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

I just had the carbs professionally overhauled after a long lay up and the machine ran well. I synched the carbs using CarbTune and the 4,000rpm sweet spot was there. However, gas mileage was very poor at 22mpg. I am used to getting twice that. The bike started easily and ran well at all revs/gears.

So first I checked the valve clearances and they were correct.

I reattached the alternator cover (I use the alternator bolt and an Allen wrench to turn the motor with the spark plugs removed) and no ignition. Starter works fine. Checking, I noted that I had managed to pinch the cable from the points to the condenser; the rubber sheath had slipped off its mounting while I was securing the cover allowing the yellow sheathed points cable to be pinched and completely severed when the alternator cover bolts were tightened. The battery was disconnected at the time I pinched and cut this wire so no short should have resulted. The cut inner wire remained partially sheathed by the yellow jacket and, I assume, insulated from the chassis.

Diode board is a Thunderchild.

I then replaced the points with a new BMW OEM set, and replaced the condenser with a new one just because I had it handy.

Then I checked the points gap - also correct.

I can get the bike to start - the starter works fine - but regardless of how I set the timing throughout the available range it will stall once I open the throttle, generally after a backfire or two. What can have caused this when all I did was check the valve clearances and replaced the points and condenser?

Any help would be appreciated.

Thomas
1975 R90/6 - second owner since 1990.

Thomas -

Guess we'll have to provide some thoughts and you can check through things. My first thought was what about air leaks. You mention that it backfires as you add throttle. Try some aerosol like spray brake clean around the connections of the carb between carb and head...also the carbs themselves. See if that causes the idle speed to change...indicating an air leak.

If everything was fine prior to having to replace your points, I suspect it's a combination of your points gap and timing. Is the backfire coming from the exhaust or carb? Timing overall (combination of points gap and timing setting), may be retarded causing the backfire (?). Good luck.

Glad that at least your motor starts and that you noticed the condesor wire problem. LOTS OF PEOPLE DONT and then have a real mystery on their hand after they button up the front cover. Anyway, for me, it's a practice that I believe most mechanics follow, to set the timing after a points adjustment/install. Did you? The symptoms you describe are clearly those of timing/points issues. Perhaps you set the timing and just didn't mention it. If so, then we have a whole different can of worms.........Good luck

Not air leaks. Nothing changed there from a well running motor. Not a variable.

I cannot get good running at ANY timing setting and the points gap is correct.

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

THOMASPIN said:

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

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Not likely. The diode board is designed to charge the battery. The bike can be run on a battery only if the diode board fails.

Usually the process is to look back at what was recently done. The most recent issue was the pinched wire and the replacement of the condenser. How was the wire fixed? It was completely severed right? If the new wire came with the new condenser then that's probably not the issue. The second issue is the carbs. It would seem that one of those two are the places to look.

Thomas..........Sorry I missed in your original post that you had timed the engine.............YOU ARE POSITIVE that the points are exactly right on and it times properly...........Then the ignition system is not at fault as far as the points, timing, condenser goes. You can button issue up.

Valves are set properly

It now starts but backfires when shutting down RPMs and stalls when variable throttle is introduced.

Previously 22 mpg AFTER carb rebuild.............but ran

I agree with Kurt to look at what was previously done. The only thing left to check if ignition is right on setting wise is the coils and wiring under the tank.

What was previously done was the carbs and at the low mileage you were getting there was something going on there..............

Turn off the lights in the garage, get the engine running with the little gas in the fuel bowls and tank off, and see if you see any sparks going on around the coils. I know it sounds strange but its an easy way to check coils............Then after you make sure all the ignition components are correctly plugged in and tight there under the tank, all that is left is perhaps switch coils...........HOW TO CHECK THE COILS THEMSELVES?????????????????

Next would be the carbs but make sure the ignition system is all perfect. YOU ARE ALMOST THERE and it sounds like you are doing good work.......God bless......Dennis

New parts have been known to be defective

If it started after a new set of points was installed with gap & timing set as before I would speculate the new points or "condenser" may be the source of your frustration. Put the former condenser back in and then if its the same reinstall the points with the damaged wire but leave the front cover off. See how it runs, if it is good order another points & condenser set.

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?

Diode board is a Thunderchild. They would certainly be the experts for how a malfunctioning diode board would affect the bike.

Is it possible that the carb choke/enrichener mechanism is reversed or not set up right? Those parts are handed, meaning they have a left and right side...not to be mixed up. If the choke were essentially on all the time, that could explain the poor performance.

THOMASPIN said:

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

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There is absolutely nothing in the symptoms you describe that would come from a defect in the diode board except and unless you were riding down the road or otherwise running the engine and it quit and would not crank to restart due to a dead battery.

The diode board is the heart of the charging system. It is the rectifier that changes the alternating current from the alaternator to direct current to charge the battery. That is all it does. It has nothing to do (obviously) with air or fuel delivery, and nothing to do with the ignition system except keep the battery charged and provide 12v DC to the system while the engine is running.

