Why does my bike stall during hard braking?

Kim Z. · Jun 26, 2016

My 1974 Commando Roadster stalls during heavy braking. Anyone have this problem and know why? What's the fix?

Rohan · Jun 26, 2016

That is possibly going to be either the fuel level is too low in the carbs,
or your battery voltage is low.
Neither of which are probably listed in the owners manual.

That is assuming that you have adjusted your idle with the engine warmed up,
to give a good consistent idle.

(Have you measured that the current draw of your brake light is only an amp or 2,
and is not shorting out the electrical system !?)
Just applying the brakes while its idling in the driveway would be a quick-n-dirty test for this...

Checking that you have adequate fuel flow out of each fuel tap could also be useful to troubleshoot this.
hth.

maylar · Jun 26, 2016

I would suspect that the float level in your carbs is low.

PeterJoe · Jun 26, 2016

A while ago I had that problem on my Norton Mark IIA. The problem started after I removed the large plastic air filter housing and installed the earlier type air filter. Unfortunately I did not realize that the original air filter housing helped support the battery. So afterwards when I would brake hard the battery would tip over which caused the negative battery terminal to short to the frame. In my case it was pretty easy to figure out however maybe something similar is going on with your bike. Possibly the battery is shifting in the battery tray or maybe the hold down strap is too close to the battery terminals causing a short or there is some loose wiring somewhere.

Peter Joe

Reggie · Jun 27, 2016

I once had a similar problem when one of the battery terminals came loose. A long shot, but worth excluding.

Reggie · Jun 27, 2016

Deleted.

acadian · Jun 27, 2016

Hard or soft stall? If the bike cuts out quickly as soon as you've closed the throttle, it's likely fuel starved. If the idle drops below your setting once you've stopped, and sputters its way to a stall afterwards... could be too rich. Either way, the plugs will tell the tale, look for indications of lean or richness on the base ring (not the insulator)

J. M. Leadbeater · Jun 28, 2016

Once had a car that would do this... I EVENTUALLY found the coil had leaked some of its internal insulating oil away and under hard braking the coil shorted out as the oil that was left shot up to one end..... A visit to my local breaker and another coil sorted the problem.

tomspro · Jun 28, 2016

if it idles fine in neutral, i would check for dragging clutch.

Kim Z. · Jun 28, 2016

More details: Let's say the stoplight turned yellow. I pull the clutch lever all the way in, then open the throttle a bit to keep the idle up. It stalls while I'm still rolling. Starts right up at the side of the road.

Deets55 · Jun 28, 2016

Kim Z. said:
More details: Let's say the stoplight turned yellow. I pull the clutch lever all the way in, then open the throttle a bit to keep the idle up. It stalls while I'm still rolling. Starts right up at the side of the road.

Try turning your mixture screws in (richer) about an 1/8-3/16 of a turn. You may have to adjust your idle after doing that.

maylar · Jun 29, 2016

The idle fuel passages in the float bowl are toward the rear of the carb. Apply the brakes and fuel sloshes forward. Check your float levels and that the bowl passages are not plugged.

tomspro · Jun 29, 2016

maylar said:
The idle fuel passages in the float bowl are toward the rear of the carb. Apply the brakes and fuel sloshes forward. Check your float levels and that the bowl passages are not plugged.

Good diagnosis!

I recently raised the float levels on my Premiers to a bit higher than recommended and find the old gal runs much better when cold, and no downside when hot.
Probably helps deliver sufficient fuel at WOT and when braking hard too.

Why does my bike stall during hard braking?

Similar threads