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Old 05-31-2015, 12:19 PM   #1
Dave Bell OP
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Carb issues on my Airhead

This is a 1980 R100RT that I'm modifying after someone else modified it by leaving it out in the rain for years. I've done a lot of cleaning and rebuilding of forks, and painted the body, etc. Has a new electronic ignition, new coils, plugs, and wires and has a good spark. I rebuilt both carbs including diaphragms because the old diaphragms were torn and gummy. The bike started and ran right away. I had some issues with learning how to set the floats but I believe that they are good now. I've had the carbs off numerous times to clean and check everything and found that the gas I was using had a lot of water in it, so I have fresh gas now and draining a bit into a glass jar does not show any water or debris.

The bike ran well yesterday for about 40 miles without a problem, and then the left cylinder quit at certain throttle positions, basically cruise speed. I adjusted some more this morning and she only ran on one cylinder at certain rpms (again, cruise), but ran even worse. I checked compression and it is 150 and 155. When I adjust idle screw and mixture it will seem to improve but then quit again.

I remembered this morning that I replaced one of the needles and not the other, and I am not at all sure they are in the same notch on both sides. That seems unlikely as the source of the problem since she ran so well for 40 miles yesterday.

I'm going to New Mexico on my Ural for a week, but will watch this spot for any helpful ideas.
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Old 05-31-2015, 12:41 PM   #2
Bill Harris
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My first guess is water in the gas. Remove and drain the tank, put in a few ounces of denatured alcohol to absorb the water, slosh it around and let it set for a while then drain and discard the waterlogged alcohol.

What does the tank interior look like? With it's history of water, it might need to be relined.

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Old 05-31-2015, 12:42 PM   #3
190e
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One slot difference in the needle positions wouldn't stop it running and in fact I've seen them run so that you wouldn't readily know in that situation. There's going to be a big difference in mixture though so it's worth checking.

If carb related the problem is more likely a blocked main jet.
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Old 05-31-2015, 12:44 PM   #4
Dave Bell OP
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Quote:
Originally Posted by Bill Harris View Post
My first guess is water in the gas. Remove and drain the tank, put in a few ounces of denatured alcohol to absorb the water, slosh it around and let it set for a while then drain and discard the waterlogged alcohol.

What does the tank interior look like? With it's history of water, it might need to be relined.

--Bill
There was water in the gas. Not any more. I'm positive. Tank was relined with epoxy.
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Old 05-31-2015, 12:45 PM   #5
Dave Bell OP
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One slot difference in the needle positions wouldn't stop it running and in fact I've seen them run so that you wouldn't readily know in that situation. There's going to be a big difference in mixture though so it's worth checking.

If carb related the problem is more likely a blocked main jet.
I've checked and cleaned the main jets numerous times. I'll go check the needle slots now.
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Old 05-31-2015, 12:49 PM   #6
disston
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I would never do stuff that I think will bite me in the ass later. On a 2 cylinder motorcycle with 2 carburetors why replace any parts in only one carb? The parts in the other carb are the same age. Just a principle I discovered many years ago and stick with it.

Many of the parts in our carbs are wear parts. That is they do wear out. Needle jets and jet needles do wear.

BTW it is absolutely necessary the jet needles be set at the same height and the correct height.

Fix these issues and maybe you will find if you really have a problem.

A thing I didn't realize about the CV carbs for many years is that there are several passages in the main carb body that will clog. Often the strongest carb cleaner doesn't seem to do it. I have resorted to manual cleaning of all the passages in the main body. Use one of those carb cleaner wire tools or thin music wire or something.
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Old 05-31-2015, 02:17 PM   #7
Dave Bell OP
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I hope I found the issue

Most of my fiddling has been on the left side carb. I took off the right side carb and found a badly shredded o ring on the mixture jet and I also found that the slide was sticking in the column on the domed top of the carb. I cleaned the slide and replaced the ring and went for a 2 mile ride and the bike ran great. I'm about to alert my rescue team and go for a 20 mile or so ride. Hopefully no issues.
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Old 05-31-2015, 02:19 PM   #8
Dave Bell OP
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[QUOTE=disston;26901609]I would never do stuff that I think will bite me in the ass later. On a 2 cylinder motorcycle with 2 carburetors why replace any parts in only one carb? The parts in the other carb are the same age. Just a principle I discovered many years ago and stick with it.

