R90/6 1975 - cannot restore smooth running after replacing points

Had a similar problem with my /5and it was a bad condenser.

20774 said:

Then you checked the valves...they were OK. As part of the valve clearance check (for which you had the alternator cover off), when the cover was reinstalled, the points wire was pinched/severed. That has been replaced. Along with new points...even the older points hasn't cured the problem.

So, the 22 mpg problem is still there because you're troubleshooting the tendency of the bike to stutter and backfire. No real road time with the bike because of this problem.

Since you've changed the points, condenser, and are quite sure the timing is correct, I just can't get past the carb issues with 22 mpg. Seems like running the bike that rich for a period of time can't do the carbs any good let alone the engine with a super rich mixture in there, possibly fouling the plugs.

I'm not seeing anything wrong with the electrics...I'm wondering about the carbs.

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I agree. I'm an amateur wrench, but when I got my current bike 3 years ago- it was getting 22mpg. (1974 R75/6) I had the heads redone and hardend valve seats put in, and I re-built the carbs myself with all new everything. Then I put it back together. It was still getting 22mpg. I talked to one of the pros at Max BMW and he told me to drop the needles in the carbs. Problem solved! Mileage immediately went up to 40 and the bikes been great ever since. I would double check the needle position in your carbs referencing the Bing book.

katcon85 said:

I agree. I'm an amateur wrench, but when I got my current bike 3 years ago- it was getting 22mpg. (1974 R75/6) I had the heads redone and hardend valve seats put in, and I re-built the carbs myself with all new everything. Then I put it back together. It was still getting 22mpg. I talked to one of the pros at Max BMW and he told me to drop the needles in the carbs. Problem solved! Mileage immediately went up to 40 and the bikes been great ever since. I would double check the needle position in your carbs referencing the Bing book.

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Beg, borrow, steal or buy the Bing Manual, and then verify that the idle jet, needle jet, main jet and needle are correct for your carbs. Then set the fuel level in the bowls precisely. Then verify that the diaphrams are sound (not spongy or torn). Then see what you get for mileage.

Unless Airhead carbs are verified to be correct it is impossible to even guess what some prior owner or tech might have done.

Turns out that the new 'BMW' points I had installed - they had languished in the tool tray for maybe 15 years - were bad. So much for my failsafe philosophy. I trailered the machine to my local Airhead expert in Scottsdale where Omar immediately diagnosed the error to the Chinese points. While these had come from a BMW dealer all those years ago, complete with BMW box and part number, Omar advised me that there was a period where the factory subcontracted manufacture to the Chinese. He showed me a box full of these appropriately marked 'Chinese cr*p - do not use'.

This is what happens when accountants take over from engineers. Some green eyeshade maven determined that 3 cents could be saved by outsourcing manufacture, successfully trashing brand equity for a few pennies on the bottom line.

New points fitted she runs beautifully again and mpg is back to 45. Yes, I bought a spare set of points while at it. For a moment there I had lost all faith in my wrenching abilities.

I recommend Omar at MotoGhost if you are in the Phoenix area as well as Dave Alquist in Mesa who overhauled the carbs. Both excellent places for Airheads.

Of course, right after I decided that the Honda could wait, the tach started screaming at me, courtesy of a stripped gear in the mechanism, so it's off to Palo Alto Speedometer, a shop well known to anyone cursed with VDO or Motometer instruments. I'm waiting to get paid for the spare kidney I donated before getting the repaired tach back .... appropriately enough the image displays upside down:

If it's not one thing, it's another!! Now, maybe we can turn that thread icon from a thumbs down to a thumbs up??

Glad you've been able to sort through things.

Tach Rebuild

You might try "Wirespokes" for the tach. He is an inmate and I have never heard anything but good about him. Might have a bit of a backlog. I understand his prices are much more reasonable.

I've been working lots of OT, sorry I couldn't contribute earlier.

Old thread > https://forums.bmwmoa.org/showthread.php?68332-Timing-and-points-problem-1976-R90-6-not-starting

NORIS


Chinese

THOMASPIN said:

Tried two new sets of points. No difference. Same symptoms with old and new condenser.

Yes, I am absolutely certain that the points gap is right. Double checked. I use the Paul Tavenier tool, as I did before this issue, and the gap checks. Paul's tool requires the advance mechanism be removed, the tool is inserted emulating the maximum points opening position and the points are adjusted using a feeler gauge. Far more accessible than with the advance unit in place.

Repeat Q: Could damage - if any - to the diode board cause my symptoms?

If it ran well before - albeit using too much gas - and refuses to run for more than a few seconds now (so I cannot do the coil test described) I'm inclined to suspect the diode board. But that rather defeats logic as I understand the function of the diode board is to keep the battery charged by rectifying AC from the alternator to DC for battery charging. Or does the diode board feed current to the coils for spark? If the latter then the diode board is the only additional variable I can think of.

Any <b>diode board experts</b> out there?

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Both new sets of points were Chinese crap?

Yes. Noris is the way to go, not Chinese.

Carb overhaul

I tend at this time to agree with Kurt as to the problem being with the carbs. I write this assuming everything has been checked and rechecked (as you say you have) with the ignition. You have also checked and set the valves.

So, what does that leave, the carbs. Sorry to say, sometimes even professionals make mistakes doing things. I think Kurt makes a good suggestion to check the chokes. Now I am going to make a suggestion that you check your choke cable settings to insure the chokes are in fact "off" when you pull the lever to turn them off. A simple mistake but if they are not fully off, it will cause poor running and poor gas mileage. If the choke cables are properly adjusted, it is possible as Kurt suggests they are installed or rebuilt improperly.


All in all, there are a bunch of things which can go wrong in carbs if they are not rebuilt properly, If your rebuild shop is local, I would take the matter up with them. I would assume they would stand behind their work and make right any problems with the carbs.
I myself would call them and talk to them regarding this. There is something not right with the carbs if you have the chokes off and it runs like junk.

Of course, one other thing ignition wise, do you have a bad spark plug wire? Again, I assume you have checked to see if you are running on both cylinders and one is not misfiring.

Sorry I can't be of more help, if you are near Rochester NY, I would be happy to give you a hand. Good luck. St.

Second page

OKAY, so I have done it again, I have not read the posts properly. LOL, I am glad the problem was solved. Believe it or not, I was going to mention the Chinese point problem but it seemed like it happened so long ago I assumed by now everyone had purged their stock and tool boxes of them.

Yep, they caused problems for a lot of people.

My last thought, Check with Rick at Motorrad Electrik, get rid of the points. There are a couple of ways to do it, I used the Boyer system on my 78RS and since it was installed, I have never had to adjust timing or mess with changing points. Rick sells a never version closer to what is now on the newer airheads. I have the electronic ignition on my 84RT and it lasted 200k miles without any problems. The only time I had to adjust anything was ignition timing after replacing a timing chain and removing the timing chain cover to replace a leaking seal in the front. Other than one or two spark plug wires conking out, I have not had a problem. One less thing to mess with.

Thanks for letting us all know what you found. St.