Engine stall during moderate/heavy braking

[iriematt]

Hi gang, interesting issue here that i'm struggling to diagnose on my '75 R75/6. Under moderate or heavy braking, the engine speed drops to the verge of stalling. I have to roll on the throttle (clutch pulled in) to keep the RPM's up and avoid a stall while braking. Once the bike comes to a stop, the engine recovers and performs fine until the next moderate/heavy braking event.

I've owned the bike for 6 years and this problem just started during the past couple months. Otherwise, the bike runs great. Rebuilt the carbs two years ago and the bike ran fine after that. i'm pretty sure the fuel floats (new) are adjusted correctly (level w/carb base). The carbs have not been synchronized for at least 6 yrs. Do i need to start there? Any ideas? Thank you!

[boxermaf]

I'd suggest that if you rebuilt the carbs, but didn't re-do the synch/adjustment on them, then you only did 1/2 the job and should really take care of that first.

That said, pretty much any carbureted bike will dip in idle speed briefly after a hard stop - the gas in the carb bowls wants to keep moving and sloshes forward and backward and forward. If the levels are adjusted right, the engine speed will likely dip under a nose-diving braking event but it won't stop dead, and idle speed will return to normal a couple seconds later when the gas sloshing calms down. If the level is too high or too low, it will either tend to force extra gas into the carb jet or it may start to suck in some air as the gas move from front to back of the carb bowls.

[44006]

confirm valve tappet clearance is ok - hot engine exhibits stalling
and lack of power off idle when valves close up - may not be
a problem with the carbs

[Bill Burke]

confirm valve tappet clearance is ok - hot engine exhibits stalling
and lack of power off idle when valves close up - may not be
a problem with the carbs
+1

[Guenther]

Maybe the ignition advance mechanic is the culprit. It may only slowly adjust back for idle rev.

/Guenther
(who only knows R75s from museums)

[mneblett]

Maybe the ignition advance mechanic is the culprit. It may only slowly adjust back for idle rev.

/Guenther
(who only knows R75s from museums)
Good thought, but that typically would cause the rpms to stay *up*, due to the excessive advance.

[RecycledRS]

What are the low speed jets set at.

[amiles]

While you are doing all of the other things suggested, pop the bail clamps and dump the carb bowls just in case you have collected some ungawa in them.

[Guenther]

mneblett,

I would not expect to see an engine rev higher with the throttle closed.

My theory...I think an engine at advanced ignition and low revs would rumble badly. And when the weighs/springs finally fold back the engine runs fine at idle again.

But that would mean the symptom has nothing to do with braking but suddenly closing the throttle.

/Guenther

[iriematt]

issue resolved after adjusting/synching the carbs! started with the easy tasks first: no gunk in the carb bowls and the valve clearance was dead on...no adjustment needed.

thanks a lot everyone!! i really appreciate the advice.

[mneblett]

I would not expect to see an engine rev higher with the throttle closed.

And yet, it does :D

A classic sign of sticking advance mechanism counterweights is the rpm idle remaining high (~2k or more) when the throttle is released. If you pull in the clutch and find the idle high when you come to a stop, try letting the clutch out a bit until it starts to drag down the engine speed, and if the engine speed returns to normal when the clutch is pulled back in (because the engine has slowed enough from the clutch drag for the counterweights to return to their base position), that's a good sign the advance mechanism needs attention.

As to why this happens, I believe the additional advance causes the engine to make more power from each combustion cycle, maintaining the higher speed and causing the engine to draw additional air past the closed throttle (which is, or course, not completely closed at the idle setting, as it is held slightly open by the idle stop screw in order for the engine to idle at all). The additional air flow then pulls enough additional fuel out of the idle jet passage (downstream of the throttle plate) to sustain the higher rpm. My experience with this condition is the engine runs perfectly smoothly at the higher rpm.

Mark

[lkchris]

And yet, it does :D

A classic sign of sticking advance mechanism counterweights is the rpm idle remaining high (~2k or more) when the throttle is released.

It's hard to believe but I've experienced it, ANOTHER reason rpm idle can remain high is one cylinder with no compression.

[Guenther]

Going by this from the original symptom description:

"... I have to roll on the throttle (clutch pulled in) to keep the RPM's up and avoid a stall while braking."

...the engine died with the throttle closed and did not turn at higher revs at idle.

Unless the throttle cable/plate prevents the throttle from closing to the idle setup that is what I would expect an engine to do when I "choke" it.

/Guenther