Point taken. Thank you.
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Old 05-31-2015, 03:09 PM   #9
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I recently had a carb problem that also turned out to be the slide sticking at the top. No idea how that can even happen, there's nothing to keep it stuck up there I think. And I run slide springs on my R90/6 carbs that weren't originally installed by BMW.

I may have fixed something else too because I took it apart but the only I noticed really was the slide stuck on the top.

Oh Well.
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Old 05-31-2015, 03:49 PM   #10
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Ok, you've already done the "water elimination" two-step, so you're ahead.

Stuff like this happens-- I recently did a clutch on mine and 10 miles into the first outing the engine suddenly went down to 20% power for a couple of seconds, then back up to normal. Turned tail, went home, checked ignition and fuel, never found the cause, and it's not happened again. But the next couple of days was uneasy.

Intermittents are a PITA.

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Old 05-31-2015, 05:02 PM   #11
Dave Bell OP
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Quote:
Originally Posted by disston View Post
I recently had a carb problem that also turned out to be the slide sticking at the top. No idea how that can even happen, there's nothing to keep it stuck up there I think. And I run slide springs on my R90/6 carbs that weren't originally installed by BMW.

I may have fixed something else too because I took it apart but the only I noticed really was the slide stuck on the top.

Oh Well.
In my case, on this bike, there was water in everything. I got about a liter out of the transmission. When I noticed the slide was sticking, it was not the actual slide but the pipe that goes up into the dome. The inside of the dome had a bit of rust in it. I got a 28 gauge brass brush and cleaned it up and sprayed carb cleaner in it and it moved freely. The rust evidently came from the steel plug in the top of the dome, since the actual carb can't rust. I learned a new spot to check. I've got two more of these bikes to rebuild.......
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Old 06-01-2015, 03:37 AM   #12
Dave Bell OP
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parts ordered

I ordered new main jets and needles, intending on making sure both needles are new and jets are in good shape. I'm curious about why the needles would have different possible positions, as well as the best method for inserting and changing. I know that the needles height being changed would change the amount of gas allowed into the atomizer, but why would that be important? Would it ever be the case that if one side was a little lean, I would raise the needle on the lean side to equalize the carbs? Or as was pointed out earlier, are they always set the same?

Also, how is the best way to raise and lower the needle?
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Old 06-01-2015, 04:09 AM   #13
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Notice the notches on the right. They fit into the spring clip that's in the slide. So the clip holds the needle on two sides inside the notch. To move the needle rotate it about a quarter turn, you can feel as the spring clip is spread by rotating the needle. Push up, or pull down, on the needle and the clip will fall into the next pair of notches, which are a quarter turn off from the last set.

I find it helps to wear some kind of rubber glove to rotate the needle for adjustment because the clips are pretty stiff and the thin needle is hard to feel turn. The normal rubber gloves, nitrile, that mechanics use to keep oil and grease off their hands may not work because they are powered or the texture isn't right. What you need is something like disher's gloves. Sometimes found in the kitchen?

When done it is still difficult to be sure you got the right notch sometimes. You can measure the length of the needles from the slide to check. If you are sure you got the correct notch but the needles are not the same length then one of the clips may have moved. The clips are held in the slide by a toothed washer. They are at the bottom of the thin tube of the slide and nothing holds them in place but friction. Before beginning it's a good idea to push the spring clips and washers to the bottom of the slide tubes with a pencil eraser end. (remember those?)
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Old 06-01-2015, 07:12 PM   #14
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If the dome leaked water through the metal plug on top, it's possible it also leaks air. Be sure to clean that plug up really well to bare metal and paint it. Make sure it's sealed well.

I'd also be concerned about a transmission that sat with water inside. I'd watch for a good used one and have it ready. Actually, I'd pull the 'wet' one as soon as possible, since it's a ticking time bomb. Definitely watch the drain magnet closely for slivers and grit.
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Old 06-02-2015, 04:15 AM   #15
190e
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I always measure the installed needle lengths. These are the figures for the smaller carbs so I expect they will be diffrent for 40mm. It gives you an idea and although I used calipers to measure them accurately the difference of approx. 1.8mm between positions could easily be measured with a steel rule.

Needle Position Needle Length
1 42.37
2 40.57
3 38.79
4 37.10
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