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Old 06-11-2012, 05:29 PM   #1
R90pilot OP
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High Idle and sputter while crusing

I am at the tail end of my restore and I have rebuilt the carbs, and now I am working the bugs out. It fires right up fine, runs at fell speed fine. But has a sputter while at low RPM's and Crusing. I searched for carb adjustments and could not find anything helpful on my isses. If anyone has any ideas or links that would help go into carb adjustments I would appreciate it.

Thanks
Tom
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Old 06-11-2012, 05:43 PM   #2
disston
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What do you consider Low RPMs and Cruising? Airheads are happiest above 3000 rpm I think. Although I can do flat ground around 2500 rpm if I'm not trying to accelerate. Airheads are not low end torque machines. Not supposed to be anyway. So...just what rpm do you consider proper for cruising? At what speed, in what gear?

Oh, What kind of an R90, S?, /6?
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Old 06-11-2012, 05:48 PM   #3
GodOmelet
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I've been going through similar issues with my 77 R100/7. First thing many veterans on this site will tell you is that if everything else isn't tuned properly, don't even worry about the carbs. If you haven't already, make sure timing and valves are set properly.

If I've learned anything with these carbs (you do have Bings right?) is that they are sensitive. Little contaminants can mess up idle by clogging jets. Likewise air leaks in the carb mount rubbers can wreak havoc on idle. The slides have to move really smoothly up and down in their cylinders or they can get hung up. Folks here recommend replacing the jet needles and needle jets if your bike has more than say 60K on the odometer or if they are out of round. Jet needles have to be in the correct position for your bike/altitude. The throttle cables should be replaced unless in really prisitne condition, and routing is important so as not to get any friction to hang up free travel.

Those are just some things I've learned/read in my troubleshooting saga that still continues with my Bings. Both Duane Ausherman's and Snowbum's (Bob Fleischer) sites have a lot of information on sorting out Bings also. Bing has a book but I haven't seen it yet. Good luck.
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Old 06-11-2012, 06:06 PM   #4
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Bike is a 1975 R90/6. Sputter is above 2000 RPMS and is present no matter what unless I am full throttle. Adjsuting the screws on the bottom of my Bings seemed to help but still not perect. My high idle is a weak spring on my Left carb. I have not synced them either as I can not find a decent write up on it
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Old 06-11-2012, 08:44 PM   #5
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The Snowbum article:http://bmwmotorcycletech.info/synchcarbs.htm

It is really long, with more parenthetical remarks than actual instructions, but it does tell you how to sync. the carbs, in a couple of different ways. The method he prefers requires shorting the plug wires using a special adaptor.

I've been told it doesn't harm points bikes to just pull one plug at a time and lay it so it doesn't arc against the cylinder, and then adjust the mixture screw of the opposite cylinder (the one that's running obviously) to achieve fastest/smoothest running and then the adjust the butterfly stop screw to about 1k on the tach. Then you plug the sparkplug wire back on and take the other one off, and repeat the procedure on the opposite cylinder.

Then once you've done that you turn both butterfly adjustment screws back equal amounts to get an idle at 1k-1.2k.
This is both a summary of Snowbums method as well as the one in the BMW owners manual. I'm writing it from memory so forgive me if I missed something. I may be wrong about the idle speed you should be shooting for on you bike.

I'm interested in building my own manometer to fine tune them once I figure out my high idle problem on one side.
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Old 06-11-2012, 11:29 PM   #6
disston
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Set the mixture screw to the spec for this carb or for this model. For the R90/6 this is between 1 and 3/4 turns out. Start at one turn out. You can leave it there for initial balance operations.

It is highly recommended that shorting rods be used for the shorting method. These can be bought or homemade. To make them use the spark plug ends that come in each new spark plug and normally get screwed onto the end of the plug but we don't use these on our Airheads. They are normally thrown away. Call them the plug nipples. At your local hardware store you will find some long screws. I believe M4 is the size, that fit the nipples. Get a pair of nuts in same size. Get the longest screws the hardware store has usually about 70 mm. Cut the heads off the screws so you have a threaded rod. Put the nipples on one end with the nuts to lock them in place. Enough thread must be left on the nipple to connect to the spark plug.

The threaded rods are screwed onto the plugs in the bike, the plug wires are placed on the other end of the rods. Now the motorcycle will run normally with the rods in between the plugs and wires. (BTW, leave these in place only for balancing. Do not ride around the neighborhood with them on the bike. They will work loose and the bike will loose connection) It is also a good idea to have a small wrench that fits these so they can be tightened.

For balancing the carbs...Valves are set, Timing is set and the motorcycle is Warmed Up (at least a ten minute ride, sometimes more) One side is shorted to a cylinder fin with a screw driver making contact with the rod. The engine should run on the other side but may die after a couple of revs. Do the same thing on the other side. The engine should tick over the same number of times on both sides but usually die after a couple of revs. If you are setting the tick over at a thousand rpm you are setting it too high. The engine may continue to run on one cylinder but at a lower rpm. Back when we had real gasoline they would usually balance out to only tick over a couple of revs and then die. They were set so the number of ticks were the same. Now a days they will continue to run but at about 500 rpm or lower.

