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Old 03-20-2013, 11:40 PM   #1
icefitnc OP
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Location: Raleigh, NC
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Carburetor adjustment

I just got my hands on a 1973 BMW R/75 and wanted to thank everyone on this site for the info that has already helped me get this 4+ year sitting bike running in one weekend. I will honestly say I would have probably never purchased an older BMW motorcycle but am already in love with it and am thinking about another one already. I am sure I am going to have many questions but right now here it is. I was to impatient to wait for a carb rebuilt kit so I dismantled everything, cleaned everything and reinstalled everything. It started first push. WHen I took it apart one (idle screw?) was 2 turns backed out and one was 3.25. Is there ever a reason this is necessary? It warms up fine after I got the enricher cleaned out and the jet in the float bowl but when I try and hold a steady (low rev), it pops and when accelerating doesn't have full power. Is this a carb adjustment, timing, or full carb rebuild needed? I know it's hard to troubleshoot but suggestions are welcome.

How do I upload picture of bike? I will say this is a one of kind BMW, was customized by Charlotte Harley Davidson.
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icefitnc screwed with this post 03-20-2013 at 11:55 PM
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Old 03-21-2013, 12:02 AM   #2
disston
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There is a Section called "fluff". In there you will find "Ask Baldy, Blame AceRPH" . And there will be many pointers for the site. amongst all this info is "How to post a picture";

http://www.advrider.com/forums/showthread.php?t=919

How many miles on said custom bike?

There are numbers on the metal tags on the side of each carb, what carb numbers are these? Check both carbs, they are not the same number.

If all the O-rings and other parts were good I guess you don't need a carb kit but I can't imagine the gaskets on the enrichners being any good. And the diaphragms are suspect too. Are these the stock Bing carbs?

BTW. This bike should be an R75/5. It is not an R75.

Do you have a manual?

Welcome to the Asylum.
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Old 03-21-2013, 12:43 AM   #3
icefitnc OP
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Carbs are 64/32/10 and 64/32/9? Mileage? No manual? I think for peace of mind I am going to do a full carb rebuild. I would prefer all stock but this is kinda fun to ride, I see getting a stock bike one day also. I have already freaked our several HD riders when they see this. It appears they put a Harley springer front end on it, fat boy gas tank, custom molded rear fender and had a custom seat made. Not sure about the whole hard tail thing (time will tell).
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icefitnc screwed with this post 03-21-2013 at 12:53 AM
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Old 03-21-2013, 12:53 AM   #4
ME 109
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Try raising the fuel level in the bowls. As a rough guide, they should be about 2/3 full.
Blocked passages may be another possibility.
Have you taken the main jet and atomizer out and cleaned?
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Old 03-21-2013, 01:22 AM   #5
icefitnc OP
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Yes I took the main and atomizer out and cleaned. One thing I did notice was the needle jets had a lot of crud on them and weren't seating properly at first, causing the bowl to leak gas. I lightly buffed them and they worked better but I did notice one of the needle jets, the little ball that detents/looked like someone had deformed it a little with pliers or something?

I guess my question is when the motor doesn't run smooth, are the carbs the culprit xxx% of the time? I have 3 people that we are planning a blue ridge parkway ride in 3-4 weeks when the weather breaks here. working on a tool kit for the bike now.
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Old 03-21-2013, 01:25 AM   #6
disston
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t least you don't have the early model carbs. The first issue Bings had a few problems. Those 9 & 10 (last digit) models are workable.

A kit will have all the O-rings. If the O-ring on the throttle shaft is replaced it gets tricky. Research this if you are going to try and do that one. New screws are needed and often new shafts.

If the gasket on the enrichner is not distorted it may be usable for now. Blue Locktight on the four screws helps.

There are a couple parts not in the kits that are wear parts and may need changing. Floats an float needles are done routinely by many. The needle jet and the jet needle wear and should be done now if mileage is unknown.

There is a Web Page that will show a blown up diagram that can be helpful;

http://realoem.com/bmw/partgrp.do?mo...85&hg=13&fg=11

There are others but a lot of us that like this site. They don't sell parts and the prices listed are not always current.

A manual would be helpful. There are a number of blogs about building these carbs.

Because of what looks like open pipes you may not have the stock jets in those carbs. If you do you may need to up the size of a few of them. I will give you the sizes of the stock jets and the needle position.

Main jet is 135

Needle Jet is 2.70

Jet Needle is 241

Idle jet is 45

Clip position on the Jet Needle is 3 (that is the third slot from the top)

The Idle Screw start position, setting before the carbs are balanced is 0.75 turns out.

From the idle screw settings you report it seems these carbs may have the original jets that have to have this increased gas because of the open pipes. 2 and 3.25 turns is a lot on a Bing Carb. None of the stock settings are this high. Could also be an indication of blocked passages. (BTW, the idle screws are a gas control, not an air control)

The easiest thing to try is raising the jet needle. One clip position though is worth two or three jet sizes so clip positions are a crude adjustment.

Often just am increase of idle jet to a 50 from the stock 45 is helpful.

Bike is better looking than some others of this style I've seen. No desire to own a hard tail myself. Good for going to the corner bar I guess?
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Old 03-21-2013, 01:30 AM   #7
ME 109
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Carbs can overflow and still idle ok.
Diaphragms can be a problem but they will eventually let the bike run at higher revs even if they are holed.
Not running just above idle wouldn't bring the advance mechanism into question.
I'm still thinking fuel supply passages other than idle circuit.

Disclaimer: What the hell would I know.
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Old 03-21-2013, 01:38 AM   #8
disston
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Before you ever seriously consider working on or getting the carbs right there are several things that come first. You will need a manual for these two things. I repeat these come before carb settings. Carbs can not be balanced unless thes two things are first in order and the process is in this order.

First is valve lash settings. The intake is 0.004" and the exhaust is 0.008"

Second is points. With new points set the Dwell and the Timing.

You may add a new set of spark plugs.

And now you are ready to mess with the carbs.
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Old 03-21-2013, 03:40 AM   #9
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I'll say this: I'm not much for fat boy tanks or chroming but that's just not a bad looking bike and I would absolutely ride it.

I'd advise heavily that Disston is correct in tuning sequence. Valves are easiest and thus first for me, then disconnect battery, pop front cover off, reconnect battery, set/inspect points gap and dwell, check advance, disconnect battery and put cover back on, reconnect battery, then do carb adjustment.

I found that even though you may monkey with the carbs quite often, unless the valves and ignition timing are on you're wasting time.
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Old 03-21-2013, 09:11 AM   #10
icefitnc OP
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Thanks guys for the info. I love the fact that I can work on this bike somewhat easily. I ordered the manual and carb manual along with extensive carb rebuild kit from bing. I'm gonna follow the tune up sequence above and do the carbs last. Yes it has straight pipes (for now), that might fall into the hard tail needing upgrade as well. I'll keep you'll updated on progress.
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