starts immediately, then shuts off just as quick (2016)

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1973x75

1973x75

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I kick it, it starts, but wants to rev too high, and then it shuts off, like it just runs out of fuel, but only after a second...not even enough time to empty the float bowls. Carbs seem clean, but maybe I'm not looking into them deep enough. Any suggestions?

Dave
 
What model bike?
If a '75, look see that your kill switch isn't turned to kill position. I've experienced just the same thing.
 
It's a 1973 750, not a Combat. Has a Boyer Bransden system that I installed in the early 90's. Amals are fairly new, less than 500 miles on them. The symptoms started after I did a brake job up front, but I can't see how taking the twist grip off and putting it back on could have had an effect. The original banjo at the float bowls has a slight crack, no drips, but wet. Battery is about four years old, but reads good on the multi meter. Less than 1000 miles on the battery. It's funny, when I was younger and rode more, there were fewer problems. I guess these old things just need to be exercised occasionally to stay healthy.
 
Fixed it. I took out the pilot air screws and worked a guitar string in there. That must have broken up a clog. Now for a test drive.
 
I am experiencing exactly the same problem as yours (starts and immediately shuts off), except for the fact that she starts only after having tickled the carb until the fuel overflows the bowl (just as we do when starting with a cold engine).

The problem occured today after 40 kilometers where everything was OK, and started with a sort of spitting then with erractic running for a few hundreds yards, then nothing.
I first suspected an ignition issue and tested a new pair of sparkplugs and even a new ignition coil, with no result.


I've read that Amal Premier (I have one single 932 on my bike) carbs may have a problem with the pilot circuit obstructed by debris or whatever cause.
So I dismantled my carb and verified everything except the two tiny holes (one in front and the other behind the groove corresponding to the bottom of the slide), and found nothing wrong.

Have you an idea of what else I should check? May the problem lies elsewhere than in the carb?

Thanks for any advice,

Laurent
 
Thks Acotrel.

"Spitting" is not the right word. It looked more like an ignition cut. Both sparkplugs colour looks OK.

I'm going to check and test more in depth the ignition system because I am a bit amazed by the fact that she starts only after having tickled the carb ,and shuts off immediately.
Could it result from an ignition issue generating a very weak spark, able to ignite a very rich mixture only?
 
When trying to fix an issue it's always best to fully complete your process of elimination before moving on to the next likely candidate, so best you complete your check of the pilot system and make sure those 2 holes are clear and also the gallery and chamber feeding them.
 
laurentdom - you don't happen to be running an older Trispark? Could be the output FET... Unfortunately to test, you need to replace the unit the points cavity.
 
The ignition system is a Pazon Sure Fire. I'm going to apply their Fault Finding procedure ASAP.
 
Salut Laurent, check your fuel flow from the petcock (robinets!), then you said you did not check the two tiny holes nearby the slide groove , so do it that's where the pilot jet send the fuel , when inserting your can of carb cleaner small tube into the pilot jet hole (removed the air screw for that !) you must see a strong jet of carb cleaner going out those two tiny holes , not only atomisation or poor leaks.......Pierre
 
1973x75 said:
It's funny, when I was younger and rode more, there were fewer problems. I guess these old things just need to be exercised occasionally to stay healthy. <> Fixed it. I took out the pilot air screws and worked a guitar string in there. That must have broken up a clog. Now for a test drive.
Click to expand...

One difference between then and now is certainly the appearance of rust, oxidation particles and some other debris in the fuel tanks. Fitting a secondary fine mesh fuel filter (in addition to the wire mesh filter on the petcocks) may save you from further carb clogging up problems, unless the carb itself produces the debris.

-Knut
 
Thanks to you all for your help.

The whole "fuel system" (tank, petcock, carb) has been carefully checked and the carb has been fully dismantled and cleaned with carb cleaner and air.
Nothing wrong was found.

I'm more and more convinced that despite looking like a fuel feed problem, Suspect N°1 is the ignition system.

I'll keep you updated once it has been thoroughly investigated.

Cheers,

Laurent
 
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