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hudson.ny24
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VBMWMO #9228
Joined: 02/01/2017
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Quote:

When properly lubed, the throttle tube (grip) on a /5, /6, or /7 shouldn't "snap' back, it should creep back. If it snaps back, it either means it's not lubricated properly (dry) or the springs in the carbs are too stiff. If your throttle grip isn't even creeping back, check that the grip isn't pushed too far onto the handle bar and is dragging on the end of the handle bar itself.

I was replacing my throttle cables on my 71 R75/5. I re-greased the cam, lined up the markings and attached the new cables. When I roll the throttle and release, it doesn't "snap" back, but slowly creeps back. Pretty much have all the slack out of the cables. I read the above on another forum, which says this is normal. I have mikuni VM carbs and the slides snap down when pulled up at the cable where it goes into the carb - so no hangup there. The stop screw underneath the throttle tube is loose (not grabbing), so that too is not holding the tube from returning to the idle position.

Bike hasn't been used in over a year due to recovering from an auto accident (old cables stretched), so I can't remember if this is correct or not. Is this right? Or the throttle suppose to snap back?

What else do I need to know?

Thanks...

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Hudson
1971 BMW R75/5

schrader7032
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San Antonio, TX
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Hudson - I don't know about

Hudson -

I don't know about Mikuni carbs but think it comes down to what you want from your throttle hand. My R25/2 throttle snaps back because of the spring in the top of the carb...I don't particularly like that, meaning I have to keep my hand constantly on the throttle otherwise I come to idle pretty quickly. My R69S used to snap back because the internal "cruise control" had been defeated by a previous owner. I have since replace the throttle tube with the cruise control working and the throttle doesn't snap back...it means I don't have to hold a death grip on the grip. My R100/7 doesn't really snap back, but will creep a bit. I have the underside stop screw adjusted to probably provide a little resistance to keep it from quickly going to idle. I have an aftermarket cruise control that lets me quickly flip on/off the control to lock the throttle in a given position.

Personally, having a throttle that snaps hard back to idle would be very tiring on the wrist. If it creeps back a little, then you should be able to relax your grip and use light palm pressure to hold the throttle where you want it.

I don't see the benefit of having a throttle rapidly go to idle. I suppose it's an issue if there's an accident and the engine could be revving. But if there's an accident, I think you have bigger problems.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

hudson.ny24
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Definitely makes sense as to

Definitely makes sense as to a quick snap return to idle. I will leave it for now and once I get the rest of the bike together, will give it a go and see how it reacts.

Thanks!

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Hudson
1971 BMW R75/5

mark_weiss
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The throttle should snap back

The throttle should snap back with relatively little resistance. This is true for the stock Bing CVs, Dell'Ortos, or Mikuni VMs. A couple of common problems are that the throttle tube does not freely turn on the handlbar. Check and clean. Use light grease or better yet, graphite. Also, the control unit could be too far up the handlbar causing the bar end to rub on the inside of the end of the grip. Easy check, just slide the pod a bit further from the bend. Could be a bent handlebar. And the last that I can think of right now, the cam teeth and tube teeth are worn and binding. The drag screw is to be used to modify how rapidly the throttle returns to ide.

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Mark
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hudson.ny24
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I will look at the throttle

I will look at the throttle tube and bar end issues you mention. It does return to idle, just not a quick snap back - a creeping return, as Kurt posted above. I appreciate all of the input. Thanks...

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Hudson
1971 BMW R75/5

mark_weiss
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I use Mikuni VMs on my R75

I use Mikuni VMs on my R75 race bike. If the throttle did not cleanly snap back to closed the machine would not even pass Tech Inspection.

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Mark
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schrader7032
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Mark - Which begs the

Mark -

Which begs the questions...do you recommend the throttle snap back for street applications...and why?

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

mark_weiss
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Yes, I recommend that unless

Yes, I recommend that unless the throttle was designed otherwise (/2, etc.) that it always be self-closing. This is basic for safety and also protects the engine in case of a fall or drop. There is a good reason that the entire world motorcycle industry has been designing throttle controls to work this way for decades. Our machines were designed with a throttle brake screw to allow a rider to adjust how rapidly the throttle will move and that is done delberately and with the rider's full knowledge and control. Rolling off the throttle at the grip and finding that the carburetors are staying open for a bit longer would be an unhappy way to enter a curve.

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Mark
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schrader7032
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San Antonio, TX
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To me, snapping back with

To me, snapping back with that much force would be annoying. Rolling off the throttle and having the slides stay open is certainly not good. For me, somewhere in between has worked fine. I especially like the updated throttle tube on my R69S which has the built-in cruise control. Riding without out it was a PITA.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

mark_weiss
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There really should not be a

There really should not be a whole lot of force from the springs. Yes, slide throttle carbs such as the Mikunis do need a bit more pull than for the butterflies on a pair of CV carbs, but not all that much. It is probably possible to change the springs. I know that my Dell'Ortos came with very strong return springs. Far stronger than was necessary, but as specified by BMW. Fortunately, Ducati also made extensive use of the same carburetors and for Ducatis there are three springs available. Switching to the middle spring made a significant difference in the grip force required to hold the throttle. So much so that I've found that I normally have the drag screw fully backed off.

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Mark
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