1983 70 ATC starts ... then stops (then hisses) ???

[maximum-karnage]

Hi Folks,
My little ride starts very easily (with choke) but can't keep running for more than 5-10 seconds. Occasionally as she takes her last turn over, I hear a short hiss coming from either the engine or the carb (not sure). It seems to die even quicker if I giver her a shot of throttle.

I've got lots of spark, a clean carb, clean fuel filter, new fuel lines, and clean new gas.

I am guessing that because she starts easily it is not the spark; and because she dies shortly after starting the engine is not getting enough fuel? The intermittent hiss sound makes me wonder if there is a leak somewhere which is causing the fuel shortage...

Thoughts and comments are most appreciated!

Thanks

[T-Sox]

This could be a lot of things. you're going to have to start digging.

I would start by adjusting the valves. that can only help. Then I would look for a cracked carb/intake boot.

Good luck!

[maximum-karnage]

Thanks T-Sox ...

The "hiss" I described could also be described as a slight "sputter" if that makes any difference to understanding the problem.

In 20 years with this little machine, I have never adjusted the valves - I am not even sure what that means because I'm not very mechanically inclined - if I've never touched them before is it likely an adjustment will help? Could someone help me understand what that means?

Second, I know what the carb is but I'm not sure about the boot ... again :( could someone explain that?

Thanks very much folks!

[tri again]

carb boot? rubber hose type thing that connects carb to airbox and maybe one to the engine too.
adjust the valves?
In the cylinder head, under big fat caps. Need to have a clearance of a
few thousandths. Maybe best left to someone who's done it? any small engine shop.
Almost nothing will tune up right if the valves are not set right.
I too, have trikes that have not had valves adjusted.

The hissing sound could easily be a gas tank vent.
I can only say that because I had a few like that.
Run for a minute and die..so I'd check the gas for the hundredth time
which would let air in the tank and it was ok for a couple minutes and them
die within seconds.
Will it run with the gas cap off?

[T-Sox]

This should help with the valve adjustment.

https://www.youtube.com/watch?v=HppcqVpnaFw

That may or may not be your problem but that's where I'd start.

[coolpool]

I'm with tri, open up your gas cap and run it. Plugged exhaust? Simple stuff first.

[T-Sox]

He was pretty sure in the OP that the hissing came from the engine or carb. He also said it dies quicker if he gives it throttle. That would tend to rule out a gas cap vent issue. It doesn't hurt to try and run it with the cap off though.

These smaller engines don't like any changes in the fuel mixture so they can run fine one minute and like crap the next. I'm still thinking it's a valve adjustment causing it to blow back through the carb a little.

[BigHarp99]

I had a suzuki that wouldn't run right either, never figured it out, sold, next thing I know, I see the dude riding it. Sometimes I think we just think to hard and miss the simple things.

[T-Sox]

+1

20 years without a valve adjustment, I would say it's due even if that isn't the problem :lol:.