Could be the centrifugal advance is stuck or advancing too soon.

I can guaranty that the diode board has no effect what so ever on the function of the ignition system. It's only purpose is to convert alternator output (AC) into direct current (DC) which your battery can absorb to remain fully charged. The only effect of a failed diode board is to allow the battery to slowly become discharged while the bike is running.

By the way, Paul Glaves IS the true expert.

Thank you Paul and James. I can confirm the Yuasa gel cell battery is fully charged using a BMW branded gel cell specific trickle charger.

As for the other point, why would the centrifugal advance, which is corrosion free and lubricated with the correct Bosch grease, suddenly cease functioning? Where is the logic behind that statement?

The process of elimination does rather point to the timing, but it defeats me why things suddenly went so awry when all I did was change the points/wire assembly.

I'm increasingly thinking there is a Honda in my future after 26 years of constant fixit with the airhead. The romance of repair work seems to be fading in inverse proportion to my age, nice as the machine is to ride.

Sorry that this has happened. Often times, one has to "get out of the car and get back in" to figure these things out. Clearly, something has gone wrong...these things don't just go wrong all by themselves.

So, I'm guessing that prior to the carb overhaul, the bike was running fine and you were getting gas mileage into the 40s. Then the carbs were overhauled and you did a carb synch. Over the course of some time, you noticed the gas mileage was 22 mpg. Sounds like except for that, the bike started and ran just fine...albeit poor gas mileage.

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.

A friend had the same problem on his R100. It took awhile, but we found the problem while checking the points while running the engine when the front cover was off. We saw sparking coming from the points, the weights were swinging out as they were suppose to, but they were also touching the lead to the points because the wire was improperly installed.

The logic is you need to check everything you touched, Snowbum has a whole page devoted to the springs on the advance mechanism, if they are not right the bike barely runs believe me I know. Runs at idle then craps out sounds like its not advancing. I have spent hours on the side of the road trying to get my R90S to run because the axis of evil aka. points, condenser, advance to work as well as trying to get the cover back on without mashing the wire. You sound like the meticulous type but sh*t happens when you take 40 year old parts off a bike and put it back together. But the whole running like crap thing sounds like ignition to me especially since thats what you were playing with, also of all the parts, condensers are the weakest, its possible you got two bad ones.

Not to whip a dead horse but a true story about new points & condeser

About 20 years ago one of my friends /2's developed a knock in the motor after an evening of hard riding and sidecar antics. He removed and sold the side car and went about trying to diagnose and cure the knock in the motor. Well he had no luck and didn't want to pull it apart and send the motor out for a lower end rebuild which all who knew of the problem were sure that it was a lower end rod knock caused by oil starvation. Fast forward to about 2011 and I finally convinced him to let me handle the lower end rebuild and crank shaft refresh. Upon reassembly of the motor and installation I got that phone call from him that upon restart the motor still had the same knock, he installed a completely different and good top end including cylinders, pistons, rings, heads, carbs and included check and reset points and timing as needed. The motor still had what sounded just like a rod knock when it came up off idle and settled back down.

Well we both lost sleep for about three days when he finally called me so I could hear the now very quiet and gentle loping of an an R 60/2. He had woke up at 2 am that morning and decided to replace the points and condenser that he had installed just prior to the knocking noise announcing itself. The $2,500 and 15 year saga of a bad set of new points and condenser.

69zeff65 said:

About 20 years ago one of my friends /2's developed a knock in the motor after an evening of hard riding and sidecar antics. He removed and sold the side car and went about trying to diagnose and cure the knock in the motor. Well he had no luck and didn't want to pull it apart and send the motor out for a lower end rebuild which all who knew of the problem were sure that it was a lower end rod knock caused by oil starvation. Fast forward to about 2011 and I finally convinced him to let me handle the lower end rebuild and crank shaft refresh. Upon reassembly of the motor and installation I got that phone call from him that upon restart the motor still had the same knock, he installed a completely different and good top end including cylinders, pistons, rings, heads, carbs and included check and reset points and timing as needed. The motor still had what sounded just like a rod knock when it came up off idle and settled back down.

Well we both lost sleep for about three days when he finally called me so I could hear the now very quiet and gentle loping of an an R 60/2. He had woke up at 2 am that morning and decided to replace the points and condenser that he had installed just prior to the knocking noise announcing itself. The $2,500 and 15 year saga of a bad set of new points and condenser.

Click to expand...

I also had trouble with a short that caused a miss. It was between the point lead and advance mechanism. They were new points I bought from Capital Cycle and found it running the engine in a dark garage without the front cover. I could hear it but couldn't see it. I was able to bend the lead and that fixed it. Capital Cycle said they'd never heard of the problem before. Didn't save anything there.