine output shaft dia.
I'll try the 11 tooth gear first and that I hope will allow the clutch to lock up and give a positive drive instead of clutch slip.

Will report back when done and tested.
 

911hillclimber

New Member
Aug 5, 2019
11
11
3
68
Greg was spot-on with his advice!
Machined and fitted the 10 tooth engine sprocket and the difference is remarkable.
Much more positive action, no clutch judder and maybe no real clutch slip.
I think the clutch shoes need much more bedding it, the contact area (where worn so far) is still about 30% max of the total shoe area, so a long way to go yet.

Thanks to Greg again for helping me sort this lot out.
Graham.

 

Jeffrotah

New Member
Oct 31, 2019
7
0
1
44
I had same issue with that clutch design its a odd ball usually came equipped in yes jack hammers.. Look up waker rammer RM75. I ended up going to afternartket 3 shoe tapered shaft from bicycle engines.com. results werea improvement but it has a higher stall speed before engagement. And after reading through your trail n error im assuming u used a 10 tooth sprocket that is usually used in a 2 cycle output? Cheers and sidewalks optional...
 

911hillclimber

New Member
Aug 5, 2019
11
11
3
68
Thanks for your post Jeff. I'll look at that, but I am also trying to get the original engine going (and failing) because of this clutch issue.
Irritating!
 

Jeffrotah

New Member
Oct 31, 2019
7
0
1
44
Thanks for your post Jeff. I'll look at that, but I am also trying to get the original engine going (and failing) because of this clutch issue.
Irritating!
Myself frustrated with a eye candy option switching 2 a 4 stroker so much info supporting that they are much smoother accross the rpm spectrum. I have had 2 engines known as the 142F family even one tapered the other straight shaft and both of them have given me same outcome. Engines have low end a thump if u will and once velocity is reached it smooths to a comfort im ok with. My overall concern is even with frames these days have welds that are robot and tedious with penetrating well im still doom and gloom and very possible to break under stress of elements we all can agree safety must not be overlooked
 

Jeffrotah

New Member
Oct 31, 2019
7
0
1
44
Myself frustrated with a eye candy option switching 2 a 4 stroker so much info supporting that they are much smoother accross the rpm spectrum. I have had 2 engines known as the 142F family even one tapered the other straight shaft and both of them have given me same outcome. Engines have low end a thump if u will and once velocity is reached it smooths to a comfort im ok with. My overall concern is even with frames these days have welds that are robot and tedious with penetrating well im still doom and gloom and very possible to break under stress of elements we all can agree safety must not be overlooked
 

Jeffrotah

New Member
Oct 31, 2019
7
0
1
44
And i have a lust pushing the envelope turning for a very healty power curve i know I'm not the only gear head here with attributes alike and duh speed kills may i mention envelope here