broke down and puzzled...

[Guest]

Check the petcock

Check the petcock

check your petcock, my 750 would do the same thing for some reason the engine vacuum couldnt over ride the spring in it and would eventually starve it for gas. I would slowly lose power and just stop. I then would pull the vacuum line from the petcock and suck on it to open up the valve to fill the carbs again and hopeful help it work some. It did for a bit though I eventually replaced the petcock.

 

[jhicok]

Sorry for the long delay on this... but it was in fact vapor locking. i installed the dyna s and it ran for a little longer than it was on the stock ignition, but in the end it was still dying out after it got up to normal running temp... so i re drilled the hole in the side of the filler neck. i've put on about 800 miles since then with absolutely no troubles at all.... plus a little more power from the dyna-s i didn't really need shhhhhhh..... dont tell the wife lol

 

[850 Combat]

You're getting too much fuel if opening the throttle makes it crank easier. QUOTE]

They always crank faster with the throttle open. Pumping losses are lesswhen it doesn't have to generate so much vacuum, I guess. Same thing with a car.

 

[argonsagas]

Sorry for the long delay on this... but it was in fact vapor locking. i installed the dyna s and it ran for a little longer than it was on the stock ignition, but in the end it was still dying out after it got up to normal running temp... so i re drilled the hole in the side of the filler neck. i've put on about 800 miles since then with absolutely no troubles at all.... plus a little more power from the dyna-s i didn't really need shhhhhhh..... dont tell the wife lol

Vapour lock is a peculiar thing. As noted in your case, the carb bowls can be full, but the fuel will not flow.

It is not the same as having the petcock shut off, as fuel will still fall due to gravity down the fuel line, through the carbs and the engine will sputter as it leans and then it will stop. There would be a partial vacuum created behind the fuel as it went down the line, but the distance is not enough to build sufficient vaccuum to halt the flow.

What occurs with vapour lock is that the fuel cannot flow in any part of the system. It cannot even escape the carbs as the partial vacuum halts all fuel movement. Once you stop the bike and it sits for a while, air can seep in the tank...slowly...and then you get a flow again.

In the mid-70's this was not a rare problem on GM cars.
When it happened to my wife's car on a weekend, we were well away from home and all stores/garages were closed. The cap was removed until we got home, then I drilled a hole in it, just as you did. Never had the problem again.

I have heard that it is not recommended on current vehicles as it can lead to other problems with computerised systems.