Can't get carburation right

[NC Steve]

I cannot get Britta to idle correctly, or more precisely, to return to idle at stop or between shifts. I've had the carbs synched, adjusted & lubed cables, checked for leaks at the intake boots, and yesterday, as the symptoms are that of a too-tight valve, got those adjusted. One exhaust was tight but the rest spot on, and now all are within spec.

She still refuses to return to a reasonably lower rpm between shifts, or drop to the usual 1K or so at a stop, and the problem only gets worse the hotter she gets. Otherwise she runs great, with plenty of smooth power.

Throttle return springs? Cable splitter? Cables themselves? Carb innards? What'm I missing here?   :-?

[Bob_Roller]

Centrifugal advance in the ignition 'bean can' .

[steve hawkins]

The carb balance is always the last thing you do and it must be done with the engine up to temperature.  I.e. after a 10 to 20 minute ride.  Since you have fiddled with something, its time to balance it again.

If you have 'had the balance done', are you sure the bike was hot when it was carried out.

I seriously recomend that you learn to do it yourself, it is not that difficult.

This issue is probably down to the sync being done whilst the bike is still warming up.

Cheers

Steve Hawkins

[Barry]

It can be a sticking advance mechanism but as Steve Hawkins said it's just as likely to be carb settings.

Only way to be sure is put the carbs back to base settings and start over.



 

[Yikes]

My bike has a similar problem when I first got it.   Turned out to be the timing, which had been set when the engine was cold.  A timing adjustment with the engine fully warmed up solved the problem without ever having to get into the carbs.  Just watch out for those hot pipes while  removing the front cover and when you're looking through the timing hole and turning the bean can.  Long sleeves and a thick glove on the left hand are highly, highly recommended.

John McC

[Ed Miller]

The timing shouldn't change with the engine temperature, unless maybe the advance unit's lube was stiff and not allowing the weights to move freely.  That's weird.

[nhmaf]

My money is on the carbs not being set with the engine fully warmed up.   The symptom has some similarity to sticking advance weights, but I gotta think that doesn't occur so frequently, especially on a machine that was used with regularity.   Possibly an air leak in the carb boots between carb and head?

[Mike V]

Steve,
Humor me with a very simple test. With the bike at idle and warmed-up spray Brakleen or Carb Cleaner liberally where the throttle shaft enters the carb body and at all the intake tube connections.  Do you notice a stumble or change in RPM? Just a shot in the dark.

Have you checked the condition of your slide and diaphram?

[NC Steve]

Steve,
Humor me with a very simple test. With the bike at idle and warmed-up spray Brakleen or Carb Cleaner liberally where the throttle shaft enters the carb body and at all the intake tube connections.  Do you notice a stumble or change in RPM? Just a shot in the dark.

Have you checked the condition of your slide and diaphram?

Thanks for the idea, Mike, but that was actually the first thing I checked, being the most simple: no change in idle. And no, I haven't been inside the carbs yet, but the diaphrams aren't all that old per the service records I have.

From what I'm reading here though, it's sounding most likely that I need to balance the carbs again with the engine good and hot, now that the valves are within spec. And no, it was not warmed up well when they were done the 1st time, so that may well have been a wasted effort.

The weekend's going to be busy, but if I get a chance Saturday I'll take the old gal out for a serious flogging, and then reset the carbs again immediately after. Hopefully that'll be my issue, so I can avoid getting into the electronics (and mo $$).

I'll post the results back here once I have them.
Thanks again for everyone's help, and wish me luck!

[Semper Gumby]

Hi Steve,

My March R65 also does this from time to time.  Epecially after I come off the highway and pull up to a stop.

I have noticed that it usually happens when the Alternator kicks off.  Without that load on the engine it just picks up and goes (1500 rpms).  I blip the throttle then gen light comes back on and the idle settles back down.

While I'm babbeling on..it may also have something to do with a long breaking stop.  During a long stop (like coming to a stop on an exit ramp off a highway) the fuel level leans forward dropping the float allowing more fuel than normal to enter the float bowl.  So when you finally come to a stop the float bowl is overfull (but not flooded).  It takes awhile for the fuel to return to the normal level (causing an elevated idle rpm).

I have learned to live with this because everything else is set just right.  It is part of the zen of the R65 I think.

All this worth usual 2 cents.

[Barry]

the fuel level leans forward dropping the float allowing more fuel than normal to enter the float bowl.So when you finally come to a stop the float bowl is overfull (but not flooded).It takes awhile for the fuel to return to the normal level (causing an elevated idle rpm).

That's an interesting theory as a higher fuel level in the float chamber would richen the idle mixture a touch and it therefore suggests an idle mixture that is set a little weak. If the idle mixture was spot on or already on the rich side a little more richness would slow down the tick over speed instead of speeding it up.

Here's another complementary theory. If the idle mixture is set too weak and this is compensated for by the throttle stops being opened more than would otherwise be the case what can happen is that the transition ports now come in to effect adding more fuel to already too much air resulting in the revs hanging up.  The objective is always minimum possible butterfly opening at idle. I think some of BMW's suggested base settings for the throttle stop screws are way too much. They suggest 1 full turn in from the butterfly being fully closed when 1/2  to 3/4 at the most is probably a better starting point.