Fellow Inmates, help a newbie get his bike running good, pleasssee.

[photorider]

Hmmmm...no. Different set-up. Disregard.

[headtube]

Originally Posted by Yachtie

the bike like before starts bogging/coughing back firing and wanting to cut out.

Fuel flow?? When the symptoms you describe begin to occur, stop the bike and drop the float bowls. Check the level of petrol in the bowl. It should be 3/4 full, or there abouts.

Question: does it bog and cough straight up or in corners?

Things to look for... (1. Be sure your gas cap is venting (breathing). You can try running the bike with the gas cap not fully closed to see if this betters your situation. (2. When the bike is NOT running - open the taps on the tank without the fuel line attached. Catch the fuel in a vessel... look for bits. Fuel should flow undisturbed. If there is a trickle or bits, you've got some cleaning to do.

If this doesn't better your problem there is a possibility that you have an electrical short that you need to track down.

[Yachtie]

Originally Posted by headtube

Fuel flow?? When the symptoms you describe begin to occur, stop the bike and drop the float bowls. Check the level of petrol in the bowl. It should be 3/4 full, or there abouts.

Question: does it bog and cough straight up or in corners?

Things to look for... (1. Be sure your gas cap is venting (breathing). You can try running the bike with the gas cap not fully closed to see if this betters your situation. (2. When the bike is NOT running - open the taps on the tank without the fuel line attached. Catch the fuel in a vessel... look for bits. Fuel should flow undisturbed. If there is a trickle or bits, you've got some cleaning to do.

If this doesn't better your problem there is a possibility that you have an electrical short that you need to track down.

thanks, i'll try that. Symptoms occur roughly 20 minutes after a start up where it sat for a while. The 20 minutes right after start up the bike runs Good. Then any time i give it any kind of throttle is starts all the symptoms and just progressively gets worse and worse to where its trying to cut out just in idle as well.

[Beamer Bum]

I've been following this with interest and now have to put my 2 cents in. If the bike runs good for 20 minutes before it starts acting up, you either have a heat issue like an overheating coil or, like Headtube suggested, your tank isn't venting to atmosphere. When it starts running crappy, open the fuel cap. If the tank develops a vacuum, you won't get fuel to the carbs. If you can hear the air rush in when you open the cap, it isn't venting. I think I saw you have new coils, so my guess (only a guess since I'm not there) is tank venting.

Don't worry with the collective wisdom here, you WILL get this sorted.

[disston]

Bike is a 1976 R90/6. The /6 bikes don't have this tank venting issue I think. I have a 1975 one and I don't know how the tanks are vented but have never heard of this issue.

It has had a lot of modifications including an electronic crank fired ignition.

[Yachtie]

Originally Posted by disston

Bike is a 1976 R90/6. The /6 bikes don't have this tank venting issue I think. I have a 1975 one and I don't know how the tanks are vented but have never heard of this issue.

It has had a lot of modifications including an electronic crank fired ignition.

Important to note, i have a R100/7 Tank.

[rufusswan]

I don't think anyone answered your question about the cable adjuster on top of the carb. In the position shown in the last photo, with the adjuter screwed all the way down into the carb, you have maximum amount of slack in the cable.

To take slack out of the cable, start screwing the adjuster out ( or up ) to remove slack.

[Yachtie]

Originally Posted by rufusswan

I don't think anyone answered your question about the cable adjuster on top of the carb. In the position shown in the last photo, with the adjuter screwed all the way down into the carb, you have maximum amount of slack in the cable.

To take slack out of the cable, start screwing the adjuster out ( or up ) to remove slack.

thanks for that!

[photomd]

If the engine is running properly for 20 minutes, and you have some valve clatter, I don't think think it's the valves. You've got compression and fuel. That's why it'll start and run for awhile. Something is changing as it gets warm that causes it to run rough. I'd concentrate on what can change as the motor gets warm: carbs, carb boots, your new ignition, fuel flow and coils.

First I'd get it warm and get it back firing and spitting. Then I'd try to figure out if it's running rich or lean: do you smell unburned fuel. If so, pull the air filters and see if it runs better. If you don't smell fuel, turn the choke on. Better with choke on? Pulling the air filter will lean out the mixture and turing on the choke will increase fuel into the engine.

Also check your diaphragms carefully. Pulling them out and carefully stretch the rubber looking for any pin holes or tears. If you find any or the rubber is hard, replace the diaphragm and recheck. I've had a bike run exactly like this: run fine cold and progressively not be able to take any throttle as it got worm and it turned out to be a hole in an old diaphragm.

Next I'd test for air leaks at the carbs: take WD-40 and spray it around the junctions at the front and rear of each carb and air box junction. If the idle changes, you've got an air leak. Fix it and recheck. Also spray around the diaphragm top and where the butterfly shaft is located on the carbs. These can leak too.

