Bike stalling out when cold?

Okay, so I found this site not too long ago and received help immediately . . . chacal was great getting me parts in a hurry (thanks again) and all the advice was great . . . I find myself stumped once again and so here I turn for some more advice.

When I start my bike (1983 XJ750 Maxim) in the morning it will fire right up - most of the time without even using the choke - and run great for about a mile, then it just quits. It's almost as though someone cut the fuel line. I coast to the side of the road, sit a moment, turn it to prime, full choke, press the starter and turn the engine for about 30 seconds, bike fires right up and is good for the day. This problem seems to come mostly when the bike is wet though it has happened on nice days as well which convinces me that the bike is not picky about weather it just likes to keep me guessing.

Any ideas, petcock maybe? - I wondered if perhaps it wasn't working just right in the on position and now and again the fuel supply would get cut off - which I guess would explain why it will start again on prime, but that doesn't explain why it doesn't happen again for a couple of days. Point of this long ramble is that I need help. I tried searching the forums but didn't find what I was looking for so forgive a new guy if I am asking a popular question over again. Thanks again for your help.

 

A little more info to add . . . the last few days I have been driving it on prime . . . no issues . . .

Any takers on whether this means a petcock issue?

 

Certainly sounds like a fuel supply issue. The fact that keeping it on prime seems to resolve the problem.

The question is...what part of the petcock is the problem. Could be the flexible vacuum operated diaphragm. I'm presuming the vacuum hose is properly connected and should be, under normal circumstances, pumping that little diapghragm back and forth, sucking petrol down the fuel line into the carbs. If this isn't happening then it might explain why when you have it on PRIME (when the fuel is flowing freely and not dependant on a vacuum) there are no stalling issues.

Wait for a few more responses but looks like you might have to check your fuel supply lines to make sure there is no obstruction and that the diaphragm is working correctly in the petcock.

Good luck.

 

Running it on Prime bypasses the Vacuum Valve.
If the Vacuum Valve has an issue, the Bike starves for Fuel.

Investigate a couple of likely scenarios.

The hose from the Manifold to the Petcock is NOT Vacuum Hose and collapses on itself shutting-off Vacuum to the Petcock.
Same Hose is not Vacuum tight.
Same Hose gets kinked or bent cutting-off Vacuum from Manifold.

Petcock Tower needs a cleaning.
Petcock has an obstruction in the ON or Fuel Position. Probe with some weedwacker line.

Membrane on Petcock Vacuum Valve is leaking and not pulling with the Vacuum at all times ... weak opening of the Fuel Valve leading to Fuel Starvation.

 

With bike cold and not running. Have a rag handy to catch fuel dribbles

Petcock on ON position

Disconnect fuel line from Petcock

Disconnect vacuum line from cylinder #3 intake

Suck on vacuum line and see if fuel drains out of petcock (as it should). If it doesn't remove and rebuild petcock.

Paul

 

sounds like rickomatics got it .The hose from the Manifold to the Petcock is NOT Vacuum Hose and collapses on itself shutting-off Vacuum .bike warms vacuum line becomes plyable and collapses on it self and shutting off your fuel.had a vacuum line leak at one time and replacing was cheap and easy

 

Guess I should chime in on this one too! I had the same problem with my 1980 650 H , it puzzled me for months till I accidently found the problem. My signals quit working so I reached up and wiggled the wires,they began to work again but then the engine quit! Turns out the wiring was getting old and cracked so it would lose power ,usually while warming up so when it warmed up enough it stopped dying on me for no reason!!Not sure if this is your problem but it could be worth a peek????

 

I apologize for bringing this one back from the dead...thought it'd be better than opening a new thread

First things first, I am a motorcycle newb. However, I have worked on my car quite a bit, so I have at least some mechanical inclination.

I developed the issue spage described. My bike (82 Seca 750) starts no problem at maybe 1/2 choke (yes, enrichener); I'll let it warm up for 5-10 minutes before I ride. Typically, it hesitates in the lower RPMs until the bike's fully up to temp (choke off) then no problems...totally responsive from idle on up.

On Tuesday this week, it was the coldest I'd ever ridden the bike so far (maybe mid/upper 30s). Again, no problem starting, let it warm up in the driveway, though I had the typical hesitation in the lower RPMs as I started out. I continued on my merry way, figuring it'd warm up further and the hesitation would disappear. No such luck. The hesitation spread across the entire rev range. As I rode, I'd have to feed it a little choke (maybe 1/4) and it would get it's pep back. Eventually, the choke didn't help, and it eventually killed.

I turned the petcock to prime, twisted the throttle a couple times, turned on the choke, and it turned over and fired fine, but the engine ran rough. I was close enough to home and just walked it. When home, I tried to start it up again. Started fine, just ran very rough (it must have still been fuel-starved, even with the petcock in prime).

Dumb Question 1:
When looking at the inline fuel filter, should it be "full"? It looks less than 1/4 full when running, or at rest, in my driveway after I got it back.

Dumb Question 2:
Could the colder ambient temperatures effect the diaphram in the petcock so that it was too stiff to "activate" in the on position?

Dumb Question 3:
Are there other things I should look for that would cause this to happen "out of nowhere", from the colder temperatures or otherwise?

Things that I've checked:
-Plenty of fuel in the tank
-Vacuum hose is sound (no kinks, etc.)
-Fuel line is sound
-Throttle cable play is fine
-Choke cable play is fine

Thanks everyone!