Engine dying when brakes are pressed at idle

Well if its not one thing, its another it seems. A couple days ago, i noticed that when i use the brakes at idle my engine dies. To be more specific, its when the brake light goes on that it dies. It doesnt die when i just turn on the headlights..that is until i brake. I know this has nothing to do with the kill switch because i can disconnect the kill wire from where it comes out of the engine and this still happens. After i've ridden about 2 miles or so and the engine is real hot, it stops misbehaving for the most part. Im thinkin that since the brake lights use the same power source as the ignition coil, there isnt enough power at idle rpms to jump the spark plug gap. i tried lessening the gap to .015" and still no luck.. what else could be wrong?

RE: Engine dying when brakes are pressed at idle

It looks like insulation is broken somewhere in the stop light circuit, so try to examine the wiring and connections for faults.

RE: Engine dying when brakes are pressed at idle

So you are saying it quits stalling once it gets fully warmed up?

In other words you come to a stop and put on the brakes and the brake light comes on and it doesn't stall? (When hot)

Then the brake light isn't causing it.

I would just turn up the idle speed a little.

Or adjust the idle mixture screw a bit to increase idle speed too.

A good carb cleaning usually helps.... and occaisonally putting a small amount of gas treatment in the gas tank to keep the inside of the carb clean is a good idea.

RE: Engine dying when brakes are pressed at idle

Reeperette /

This is almost positively an electrical short or misconnection.

Check the wiring for worn insulation, but also check and see exactly how the system is grounded, because if it's wire-grounded (using a ground other than the frame of the 'ped itself) and the brake light happens to be body-grounded, even accidently, therein lies your problem.

My personal preference is a body-grounded 12V system, but not all 'peds use that.

Most likely, somewhere along the line, when you bress the brakes, you've got a short...borrow a DVM (Digital Voltage Meter) and trace the circuit, or have someone do it for you.

If it's not electrical, check, in order, idle speed, fuel mix, throttle cable, compression.

Let us know if ya need more help.


RE: Engine dying when brakes are pressed at idle

well.. it wont stop doing it until the idle has been set so high that it becomes hard to keep the moped from rolling forward. my normal driving distance is from home to school or home to work and it wont stop doing it in that distance (about 1.5-2 miles). I have to go about 3 miles i think before it stops..

RE: Engine dying when brakes are pressed at idle

If it quits when its warm then It's probably not electrical.

If it was mine I would clean the carb by taking it all apart and using compressed air and carb cleaner... especially the idle air and fuel passages.

Then install an inline fuel filter (if it doesn't have one), and put a bit of gas treatment in the gas tank.

You didn't mention how old it is... most of the mopeds here are 20 years old... they have had 20 years to build up small amount of crud in the carbs.. and it causes 75% of the problems you read about here.

RE: Engine dying when brakes are pressed at idle

yeah..its a '78, i cleaned out the carb real good a month or two ago, though it wasnt very dirty at all.. but i took it apart two days ago and replaced the air filter element. Its got two fuel filters and those are clean... im thinking its something like the spark is having trouble jumping the gap when its cold. i have to check the plug after its only been running for a minute or two to see whether its fouled or not. im thinking it might be getting fouled and then burning off the stuff when it gets hot enough. i just dont know why it started doing this out of nowhere..i havent made any changes in a while..

RE: Engine dying when brakes are pressed at idle

It is not the dirt that you can SEE

It is varnish deposits INSIDE tiny little fuel passages

It stands to reason that if a moped gets 100 mpg that it is sipping gas through tiny holes.. the holes for idling (your problem) are even TINIER.. that is why they are easy to obstruct (they are as small or smaller than a pin point)... they absolutely have to be blown out with 100 pound compressed air and carb cleaner.. several times in each direction... you cannot see into these holes.

Shops who do a lot of this work have a "dunk can" of carb cleaner and disassemble the carb and put the parts in a basket and submerge it overnight to dissolve the deposits... then the next day (or two) he removes it and thoroughly blows it out with high pressure air.

To convince yourself it is not the electrical system being overloaded by the taillight drawing power.. try removing the tail light bulb.. if it still stalls.. what does that tell you?

RE: Engine dying when brakes are pressed at idle

Just had the same problem with a Vespa Grande. It had a bad taillite ground that would kill the engine when the bulb got it's brightest.

On many mopeds this is an unknown secret. for some reason the ignition circuit is grounded thru rhe stoplight and if the stoplite filament is burned out, the engine will stop when the brakes are applied.


RE: Engine dying when brakes are pressed at idle

the cimatti City Bike shop manual claims that the reason the ignition groung circuit is run throught the brake light (no the tail light) is for safty reasons...but after 1977 the decided that that was not a great idea an included a bypass switch under the headlight. so for those of you who have a city bike and dont know what that little switch under the head light is for....there ya go.

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