To test the ignition, simply remove the new one and put the old one back on. Some electronic ignitions don't work and you need to check the engine with a good one. If this fixes the problem, get a new electronic ignition. Your timing could be off, but it should run a little rough cold as well as hot with incorrect timing. Also I don't know if your new ignition has a mechanical advance or electronic. Again, mechanical one will not change going from cold to hot. An electronic one can fail as it gets hot if there's an intermittant short the opens with heat.

Test the fuel flow is easy. Ride for 20 minutes and get it running rough, crack the fuel fill and see if it runs better. All these bikes have some sort of fuel vent. You've got to replace the fuel in your tank with air or it won't flow fuel to the carbs.

As for the coils, replace them with the old ones and see if the problem is corrected. Also make sure your ignition leads are the correct ones for the bike. Some run resistence leads, some don't. I don't keep that in my brain. You need a manual to help get things set up correctly.

Do these things and let us know what you find. It sounds like a fun project.

[disston]

All good ideas. I still think that the proper adjustment of the carbs can be at fault. I'm not sure about this. Just think you haven't gotten them right yet. And these bikes always run weird after twenty mins if the carbs aren't properly balanced.

Do you have the timing light? You said you would move to checking the timing. That's a good one to get out of the way.

Do /7 tanks have vents? It might say in the manual. Do you have a manual?

I wanted to add my small bit about the cable adjusters. Throttle cables are manufactured to a certain standard so they match and will fit the application they were made for. Those standards are not an exact measure so some inner cables are a little longer or shorter, some outer cables are not exact also. The cable adjusters are here to make up for this little difference found. There should always be some bit of slack in the cables. Don't confuse with slack in the twist grip, that's there also but the cable should not pull the carb throttle while the bike is at idle. Idle is effected and adjust only by the big screw on the bottom of the carb. When you twist your wrist the cables take over and their balance comes into play.

In theory the cables should act at the same time, they should have the exact same amount of free play. In practice this can be hard to see, or achieve. Sometimes one cable will show a definite movement when the throttle is twisted and the other cable will seem just soft.

But check the timing and the tank vent(?) and then you'll be back at carbs again.

[Yachtie]

Originally Posted by disston

All good ideas. I still think that the proper adjustment of the carbs can be at fault. I'm not sure about this. Just think you haven't gotten them right yet. And these bikes always run weird after twenty mins if the carbs aren't properly balanced.

Do you have the timing light? You said you would move to checking the timing. That's a good one to get out of the way.

Do /7 tanks have vents? It might say in the manual. Do you have a manual?

I wanted to add my small bit about the cable adjusters. Throttle cables are manufactured to a certain standard so they match and will fit the application they were made for. Those standards are not an exact measure so some inner cables are a little longer or shorter, some outer cables are not exact also. The cable adjusters are here to make up for this little difference found. There should always be some bit of slack in the cables. Don't confuse with slack in the twist grip, that's there also but the cable should not pull the carb throttle while the bike is at idle. Idle is effected and adjust only by the big screw on the bottom of the carb. When you twist your wrist the cables take over and their balance comes into play.

In theory the cables should act at the same time, they should have the exact same amount of free play. In practice this can be hard to see, or achieve. Sometimes one cable will show a definite movement when the throttle is twisted and the other cable will seem just soft.

But check the timing and the tank vent(?) and then you'll be back at carbs again.

Fuel vent wasn't it. Ran it without the cap and same thing. 20 minutes of good riding and then put put cough cough boom.

[Yachtie]

Where can I find the diaphragms?

[disston]

It is part #11. They are large rubber things under the top cover of the carb. If you haven't had the top covers off yet be careful with it. Use a good proper fitting screwdriver. Anti-seize the screws when you put them back together. You may consider new screws if the ones you have are beginning to show their age. It is best to have the carbs off to take the tops off.

Once you get the slides out hold them up to a strong light and stretch them. You are looking for holes in the rubber.

I'm sorry but I don't think poor running after twenty mins is a symptom of diaphragim problems. Oh, and I don't know how well you will do from other sources but I found one that was almost $40 each.

[headtube]

Originally Posted by Yachtie

Where can I find the diaphragms?

At your BMW Motorrad dealership. If you don't know about this, you should. It's a good resource. Make it your friend.

Also this... Bing. I would suggest that you purchase the Bing carb rebuild DVD. A good investment. Seeing how things are done makes it sooo much easier. Also... grab yourself a workshop manual. Make it your bible.

[photomd]

Originally Posted by Yachtie

Where can I find the diaphragms?

Look at your carbs. The round, dome part on top holds the diaphragm. Undo the 4 screws SLOWLY. There is a spring under there that will push the top off. I usually hold the carb top down with one hand and remove the 4 screws with the other. Then slowly let the spring push the top up and remove it. Look at the diaphragm and how it fits in the carb body. The outer lip should seat in a grove and the diaphragm will have an indexing mark on one side that will fit in a corresponding grove. Make sure you see this, you'll need this info to reassemble correctly. Other than that, lift it out and take a look.