The end product should be even. This is why we call it balance I guess. I personally don't like an idle any higher than one thousand rpm and prefer lower than this, 7-8 hundred rpm. I have the same bike as you, a 1975 R90/6. I've had much better idle since I upped the size of the idle jet recently. The stock idle jet is 45. I have now a 50 in mine. This seems to be needed once again because of the gasoline. BTW, it's the alcohol in gas. Modern gas does not pack the punch that we had when these bikes were built.

Removing the plug wires with out using the shorting rods was how we did this years ago. You may get away with it or you might end up with coil problems and have a hard time figuring out what went wrong. If you have any kind of electronics, either OEM or after market spark box, then you must use shorting rods or some other method of balancing. Removing the plug wire on an electronically sparked bike will blow the box. That's not a maybe, it will. With simple points/coil it might harm the coils.

I also rarely use the shorting method these days. I have a Harmonizer that does a better job than any other method so far developed. It was designed and is built by an inmate here at Adventure. His name is Grok. This tool can be found in the vendors section. It's over a hundred bucks but cheaper than other tools, except for the homemade manometer. (btw, I have a mercury manometer and it bounces around so much I think it's useless)

At the beginning I said to set the mixture screw at one turn out. You might have better ears than me and can learn to mess with the mixture screws but I look at the plugs and if they are black turn the screws in a bit. If they are white I turn the screws out. I want them brown.

I hope this helps.
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Old 06-12-2012, 12:38 AM   #7
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Thanks for that explanation of the adaptors. I didn't know you could get screws of that length and diameter so easily!
I'm a little confused about the part about tickover and the bike dying after a few revs- you mean revolutions, correct? I'll reread it again and see if it becomes clearer.
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Old 06-12-2012, 12:50 AM   #8
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Quote:
Originally Posted by R90pilot View Post
Bike is a 1975 R90/6. Sputter is above 2000 RPMS and is present no matter what unless I am full throttle. Adjsuting the screws on the bottom of my Bings seemed to help but still not perect. My high idle is a weak spring on my Left carb. I have not synced them either as I can not find a decent write up on it
That's interesting about the weak spring. How did you determine that was the cause? You are talking about the slide spring, correct?
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Old 06-12-2012, 08:21 AM   #9
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You should not be setting the idle on one cylinder at 1000 rpm. (BTW, rpm, RPM and revs all mean the same thing. Tick over is sort of the same thing). If you have each cylinder set to rev at 1000 rpm by it self, the bike will idle close to 2000 rpm on two cylinders.

It really does not mater what rpm the bike runs at on one cylinder. They should just be the same, that is balance. The rpm of the bike on two cylinders is important. I believe the official figure is too high if it is 1100 rpm. I will not accept more than 1000 rpm on mine and have it currently idling at about 500 rpm when cold and 750 rpm when in the neighborhood. After a highway run I will have an idle of about 900-1000 rpm.

When we had good gas then we called it tick over because the engine would die after one or usually two ticks over. That is I would remove one plug wire and the engine would fire on the other side once or twice then die. The gas was better and this provided a balanced idle of about 800 rpm. No longer true...I can not get my bike to idle unless I have each cylinder idling high enough to keep the bike going, running on one cylinder. Of course the rest of my mechanics are not in the same state of repair as yours but all I'm saying is that it doesn't mater if the bike runs on one cylinder or it dies on one cylinder. Balance them so it does the same thing on either side and the end result is an idle you can live with.

Now something about a high speed miss. I have found this to be a function of the points dwell. Mostly we tell you to set the points gap. What you a doing is setting the points dwell. This is the degrees of angle that the points are closed. It can be measured with a Dwell Meter and usually is on larger engines but the gap method is perfectly usable on our bikes. (if you want to learn more about using a Dwell Meter to more accurately set points dwell you will find the info on Snowbum's site). The one problem with depending on the gap method of setting points dwell is it is not accurate when the points are worn or for that mater even used.

The points are either open or closed. When they are closed the coils are being charged. Points not closed long enough and there is no miss at lower rpm but when the rpm rises there is not enough time to charge the coils. End result is a miss at higher rpms.

I can't say this is the source of the miss you have but it is a possibility. You probably don't have a Dwell Meter so, get new points, install and gap. Might be source of miss. I would also use new plugs.
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Old 06-12-2012, 12:16 PM   #10
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"I have a mercury manometer and it bounces around so much I think it is useless." They sure are useless if you don't dampen them. Yours isn't or it wouldn't be bouncing around so much. I see and here of this ALL the time. Sometimes even long time mechanics with a mercury manometer with little or no damping. They have to be damped. How much? To where ever you like it the most. How? Simple. With holed line plugs per the instructions! When they are set up to your tastes what ever that may be, there is no better tool for the job. They are THE scientific standard for good reason.
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Old 06-12-2012, 03:03 PM   #11
R90pilot OP
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I checked all my external items and all seems well so I went on to my carbs, I let my Dad rebuild them for me while I was doing paint work and he left out a few pieces in the middle of the carbs. The atomizer and the other jet thing it goes with. My needle would just flop all around. I am going to get those items installed and try again
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