Engine won't stay running (1971 50 Special)

[cosmicsheep]

Hi all, i have lot's of questions about why my Vespa isn't running properly but since this is my first post i'll start by giving you all the details and what i've already tried.

OK... first off... i have a 1971 50 Special which i bought some years back in running order but needing some general repairs and tarting up.
It got through it's MOT and i rode it to work a few times before the kickstart spring gave out. A mate of mine is pretty handy with engines and offered to strip the engine and replace the spring.
<skip several years>
Upon rebuilding the engine he noticed that the piston pin looked worn and suggested i replace it or upgrade. So i went ahead and bought the DR 75cc kit which he fitted.

All was well for a few days, it passed MOT again and took me to work a couple of times before stalling one day (half way to work) and refusing to start again.

Now, it seems ok starting up but not for very long and when it stops it doesn't seem to start again until it is cooled down.

Here's what i've already tried and checked:
new spark splug B8HS
spark plug not oily or white
petrol cap breather hole seems ok
carb and float cleaned
fuel filter cleaned
air mixture adjusted 1 1/2 turns up to 2 turns
new fuel

the jet fitted by my mate is a 68. he wanted to fit a 63 but didn't have one.

I'm at my wits end and desperately want to get out on her.
Can anyone please give me their thoughts on this one or any ideas what else to try?

[ben]

Hi.
I've got a 1983 50 special and had similar problems. Here's a few ideas that I had to get around.

Is it stalling when you're flat out? If so chances are that the carb is being starved of fuel due to the big bore kit. I rode around a little slower til I fitted a fast flow fuel tap (which is a right royal pain to fit but did the trick).

When mine stalled I too wouldn't be able to kick it over til it was cool. I got round that by giving it a minute or so then bumping it ( just stick it in third release the clutch and run). I've got a 74 main jet on mine with a 125 kit on it. And I'm considering up jetting that so you may want to think about that.

But the easiest way to find out if it's being starved of fuel is to run it slower for a couple of trips and see if it doesn't cut out. Say around 30 to 35.

Next on my list is a 4 speed box to get the extra speed without revving it to death.

[ben]

If you do go down the fast flow fuel tap route here's a few pointers.

Get a second person to help. Then take the tank off and drain it. Then get a long 1/2" square drive extension bar. Stick it in the tank and wedge it against the nut whilst your helper twists the tap off from the bottom. This will snap the breather pipe. Then you'll find the filler hole is too small to get the nut out so you'll need to bend this out then shake the nut til you can grab it and work it out. Same with the broken breather pipe.

On the new one. Cut the breather down by about 2 inches or so (just above the bend) then feed the new nut into the tank and guide it above the hole with your extension bar. Feed the new tap in. Wedge the nut. Tighten and then bend the filler hole back til you can close the cap.

It's very fiddly and tests your patience ( unless you know a welder who can cut the top off the tank then weld it back on).

But the results are worth it.

[dave1967]

Hi
You dont say if the plug has fuel on it or not, when its stops do you still have a spark, is your HT cap the metal type they use to fit to 50 specials or the later ceramice type, have you drained the gear oil and has the oil got traces of petrol in it, have you changed point and condenser, i would look at all this before playing with the tank i have built many small frame engines and put kits on them and never needed to change the fuel tap in anyway but if you decide to do as suggested above buy a tank spanner they cost about £5.00 and save loads of agro.

[ben]

Tank spanner???

oh how I wish someone had told me that there was such a thing called a tank spanner before the 2 hour fiddle session. :-(


I'm gonna get me one of them.