New Build, possible clutch problems?

[Evmo]

Hi there everyone,

I'm new to the forum, I've introduced myself in New Members. I've just built up a 70cc HT with expansion chamber and 'performance' carb, and had it running a couple of times- but I have pushed and pedalled for about fifty kilometres in between with no luck!

After familiarizing myself with the engine and carrying out a process of elimination, I'm pretty dang sure it has spark, gas, and compression. All the head bolts, intake, exhaust, and spark plug are torqued down tight- the spark plug is replaced with NGK and shows a bright blue spark on test- and I can smell gas in the cylinder and exhaust after I've tried to get it going, so the carby appears to be doing its job.

The clutch appears to engage- there will be a large amount of resistance on the pedals and I get a very occasional 'thwunk' from the cylinder as the piston reciprocates- with no joy. However on the two times it has started, I can hear the piston reciprocating quickly and then it will ignite and run fine- then when I turn it off after a brief run, back to strange semi-clutching again.

While running, I can't get more than about 15-20kmh out of it, and it will bog heavily going up hills. I've got the clutch adjusted so the arm reverts to flush with the crankcase after releasing the lever, but I get the impression it's not engaging properly- it will take a second to engage once the lever is released too.

Has anyone else experienced clutch problems like this right out of the box, or any ideas on what to check for if I open it up?

Thanks everyone, loving all the great info here on the forum!

 

[HeadSmess]

time to remove covers.

clutch itself has a nifty lil "flower" nut, locked in place with one tiny lil phillips head screw. remove screw, tighten nut, replace screw. that adjusts up the plate so it can squeeze in far enough.

occasionally theres interference somewhere and something needs grinding back a touch. rarely.

if the plate cant possibly be adjusted any further... and there just isnt enough pressure...


by removing the sprocket, the entire clutch shaft can be pushed out of the engine (the kit should come with a puller!) and then...the little nut on the shaft, that adjusts the spring pressure, can be tightened.

if youre sneaky... you can remove the clutch cable post, the bit that screws into the crank case... and adjust the nut on the clutch shaft through that hole with a thin screwdriver...

 

[Evmo]

Thanks for the help there HeadSmess, I ended up just diving in anyway and taking apart the clutch, roughening up the friction plate a bit with some wet & dry sandpaper, and re-assembling with a tightened-down flower nut. Now it starts every time without fail and is a real pleasure to ride!

I took it for a good 30km run along some mountain ridges last Sunday and it performed admirably. I'm getting about 50kmh out of it on the flat, hoping to improve this with a new Mikuni carb, some port work and a reed valve.

However it's already a really useful little off-roader which is light enough to carry across rivers and push easily out of trouble etc- one thing I would recommend is carrying a small pre-mixed flask of fuel on the bike somewhere; there is nothing more welcome if you run out in the middle of nowhere.

So far I am really enjoying the mechanical accessibility of these little engines, and the fact that there is nothing too overly complex for my amateur skillset- makes for a real connection between engine and owner which you just don't get with big bikes.
Thanks again everyone-

Evan L