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Old 02-14-2013, 11:58 AM   #1
Bluethumb OP
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Cold Natured R100 - rejetting maybe?

My 82 R100 is very cold natured, takes an extremely long time to warm up. My 83 ST warms up much faster. On my ST, from a cold start, by the time I weave out of my neighborhood and get to the hi-way, 5 minutes or so, the bike's warmed up. The R100 on the other hand is still surging and acting as if it's starved for fuel for the first 15-20 minutes. Once warmed up, runs strong and smooth. But it seems to take an incredibly long time.

A friend told me that he once had a 83 R100CS that was the same.

Do I need to look at the jetting? Change to bigger jets, different needles, raise the needles?

The gas recirculating plumbing's been removed.
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Old 02-14-2013, 01:45 PM   #2
supershaft
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That depends on if you want your bike jetted for warmup or for running the best it can when it's good and hot. Personally, I spend a lot of time jetting my bikes to get them to act about like yours is. That's what I call pretty close to perfect. They run stronger for it too. Best power AND best mileage. When they are hot that is. I have helped with many a damn cold natured race bike for the same reasons. We didn't care what it ran like cold since we were focused on what it ran like hot. You can not have both at the same time but that is what chokes are for in the first place.
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Old 02-15-2013, 08:08 AM   #3
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Also consider how clean and what condition all the internals of the carbs may be. When I got my ST I had all of the various cold start/run issues at one time or the other. Certain tuning rituals would provide short term fixes at best.

I finally dug in and did a thorough cleaning of everything short of pulling the butterflies and shafts. Main jets were literally crumbling from storage up north by a PO where apparently moisture was trapped in the small space between the jet proper and the jet holder (Hans Lowe at Hucky's explained that problem ot me when I went down to pick up parts and meet him.) Idle jets went up from stock #40 to #45. Not sure which notch my slide needles are in but both measured 40mm stickout from the slide. Made sure the little jet in the corner of the float bowl was clean(oxy acetylene torch tip cleaners are great carb tools because they are designed not to ream out a hole any larger than it is).

After that cleanup and a straightforward re-tuning and balancing the carbs startups and running became the so called textbook example. Now an annual check and cleanup keeps the carbs working pretty much as they should.

The only other problem I encounter, and its related to cleanliness - I still run the stock breather system and the left carb gets more than its share of oil vapor when I have to run at highway speeds in hot weather for any length of time. Eventually the oil seems to condense and partially clog some air passages located in the bottom edge of the carb inlets. And starting/idle issues pop up. A quick shot of carb cleaner with the engine running usually fixes that issue. Try to keep carb cleaner from rubber bits as much as possible - in my experience ethanol in modern gas affects rubber in a way that makes it way more susceptible to damage from carb cleaners. (I run ethanol free gas in some things, but not the bikes. And I have noticed differences in rubber parts big time).
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Old 02-15-2013, 08:33 AM   #4
100RT
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My 81 is cold blooded in the sense that It almost always requires a choke to start even during the summer. I can turn the choke off after 1/4 mile. Years ago I went up one size on the needle jet and needle but keep the needle in the original notch.
If you have a owners manual it will tell you what needle and jets the bike had. I removed the emissions plumbing also at that time

I'm contemplating going up one size with the idle jets also. Of course the carb should be clean and all passages free of crud.

Is there an icon for spell check when replying?
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Old 02-15-2013, 12:04 PM   #5
supershaft
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Mark, I figured out that carb cleaners were real hard on rubber long before there was any alcohol in the gas around where I lived.

IMO about the only place our carbs might need richened up is with the needle jets. The '70's mains are almost always too rich. I have found that richer idle jets almost always do nothing but have you setting your mixture screw further in for the same mix. One size jet needle? There is only one size as far as I know. They often do need richer needle jets but not if they are running perfect once warmed up. If they are very cold natured and then run perfect once they are warmed up, that just means you have the jetting about perfect. If they surge or cough when cracking the throttle open once warmed up, THAT'S when they need a richer needle jet. You want the needle jets as lean as possible. You are on those jets a lot and your mileage and whatnot goes down hill real quick getting richer than you absolutely need there. Same advise with the jet needles. My advise is to never run them richer then the third clip and use bigger needle jets if you need it richer just off idle. The carbs that come with the needles on the second clip? Second or third clip ALL depends on the needle jets/clip position combo but going down to the forth clip on the needles is always not the answer in my experience for them then being too rich in the upper midrange. Go ahead and jet your bike for better warmup but it will cost you power, mileage, and carbon buildup for the entire rest of the ride.

supershaft screwed with this post 02-15-2013 at 12:15 PM
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Old 02-15-2013, 12:12 PM   #6
bmweuro
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Loosen the clamps and rotate the carburetor outward and make sure the choke body screws are tight. If this helps but does not fix it 100% replace the choke gaskets.
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Old 02-15-2013, 12:45 PM   #7
100RT
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Quote:
Originally Posted by supershaft View Post
. One size jet needle? There is only one size as far as I know.
Are you serious? Somewhere in my posession is the parts listing for all of the jets and needle sizes
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Old 02-15-2013, 01:52 PM   #8
supershaft
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I have the listing from Bing. You can get a wider selection of jets from Bing versus BMW. For our carbs? There is only one jet needle available AFAIK.
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Old 02-15-2013, 02:55 PM   #9
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Let me see if I can locate the listing. It has been a few years since I put it somewhere. I am possibly mistaken, wont be the first or last time.
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