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R51/3 Fix or repair for sticking carb slide in throttle body

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101-Scout
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Joined: Sat Aug 13, 2022 9:09 am

R51/3 Fix or repair for sticking carb slide in throttle body

Post by 101-Scout »

I am new to this forum and vintage BMs - my first post!
I'm very grateful for the volumes of technical and helpful information on your forum. Congrats to all.

I'm just starting a project to restore ( fairly complete R51/3) that hasn't been started for over 10 years.
I have the Barrington Motor Works BMW R51/3-68 Motorcycle R & S Manual (excellent reference for newbie).

My first question (perhaps of many) is how to repair/fix stuck throttle slides in my throttle body chambers (both LHS and RHS).
Carby and all components I have checked look in very good nick IMHO. 1/12/62 ID on Bing carbs - appear very clean inside and out. Both slide and bore no obvious damage, look concentric - just a very tight fit.
The top 1/4 of throttle slide jams/sticks fast near the top of the body when trying to remove from chamber. Note it has a " firm fit through the "normal usage area" of throttle body. RHS no clearance across length of movement, LHS I guess 1-5 /1000" (Both LHS & RHS stick at top of chamber).

What is clearance spec for gap around slide inside the bore?
Advice on how to alleviate sticking? Identify high spots? Hone bore or reduce outer face of slides?
If there is more information I can provide to help diagnose, please ask.
De
R51/3
NSW
Australia

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schrader7032
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Re: R51/3 Fix or repair for sticking carb slide in throttle body

Post by schrader7032 »

Welcome to the forum! I had the slide in my R25/2 stick in the bore earlier this year. Through a series of sanding and replacing a spring, I have the bike running again although I'm always thinking of what happened in case I should sense that it's doing it again. My discussion was here:

viewtopic.php?f=7&t=17270
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.

101-Scout
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Re: R51/3 Fix or repair for sticking carb slide in throttle body

Post by 101-Scout »

Thanks Kurt for your prompt reply and help.

Interesting issue you've encountered on your restored and running bike. First day out for the year (cold) and throttle slide decides it's happier to stay locked inside and refuses to budge ;)
Does this issue regularly happen to other riders of 50s plungers?

My situation is a little different; in that my bike has not been riden or started, maybe even dealing with unused carbs?

I measured the slide diameter around skirt at base and top - consistently approx 27mm. The top of throttle body bore also 27mm - no clearance.

Where do I start with lightly removing this soft material? Cautions and suggested setup very welcome.

What is the recommended approach and technique to using 600grit on top area of bore? Cirles, cross hatch, angles? What clearance am I aiming for? Best way to meausre?

In your reply "slide essentially falls down on its own weight": when cold - is there some extra allowance as temperature increase (i.e. hot day, hot motor) ?
De
R51/3
NSW
Australia

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jwonder
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Re: R51/3 Fix or repair for sticking carb slide in throttle body

Post by jwonder »

I have had this happen with motorcycles that were not run in some time and I needed to free up the slides. Here is what I have done with very good results:

1) Do not force the slides out as it will score the surfaces.
2) Spray wd40 in the bore and let soak overnight.
3) Pull the slides out, if you need to force them after a soak of wd40 so be it, but I usually wait longer or try to move them up and down in small increments to get the wd40 in all the areas.
4) Spray wd40 on a clean bounty towel and rub HARD the carb bore. It should get VERY dark when rubbing. Use a clean towel and rub until there is not any dirt coming off.
5) Spray wd40 on a bounty towel and rub the slide and clean it off completely.
6) The slides will usually be much better after this and typically function normally.
7) If the slide is still sticky I use a piece of red scotchbright soaked with wd40 and LIGHTLY rub the carb bore in the direction of travel (in and out).
8) If it is still sticky rub the slide in the direction of travel lightly with scotchbright soaked in wd40.
9) Go back and forth between bore and slide until it slides up and down nicely.

The moral of the story is that once it is clean it SHOULD slide without the scotchbright treatment but I have had some that needed it.

No, I am not sponsored by wd40, Bounty towels or scotchbright......

P.S. I have heated my carbs up with a heat gun to about 120 degrees on the rare occasion that the slides are very finicky, and the slides typically come out easily after heat is applied.
James Wonder
Vice President, Vintage BMW Motorcycle Owners
2022 BMW Friend Of the Marque
Long Island, New York

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Flx48
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Re: R51/3 Fix or repair for sticking carb slide in throttle body

Post by Flx48 »

Hi De-
Carb bodies are generally made from weak material, this because they can be, because they don't need much strength to function, and because it helps keep the production cost low.
Bings don't suffer much for this, aside from possible distortion at the mounting flange.

Being from a commonwealth, you may have experience on this with the old Amal concentrics?
They sometimes suffered distorted flange mounts to the point of distorting the slide bore, and sometimes requiring a brake hone to return the bore to a concentric shape.

With Bings, while I have heard of the bore being similarly distorted by mounting flange distortion, I've never seen that for myself, so at this point in time would personally have to judge it as a very rare occurrence.
And so I'd definitely follow James' advice; after thorough cleaning, it generally takes very little, in terms of actual material removal, to return the slide bore to good function.

As to warped flanges, after checking/correcting the flatness, two things: don't overtighten, and only use thin gasket paper.
In the past I've found double gaskets here, probably an effort to stop the vacuum leak caused by bent flanges, which in turn only exacerbated the problem.
If the gasket paper used is thick enough to get some crush, robust tightening will again bend these weak flanges.

101 Scout- a great bike!
Best-
George

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vechorik1373
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Re: R51/3 Fix or repair for sticking carb slide in throttle body

Post by vechorik1373 »

Rather than overdue it using sandpaper, you can buff the slides on a cotton cloth buffing wheel, and a bit of white rouge.

Buffing does remove material, but not very much, and certainly not as fast as sandpaper. I have used the buffing method on new slides that were tight in the bore, successfully. Just be careful, and don't let the slide get away from you when buffing.
If it gets thrown across the room violently, it can ruin the slide.
Vech
Technical Adviser, Former owner, Bench Mark Works
662 312 2838 cell 9 am to 4pm CST PLEASE!

101-Scout
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Re: R51/3 Fix or repair for sticking carb slide in throttle body

Post by 101-Scout »

Thanks Kurt,James, George & Vech for your help; comforting to know of your experiences with these slides.
All good advice. I'll start with the wonderful "James method" and let you know of success- I'll also check/repair flange face.
De
R51/3
NSW
Australia

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